Motor Theory

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Here is a couple of Articles some may find interesting. They are written by David Reher from Reher-Morrisson racing engines. He writes for National Dragster each month. They are about car engines, but it does relate to bikes, if you think about it.


Summertime Blues: How to Live with Bad Air

Written by David Reher

As I write these words, we’re loading our Speedco Pontiac Grand Am into the transporter for the annual trek to Denver. Preparing to race at a mile above sea level brings home the reality of racing under adverse conditions. While we make many adjustments for Bandimere Speedway, one of the items that is definitely not on our to-do list is to change carburetor jets.

Some racers just about wear out the threads on their carburetors trying to adjust for altitude and weather conditions. The truth is that you really can’t compensate for bad air by changing jets. When it comes to carburetor-equipped racing engines, you can’t fight Mother Nature.

It is a misconception that you must lean out a carburetor at high altitude. The fact is that a properly tuned engine will use the same jets in Denver as it does at sea level.

So why do cars run so much slower in bad air than they do in good air? The obvious answer is that the engine is making less power. When an engine is tested on a dyno, a correction factor is applied to the raw numbers to adjust the observed power to standard conditions. This allows us to compare the dyno test results that are made at different times of the year and under very different conditions. But when you are running a car down a race track, the correction factor is irrelevant. The only power that is available to accelerate the car is the engine’s actual output at that particular moment in time. If the engine is producing more or less power than it would at standard conditions, that’s what you’ve got to work with.

In this age of digital everything, carburetors have an undeserved reputation as low-tech devices. In fact, a racing carburetor is a very ingenious system. A carburetor responds to differential pressure, and therefore it self-compensates for changes in barometric pressure. The gas in the float bowl is always subject to the prevailing atmospheric pressure; the jets deliver fuel in proportion to the differential between the pressure in the float bowl and the pressure in the induction system. So when the barometric pressure falls, as it does so dramatically in Denver, there is less pressure differential and therefore fuel flow is reduced accordingly.

You don’t have to go to Bandimere to experience the effects of thin air. Even if you don’t travel, the changes in your race car’s performance at your local track from February to August will be substantial. On a typical summer day with 90-degree heat, the relative altitude can easily approach 4,000 feet. The unfortunate fact is that there is very little you can do to regain the missing horsepower by tuning the engine. While you might have a zero correction factor in January, it’s common to see a correction factor approaching eight percent in the summertime – and in Denver, we see 22 percent!

The harsh truth is that you’ve got a car with less horsepower in the summer, so you must figure out how to race it. What can you work on? You can work on the car – the torque convertor or clutch, the transmission ratios, the rearend gears, the tires, and the chassis – to work around the power deficiency. That’s really what we do in Pro Stock, and that’s why you seldom see Pro racers working on engines at the track aside from routine maintenance. We simply take the power we’ve got and try to make our cars use it as efficiently as possible.
In general, drag racers tend to be more engine-oriented than racers in other forms of motorsports. Perhaps that is because we spend relatively little time on the track compared to oval-track and road racing drivers. In my infrequent visits to NASCAR events, I find that most teams regard the engine as a small variable at the track because their engines are developed and tested at the shop. They spend the majority of their track time adjusting the chassis and working on suspension setups. I’ve seen competent teams qualify their Busch Series cars faster than a Nextel Cup car – despite the fact that the Busch Series engines have about 110 horsepower less than the Cup engines!

That simply shows how important the chassis setup is in circle track racing – and points out that drag racers could benefit from spending more time on chassis adjustments and less time on carburetor jets when the weather and altitude conditions are bad. The best place to work on an engine is on a dyno; the best place to work on a car is at a race track. As a professional engine builder, that’s a difficult statement for me to make, but I think it’s the truth.

In reality, the first 1/8th mile pretty well determines a drag race car’s elapsed time. If you’re competing in “go-fast” class such as a Quick 32 or Top Sportsman eliminator, the setup that made you fast in February isn’t going to make you a winner in August. When the relative altitude has changed 3,000 or 4,000 feet, you’re not going to be successful using the same convertor and the same gear ratios that you used in winter.

My recommendation for running in bad air is to target the engine’s rpm range and then make the necessary changes that will allow the engine to achieve that range. A racing engine’s peak power and torque is fixed by the intake manifold’s runner length, the airflow capacity of the ports, the camshaft timing and other factors. The engine is going to perform at its best when it runs at the speed that it was intended to run. Therefore if you go to a high-altitude track, or if you encounter high relative altitude conditions, you have to gear the car to allow the engine to reach its optimum rpm. When you’re missing 300 rpm at the top end due to a change in weather, you must work on regaining the engine speed to maximize performance. Maybe it’s a set of shorter rear tires (if the available traction permits), or perhaps a numerically higher rearend ratio. You still won’t run as fast as you did under ideal conditions, but you will close the gap.

