Exhaust Backpressure Faq

[fritzfab]

its true about gases heating up because i took the heat shield off my headers and now my car smokes and my engine bay is ungodly hot all the time with the fan running .... i just have stock 2 1/4 it looks like and no cat, but i got alot of pick up the way mine is set up i noticed a difference but i have a really raspy sound when the rpms hit over 3 grand any idea why?

[wytbishop]

It's also possible to have too much velocity. The [backpressure] pulse helps keep the new gas mixture in the chamber, since the valves are briefly open @ the same time. Someone built a system that created a slight vacuum, and he lost power because it sucked some of the new gas out before the compression stroke. A perfect system would give a slight backpressure pulse just for that split second. That's how and why Flowmaster mufflers were developed, and can (in the right position) have more HP than straight pipes.

I know its like a year and a half since this guy posted...but NO...WRONG!!!

More velocity is always better until the volumetric flow capacity of tHe pipe becomes limiting. The harmonics of the collector creates an effect called scavenging which is intended to create vacuum in the area just outside the exhaust seat area and pull the exhaust out of the combustion chamber. This in turn gives the incoming air/fuel charge greater momentum so that it will rush into the combustion area filling it more fully. It is accepted that because of valve overlap some air/fuel mixture will be taken out through the exhaust before the valves close. Hence the existance of the EGR system. The amount of air/fuel charge lost is controlled by limiting the valve overlap. The cam timing is designed largely to ensure that the exhaust valve closes just after some fresh charge has escaped, ensuring that all burnt exhaust gases from the previous cycle have been evacuated. The loss of a small amount of fresh charge is greatly outweighed by the adverse effect of leaving burned exhaust gases in the combustion chamber for the following cycle.

NO AMOUNT OF BACKPRESSURE IS EVER DESIRED...EVER.

I have a bit of an unreasonable reaction to this issue. I've been having this argument for years.

[fowljesse]

You're right about how the system works. Appearantly pulse technology is beyond your research. I saw the aformentioned guy build an exhaust system that created too much vacuum, believing that "the less backpressure, the better", and it sucked too much of the new charge of air/ fuel out, and of course, with less fuel to burn, he lost power. What I said about Flowmaster was almost a quote from their advertisement, and at that point they were (and probably still are) the leaders in the industry due to superior research and developement. Maybe the pulse is not technically considered backpressure. If that's the case, I apologise for being misleading. I do know for sure that Flowmaster(s) installed at the right point in the system create more HP than straight pipes, with the exception of systems that don't go to a collector. For the sake of anyone trying to build an exhaust system that creates vacuum, I hope they research pulse technology. I only wrote to help those who are capable of going beyond common understanding of the exhaust system, and to prevent wasted time and money due to making the same mistake as my fellow drag racer.

[bbox]

I have found that with almost any vehicle, the stock size will work out fine. I also agree with the 1 inch per liter... to an extent. The size isnt what should count, its the flow. A smooth, full flowing exhaust of 2.5" will do alot better than a stock, restricted 2.5" exhaust. basically, run high a flow cat(s), a free flow muffler(s), and try to make the piping more of a straight direct flow.

[wytbishop]

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