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==Collector size== ===Argument for smaller collectors=== Ever see a car header with 1 5/8" primaries that had a 2 1/2" collector? Hooker makes them for trucks, but you don't see them for cars. But that is the optimum diameter for many street headers. Run that right into a 2 1/2" exhaust and you have a sweet system with lots of torque where you need it, and better fuel economy to boot. The collector should have a smooth, gentle shape from the four tube area down to the final diameter to keep things moving smoothly. If you have very healthy big block, are running a blower, turbo, or nitrous, a 3" header may be appropriate. However, about the only street application of the monster 3" diameter pipe is when you count on having an engine with a lot of top-end power. Otherwise, velocity is king in exhaust and 3" is probably too big to keep the speed of the gases up in the exhaust, and there goes that bottom end torque again! Many recommend either 2-1/4" or 2-1/2" diameter pipe for street V-8's. ===Argument for larger collectors=== A larger-than-needed exhaust system won't necessarily "kill" your low end torque. Any engine that makes respectable low end power will continue to make low end power regardless of a sewer pipe sized exhaust. SOME power may be lost, yes. Usually not enough to make a difference on MOST V8 engines that are even close to being tuned right. Even a stock big block can make complete use of a 3" single exhaust system. Some even come with that size STOCK. The majority of torque "lost" from the bottom end of the power band has actually been moved upwards in the powerband, into the midrange. This can help performance during highway driving and towing, and may provide better passing power when needed. A more powerful midrange tends to make the low end seem less powerful, simply because the engine pulls better in a different part of the RPM range. Most people that think they lost low end power are simply noticing that the engine pulls better in a different way, even if NO low end power was ever lost.
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