Header size calculator
 

...found this calculator on line. For those who "know", how does it look? ...not quite sure about collector dia (seems pretty big).

76.4 stroke, 102.7 bore, 42.5 exh valve, timing from DC-85-106 cam.

Primary Header Tube Length and Diameter Calculator

http://forums.pelicanparts.com/uploa...1684793485.jpg


http://forums.pelicanparts.com/uploa...1684793609.jpg

Your volumetric efficiency should be higher than that. Likely closer to 100%, if not over.

Otherwise, the calculations seem really close to what I'd recommend.

I’ve wondered about collector size as well. Calculators seem to recommend substantially larger than any of the 911 race header manufacturers produce. Largest I usually see is 2.5” for 911 headers.
Thanks for posting about this. I’m about to purchase new headers for my 3.5L race engine and am also curious.

One minor thing - I’d bet the calculator assumes duration measurements at .050” lift. I see you used the 1 mm value.

That particular calculator uses "advertised" duration and likely assumes that you don't have a merge collector. With a merge collector, you can decrease the diameter.

Another thing it assumes is open exhaust. Many of the aftermarket Porsche headers (even sold as "race headers") have flanges to fit various mufflers. That'll make a difference in what we see available in the marketplace.

One thing that I've rarely seen over the years on even race Porsches was stepped headers. Practically every other brand has two and three step headers available. Just an observance.

Dan -- Would you say the primary tube diameter is ID?

That's in line w/ the rule of humb usually bandied about, 1.75" OD which can vary but is often 1.625" ID

911 collectors will be smaller than on some other cars because the 911 engine is even fire. Each collector receives a pulse every 240° +/- depending on cam timing
http://forums.pelicanparts.com/uploa...1684869029.gif

American V8s are of=dd fire, each collector needs to be larger because each bank gets 2 consecutive pulses in the rotation
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Tube diameter is almost always referenced as O.D. along with a wall thickness. Only the folks that created the calculator could tell you if the output reflects I.D. or an O.D. with an assumed wall thickness.

As Scott mentioned, tubing is always referenced by O.D. Pipe is referenced by I.D.

BTW, you can order tubing in many different wall thicknesses, while the O.D. remains the same. It's not too critical unless you're worried about weight or when stepped headers are involved and you want one tube to fit inside the other. Or if the tubing is supporting a turbo with minimal brackets.