Most of us race to go fast. We love to see a good number on the scoreboard. When the conditions are bad, the e.t.s aren’t as satisfying to our egos, but we can use those times to work on the race car and to learn how it responds. Challenge yourself to regain as much performance as the conditions will allow. The payoff will come when the “good air” returns because your race car will be faster and you will be smarter. Gaining an understanding of how a car responds to different conditions will make you a more formidable racer.

Airflow Fallacies: Avoiding the Pitfalls of the Flow bench


Written by David Reher

“What’s it flow?”


Whenever a conversation about cylinder heads begins with that question, I cringe. I know where this discussion is going, and it’s not good. When a racer wants to distill the performance of a highly developed cylinder head down to a single number, I know I’m dealing with someone who is fixated on the flow bench.


I can speak from hard-earned experience, because there was a time when the flow bench was the center of my universe. When my partners Buddy Morrison and Lee Shepherd constructed our first flow bench in the ’70s, it was a revelation – or so we believed. We were addicted to airflow, and like three flow bench junkies, we convinced ourselves that big flow numbers translated to quicker elapsed times. But that was more than 30 years ago, and since then I’ve learned to avoid the pitfalls of flow bench testing.


Unfortunately many racers coming into the sport haven’t been taught the lessons that Buddy, Lee and I learned the hard way. Cylinder head manufacturers, porting shops, and engine builders constantly advertise flow numbers – and I confess that I’m sometimes guilty as well. In this environment, it’s understandable that some racers think it’s all about maximum airflow. They shop for the biggest cfm number at the lowest price, like finding a screaming bargain on a 52-inch TV at WalMart.


The strategy to win the “Biggest CFM Contest” is simple: Grind the largest port that will physically fit in the head, use the biggest valves that will fit the combustion chambers, and test it on the biggest fixture you can find. That head might win the prize for airflow, but it won’t win on the dyno or on the race track.


The factors that determine the performance of a cylinder head are complex. A head that is ported without considering air speed, the size of the engine, the rpm range, the location of the valves, and a dozen other parameters isn’t going to be the best head, regardless of its peak airflow. And yet I see racers who are seduced by big cfm numbers bolt a pair of 10,000 rpm cylinder heads on a 7,000 rpm short block and then wonder why the engine won’t run.


The most critical area in a competition cylinder head is the valve seat, and the order of importance works its way out from there. There are many questions that are much more important than airflow: How far are the valve heads off the cylinder wall? What’s the ratio of valve size to bore diameter? What’s the ratio of the airflow to the size of the valve? What’s the size of the port, what’s its taper, how high is the short-side radius? The answers to these aren’t as simple as comparing a flow number, but they are what really make a difference in an engine.


Airflow is simply one measurement among many that influence engine performance. With the availability of affordable flow benches and computer simulation programs, it’s easy to fall into the airflow trap. A builder works on a cylinder head, sees some bigger cfm numbers, and keeps working for more flow. But if he doesn’t stop and test the engine on a dyno and on the drag strip, it’s very likely he’s gone down a blind alley. What the manometer on a flow bench sees at a steady 28 inches of depression is not at all what the engine sees in the real world. The pursuit of a big cfm rating has ruined countless cylinder heads in terms of what will actually run on an engine.


I put more faith in dyno pulls and time slips than I do in flow benches. I’ll cite an example from back in the day when Buddy, Lee and I were winning Pro Stock championships. Lee came up with an idea for a tuliped exhaust valve. He filled in the back of the valve with Bondo, and tested the new design on our flow bench. It was killer. We instantly saw a tremendous improvement in airflow with a small exhaust port, a nice tight radius below the seat, and much more stable flow. So we had some titanium tulip exhaust valves made and tested them on the dyno – and the engine didn’t run well at all. We had great airflow on the bench, but the engine didn’t care.


We were working late one night, and Buddy decided to yank the heads off the block and have Lee open up the exhaust throats. Well, Lee kept grinding and Buddy kept taking the heads on and off, and eventually we picked up 30 horsepower that night. We were porting from the dyno and not from the flow bench. When Lee finally flow tested the heads the next day, they were down 30 or 40 cfm, but that’s not what that engine saw.


The final test of a cylinder head is on the track. Frank Iaconio was our chief Pro Stock rival, and he was a smart racer. Frankie used to change valves at the track — he’d make a run, come back to the pits and switch from valves with a 30-degree back angle to a 20-degree back angle. We did similar tests on the dyno, but he did it at the track. I was impressed.


I’m not dismissing flow benches. In fact, we use them daily at Reher-Morrison Racing Engines. But a flow bench is a tool, and it’s really not much different than a micrometer. A micrometer can measure the diameter of a piston, but you have to run the engine to learn the correct piston clearance. Knowing the sizes of the piston and cylinder bore doesn’t tell you if the piston is going to gall or collapse a skirt until you run it. And knowing the airflow of a cylinder head doesn’t tell you whether it will make good power on a given engine until you race it.


Experience is the most important tool in cylinder head development. A person with extensive dyno and track experience has been through it all before, and knows how to avoid the flow bench fallacies.

