firedawg said:
Ken, Would running dual straight pipes auctally decrease performance due to loss of back pressure?
Most schools of thought accept the idea of one bank of cylinders scavenging the exhaust of the other as a good thing, some prefer the old school of "true dual" exhaust. It's a matter of personal prefernece most of the time.
In the end though, it all boils down to the system as a whole and how you have it set up. If you have a 3-inch exhaust system (given the premise that you have an engine that requires a 3-inch exhaust), you don't want to restrict it with small or otherwise less-than-desirable mufflers. The same applies that if you have a restrictive exhaust pipe (hence the "mandrel bent" trend you see nowadays), no free-flowing cat, muffler, or "X" (or "H") pipe will help you. Either way you'll be stifling the engine's ability to breathe.
As for whether there is a need for backpressure, here again you're going to get differing views on what works best. Some say that a properly tuned motor/exhaust has no need for backpressure and that it only causes a loss of horsepower. The need for "backpressure" should more accurately be termed
"a need for gas velocity optimization".
Backpressure is an often misused term.
Port velocity is better suited and should probably be used. Simplified, velocity is the rate of change with respect to time. With a poorly designed exhaust, you loose your low speed port velocity, meaning the engine has to use some of its power to
"push" the exhaust gases out of the cylinder, leaving room for the fresh intake charge. The idea is to get the exhaust gasses out of the cylinder; backpressure in the exhaust system will prevent complete clearing of the cylinder. There is also a tuning effect of sound waves that cause pressure fluctuations in the pipe and manifold. An engine with higher average backpressure, but properly tuned exhaust where there is a below atmospheric pressure condition at the end of the exhaust stroke, will perform better than an engine with lower average backpressure that happens to be poorly tuned in the exhaust so that the pressure is high at the end of the exhaust stroke, thereby causing residuals to remain in the cylinder. Got that? :L
The easiest (read cheapest) way to quiet an engine is to muffle it with backpressure, but there are systems designed for low backpressure and sound reduction.
Also, a tuned exhaust tends to work best at a certain rpm range, as do tuned intakes. Generally the larger the pipe diameter, the higher rpm for the torque peak and the longer the pipe (within reason) the more torque below the peak, the shorter the pipe, the more torque above the peak.
In the worst case a poorly designed manifold will have pressure from one cylinder beginning its exhaust stroke backing up into another cylinder that is just finishing its exhaust stroke. Any back pressure will push against the crank, reducing power output, and will contaminate the intake charge with what is left in the cylinder, again reducing power output. Torque (and consequently horsepower) are strongly related to volumetric efficency, which is how well the cylinder is filled with fresh fuel-air mixture. Any residual exhaust gasses trapped in the cylinder reduce volumetric efficency.
Rather than more typing (which involves more
work), I'll just provide you with some links to information regarding backpressure:
Destroying A Myth - "An engine needs backpressure to work correctly." Is this true?,
Exhaust Tubing Selection Guide, and one that will provide you with countless hours of reading pleasure, and much worthwhile information,
The "Performance Professor", Jim McFarland - Lecture Archives.
(I love cars!)
