Cam Overlap Question

[Rslevin]

I rebuilt my '73 2.4L using S pistons from a 2.2 and E cams. Spec books seem to have 2 overlaps listed for E cams. Anyone have any thoughts about best overlap? Thanks,

[tom1394racing]

Rich

Overlap is built into the camshaft geometry. There is one standard for factory E cams.

Are you asking about cam timing?

[Rslevin]

Understood, but there are two mitigating factors; one the engine is modified with 2.2S pistons, and two, there are two E cam overlap specs, depending on the year. If I read what you are saying, the former, 2.2S pistons don't matter. But, can you shed any light on the two differing specification in the Porsche spec books? I don't have the two books in front of my now, but I recall the earlier was 3.1mm overlap and later, 73 book showed 2.9mm. Any thoughts as to what is going to give me the most kick? Thanks, Rich

[Walt Fricke]

Rich

That's not overlap per se you are talking about. As Tom said, true overlap is the time (well, duration in degrees) when the intake is opening while the exhaust is still open. You can only change that by regrinding a cam, or using one which is ground differently.

You are talking about cam timing, which is how high the intake valve is lifted at TDC. The 3.0 and 3.2 cams are the same (not sure if the 2.7S is also, but different bearings), but Porsche over the years used lower numbers 0.9mm and higher numbers 1.7mm, and ended up somewhere in the middle for the 3.2s. Bruce Anderson's books go into this. The 3.1 will give you a little more torque down low, and the 2.9 will give you a little more HP up high, and you decide which you want. The motor will run fine either way. In fact, the difference of 0.2mm between the two specs is within the spec tolerances for these cams, which run from 0.2 to 0.3mm allowed.

And no, I don't know why Porsche decided to change the spec. But it gives you the choice.

And this is often called overlap, too, even if it has nothing much to do with that. Anderson calls it TDC overlap, which serves to differentiate it from the TDC where the spark plug has just fired.

[Rslevin]

Thanks Walt. This is exactly what I was looking for. -Rich