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Old 10-18-2012, 04:28 PM
oldbogie oldbogie is offline
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Quote:
Originally Posted by hcompton View Post
I dont think he can pick a cam since the heads dont fit the block. He is going to end up with diff heads or pistons before its right. At the very least he will need the compression ratio. If it super high those small cams will be more trouble than the lack of hp is worth. If you get my meaning.
I guess I've been at this too long, I start with a general idea of how much power we want and the RPM range we need that happen in. I have at that time a general idea of pistons and heads which probably comes from doing this more than once but never-the-less the cam timing is in large measure going to drive the compression ratio which will drive the combustion chamber and piston crown sizes and shapes. So once I'm settled on that choice and know where the intake valve will close in crankshaft degrees of piston position in the bore, then I compute the DCR and SCR that the cam needs to be efficient and effective. Then the heads and pistons start to get narrowed down as to port, valve and combustion chamber size, and piston crown shape, deck clearance, and gasket thickness that are needed to dial in the compression with minimal squish/quench clearance along with adequate spark plug, valve and chamber contour clearances. The head selection will drive on the piston crown shape and volumes as to flat top, dome, or dish for best fits and clearances that minimize having to mill the piston to shape as most catalog pistons don't have enough head material, nor should they, to go change the crown shape. If you're getting into that then you need to order a custom semi-finished slugs and machine the crown to suit myself. This obviously is on high buck motors; certainly that kind of effort isn't done on street or claimer engines. I realize, especially with drag engines that very wild cams end up driving you into domes alternate fuels can drive you into large open chambers which also take you to domes, but I bust my hump to avoid domes as best I can because they donít play well with flame front travel and create a lot of surface to volume which makes the whole chamber run like itís a giant squish/quench deck, this drags you to other compromises in fuel mixture ratios and ignition advance that drive sufficient perhaps but certainly less than optimal tuning solutions.

I'm always amazed how guys just order a bunch of parts without thinking this through. For the hobbyist and tinkerer their best strategy is to copy one of the many magazine and soft cover book builds or cruise the web for similar information, this can save the average guy a lot of money and time while letting him/her build a very nice engine they can be quite proud of.

Bogie
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