how to calculate cam overlap?

[bitpusher]

i see a few overlap calculators out on the web but they all seem to want lobe separation angle which i don't seem to see in all the charts...

specifically, i am interested in the overlap of N4's vs D grinds (the cams i have on hand).

i suspect that for my application, turbo, that less overlap is going to be my friend at this stage and that D grinds have less than N4.

but what about mixing the intake from one with the exhaust of the other? perhaps i can get a good compromise in overlap in the interest of giving the turbo more of what it wants, exhaust....

so i guess there are 2 questions here....

[Gcram399]

Need a degree wheel and an indicator set-up would be my guess as the best way to measure anything with cams.

[bitpusher]

Originally Posted by Gcram399

vht paints

is this a shot at the king of non sequitur?

[Screw Loose Dan]

I'll say, I'm not really 100% sure. I've read and re-read this passage on Hammer's website trying to grasp it:

The next important spec to look at is the overlap. Overlap is the sum of two of the timing numbers, exhaust close and intake open. Add these two numbers together. Overlap is the number of crankshaft degrees in which both valves are open at the same time. Why is this so critical? It happens as the exhaust cycle is finishing and the intake cycle is beginning, with the piston passing through top dead center. While both valves are open, the exhaust and intake are effectively connected together. If the exhaust pulls right then, it'll get the next intake charge moving before the piston even starts pulling on it, giving the cylinder fill a huge head start. But if the exhaust pushes back right then, it really messes up the cylinder fill and fuel mixture (we call it a "reversion"). So in a nutshell, more overlap turns over more control of your engine's powerband to the pipe. When using cams with a lot of overlap, your torque curve will essentially be a map of how the pipe behaves at different rpm's. Overlap with a good pipe really helps you. Overlap with a bad pipe really hurts you. If you've got a pipe with the sound or looks that you like, but it doesn't work well at the rpm you like to ride, choose cams with less overlap.

This blatantly copied from this specific page on Hammer's website.

[Ireeman]

It is a rotating circle, 360 degrees is all you will have. Pick your lift in height and time.

[Folkie]

Yep Dan's got it right: intake open (° BTDC) + exhaust close (° ATDC). It's the angle during which both valves are open.

[maru]

The se .551's make a great turbo cam. Your N4's will be fine. The custom ground turbo cams I have seen have more overlap than the N4's so i would not get overly technical. I am sure the D's would work as well, but I would use the se .551s in your application.

[bitpusher]

thanks guys. it seems simple enough but the calculators made it seem like i was missing something.

".... so i would not get overly technical" - if anyone believes that steve would not get overly technical i have a bridge in alaska you might be interested in...

at risk of getting overly technical why do you find .551 attractive in a turbo app?

[aswracing]

Originally Posted by Folkie

Yep Dan's got it right: intake open (° BTDC) + exhaust close (° ATDC). It's the angle during which both valves are open.

Yep, HAMMER Dan is a genius. But I wrote that page

The intake valve actually opens at the tail end of the exhaust stroke. Likewise the exhaust valve closes in the early stages of the intake stroke.

So overlap is how early the intake opens plus how late the exhaust closes, both relative to overlap TDC.

Pressure in the cylinder and exhaust (they're the same thing when the exhaust valve is open) is a very bad thing when you open the intake valve. It makes everything go the wrong direction.

This is why pressure differential is so important in a turbo application. The turbo is an exhaust restriction, which creates pressure upstream from it, i.e. in the cylinder. If it's not creating at least as much pressure for the intake side at the same time, overlap makes the flow go the wrong way. And from what I can gather, most turbo's don't.

So you have to take overlap out the cams.

The lower you can get that exhaust to intake pressure differential, the more your cams can start looking like NA cams.

There's lots of good reading on this subject.

[Folkie]

Originally Posted by aswracing

Yep, HAMMER Dan is a genius. But I wrote that page

I meant Screw Loose Dan