The CNC Fallacy

Written by David Reher

Every decade has its buzzwords. In the ’30s, it was “streamlining;” in the ’50s, anything “atomic” was cool (except perhaps The Bomb). The ’80s were all about “turbo”, and ’90s were the Digital Decade. For many drag racers, the buzzword for the 21st century is “CNC,” an acronym for Computer Numerical Control.


Some racers have the mistaken belief that a CNC-machined part is superior because it is made by an infallible computer. When you’re spending money on race car parts, it’s important to understand both the promise and the pitfalls of CNC machining.


It seems I can’t pick up a racing newspaper or magazine without seeing “CNC” in advertisements and articles. Everything from motor mounts and throttle linkages to cylinder heads and engine blocks are touted as “CNC machined.” It’s become a stamp of approval. Some racers mistakenly assume that anything that’s CNC machined has to be perfect and therefore is a surefire performance enhancement. Unfortunately, that’s not true.


I’m certainly not an expert on CNC programming. About all I can do is hit the red “STOP” button. But I’ve been around these machines for decades, and I understand their advantages and limitations.


I’m an advocate of CNC machining. Like affordable flow benches and dynamometers, the advent of reasonably priced CNC machines has had a positive impact on racing by making advanced technology readily available. We installed our first CNC machining center at Reher-Morrison Racing Engines in 1988, and currently there are three CNC machines in residence at our shop. CNC machines are excellent “employees” – they don’t call in sick, they’ll work 24 hours a day without a break, and they don’t need medical insurance.


On the other hand, a CNC machine is incredibly stupid. It will do exactly what it’s programmed to do – which is not always what you intend it to do. It will cut right through a port wall or drill a bolt hole in the middle of a combustion chamber if there’s an error in the programming. That’s why we use a chunk of foam instead of an expensive new head casting when we’re testing a new program.


More importantly, a CNC machine can’t distinguish between a good part and a bad part. It doesn’t know whether a cylinder head port is efficient or a combustion chamber is shaped properly. It’s not capable of doing research and development, testing a part on a dyno or running it down a race track to determine how well it works. All it does is machine metal following a prescribed tool path.


Computer programmers have a name for this phenomenon: GIGO, which translates as “Garbage In, Garbage Out.” A CNC machine is just a big, fast tool, without intelligence or reasoning power. A human operator can recognize that it’s a bad thing to cut a cylinder head in half; a CNC machine can’t.


There are many types of CNC machines, from vertical mills and lathes to camshaft grinders and tubing benders. Not all CNC machines are created equal. There are giant CNC machines with 120-foot long gantries that can machine a spar for an aircraft wing from a solid piece of metal. There are CNC machines the size of a toaster oven that make precision subminiature parts. The point is that you wouldn’t machine a cylinder head on a desktop CNC, and you couldn’t economically machine header flanges on CNC mill that’s designed to make landing struts for F16s. Like any machine tool, a CNC machining center must be rigid and powerful enough to do its job, but not so massive and expensive that it can’t be operated profitably. As with any complex device, a CNC machine and its tooling must be serviced and maintained religiously.


There are several types of CNC machining centers. A three-axis machine is like an automated Bridgeport vertical mill: it machines side-to-side (X axis), forward and backward (Y axis) and up and down (Z axis). A four-axis machine adds a rotary feature, and a five-axis CNC has the ability to tilt the machining head. A four-axis CNC works well for machining relatively simple parts like exhaust port plates and carburetor spacers; a fifth axis is a necessity to machine complex shapes like cylinder head ports. We currently have one four-axis CNC machine and a pair of five-axis CNCs in our shop.


A CNC machine has the ability to make dozens, hundreds or thousands of identical parts. Unfortunately, if the original design is flawed, it repeats the same mistake over and over again with blinding speed. For example, I’ve seen CNC-machined connecting rods that have exposed bolt threads in the forks; these threads are an open invitation to stress risers and potential catastrophic failure. The CNC machine did exactly what it was programmed to do: it left exposed threads all the way through the bolt holes. Just because these rods were manufactured on a CNC machine didn’t make them right.


It’s a relatively simple process to digitize a cylinder head port and then duplicate the runner shape with a CNC machine. But if the original port shape isn’t good, the result is a series of identical and equally inferior clones. I’ve seen CNC-machined cylinder ports runners with sharp intersections and nasty tool marks. The ports may have been machined under computer control, but an experienced head porter with a hand grinder could have done a better job.


What’s important is not whether a part is CNC machined, but how well it performs. Whether it’s a washing machine, a ratchet or a competition cylinder head, the best buy is always the product with the highest value, not the cheapest price.

Raisin the Redline: Why RPM Matters


Written by David Reher

Looking back at the 2004 season, I can attribute much of the performance improvement in Pro Stock to faster engine speeds. It’s difficult to believe that 500cid Pro Stock engines now routinely turn 10,000 rpm, but the truth is plain to see on the data recorders and on the time slips.

