Air restrictor camshaft design principle?

[knighty]

I'm interested to know information about the basic principles employed when designing a camshaft for a normally aspirated race engine that runs air restrictors.........an f3 engine would be a fine example...........my thinking is that overlap is generally to be avoided, because the depression in the intake (due to the restrictor) could potentially pull the exhaust gasses back into the combustion chamber during the overlap period - this in my mind would be a bad thing........I'm thinking a good set of restrictor camshafts would be much like a set of turbo cams, whereby turbo cams also have no overlap, otherwise the excessive pressure in the intake manifold during overlap, would blow too much air out of the combustion chamber - which is a tad inefficient.

[Dennis.Doyle]

I've no answers, but some questions. First what does the pressure curve over time in the inlet manifold actually look like? I don't know, but I'd guess that it is actually positive during some of the overlap period - isn't that why we get "stand-off" at carb trumpets? Hence, won't a restrictor actually make the pressure rise more? I'd also ask how much effect does a restrictor have in the overlap period - remember valves are not not fully open & the piston travelling relatively slowly?

Next, 2 real-world thoughts. First, F1300s ran with a single 28mm restrictor & revved to 9K. People used standard race cams (A8s, 9s etc). Mind you they probably didn;t have the resources to get special profiles made - so maybe that's irrelevant. Second, get hold of a set of F3 cams (from anyone who put one out to grass in hilllclimbs) & see what they actually look like.

My guess is they are lowere overlap than non-restricted cams, but mostly because that optimises power/torque lower down & the restrictor makes it pointless to look for peak power at a zillion revs. I could be completely wrong of course as I am guessing.

[AU N EGL]

Originally Posted by Dennis.Doyle

My guess is they are lowere overlap than non-restricted cams, but mostly because that optimises power/torque lower down & the restrictor makes it pointless to look for peak power at a zillion revs. I could be completely wrong of course as I am guessing.

Dennis you quite right.