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I think they did an X pipe vs H pipe vs straight through on that motor trend show with Frieburger at Westtech dyno. Showed really no improvement and only minimal sound change 8f I remember correctly
 

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So are you saying from experience that dual 2.5" will not be a restriction on a 650hp Small block? If so, do you think it would be better if the headers were also made with 2.5" collectors?
No, 3.0" collectors are the best choice for this system. When the exhaust gas "plug" (aka pulse) enters the collector from the primary tube, the larger collector diameter mimicks the exhaust emptying into the atmosphere (to some extent) whereby the exhaust pulse loses significant pressure and velocity (and sends a pressure wave back up the primary tube towards the exhaust valve). Once the exhaust pulse has lost some pressure and velocity (as well as heat) the exhaust will travel very easily through the 2.5" exhaust system. Upon the reduction of exhaust pressure/velocity/heat in the mufflers, a 2.0" exhaust pipe will also work very well.
 

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So are you saying from experience that dual 2.5" will not be a restriction on a 650hp Small block? If so, do you think it would be better if the headers were also made with 2.5" collectors?
You have been saying this on every forum you belong to for years. I would love see the data that backs your claims up.
Just read this quote from him and you will ignore everything he says.

If the engine produces more torque/horsepower after advancing or retarding the camshaft, it is a signal that you do not have the optimum cam for your combination. Consult with a camshaft designer, or major camshaft manufacturer for advice on selecting the best cam for your application.
 

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You have been saying this on every forum you belong to for years. I would love see the data that backs your claims up.
Thank you for noticing and mentioning the consistency of my views on this subject over the years . . . .
 

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Thank you for noticing and mentioning the consistency of my views on this subject over the years . . . .
Even being incorrect, at least your consistent! ;)
 

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Ok thinking about this for my new car, but 433 inch BBC. About the same N/A Hp though. I have 3.5 collector to 3" pipe and mufflers. Looking at going 2.5" for tail pipe. So looking at this I should be fine????
 

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I picked up 4 MPH in the 1/8th switching from 2.5" exhaust to 3" exhaust on a 450 HP SBC. 2.5" exhaust is too small for 650 HP.

I have run 117 MPH @ 3650 lbs in the 1/8th with 3.5" collectors and headpipes into a 3" muffler using Flowmaster 3.5" to 3" long reducers right before the mufflers.

Switching to 3.5" mufflers on the same system gained nothing. ~900 HP N/A SBC.

I'll be testing 3.5" vs. 4" exhaust along with merge collectors on a 1000+ HP N/A SBC this year.

Jason G.
 

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I tried posting these images earlier, but the forum still doesn't show tinypick images so let's try imgur:


Flow table for straight pipes:



Engine master resluts:



Engine master h-pipe vs straight pipe:



Engine master h-pipe vs x-pipe:
 
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