The trend toward higher and higher engine speeds was also evident in NASCAR stock car racing until the rulemakers applied the brakes with new restrictions on rearend and transmission gear ratios. Now the growing interest in fast bracket racing, Top Sportsman, and Top Comp eliminators is bringing this same high-rpm technology to sportsman drag racers.

Why does turning an engine higher make a race car run faster? This is my final column of the year, so I’ll offer my ideas and hope that they give racers something to think about over the winter break.

The simple explanation is that raising rpm effectively increases an engine’s displacement. This might seem nonsensical because the volume displaced by the pistons doesn’t change, but consider the effects of filling and emptying the cylinders faster in real time. An internal combustion engine is an air pump, and if we turn that pump faster, we can theoretically burn more fuel in a given amount of time and consequently produce more power. For example, an eight-cylinder engine running at 6,000 rpm fires its cylinders 24,000 times in one minute (assuming perfect combustion). Increase the engine’s speed to 8,000 rpm and it will fire 32,000 times per minute, a 33 percent increase. The volume of air and fuel that moves through the engine is now equivalent to an engine with a much larger displacement. There are also 8,000 additional power pulses per minute transmitted to the crankshaft that can be harnessed to turn the wheels and accelerate the car.

Raising engine speed is analogous to supercharging or turbocharging a motor; the goal is to increase the volume of air and fuel that moves through the engine. The airflow is increased with a forced induction system by pressurizing the intake system; in a naturally aspirated engine, the airflow is increased by raising rpm. If done correctly, both approaches will increase power.

A higher revving engine also permits the use of a numerically higher gear ratio to multiply the engine’s torque all the way down the drag strip. Let’s say an engine that produces 1,000 horsepower at 7,000 rpm is paired with a 4.56:1 rearend gear ratio. If this engine is then modified to produce 1,000 horsepower at 8,000 rpm, it can now pull a 4.88:1 or 5:14:1 rearend gear without running out of rpm before reaching the finish line. The numerically higher gear ratio gives the engine a mechanical advantage by multiplying its torque by a greater number to accelerate the car faster – in effect, it has a longer lever to move the mass.

I learned this lesson many years ago when I started drag racing. I raced my little 302cid Camaro against 426 Hemis and 440cid Max Wedge Mopars. The big-inch engines had thunderous low-end power, but my high-revving 302 with a 4.88:1 rear gear would just kill them because they were all done at 5,800 rpm. My small-block had much less torque and horsepower, but I could multiply the power it had with a steeper gear ratio. The same principle applies to racing a Pro Stock or a Top Sportsman dragster. By turning more rpm, we can use a greater gear ratio to produce more mechanical advantage to accelerate the car.

There are limits to engine speed, of course. Higher rpm increases parasitic losses from friction and windage. The stability of the valvetrain also restricts engine rpm. However, with the technology developed in NASCAR and in Pro Stock, racers are learning how to build engines that operate reliably at high rpm. Research and development on valve materials, springs, rocker arms, and pushrods are now being applied to serious sportsman drag racing engines. In fact, I wish that I had some of the parts that we now install in our high-horsepower sportsman engines for our Pro Stock program a few years ago!

While increasing rpm is generally a good thing for a racing engine, it also puts more responsibility on the owner. A high-rpm combination requires more vigilance and more maintenance than a low-rpm motor. It’s important to check the valve lash frequently and to look for early warning signs such as weak or broken valve springs. Neglecting these parts in a high-rpm racing engine can produce some very expensive problems.

Raising an engine’s operating range also requires complementary changes in the drivetrain and chassis. A high-rpm sportsman engine really needs a high-stall torque converter to realize its potential. With an automatic transmission, the engine speed should ideally drop 1,000 to 1,300 rpm after a gear change. For example, if the converter stalls at 6,700 rpm, the engine should be shifted at around 8,000 rpm. Shifting this engine at 7,000 rpm would simply put the engine back on the converter, causing the converter to operate inefficiently and wasting horsepower by heating the transmission fluid.

I’m excited about the emerging trend toward fast sportsman drag racing. I enjoy working with customers who want to go fast because it gives me an opportunity to deliver the benefits of our Pro Stock R&D to other racers. Not every racer wants or needs a high-rpm engine, but if the goal is to have a fast car, raising the redline is a proven approach

The Fundamental Things Apply


Written by David Reher

Sometimes technology can go too far. I recently purchased a cell phone that came with an instruction manual as thick as a brick. I use a telephone for one reason: to make calls. I didn’t buy a cell phone to play video games, take fuzzy photographs, download disco music, get directions to my house or monitor the stock market.

I know how to punch in a phone number, so I threw away the manual. I do not regret for a second that I am using only one percent of the capabilities of this technological marvel.

Sometimes racing engines turn into the four-stroke equivalents of cell phones. Racers who are dazzled by the latest gadgets, tricks and technology can lose sight of the basics. The purpose of a racing engine is to produce maximum power efficiently and reliably within the limits of the rules. Period.

