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How big a exhaust pipe do you really need for a 4bt? I am accumulating parts for a conversion now. I want to put a 3200rpm spring in my pump. my chevy 400 had 400+hp and had dual 2.5" exhaust good enough to 6200rpm. 2X2.5" pipes have the same area as 1X3.125 pipe. 400CI=6.5L. 6.5LX6200=40655LPM=/needs 3.125 pipe.

4bt numbers: 3.9LX3200=12480lpm/40655lpm=3.2 or about 1/3 the LPM flow requirement for exhaust............................................So lets now double this because of the "unknown factor" and say the 3.9 needs 2/3 the flow of my 400. This equals approximately 1X2.5" pipe.
I now have some 2.5" pipe from my old exhaust and 2X flowmaster 50 series with 2.5" in/outs. The flowmaster 50's are too loud by themselves. If I have a 2.5" pipe leading to 2Xflowmaster 50's in series I think it will be quiet enough and not too restrictive.
Anyone else got a different opinion or think 2.5" is not big enough for a 4Bt?
 

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2.5 should be fine.

My Isuzu revs to 3600 at at 15psi the backpressure in front of the turbo doesn't rise from 1500rpm to the rev limiter.
I haven't measured backpressure after the turbo (exhaust only backpressure) but it stands to reason that if turbo+exhaust doesn't increase noticably with rpm then the exhaust only isn't significant.

The more fuel you put in, the hotter the exhaust gas and the bigger volume you have to get ride of.
 

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I have just put a 4" on my 6BT, I would not have been happy with 3" as I didn't want EGTs to be an issue. A 3" pipe is only just over 1/2 the surface area of a 4" pipe yet a 4BT is 2/3 the capacity. If you are going to be bombing at all go 3", you don't want to be doing it again do you. Diesel's require NO backpressure unlike a gasser so the BIGGER the better upto a point. Hell put a 4" on and be done with it.

Gaza
 
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