Exhaust back pressure

Good afternoon, well it is another not so wonderful afternoon here in Alberta, so I decided to tear my bike apart yet again. I was wondering if I remove the pipe off of my header and replace it with a custom lighter piece would I have any back pressure problems? I assume I would have to do some carb adjustments, but other then that would what problems would I face?
Also I have started to poke around the engine and according to the compression test I'm sitting at about 100lbs on all 4 cylinders rather than the stock 160lbs. I have not noticed any oil burning, but have not ridden the bike to really know. I did notice that the bike has no where near the go that people claim this type of bike had. Any idea what I might need to do to get it back to normal? Or is it too hard to tell unless I open it up?

Whether or not it has too much back pressure would depend on how well it flows, the less back pressure the better.

I suggest checking the valve clearances before doing much else to the motor, insufficient valve clearance will drop compression considerably.

"the less back pressure the better." If this is true, then wouldn't removing the entire exhaust system make the engine run its best? But we know that's not the case, so some backpressure must be a good thing, the tricky part is finding out exactly how much. I would suggest that the folks who designed the engine or engineers with $$ for R&D (i.e. Yosh., etc,) had a pretty fair understanding of backpressure, scavenging dynamics, etc. and that any of us mere mortals would be relying on a fair amount of luck to stumble on a combination of pipes & cans that would exceed their performance levels. I'm not trying to be a wise guy, it's just my $.02. Ed

I agree that using an existing can is the best idea, which is what I have. However, I want to lighten as much weight off it as I can. I have seen people use their own exhaust pipe though. How do they make up their own exhaust systems in custom HD and get it to work?

You do realize that there is a world of difference between back pressure and tuned exhaust, right?

As far as "mere mortals" vs "engineers", I believe the engineers are just as mortal as anyone else

When engineers design an exhaust system, they have to deal with considerations that we don't have to worry about, noise and exhaust life expectancy for example. That's why most KZ exhaust systems weigh a lot more than aftermarket exhaust.

Most people can design an effective exhaust that will increase the hp in a particular rpm range if they are willing to do the research, there is no magic involved, it's all numbers and metalworking skills.

To put it in simple terms, back pressure bad, exhaust tuning good

The Scientific Design of Exhaust and Intake Systems' by Phillip H. Smith is a good place to start.

For a quick start, try a search on Google for "Wave Tuning".

XL77, the exhaust pipes are so long on a KZ, the exhaust tuning is pretty much over by the time it gets to the muffler. You can get sport bike takeoffs for next to nothing, just find one you like the looks of, they will flow well enough to not cause you any problems.

I hate it when people think that you have to have back pressure for an engine to run, there are a lot more factors involved than that.

One of the biggest thing is scavenging. This is where one exhaust pulse is used to help draw exhaust out of the other ports. Different pipes use different lengths and configurations to tune the powerband. Whether it be for a low rpm powerband or way higher towards redline.

I think most of the confusion comes from the fact that more back pressure usually increases port velocity which in turn gives more lowend power.

Thanks for the input. What I would like to do is keep the stock header and just add a slash cut exhaust pipe rather then the current can on it now. If I were to do this what should I know?

I think most of the confusion comes from the fact that more back pressure usually increases port velocity which in turn gives more lowend power.

How about looking at it this way?

Back pressure is the undesirable side effect of reducing pipe diameter to increase exhaust velocity.

I think it's the velocity scavaging the exhaust from the combustion chamber that increases the low end torque, not back pressure.

If you want to see the pinnacle of exhaust and intake tuning, look at the Formula One motors, those guys squeeze 600-700 hp out of 1500cc motors :woohoo:

Thanks for the input. What I would like to do is keep the stock header and just add a slash cut exhaust pipe rather then the current can on it now. If I were to do this what should I know?

You will have to re-jet the carbs or risk burning holes in your pistons for one.

Number two, if you happen to live or ride through an area that has a noise ordinance, it may get expensive.

Number three, you will piss off some people, and one of them may be crazy enough to try and do something about it. I live in a rural area, and every time a Harley with straight pipes rattles my windows I want to grab my shotgun.

A little noise reduction would be nice, at least get some with baffles

My take on this...kinda blunt...its late im tired, Ok replace muffler with pipe of your choice. Run/ride bike see how it runs. If adjustment is needed learn enough to know if your rich or lean and approx where in the rpm band. Adjust mixture pertaining to that range in the carb.You should learn to read plugs ( do a plug chop) correctly determine what circut needs to be adjusted, be able to make the changes. all that said , its pretty likely that a mixture screw adjustment or perhaps a bit bigger main jet is all that will be required.There are things to learn and understand about carb tunning but its really not rocket science. take care all. jonnybiker

XL777 wrote:

Good afternoon, well it is another not so wonderful afternoon here in Alberta, so I decided to tear my bike apart yet again. I was wondering if I remove the pipe off of my header and replace it with a custom lighter piece would I have any back pressure problems? I assume I would have to do some carb adjustments, but other then that would what problems would I face?
Also I have started to poke around the engine and according to the compression test I'm sitting at about 100lbs on all 4 cylinders rather than the stock 160lbs. I have not noticed any oil burning, but have not ridden the bike to really know. I did notice that the bike has no where near the go that people claim this type of bike had. Any idea what I might need to do to get it back to normal? Or is it too hard to tell unless I open it up?

after checking valve clearance .the only thing u can do to increase compresion is re-ring. but most likely u will need to rebore and go up 1 size of piston as well.
if u remove back preasure u will notice less bottom end torque (i did when i removed my baffle)witch u say u have little of now.

steell wrote:

You do realize that there is a world of difference between back pressure and tuned exhaust, right?

