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The advantage of "straight pipes" as opposed to a muffled exhaust is the lack of restriction. The engine is basically an air pump - piston down, air in, piston up, compress it and add fuel, ignite it, hot air expands and the piston goes down, piston up, air out, repeat several thousand times a minute. To increase performance you need to burn more fuel more efficiently, to achieve this you need more air to mix with the fuel, well you can't get more air in unless you can get more air out.
So the exhaust is the first step in performance gains, get the air out. Mufflers quiet the engine but they add a lot of back pressure and restrict the amount of air that can get out of the engine. Hi-flow mufflers try to maximize the amount of air you can get out of the engine with the minimum amount of sound. Unmuffled engines are extremely loud.

The disadvantage to straight pipes is that they are illegal for road use anywhere on the face of the earth. They are for track use only. If you are going to track the car, this is the exhaust for you. If you are going to drive it on the street, no. Get a good tuned exhaust system like a Tubi that will balance your car on that fine line between cool street performance and obnoxiously loud bad neighbor.
 

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Discussion Starter #3
Thanks, so that's why people use sports air filters with sports exhausts as it allows more air in.

As the mufflerless exhaust has no back pressure, is this a good or bad thing?
Will it increase performance against a muffler exhaust?

Cheers
 

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Exhaust systems aren't as simple as many people think. There can be times when the exhaust flows TOO good. This is what happens: All engines are set up to run with a somewhat restrictive exhaust system. The most important single component in this tuning is the camshaft lift, timing, and duration, for the exhaust valves. If an engine is running about as good as can be with a restrictive exhaust system, then chances are if the exhaust system backpressure is reduced to zero the engine may actually lose power. The optimum idea is to get all of the spent gases out of the combustion chamber, and as much air/fuel into the chamber as possible. The exhaust valve must shut at a specific time to stop the flow of gases....but it doesn't shut instantly. The backpressure actually acts like a wall or barrier and sorta keeps the fresh charge of air/fuel from following the spent exhaust gases right out of the head. If this backpressure is removed...many times some the incoming air/fuel charge will be lost with the spent gases...resulting a slight reduction of horsepower. This is often compensated for by richening up the fuel system a bit. If the engine has variable valve timing then the reduction in backpressure will be compensated for and there will be an increase in power....though maybe not as much as some manufacturers claim.

That system in the original posting without mufflers still does allow the use of catalytic convertors. So it would pass emissions testing, and be legal....if the cats were kept in place.

My Testarossa can be tuned to pass emissions without cats....but even if it does it's illegal not to have them on the car. I think they should modify the laws so that if a car can be made to run clean enough to pass testing it shouldn't matter what exhaust system it has.
 
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