For 'reasonable & possible' changes to R/S ratio (for instance, +/- 0.5" on rod length), piston-to-valve clearance differences @ about 12 degrees from TDC (where clearance is usually minimal) is negligible. Unless I screwed up the "geometry exercise", it would be on the order of a thousandth or two. Please show the math that supports your "dwell" concern.
Hi Jack,
It is not the piston position itself at a particular crankshaft angle that is my big concern. As R/S increases, dwell time around TDC is expanded, ie, it takes more crankshaft angle in degrees to begin moving the piston away from TDC. This, itself isn't "bad". Coupled with a "high velocity" valve train, problems occur. I want to use a cam that will give something like .0052"/.0055" lift increase, per degree. And I want to design in some "freedom" to advance/retard the cams a bit, on the dyno.
Indeed the position of 10-12* before and after TDC are often quoted as the spot of closest P-V approach ( and I have seen a good engine building book suggest checking at 10*-only-) you have to check. My engine with high R/S ratio, big valve, high lift cam had closest P-V at 20 degrees. I checked at every 5 until I was sure where that occurred. If you rely on one "generally accepted" spot, I humbly suggest Mr Piston and Mr Valve may become closely acquainted.
If the point of closest V/P clr is altered by 4/6 degrees, the valve notches have to be .022"/.033" deeper for the cam(s) I'd like to run. I want this built in. I don't want to take the engine off the dyno to machine pistons or change pistons, just to be able to change cam timing.
On "monstrously heavy" pistons, machining an extra .030" out of the valve clearance notch is probably of no concern. On the MM's BMC, we had to compromise on the cam, because the lightweight piston would not tolerate .020" of valve clearance machining. The dome was already very thin, .180" or so. Why? Because it had been designed for minimum weight to begin with . . . . . . I do not want to be in this position again . . . . . . .
On larger displacement engines, there is typically "room" or material, to alter (or "massage") the parts. On very small displacement engines, (61 cubic inches) there is much less leeway for "errors". I'd rather spend the time calculating out what is needed, than have a manufacturer send me what they "think" I need. It is always "a crapshoot".
And if mistakes are made, compression ratio suffers . . . . . . .
I'll get to the math on this for you later.
Mark