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LS1 powa!
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I'm doing some research and pre-planning and want to know if what I have right now will be sufficient. I currently have the Flowmaster force II cat back, the pipes measure 2.5" OD. Will this be too restrictive, or good to go?
 

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Unless your building a high compression engine, with mods, that will be fine. Dual 2.5" supports just over 500hp if I remember right.
 

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I am also interested what others have done.

I am thinking a Dynomax VT 3 inch single all the way back or 3 inch in a dual 2.5 out. Its a daily driver so max HP is not a issue.

Anybody done either one of these ??
 

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It's a 4. something
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I have true dual 2.5 mandral bend no cats with sanderson shorties and borla pro xs mufflers. It's good but I think a tad large for my stock internal, stock tune 4.8. I wish I would have went 2.25 into a single 3" exit based on what I've seen. I dunno. I'm no expert, that's just my 2 cents
 

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I will be doing 2.5" off of each manifold to a single 3" through some sort of turbo muffler out the factory exit as well.
 

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SOmething i read on ls1 forums . This is information written by someone else take it for what its worth . the replies on the thread was that this right on the money


"Hey guys I just got done reading a good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.). From reading the article I think that a lot of people that see gains when going from a 2.5” exhaust system to a 3” dual system see them because the muffler on the 2.5” system didn’t flow enough for their application. David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss

A 2.75” (stock) single system is good for a 310hp engine with zero loss…
A 3” Single system is good for a 370hp engine with zero loss…
A 3.5” Single system is good for a 503hp engine with zero loss…
A 4” Single system is good for a 657hp engine with zero loss…
A 2.25” dual system is good for a 457hp engine with zero loss…
A 2.5” dual system is good for a 513hp engine with zero loss…
A 3” dual system is good for a 812hp engine with zero loss…

Now these numbers are assuming that everything else is set up perfectly. The muffler must flow as much as the open pipe or more to get zero loss at the listed hp levels. He also states that using a muffler with a larger inlet/outlet diameter than your exhaust pipe is a great way to get more out of a smaller diameter system since the muffler flow will be able to match the straight pipe flow. Now there are many other things to consider when designing an exhaust system but I figured this would give a great foundation to build on."
 

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true duals 3" with cats an h-pipe and two dynomax 3" vt mufflers
 
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