Increase stroke, same cam.....

[A to Z]

Here's a bit of a complicated question. My L6 has a diesel crank and H beam 240Z rods, and Rebello custom pistons set at 10.0 to 1 compression at zero deck height with my E31 head. The cam is the stock "A" cam. Because I am adding basically 1/2 inch of stroke, how will the stock cam act? I am thinking the cylinder pressures will be higher and port velocity will probably go up, but wondering what to expect.......will the timing have to be backed off? Opinions/comments please.

[Brad-ManQ45]

Cylinder pressure will be no higher than any other engine with 10:1 compression ratio.  Port velocities will be higher than with stock stroke and max rpm will be lower with same heads and cam.  You will have more power under the curve but max rpm lower because of port cam limitations due to larger displacement.  No reason to think timing would be unduly affected.

[A to Z]

Since I started this thread, I have learned a lot about this.  The basic idea is that when you increse the size of an engine, you need to incresae the size of the cam as well.  Adding stroke changes the STATIC compression, and too high of a static compression becomes an issue for several factors, one of which is pump gas will be harder to run......timing has to be pulled back, etc.  you have to find a fine line on what your usage is.....pump gas?  93? 91?  elevation?  carburetion adequate to feed the beast?  compression adjustments if necessary with a thicker cometic head gasket?....but you can't go too thick before quench becomes too much and affects the cylinders ability to burn the mixture efficiently.  When all is said and done, richening the mixture or even going 1 step or 2 stewps with a colder spark plug can help you live with pump gas with a higher compression ratio.

[clarkspeed]

Static compression is calculated based on bore, stroke, and volumes. Dynamic compression takes into account valve opening events. Longer duration cams reduce the dynamic compression. I have read you can have too much dynamic compression, say a low duration cam on a high compression engine, but I have never never seen any real numbers on what the limit is.  My my guess would be less power at higher rpm's.