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Well, thank you for going through all that for me, and many other I assume. Now I have a slight understanding of why logic wouldn't hold true in this circumstance.

This should be in the FAQ for both 4 and 6 cyl.

Hey Gro (if I may be so bold), to revisit the entire point of my post...

The word "backpressure" was coined to describe a phenomenon that was not clearly understood by the person who first tried to describe it.

There is no backpressure; only efficient or inefficient flow. The word backpressure conjures the image of some sort of reversal of flow or obstacle within the exhaust system upon which the exhaust gases are building up, like water in a pipe. This is not the case except within a poorly engineered muffler or collector (which is undesirable and to be avoided as much as possible).

What you think of as backpressure is really just the correctly chosen diameter of pipe or collector to maintain favorable flow characteristics. The whole point is that, contrary to what you may believe, the best possible flow does not occur within the largest exhaust cavity or in the total absence of any exhaust system.

When we are told that "some backpressure is necessary" what they are really telling us is that for the best flow, the correct pipe diameter and so on and so forth are better than "a big ole 3" jobby" for a 1.6L engine.

That's my point. And that I have thoroughly enjoyed the spirited discussion.

Wyt

The only engine i know that does better with free-flow exhaust (N/A) are rotary engines.

True to a point, but in general if the pipes are too big they can have the same issue as piston engines, a decrease in low end torque. And not be belabor the point, but larger pipes does not mean better flow. When it comes to exhaust pipes, regardless of what your girlfriend tells you, bigger is not always better.

If you look at aftermarket exhaust systems for Rx7's, the good ones have smaller pipes for the n/a models and larger pipes (and different mufflers, a more free-flow design) for the turbos. I used a turbo exhaust on my '86 n/a, it behaved pretty well all things considered. About the same bottom end, very mild increase mid-range, big increase on the top end, which is about what you'd expect with a too-large exhaust. The manufacturers info expressly stated to not use the turbo exhaust on an n/a due to its effect on the low end, but I figured what the hell, there was no low end torque to speak of anyway.

Mazda rotaries are a different beast. They are similar in setup to a big bore, short stroke piston engine. They love to rev, everything is top end and any differences an exhaust is going to have on the bottom end are hard to detect because there is so little bottom end torque to begin with.

Wytbishop hit the nail right on the head with his explanations. This should be required reading for all the little ricers terrorizing the streets with their fart-can mufflers.

<small>[ October 28, 2004, 01:28 PM: Message edited by: guyaverage ]</small>

Just saw this thread and thought id reply a bit. Im no engineer or rocket scienctist but I do have some info to offer up. Recently I ran over somone exhaust tip that was in front of me on the highway going about 70mph. Whats the use of tips? anyways my car is low with the setup I have and it hit the underside of my car pretty hard. I noticed my car was getting really loud so looked at the exhaust and my flex pipe is completely gone. i comes right out of the headers through the small 1 foot bend if that, and thats it. The car doesnt act weird at all. The check engine light comes on after about 3 min of running for 15 sec and im sure its the 02 sensor. if I shut the car off during those 15 sec ith as a hard time starting up. However after that its loud as a bat out of hell and pulls pretty hard up to redline. My .02 cents

i'm not an engineer or anything, but my guess is that one of the main reasons you need a certain ammount of back pressure is so that the exhaust valves can close faster since there will be an almost balanced pressure in the cylinder and in the exhaust as opposed to only pressure inside the cylinder which will make it hard for the spring to close the valve. It does make since, its like closing the door in a building w/ the ac blasting... you can close it, but all the air pushing the door open doesn't help you close it. since we cant just eliminate the pressure in the cylinder when its time for the valve to close, we can try to balance it a bit to make the valve easier to close.
WHY would we want to close the valve so fast? probably because we dont want the engine to backfire. Also because- after the exhaust cycle (RIGHT AFTER), the engine goes back into the intake stage, where it sucks up air. we all know (i hope) that the engine sucks air through the intake valve (when it opens)... if the exhaust valve still hasn't completely been shut, the engine will posibly suck in air through the exhaust valve too (try hooking up your muffler to your intake, see how that goes). This SHOULD theoretically give you :
1- Poor air/fuel mixture
2- Poor performance
3- Hotter air temps inside the cylinder (might detonate)

Of course that it all goes in cycles, because when the engine begins to suck in air, the vacumn will help close the open exhaust valve, but that very little bit of exhaust getting sucked in could be your couple of HP or Lb/ft of torque missing