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Do we need back pressure?


jumpinjivinjoe
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Absolutely. A lack of proper (at least some) backpressure will result in valve damage pretty quickly. Saw your other post and would agree, the flex stuff will fail in short order and while it may not cause damage right away I think you are ahead of the game to do it right the first time, especially if you have a sound engine and want to maintain it in good order.

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Back pressure is common in all engines

I disagree. Some folks have a need for people to "notice" them. The more annoying the better. So they will remove mufflers, cut down "header" pipes, ANYTHING to make more noise. They have decided in their infinite wisdom that this is "good" for the motor and makes more power. Ask them about it, and they will assure you it is silly to bother reading SAE/ASTM tech. papers. After all, why bother with precise, logical analysis when you "know" what you want !

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Could somebody actually explain this in real technical language? I had a professor in college when I was an automotive engineering student (not for very long) who assured me that all back pressure did was limit power. He as a fairly serious drag racer and ran the SAE group at the college. He would run open headers on his Barracuda dragster (9 second car).

I also remember working at a small engine shop, and seeing Briggs & Stratton engines come in missing mufflers, and having burned exhaust valves as a result.

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So they will remove mufflers, cut down "header" pipes, ANYTHING to make more noise. They have decided in their infinite wisdom that this is "good" for the motor and makes more power. Ask them about it, and they will assure you it is silly to bother reading SAE/ASTM tech. papers. After all, why bother with precise, logical analysis when you "know" what you want !

Packard,

You are correct. But, there is one thing you forgot to mention. THOSE are the people that keep engine rebuilders in business!

They are like the people that put absurdly large or small wheel/tire packages on their car. Those people keep suspension and alignment shops in business.

Joe

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Exhaust is simply the movement of air or gas if you prefer. Air has mass and therefore inertia and harmonics. A properly tuned exhaust will make the engine breathe much better. Observe the increasingly rare two stroke motorcycle expansion chambers. They help scavenge the cylinder of spent gases. In the older days, power boat racers who utilized two strokes would actually have a moveable "stinger" on the chamber to change the RPM range where the engine would produce max power. Also older dual exhaust cars had what was called an "H" pipe which was a crossover between the two sides of the engine, once again to aid in flow rate by the use of the harmonics.

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Even with a very high performance drag racing engine, if you will observe the pit crew, at the end of the run the first thing they do before towing the vehicle back is to plug the exhaust pipes to keep the relatively cool air out. Also, almost all of the engines are either supercharged or turbocharged. This brings a whole new set of numbers for what will work as a exhaust system.

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I remember the 'H' pipe on a 50tys dual exhaust car. I also remember replacing the exhaust system and ignoring the 'H' pipe. Paid for that wisdom/mistake with fried valves. Oh, to be 17 again and know it all!

By the way, yanging out the tailpipes on an air-cooled VW muffler to produce that 'BIG' car sounf results in burnt exhaust valves. I learned the hard way once again.

Now for the 'HOT' question of the day. What happens if you decide to eliminate the catalyic converter from the system? I'm thinking about eliminating a 25 plus year old CAT that is probably just about useless by now. Thinking about dropping a Cherry Bomb, or Thrush glass pack into the car in place of the stock stuff. I guess I'm getting to old to grow up.

Any thoughts, other then telling me to grow up, are appreciated.

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1910_ANON: I had not read your post before I made my first reply. And now I have a question. I had a 1962 Corvair Spyder once upon a time, and you may recall that it was the first year for the turbocharged Corvair. The turbocharger was powered by the exhaust system flow, and I am only guessing, but won't that result in rather low back pressure? When the turbocharger engaged the horsepower when from 110 to 150 in the blink of an eye, or so it seemed. The car just about flew!

I am a bit in the dark as to back pressure effects/benefits.

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