An engine is an induction system, cylinder heads, a short block and a valvetrain. It doesn’t matter whether the engine competes in Formula One, Indy cars, NASCAR, or drag racing. If I’m talking to an engine builder in another form of racing, those are the parts that we talk about. All of the high-tech electronics, exotic materials and expensive trinkets are meaningless if an engine isn’t built on a solid foundation.

There are certain fundamental truths about racing engines. The first is that the cylinders must be sealed. That seems simple, but it is difficult to achieve. Engine simulation programs that boldly predict a certain level of performance assume that the cylinders are properly sealed, but that is sometimes an unwarranted assumption. It’s much more important to optimize cylinder sealing than to worry about 10 cfm of airflow.

A cylinder hone may not seem very sophisticated compared to a computer-controlled flow bench or dynamometer, but it is an essential tool of engine building. It takes years of experience and decades of data to learn how to hone for maximum power. Savvy engine builders have information on block hardness that extends back to the days of production castings. They know the different honing techniques that are required for grey iron, compacted graphite and ductile alloys. They recognize that ring material and profile make a huge difference when honing.

Oil ring tension does not necessarily determine whether a racing engine has powder-dry exhaust ports or black, oily ports. A dry engine is the result of honing the cylinders correctly and selecting the best ring package for the application. Of course, ring tension can be increased to the point that it will cover up an incorrect hone, but the motor will barely turn over, and it certainly won’t make good power.

The quality of the pistons is also crucial to cylinder sealing. I scrutinize ring grooves carefully because I know that not all pistons are created equal. Machining a first-rate piston begins with the blueprint; are the ring grooves located properly so that the lands are thick enough to support the rings? I’ve seen too many “trick” pistons with the ring package squeezed into a narrow space that’s dictated by the piston pin height. If the land under the top ring deflects and breaks the cylinder seal, then the engine simply can’t perform to its potential.

Exotic lubricants and expensive coatings are a waste of money if the short block clearances aren’t right. When the bearing and piston clearances are wrong, the parts are going to fail. Before spending big money on costly treatments, invest in a good set of micrometers and make sure that the clearances are correct.

The most important characteristic in a cylinder head is the ratio of the throat size to the valve size. That’s never mentioned in ads and articles that focus on flow numbers, and it’s not even considered in engine simulation programs. The fact that a port moves a certain amount of dry air in a steady-state flow bench test has only a tenuous connection to real world operating conditions. In a running engine, the flow is constantly in a dynamic state as the valves open and close and the piston rises and falls. The fact that a port flows X cfm at a predetermined depression has little relevance; the true test is whether the port develops a signal quickly in real time as engine rpm increases. And that can’t be measured on any flow bench.

A software program may calculate that an engine needs bigger valves, and the flow bench might confirm that larger valves indeed produce more airflow – but the engine may not care. In fact, it might not even accelerate as well as it did with small valves. Why? Because stuffing bigger valves into a cylinder can pinch off the airflow between the valve heads and the cylinder walls. The valve sizes must be in proportion to the bore diameter. Increasing the diameter of the valve 10 percent to pick up a 5 percent increase in airflow is never a good bargain.

Why does an engine with standard parts run better than one with all of the latest tricks, gizmos and gadgets? It’s usually because the builder took care of the basics with good cylinder seal, correct clearances, and a properly matched induction system. If you want technology for technology’s sake, my advice is to buy a cell phone.

This Guy sometimes, is his fill-in.

The Simulation Situation


Written by Darin Morgan

With four national events in four weeks, including two round trips to Maple Grove Raceway, David Reher has been spending more time sitting on airplanes than on writing his column for National DRAGSTER. David asked me to pinch hit for him in this issue, and since my subject is computer simulation, you might think of this as a simulated David Reher article.

Engine simulation software is a hot topic in the automotive industry. Sophisticated programs such as WAVE and MANDY cost millions of dollars, which limits their customer base to auto manufacturers and Formula 1 teams. For the rest of us, there are engine simulation programs with prices that range from less than $100 to several thousand dollars.

I admit that I’m addicted to engine simulation software. I’ve played with numerous programs, and I’ve spent hours running “what if?” scenarios. But as a professional engine builder, I also understand the limitations of these programs. Unfortunately, some racers don’t.

In my conversations with simulation software designers and code writers, I’ve learned that these programs were never intended to design an entire engine. Rather, their primary objective is to note specific trends of individual changes. In other words, a program can tell you with reasonable accuracy what the likely result will be if you change the bore, stroke or runner length in a Corvette LS1 engine. It can’t tell you how to build a Quick 16 engine from scratch.

My specialty at Reher-Morrison Racing Engines is cylinder head development. I’m frequently asked about equations or formulas that can determine specific engine design criteria. Customers want to know how to calculate the perfect port volume for an engine, how to select the ideal intake manifold, or how to determine the optimum valve diameter for a runner. They want a magic formula that explains how a racing engine works – but such a shortcut simply doesn’t exist.

