header size
 

For a track car, 3.0 engine, 11.8 to 1.0, D60 cams, MFI, twin plug. What size header do want for that set up, OD/ID. Thanks, Bob

I'd go with the 1-3/4" primary pipes or at least 1-5/8".

My thoughts; 1&5/8. I'd think 1.75 might be a tad large for a 3.0?

1 5/8" for sure. 1 3/4" is too big for a normally aspirated 3.0L.

Your engine has a pretty healthy build spec 1-5/8" would work, ID is ballpark 1-1/2"

I have a 914 with a similar spec 3.0, except webers with big ports and 1-1/2" headers. It's a street car so I haven't been chasing HP, but I picked up a soup to nuts twin plug EFI system so I'm going to 1-5/8" headers.

What's the rpm range, muffled or not

http://forums.pelicanparts.com/uploa...1651879825.gif

but most likely 1 5/8" ID which is ~1.75" OD

No. 1 5/8" O.D. is the way to go here. Theory versus reality here.....

Let me give more perspective:

I had a 3.6L race engine with stock 1995 993 EFI intake system and 1 5/8" O.D. headers (not with overly thin wall tubing) make over 320 HP. Testing indicated that the intake was the limiting factor as more power could be made with a larger throttle body. I now have a 3.6L race engine with 45mm AT Power ITBs and 1 3/4" O.D. headers (again, not with overly thin wall tubing) make over 430 HP.

There is no way the 3.0L engine described needs more exhaust flow than what 1 5/8" O.D. headers would provide. In fact, the 1 3/4" O.D. headers would hurt power output especially under 6000 RPM.

header size
 

72-7500 RPM and a set of Burns megaphones. A 2''straight shot through a muffled shell. Bob

Bill. How does this flow graph relate to pipe size? What am I missing?

diameter is only part of the equation. That said, a 3.0 NA motor should not need more than 1 5/8....

Pipe diameter goes up with the square root of flow, so the graph shows you how you need bigger pipes if your redline and desired power band are higher than if they are lower.

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Right, but I don't see any relationship to pipe diameter in the graphs.

Pipe diameter required changes with flow, so I think the point is, until you state your redline and power band, nobody can answer.

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You'll notice that 8k a 2.7 processes the same volume of intake air as a 3.8 @ 5.5k under theoretical condition of 100% volumetric efficiency

the 2.7 operating up to 8k may want the same exhaust as a 3.8 operating to 6.5k

general rule of thumb for the eigen values of stockish street motors all ~31" primaries +/-
to 3.0 1 3/8" ID
to 3.4 1 1/2" ID
to 4.0 1 5/8" ID

Maybe on the back of a napkin….but never in real life.

That makes sense. Although I have seen formulas that predict flow and velocity related to RPM and cylinder volume.