Originally Posted by Silver Surfer
I have not thought about the relationship to vehicle weight & gearing as it relates to quench. So apparently motorcycles have it easy based on what you are saying. But many of the newer autos with them fancy overhead cams have 4v per cyl. Granted they tend to be lighter vehicles and run at higher RPM's than old Detroit iron, but they are a far cry away from inherent advantages of motorcycles. The intent of my post was not to single out motorcycles either, I was mainly focused on autos, but I do thank you for pointing out that comparing engines to engines is not apples to apples in this case.
So if I understand, you are saying there outer rim of the piston is the squish band (can't be too much surface area here to be effective?), and the domed piston plays a part of the squish operation in the hemi type head? I would think that to get any type of decent squish with the domed piston/hemi head you would have to get so close that the piston and valves would occupy the same space-time...unless these newer 4v engines have significantly less valve overlap?
Often the squish band isn't as effective, detonation with pent and hemi chambers is a problem, they like to call it combustion rumble. Its admittedly a little different but left unabated the results look a lot the same. A factor the pent and hemi have going for them is the spark plug is usually much closer to the center of the chamber so the flame front radiates out from there getting to the wall side much faster than a wedge with the plug off to one side causing the burn to have to spend time to get across the bore so the far side (sounds like a cartoon) sees a lot of heat and pressure before the fire gets there so sometimes it goes off by itself ahead of time. The rather subtle changes of the Vortec type chamber compared to those small or large that came before is astounding in effect. It moves the plug in as far as practical so the burn isn't so far, controls the incoming mixture flow with a relief between the the valve and plug and puts the old plug side squish/quench step back in if only by the exhaust valve short turn side.
One also cannot rule out the effect of fuel injection where the unequal mixture ratio and volume distribution of the carb and manifold is eliminated. Additionally with FI, the puddling of fuel immediately behind, if not on the valve backside, puts the heat absorption of vaporization into the combustion chamber instead of within the venturi. Plus, the incoming air (mixture) doesn't need to be heated to force vaporization of the fuel, so the injected engine sees a much cooler mixture at the valve. This forces a better combustion chamber design but that also works better where a carburetor is involved.
Yes detonation sensitivity is related to the load on the end of the crankshaft. The more advantage the engine has in lever arm against those loads the less it tends to detonate. That backs up into the root causes in the cylinder of too much heat over too much time. In and of themselves gearing isn't a cause or solution it just introduces less cylinder heat by lowering how hard the engine has to work.