Consider the Space Shuttle. It’s just an airplane with rocket motors, right? But when you look into the details of launching, flying and recovering the Shuttle, it becomes apparent that this is a task of mind-boggling complexity. The variables are almost infinite.

Comparing the Space Shuttle to a racing engine may seem like a leap, but in fact the underlying variables in an engine combination are equally mind-boggling. If you don’t understand what these variables actually do, it’s tempting to plug numbers into a simulation program until you get the results you want. One likely consequence of this approach is to design an engine that’s not applicable in the real world.

Here’s an example. Recently a racer questioned me about all of the components in one of our Super Series bracket racing engines. I gave him the information he requested, and he modeled the engine with a simulation program. His results were fairly accurate, with an error of about 2.5 percent (20 horsepower), which I thought was reasonable. But the software stated that if the exhaust duration were increased 10 degrees, the engine would gain 25 additional horsepower. I just wish it were that easy!

We’ve built and dyno tested dozens of these engines. They’ve logged thousands of runs on drag strips. Now a customer tells us that we left 25 horsepower on the table. But what the simulation software didn’t allow him to do was input the discharge coefficient of the exhaust port. In other words, the program didn’t “know” the design specifics of the exhaust system. It based its calculations on simple airflow, and therefore didn’t have enough information to generate a realistic answer.

You still need experience and knowledge to get it right. The more complicated the software, the more essential this real-world database becomes. When dealing with ultra-high performance engines, the details become so subtle that no software can simulate them. In a high-end racing engine, a tiny change in the approach to the valve seat or the contour of a piston dome can produce a measurable difference in performance on the dyno and on the drag strip. Even Ferrari, with its whiz-bang Formula 1 simulation software, still has to build, test and validate any change in hardware before taking it to a race.

It’s a mistake to put blind faith in a computer program. Some programs are certainly more accurate in their predictions than others, but they all have shortcomings. Building an engine on a computer screen is no substitute for bolting together the parts and running it down a track. To my knowledge, a virtual engine has never won an NHRA national event.

Big Bore or Long Stroke: Which Is Better?

Written by David Reher

Recently I’ve had several conversations with racers who wanted to build engines with long crankshaft strokes and small cylinder bores. When I questioned them about their preference for long-stroke/small-bore engines, the common answer was that this combination makes more torque. Unfortunately that assertion doesn’t match up with my experience in building drag racing engines.


My subject is racing engines, not street motors, so I’m not concerned with torque at 2,000 rpm. In my view, if you are building an engine for maximum output at a specific displacement, such as a Comp eliminator motor, then the bores should be as big as possible and the stroke as short as possible. If you’re building an engine that’s not restricted in size, such as a heads-up Super eliminator or Quick 16 motor, then big bores are an absolute performance bargain.


I know that there are drag racers who are successful with small-bore/long-stroke engines. And I know that countless magazine articles have been written about “torque monster” motors. But before readers fire off angry e-mails to National DRAGSTER about Reher’s rantings on the back page, allow me to explain my observations on the bore vs. stroke debate.


In mechanical terms, the definition of torque is the force acting on an object that causes that object to rotate. In an internal combustion engine, the pressure produced by expanding gases acts through the pistons and connecting rods to push against the crankshaft, producing torque. The mechanical leverage is greatest at the point when the connecting rod is perpendicular to its respective crank throw; depending on the geometry of the crank, piston and rod, this typically occurs when the piston is about 80 degrees after top dead center (ATDC).


So if torque is what accelerates a race car, why don’t we use engines with 2-inch diameter cylinder bores and 6-inch long crankshaft strokes? Obviously there are other factors involved.


The first consideration is that the cylinder pressure produced by the expanding gases reaches its peak shortly after combustion begins, when the volume above the piston is still relatively small and the lever arm created by the piston, rod and crank pin is an acute angle of less than 90 degrees. Peak cylinder pressure occurs at approximately 30 degrees ATDC, and drops dramatically by the time that the rod has its maximum leverage against the crank arm. Consequently the mechanical torque advantage of a long stroke is significantly diminished by the reduced force that’s pushing against the piston when the leverage of a long crankshaft stroke is greatest.


An engine produces peak torque at the rpm where it is most efficient. Efficiency is the result of many factors, including airflow, combustion, and parasitic losses such as friction and windage. Comparing two engines with the same displacement, a long-stroke/small-bore combination is simply less efficient than a short-stroke/big-bore combination on several counts.


Big bores promote better breathing. If you compare cylinder head airflow on a small-bore test fixture and on a large-bore fixture, the bigger bore will almost invariably improve airflow due to less valve shrouding. If the goal is maximum performance, the larger bore diameter allows the installation of larger valves, which further improve power.


A short crankshaft stroke reduces parasitic losses. Ring drag is the major source of internal friction. With a shorter stroke, the pistons don’t travel as far with every revolution. The crankshaft assembly also rotates in a smaller arc so the windage is reduced. In a wet-sump engine, a shorter stroke also cuts down on oil pressure problems caused by windage and oil aeration.