As far as "mere mortals" vs "engineers", I believe the engineers are just as mortal as anyone else

When engineers design an exhaust system, they have to deal with considerations that we don't have to worry about, noise and exhaust life expectancy for example. That's why most KZ exhaust systems weigh a lot more than aftermarket exhaust.

Most people can design an effective exhaust that will increase the hp in a particular rpm range if they are willing to do the research, there is no magic involved, it's all numbers and metalworking skills.

To put it in simple terms, back pressure bad, exhaust tuning good :)s.

That was what I was raised to believe until I did some research on it recently. If reducing back pressure was always better, straight pipes could always be made to run better. In reality, they often run a lot worse.

The mile high view that I learned was:

The intake and exhaust flow of a properly designed engine have to sing at the same key. Screwing with one will usually make it worse.

Bike makers are pretty smart and do a very good job wringing HP out of the engine. Not to say a good exhaust (like Kerker) can't squeeze out a few more, but mediocre exhaust (like MAC) probably do worse than stock. At least on my bike they do.

Engines are modeled basically as big air pumps. The air (or exhaust) can be modeled as fluid flow (fluid is not necessarily liquid). The key to getting more HP is squeezing more air/fuel into the cylinder when the valve opens, and exhausting it efficiently. To that end, the intake is tuned to play with the exhaust.

It may seem odd the intake valve opens before TDC and actually stays open after BDC (even as the piston is rising). The reason is, the exhaust flow creates a vacuum (scavenging effect) and the intake valve opening before TDC aids in getting the fuel/air mix going in. The inertai of the fuel/air mix flowing means you get some extra in leaving the valve opwn as the piston starts back up. Fuel/air will still flow in a bit after BDC.

That is the main reason you want to get the flow velocity in the intake as fast as possible before other effects like turbulence start screwing up the flow. Velocity increases inertia as the square of the velocity (1/2 x m x Vsquared). There are side effects of increasing velocity, so you need to get the sweet spot.

Some of the oddities I learned searching the net:

A guy who set a record at Bonnevile used flow modeling to increase the flow rate through the intake and exhaust... and the equations said they needed a rather thin exhaust pipe. The car broke the record. A veteran looked at the skinny exhaust pipe and said something like: "It will go faster if you get a bigger pipe and reduce the backpressure." They tried it and it went slower.

Bigger valves don't always mean better breathing. One bike (Harley davidson) went to bigger valves and screwed up the engine HP. If you wonder why so many bikes use multiple smaller valves instead of big valves: they flow better. I'm not sure why. There are also specific angles for valve seat cuts that work better and nobody really knows why.

The guys who tune intake systems admit it's a lot of experience and black magic. In one case, the guy polishing an intake manifold intentionally leaves rough spots at locations to create turbulence to slow the flow where it has to turn to get a smoother overall flow result. You can bet there are no textbooks that tell how to do that.

The only thing I can tell from personal experience is this: I cut the mufflers off my pipes and put on different ones and totally screwed up my bike. I bought a designed header/muffler set and it went like a cruise missile.

650ed wrote:

"the less back pressure the better." If this is true, then wouldn't removing the entire exhaust system make the engine run its best? But we know that's not the case, so some backpressure must be a good thing, the tricky part is finding out exactly how much. I would suggest that the folks who designed the engine or engineers with $$ for R&D (i.e. Yosh., etc,) had a pretty fair understanding of backpressure, scavenging dynamics, etc. and that any of us mere mortals would be relying on a fair amount of luck to stumble on a combination of pipes & cans that would exceed their performance levels. I'm not trying to be a wise guy, it's just my $.02. Ed

The pros who race bukes can change the torque characteristics of the engine just by putting "tunning pipe shorties" at the outlet of the exhaust canister.

On mine, the muffler was so quiet I couldn't even hear the engine. Me (and everybody else around here with twins) would take a 1/4" drill and rill four holes in the rear face of the muffler into the inner inlet chamber, bypassing the muffler. It made the bike run noticably worse. So, I bought new aftermarket mufflers and it ran WAY worse. New headers set with muffler and it ran better than stock. The guys who design these dyno tune them and know how to do it. A fool with a drill or a hacksaw is surely going to get it wrong.:laugh: