exhaust sizes versus horspower/torque?

Question

Is it true that putting too big of an exhaust on a lower-horspower car will actualy cause substantial reductions in performance? I know that bigger exhausts and headers can help a stock engine breath better, but how do you know what is too much? Is there some formula to determine what size of exhaust is needed for a certain amount of horsepower/torque?

Answer 1

Yes, too large an exhaust system will reduce low rpm torque. Too large a pipe slows down the exhaust gasses as it exits the exhaust port and into manifold/header. The exhaust flow reaches a large area (the pipe) and slows down and diffuses to fill the pipe. As the velocity of the exhaust gas is reduced, scavaging during overlap is affected.

Cylinder scavaging is the process where the flow of exhaust gas leaving the cylinder helps pull in fresh air/fuel during the overlap period, when both intake and exhaust valves are open. The exhaust gas kinda siphons the cylinder as the exhaust valve is closing. This helps low-rpm efficiency. This is why properly sized long-tube headers make more torque than manifolds. Then the header must be large enough to flow at higher RPMs, when the volume of exhaust gas increases.

I don't know of a formula, more of a rule of thumb. Header primary tube size and design affect cylinder scavaging more than the exhaust system. Stock "log" manifolds need the proper exhaust pipe sizing to help scavaging. For V8 street engines up to 350 cubes, a primary tube should be 1-5/8"s or so. 396-455 engines like a 1-3/4" header tube. Larger than 455 is race/pro street, header tubes of 2" and larger are common.

Also depends on engine design. The flow charateristics of the exhaust ports, bore/stroke ratio, RPM range, etc determine what header/exhaust to use. Race engines are actually easier to set up, because they always run at high RPM. Low speed response and noise isn't an issue. Street motors have to handle the trade-offs of low-speed response, high RPM flow, noise reduction (mufflers), etc.

Answer 2

Trial and error on a dynamometer is the only way. That's why most people buy their headers and systems from performance manufacturers.

It is not just free flowing gasses that create horsepower. It is also the tuning frequency of the pulses of gass that will help suck the next pulse out of the cylinder. There is a tremendous amount of research and technology that goes into extracting the most from an exhaust system.

There are a lot of fools out there who have put righteous sounding pipes or mufflers on their cars and bikes, but don't realize that the same vehicle with a stock system can out run them. Makes for interesting results when money or pinks are on the line.

Answer 3

Generally the smaller the engine the more backpressure is required to have the engine run properly, so essentially these people who run the 3"+ exhausts on the honda's etc. because they think it adds power are total morons. The engines that benefit the most from free-er flowing exhausts are larger engines (V8's) and diesel engines. Look up some videos of diesel pickup trucks at a pull track, you are going to see an exhaust port coming through their hood because they are trying to vent that exhaust as fast as possible...