The big-block Chevrolet V-8 is an example of an engine that responds positively to increases in bore diameter. The GM engineers who designed the big-block knew that its splayed valves needed room to breath; that’s why the factory machined notches in the tops of the cylinder bores on high-performance blocks. When Chevy went Can-Am racing back in the ’60s, special blocks were produced with 4.440-inch bores instead of the standard 4.250-inch diameter cylinders. There’s been a steady progression in bore diameters ever since. We’re now using 4.700-inch bores in NHRA Pro Stock, and even bigger bores in unrestricted engines.


Racers are no longer limited to production castings and the relatively small cylinder bore diameters that they dictated. Today’s aftermarket blocks are manufactured with better materials and thicker cylinder walls that make big-bore engines affordable and reliable. A sportsman drag racer can enjoy the benefits of big cylinder bores at no extra cost: a set of pistons for 4.500-inch, 4.600-inch or 4.625-inch cylinders cost virtually the same. For my money, the bigger bore is a bargain. The customer not only gets more cubic inches for the same price, but also gets better performance because the larger bores improve airflow. A big-bore engine delivers more bang for the buck.


Big bores aren’t just for big-blocks. Many aftermarket Chevy small-block V-8s now have siamesed cylinder walls that will easily accommodate 4.185-inch cylinder bores. There’s simply no reason to build a 383-cubic-inch small-block with a 4-inch bore block when you can have a 406 or 412-cubic-inch small-block for about the same money.


There are much more cost-effective ways to tailor an engine’s torque curve than to use a long stroke crank and small bore block. The intake manifold, cylinder head runner volume, and camshaft timing all have a much more significant impact on the torque curve than the stroke – and are much easier and less expensive to change.

Note- The fill-in guy.

Wet Flow Revelations: The Monsoon Inside Your Motor


Written by Darin Morgan

If you expected to find David Reher’s column on this page, I hope you won’t be too disappointed to find my words here instead. I’m in charge of cylinder head development at Reher-Morrison Racing Engines, and David asked me to write about the extraordinary impact of wet flow testing on our R&D program. I’ve given presentations on our wet flow development at the Performance Racing Industry (PRI) trade show and the Advanced Engine Technology Conference. Since most National DRAGSTER readers can’t attend these events, David thought that Technically Speaking would be a good way to spread the news. He also promised that he’d be back on this page in a few weeks.

Every racer is familiar with dry flow testing; it’s become commonplace in every form of motorsports. In wet flow testing, we add a liquid to the airstream to simulate the behavior of atomized fuel in the engine. Many professional engine builders have built there own wet flow benches. They have told me that after hundreds of hours on there benches, they learned nothing. That does not mean that wet flow testing is useless. It simply means there testing procedures where useless. That all changed last summer when Joe Mondello and Lloyd Creek came up with the fabulous idea. Design a wet flow bench that everyone could use, Make it affordable and make it useful. They have accomplished there task in a BIG way! I was the lucky benefactor of one of the third machine they built. Joe Mondello called and asked me to work with there new bench using our current Pro Stock heads. Since then we have learned so many new and wonder full things about wet flow dynamics and continue to learn more every day.

The mechanics of wet flow testing are straightforward: the intake port is pressurized and liquid is introduced into the air stream with an atomizer. A clear plastic cylinder sleeve allows the operator to observe and record the behavior of the fuel droplets in the valve bowl and combustion chamber. The liquid is then separated from the air and captured in a recovery canister.

Watching wet flow in a cylinder is like trying to see individual rain drops in a hurricane. Adding dye to the mixture helps, but only slightly – the dye reveals gross trends, but not the important details. The breakthrough came when Mondello and Creek came up with the idea to add fluorescent dye to the mixture and observe the motion with an ultraviolet light. What had been invisible suddenly became perceptible – and what they saw was a revelation!

The fluorescent dye allowed us to see the behavior of the air/fuel mixture in fine detail. We could watch as a vortex would form, grow, and move around the chamber like a miniature tornado as we adjusted the valve lift. We could spot areas where the vortices joined to form a cyclone of fuel and air. We could see where flow was turbulent, and where it was stagnant. We felt like blind men who had been given the gift of sight.

The first thing that came to light was the myth of fuel wash. Like most racers, I believed that clean areas on the chamber walls and piston dome indicated where fuel had fallen out of suspension and cleaned off the carbon. Wet flow testing revealed the truth: The shiny areas are where there is the least amount of fuel. This lean mixture burns quickly and completely. In fact, the most fuel fallout occurs where there is a dark, sooty burn pattern on the chamber and piston. That is where the fuel falls out of suspension, creating a vortex. In this condition, the fuel burns, but it burns too slowly and too late in the cycle to create usable cylinder pressure.

Usually the burn pattern that we see on the chambers and pistons mimics what we see on the wet flow bench at the upper ranges of valve lift. For example, our Pro Stock engines use a cam with 1-inch valve lift, and the burn signature in the chamber is almost identical to what we observe on the wet flow bench at .800-inch valve lift.

