can't decide on exhaust

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Aug 5, 2007 | 09:05 PM

quickspec

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From: Louisiana

can't decide on exhaust

Guys I have a question, right now I have megan racing exhaust and I plan on getting a vibrant header, will running a vibrant exhaust help me out if i'm making over 300whp? I'm set on buying the vibrant header, but will going from a 2.5'' header to a 3'' exhaust be better than 2.5'' header to 2.5'' exhaust? Sorry if it's kind of confusing.

Aug 5, 2007 | 10:29 PM

ebristol

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From: WI

Quote:

Originally Posted by quickspec

Im not sure about hp. Here is some usful info. I will wish that everyone would read this.

Quote:

Originally Posted by jmc007

Hi ! Here are my flow calculation for some LSJ engine parts :

Stock Throttle Body (67 mm) : 625 CFM.
Stock Head : 600 CFM.
Stock Exhaust manifold ("Header") : 425 CFM.
Stock Downipe (DP) with cat : Plus or minus 370 CFM or less.
Stock Cat-back Exhaust (2,5" inlet to 2,25" outlet with stock muffler) : 300 CFM.

Now here are some replacement parts :

GMPP Exhaust manifold ("Header") : (30 more CFM than stock) : 455 CFM.
Shorty or long tube Header with 1,75" primaries and 2,5" outlet : at least 500 CFM.
Shorty or long tube Header with 1,75" primaries and 3" outlet : at least 725 CFM.
Shorty header with 1,5" primaries and 2" outlet : Waay not enough, who cares LOL.

Stock DP w/o cat : 425 CFM.
2,5" DP with VERY HIGH FLOW cat : plus or minus 450 CFM.
2,5" DP w/o cat : 500 CFM.
2,75" DP with VERY HIGH FLOW cat : plus or minus 475 CFM.
2,75" DP w/o cat : 600 CFM.
3" DP with VERY HIGH FLOW cat : plus or minus 500 CFM.
3" DP w/o cat : 725 CFM.

Stock catback with only the rear muffler replaced (the last portion is still in 2,25" tubing) : 350 CFM.
Stock catback with last portion replaced for 2,5" tubing and straight through muffler : 450 CFM.
2,5" catback : From 450 to 500 CFM (must be a straight through muffler).
2,75" catback : From 550 to 600 CFM (must be a straight through muffler).
3" catback : From 650 to 725 CFM (must be a straight through muffler).

Hope this can help ...

Regards

)

Basically if you plan on keeping your Megan 2.5" exhaust a 3" header is not going to do anything for you without replacing your dp.

Do you know the flow rate of your megan exhaust? I was looking for it but couldn't find it. Bascially here is your current setup.

Stock Manifold 425 CFM
Stock DP 370 CFM
Megan Cat-back 450-500 CFM

So right now your dp is your bottle neck. If you want your cat-back to perform at its highest potential I would recommend picking up a 2.5" header/dp combo from zzp. There seems to be a lot of people going with that setup.

Aug 6, 2007 | 02:59 PM

quickspec

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From: Louisiana

I plan on getting the vibrant header, but I don't know if it would be better to keep my 2.5'' exhaust or should I order there 3'' exhaust since i'm making over 300whp?

Aug 7, 2007 | 02:54 PM

ebristol

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From: WI

Quote:

Originally Posted by quickspec

I plan on getting the vibrant header, but I don't know if it would be better to keep my 2.5'' exhaust or should I order there 3'' exhaust since i'm making over 300whp?

That is a choice that you are going to have to make yourself. Personally, I think a 3" exhaust is a waist of time. Why else do you think there are so many used sets for sale?

1. They are way too loud
2. They are not worth it.

But on the other hand some people swear by them. So its really your choice.

But don't get one so you can say "I have a 3" exhaust on my 4 banger!"