Exhaust Physics Questions!

If you have a pressure source (such as... a cylinder in the exhaust stroke), a conduit (like perhaps... an exhaust header pipe?), and a restrictive orifice (not perhaps... a muffler with baffles...)

Pressure on the left side of the orifice is going to be higher than that which is open to the atmosphere. FLOW can be described and is measured by this difference in pressure. A bigger pipe, and/or a bigger opening in the orifice will yeild a higher FLOW, because it is less RESTRICTIVE.

What units are used to describe the restriction measurement?

Are mufflers described in this way by the engineers? A less restrictive muffler will produce less back-pressure, and a more restrictive muffler more back-pressure, but is this restriction measured and advertised by the muffler manufacturers? I would think that anyone would want to know the specs on whatever muffler they were buying so they would know what effect it would have on back-pressure! Or if you were making a home-made muffler out of a folgers-can, how could you measure how restrictive it was and match that up to some industry standard?

Any ideas? Am I totally out in the left-field here?

Post edited by: indytwin, at: 2005/12/27 15:13

Post edited by: indytwin, at: 2005/12/27 15:24

consider this diagram

Post edited by: indytwin, at: 2005/12/27 15:34

Here's an article on it. Pay attention to the part about exhaust pulses. It seems that the length and diameter all play a role. But getting the dimensions right to have the exhaust pulse actually "scavange" and pull the exhaust out is the big thing.

I think it's just a trial and error thing. Maybe that's why pipes cost so much;)


home.jps.net/~snowbum/InExTuning.htm

Post edited by: shaner, at: 2005/12/27 16:44

Holy crap, that's a lot of information. Thanks for posting the link!

yes it is. lmao.

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...The road to hell is paved with good intentions...

I have built a lot of car engines, mostley for the street and exhaust plays a big part, headers in particular, I have found the best street header to be of the tri Y design. This design actually scavanges the burnt gases from the cylinders, By pairing the cylinders correctly the exhaust pulse will create suction at the Y to draw the burnt A/F out so a new fresh/clean charge will fill the cylinder. In a typical 4 cylinder where the fireing order is 1342 cylinders 1&4 and 2&3 would be paired off then joined again farther back for a single exit.When number 1 is fired the exhaust pulse moves down the pipe till it crosses into the Y,this makes vacume on #4 which pulls the burnt gases out, the same happens with 2&3 till they meet the second Y which makes more vacume scavaging all cylinders. The power band is determined by lenth of primaries, the lenth of the powerband is determined by the diameter and lenth of the secondariy pipe

To answer one of your questions-We,(Mercedes Benz),have a test to measure exhaust back presure.The gauge measures in BAR. Most of the time we use it to diagnose plugged catalytic converters and trap oxidizers.
Designing exhaust systems is an art.The amount of science that goes into them is amazing. Two stroke pipes are even more interesting and mysterious.

I've read that article posted above before, and a few others on the subject.

The short version of what most articles have to say is that the exhaust comes out in pulses which correspond to the exhaust stroke of your engine. This pulse travels through the exhaust pipe until it hits something that will cause it to reflect. The end of the pipe, despite being open to the air, will still cause some reflection in the pulse.

The point of tuning is to make sure that the reflected pulse doesn't arrive at some point when the exhaust valve is open, since the pulse would continue back into the combustion chamber!

I'm not really sure why crossovers help except to perhaps provide a more consistent pressure at the back end of the pipe, which would mean less turbulence and probably smaller pulse reflection. I may be wrong on this one.

You'll hear a lot about backpressure among shadetree tuners, but I'm not convinced that backpressure has as much to do with it. There has to be a reason why car makers don't just put 3" pipes on all the way, and I doubt it's the cost of the extra metal it would require to make the pipe. It's probably something to do with pulse regulation.

In effect this all means that even with specialized equipment to check flow inhibition, there's no way to compare pipes using published flow rates. From the consumer standpoint it's all guesswork until you put it on a dyno or take it to the track.

Suggest a visit to:
pulse-jets.com/

In particular, the section on valveless pulsejets...

-Duck

I believe in/hg (inches of mercury) is the correct unit of measurement. Any restriction in air flow would be measured by it's pressure drop across it. HVAC systems measure pressure drop across filters to determine if they are in need of replacement.

A positive pressure wave travelling through a pipe will be reflected by a closed end of the pipe. An open end will reflect a negative pressure wave. I believe restrictions and openings will operate similarily. This is the process used in 2-stroke expansion chambers. Any one in the HVAC industry may remember the Lennox Pulse Hi-efficiency furnace. It used negative pressure waves to draw natural gas into a combustion chamber where a glow plug ignited it.

Cylinder head flow benches work on measuring pressure drop across a known orifice and the cylinder head to determine flow rates in CFM.

you could take a look at kamware, which can tell you the dimensions you need to get power at different engine speeds, try googling it

i was allways lead to believe the cross-over pipe was used to equalize the pressure between the two pipes.as far as the 3"exhuast goes it kills off alot of torque,it would work great at higher rpms its like pulling the baffle and endcap out of a header it turns into a slug at lower rpms but is great on the higher end of the scale but if you put just the endcap back in it creates some backpressure which greatly improves the low end response!another reason they dont use big exhuast on most production cars is noise/comfort levels and there also isnt enough room with everything else they pack in there!tri-y headers were ahead of their time back in the 60s when doug thorley and some other people were making them people were too busy drooling over the "big tube"headers the drag racers were running to realize just how good these headers work on a real world street machine,i just wish they would make them for more applications!everybody used to think to build a fast big block chevy you had to use all the stuff the prostock guys use 4-bolt main blocks,steel cranks,big rods rectangular port heads and dual 750s,we figured out a long time ago you can do the same thing with a 2-bolt block,steel crank,small rods ,oval port heads and a single 4-barrel and whip their asses on the street!the same holds true for these kaw motors if you build it to run in a realistic rpm range youd be suprised what you can do with mostly stock parts.goodluck,happy wrenching!