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I frequently get asked if adding headers and a low restriction exhaust is necessary or worth the money, well the answer depends on what your trying to accomplish, the stock corvette exhaust and shorty headers are not all that bad on a stock application, but theres always room for improvements to be made.
being a bit pragmatic here, Id say that its rather rare for a hot street/strip car combo with 400 cubic inches or less and under 500 hp that probably 75% of us drive to need an exhaust larger than dual 3" pipes with an (X) and that's usually what Ive found works on most street strip cars.
you don,t want too assume your exhaust is restrictive , or that its adequate, you need facts to base your component selection on.
now obviously if your building something significantly larger or more powerful you'll want to do some calculation before selecting an exhaust design, but what always amazes me is how few people bother to buy and use a simple vacuum/pressure gauge to actually test and measure back pressure.
without, testing and having the facts,and knowing what your dealing with it seems foolish to rush out and spend hundreds of dollars on a new exhaust that may or may not solve your problems or to ignore a problem that you can easily test for and verify before spending any money on a problem you can prove is caused by a restrictive exhaust system,
drilling a small easily re-welded test hole in the header collector gives you access


http://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html

http://forum.grumpysperformance.com/viewtopic.php?f=56&t=495&p=613#p613

http://forum.grumpysperformance.com/viewtopic.php?f=56&t=1166
 

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A vacuum gauge is also useful in determining if you need a bigger throttle body. If your making vacuum at wot under load, you can use more air flow there.
 
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