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Darth Luder · 07-19-2001, 12:39 PM

Quote:

Originally posted by 71dsp
However, I've also seen graphs where the engine definitely lost low RPM torque after restriction was removed from the exhaust, so I know this isn't an illusion. Low end torque loss is a possibility, but not a given.

You should check the diameter of the aftermarket exhausts to make sure that the tubing isn't so big that the engine is working to push the exhaust gasses out the exhaust at low RPMs. That could also be what's causing the problem, not just lack of backpressure. The larger tubing could be causing MORE backpressure than the stock exhaust.

You left out the sentence before that one: Sometimes, an exhaust mod added low end torque but moved both the peak torque and peak power up a little in the rev range, making the engine feel like it had comparatively less low end grunt - even when it was really pulling harder at low low RPM too. It is just a matter of how good the design really is. With a smaller diameter pipe, you get greater velocity of gases with some added pressure at the beginning of the pipe. With a larger diameter pipe, you get a slower moving velocity, but there is lower pressure drawing more air out of the cylinder which helps the engine work.

As you may have guessed, there's a second effect here at work. Remember what I said about camshaft overlap and low backpressure letting the exhaust gasses pull in the air/fuel mixture? Well, at low RPM with a cam with large amounts of overlap, the momentum of the exhaust doesn't just pull the air/fuel mixture into the cylinder; it can carry it straight out the exhaust port if the pressure at the exhaust port is lower than atmospheric (or lower than your intake pressure, if you're running forced induction) and the exhaust valve's open long enough (either low RPM or way too much overlap). Obviously, this reduces the amount of air and fuel that do anything useful.

There are other ways to tune this besides adding backpressure, like decreasing overlap, retuning the header, etc.

But who in the world is really talking about adding backpressure??? I am not saying, go and add more backpressure by getting a smaller exhaust.... Are you refering to the jokingly comment I made by saying "imagine all of the backpressure..." or whatever...

True there are many ways to tune everything... You are not going to add more back pressure by adding a freer flowing header, come on, give me some credit here. I mean, in the next paragraph or something I said that a VTEC engine already has very little overlap in the lower rpms...