What had appeared to be chaos in the cylinder now became a fluid and predictable motion, thanks to the fluorescent dye and ultraviolet light. We saw how the vortices spin in a clockwise or counterclockwise direction depending on the port orientation and the direction of the airflow. We watched as they started small, gained size and speed, and rotated clockwise or counterclockwise around the chamber.

One of the variables that significantly affects power is spark plug wetting. If there is a vortex of raw fuel droplets near the plug gap, it is very difficult to ignite the mixture. The wet flow bench allowed us to see whether a vortex formed near the plug gap and then take steps to move it away.

We also discovered that there is a strong tendency for vortex generation around the exhaust valve. The fuel converges in a big rotating ball, and its energy is wasted because it burns too late and too slowly to create useful cylinder pressure on the power stroke. In an inefficient engine, this fuel is expelled into the exhaust port while it is still burning, raising the exhaust gas temperature and superheating the valve guides. This isn’t a simple problem to solve, but now we know where to focus our efforts.

Wet-flow technology is already migrating to sportsman engines. The shape, cross-sectional area and velocity characteristics of the ports in our Raptor big-block sportsman cylinder head came directly from our Pro Stock research. There is a small “wing” in the Raptor’s intake runner that takes the wet flow off the floor and literally launches it back into the airstream. The port has to be designed around this wing; you can’t just put a wing in a port and gain power. That’s one example of how we have applied wet flow technology to sportsman engines.

In a perfect world, the air/fuel mixture would be 100 percent homogenous. The atomized fuel droplets would all be the same size and surrounded by an adequate number of oxygen molecules for complete combustion. Unfortunately, we don’t live in a perfect world. Thanks to Joe Mondello and Lloyd Creek the wet flow bench is starting to show us how imperfect the world inside a racing engine really is.

great read! Thanks for sharing!

Yes very good reading.
But as far as altitude carb jetting, if no adjustment to fuel mixture is needed, then why are manufactures from cars to bikes to snow mobiles etc. All produced altitude compensating systems for their fuel systems. These would lean the mixture, and today's fuel injection systems to the same. Of course this is not a drag race environment some would say. But what about a 200hp 3 or 4 cylinder 2 stroke engine that is running say 40mm Mikuni carbs? I can tell you that these are real finicky, and will either fowl a plug if two rich, or melt the piston if lean. Just going from ;lower elevations to climbing a set of mountains for the day requires setting the jets Leaner or it will fowl plugs. Really the same holds true for temp changes, what works for 0 degrees F is too lean at -30F and a fun extended run on a river can burn the piston, it will run really good, then your toast.

Motor Head wrote:

Yes very good reading.
But as far as altitude carb jetting, if no adjustment to fuel mixture is needed, then why are manufactures from cars to bikes to snow mobiles etc. All produced altitude compensating systems for their fuel systems. These would lean the mixture, and today's fuel injection systems to the same. Of course this is not a drag race environment some would say. But what about a 200hp 3 or 4 cylinder 2 stroke engine that is running say 40mm Mikuni carbs? I can tell you that these are real finicky, and will either fowl a plug if two rich, or melt the piston if lean. Just going from ;lower elevations to climbing a set of mountains for the day requires setting the jets Leaner or it will fowl plugs. Really the same holds true for temp changes, what works for 0 degrees F is too lean at -30F and a fun extended run on a river can burn the piston, it will run really good, then your toast.

If I were to take a guess??? Cars you have the EPA to deal with. Most people when taking a trip aren't going to stop and tune on the motor(timing, gearing, etc.) It needs to be done automatically. No thinking required!

Please bare in mind, this guy tunes on high horsepower motors. They do motors with 1,400hp with carbs.
I'm not saying he is wrong, just be open minded.

Two strokes, now that another issue all by it's self. Some are very sensitive to humidity, temperture, altitude (air density), time of day & what color the sky is, on a given day. They are a whole different animal all together. Most of the time you don't make timing changes on them. Some dirt bike teams don't change the jetting, they adjust the oil mixture to richen or lean the motor.

Yes adding oil can do it partly for a 2 stroke. But these days the muscle sleds are oil injected first, then I suppose you could add some to the tank mix. Your right I think even a 2 stroke race bike, like the GP smaller class these days, would still be remapping spark/ fuel for different tracks and conditions.
Also this 2 stroke area is a good idea for a ceramic piston crown coating, not a cure for a melted piston, but a wider window of temperature the thing will take before turning into lava.
The EPA, and the fuel economy #'s that are government mandated,along with the higher fuel price have been much of the driving force in engine and fuel designs in your road car. If it was completely left up to the public we wouldn't have stopped at V10 pickup's, or maybe we haven't? But still the mixture/ air fuel ratio is constantly monitored for both converter operation and efficiency of power/ fuel economy.
At the track, with the Weber's and Holley's I've run, jetting set even at dyno time changes when the load/launch is there. Maybe I'm one of the ones who spends way too much effort on the jetting as he talked about?