Quote:
Originally Posted by josh18_2k
2.5" at most. 2.25 would be better.
too big slows exhaust velocity, creating backpressure.
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To correct/clarify your statement: To large of a pipe will not cause back-pressure. What it will cause is a low exhaust gas velocity, and an obnoxiously loud buzzing exhaust note as the noise reverberates in the overly large pipe.
Gas velocity is important. Think about it this way: In a properly sized exhaust system, your exhaust port opens, and the pressure in the combustion chamber equalizes with the pressure in the header, and sends a high velocity pulse down the pipe. This column of gas moving down the header has
momentum, which a properly tuned engine will use to it's advantage.
Toward the end of the exhaust phase, when the intake is opening, there isn't really any more exhaust to push out; the pressures are equal. However, that column of gas moving down the pipe isn't going to immediately stop, it's going to continue traveling down the pipe, which creates a negative pressure area that, upon reaching the exhaust port, pulls out some more spent exhaust gas that would otherwise have made another trip around the cycle with the intake charge. The real beauty is that now there's negative pressure in the chamber, which works against the intake port and pulls some of the air/fuel mixture into the chamber, and helps get that column of gas moving.
Too large an exhaust pipe will prevent the gas from reaching a high enough velocity to create enough momentum to pull this off; the gas has very little energy and stops soon after the exhaust pressures equalize, and in extreme cases, will even backflow after the momentum is spent.
On the other hand, too small a pipe will prevent the gas from equalizing pressure with the outside atmosphere before the exhaust port closes, resulting in a diluted air/fuel charge. In other words, back-pressure.
The same concepts apply to intake tuning; that column of intake air charge also has momentum that can serve to ram more air into the chamber after it's "full", achieving over 100% VE in some cases. All of this relies on properly sized runners, both in diameter and in length. It should be obvious to anyone versed in engine dynamics that the effects I've talked about rely heavily on the time between the gas equalizing pressure and the closing of the exhaust port/valve. That's where the runner/system length comes into play; the longer the system length, the lower the RPM where you'll see your benefit from this sort of tuning. If the system is too long then the system won't have time to make use of the vacuum created before the exhaust closes, and if the system is too short, the momentum will be gone before the intake is opened. In general, tune all of your runners towards your peak power band. Some race engines have made use of varying runner lengths between cylinders to "spread out" the power increase (one cylinder reaching it's peak effect at 6000, the next at 6250, the next at 6500, etc) to provide a wider torque peak, but peak power from this relies to some extent on individual cylinder fuel/spark tuning, and is therefore out of reach of most of us mortals.
The specifics for your application are, as usual, left as an exercise for the reader.