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Discussion Starter · #1 ·
This is taken from Hohn over on the TDR.

Generally, an open tube will flow about 115cfm per square inch of cross-sectional area.

The stock 3" pipe yields an area of 7.07 inches, and therefore flows about 813CFM.

A 4" pipe yields an area of 12.57 inches, and therefore flows about 1445 CFM.

A 5" pipe yields an area of 19.63 inches, and therefore flows about 2258CFM.


So there you have it:
3"= 813 CFM
4"= 1445 CFM
5"= 2258 CFM

Also note how increasing the pipe diameter affects AREA. When we go from 3" to 4", we've increase diameter by only 33%. But we've increased flow by 78%!

Going from 4" pipe to 5" pipe is a 25% increase in pipe diameter. The flow increase is 56%.

Finally, note that the difference between 4" and 5" pipe is EXACTLY that capacity of the 3" pipe by itself. So stepping up from 4" to 5" is exactly like ADDING another 3" pipe worth of flow capability.

The numbers also tell us that many of us cannot get to the point where 4" is a restriction, so stepping up to 5" will yield no performance benefit-- exceptions being those with large twins.

Consider that with no boost, the 359 Cubic inches of your CTD will need 332 CFM at redline of 3200 rpm. The first bar of boost doubles that to 664 CFM. The next bar (29.4 psi of boost) gets us to 996 CFM. Going up to 3 bars takes us to 44.1 psi of boost and 1328 CFM.

So on paper, a 4" exhaust isn't even nearing it's flow limits at ~45psi of boost. In REALITY, though, the actual flow will be MUCH lower, further increasing the margin that a 4" exhaust has.

This is because our above illustration assumed that as intake pressure was raised, we would get a linear increase in airflow. NOT TRUE. With each succeeding bar of boost, we gain less and less actual airflow. Eventually, you get to the point of stagnation, where no matter how much higher you raise the boost, you will NOT see any more airflow through the engine.

So while the "paper" example shows that 4" is more than adequate up to 45psi of boost or so, in the real world, the flow limit of 4" is probably much higher-- in the mid 50s or so.

Just something that occurred to me last night laying in bed.
 

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Cooter, maybe you should take up counting sheep???

Good info though.
 

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Discussion Starter · #3 ·
LOL

I know Exactly how Hohn over at the TDR feels. I'm the same way.

I took that bit of information from the him over there.

I hate when I get a great idea laying in bed......have to get up and write it down.....next thing you know the alarm is going off!

Josh
 
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