Originally Posted By: PeterPolyol
Sorry I didn't see. I get too excited to respond to threads sometimes
Not sure if I totally agree that it's due to manufacturing processes resulting in sloppier clearances though. Plenty of engines, specific engines, suffer slap. Honda 4 cyls are ready examples- not sure if their manufacturing process leads them to wide ranging clearances but as horrible as slap sounds, it doesn't hurt.
The piston expansion was a good point. It's why many OEs choose high-silicon hypereutectic metallurgy for their pistons, which controls contraction/expansion. Downside is that it makes for a more brittle piston- less tolerant to severe detonation- but not exactly a problem for grocery getters and commuters.
Why would some engines like the LS and several series of Honda engines have such disagreeable NVH/chronic piston slap? Most engines never slap at all and aren't exactly cracking pistons or losing power to thrust
Short pistons/short skirts.
Examples with forged pistons are even louder, FWIW, because of the greater expansion/contraction when compared to a hypereutectic.
I wish I had some here to show you, but if you compare an old Ford 302 piston (an engine that never slapped as far as I'm aware, even with forged pistons) to a Modular, of which there have been examples that have slapped, you can readily see the rather massive difference in piston height. Now of course the older engines had hand-fit pistons/bores too, but even on the "el cheapo" cast slug non-HO versions, which didn't get the factory balanced rotating assemblies and I assume far less attention to detail on piston-to-bore clearance than their forged-slug HO siblings, they were not noisy because of the piston height.
Just going to short pistons doesn't make noise. That's why not all examples slap. But short pistons in conjunction with wide piston-to-bore clearance will make noise. It's a cumulative result.
Some stock pictures which may help:
Stock 302HO piston/rod:
Stock 4.6L piston/rod (not in the best of shape but conveys the difference):
Stock GM LS1 piston/rod: