people, cylinder pressure is not terribly relevant to octane requirement.

i know that there are all sorts of websites out there that use digital "slide-rules" and fancy, all-encompassing guidelines for octane choice, but the fact is, it doesnt mean squat in a V8 pushrod engine with inline valve heads.

whichever octane rating, or anti-knock index, you require is defined by several things, as most of you have already mentioned. the most important, and unstated factor is PISTON DWELL TIME. with a typical O.E.M. chamber and valve layout (which most aftermarket heads use as well, despite claims to the contrary), a long rod or large stroke will enable the engine to resist detonation more easily. a 351 with a 6.35 inch rod can take nearly 11.7 to 1 compression with aluminum heads and pump gas. timing values have to reflect the engine, not the cranking pressure or octane. regardless of what the side of the gas pump says, an engine will always be happiest with ONE timing curve. nearly identical engines can take totally different curves. this is something that cant be accurately estimated over the internet. the only way to find out the optimum curve is through testing. the better your cylinder head and camshaft, the less total timing youll need.

air intake charge is not terribly important in a N/A engine. the air temperatures do not reach the heights they would in a pressurized manifold, and therefore are fairly consistent. more to the point, a fluctuation of 50 degrees of N/A intake temperature rarely requires a tenth of a point of octane to remedy. while colder air is denser, it is not less prone to detonation. the chamber is ridiculously warm, and prior to ignition, the air charge is too. emulsified fuel cools that chamber, but that just happens to be a constant. regardless of aspiration, the fuel is always there. an isolated plenum manifold wouldnt stop a pinging motor from pinging, no matter what. coating the headers wont help either. exhaust pipes can get a million degrees, and it wouldnt matter. they are after the fact, in the combustion cycle. they dont heat soak a head enough to matter either.

dynamic compression is crucial of course, but most aftermarket camshafts are excellent as far as compression retention. lobe seperation is not the deciding factor in wheter or not your cam is pissing away compression though. valve timing is far more important.

and please, never use Av gas. it was engineered for engines that dont see fluctuating rpms or high atmospheric pressure. Av gas is not a stable race fuel.

 

I can't agree with everything you are saying. The longer rod to stroke ratio, the higher chance of pre detination. The longer rod ratio creates more piston dwell time at TDC. This is bad for detination. The flame travel will peak without the piston moving away fast enough. This equal detination. I don't beleive the stroke length has anything to do with detination. It's the ratio of the two.

 

dwell time is controlled by rod length and stroke, with stroke being the more effective of the two.

whenever you increase your dwell time, your camshaft timing should be changed to reflect this. what we then have is a piston that spends more time at top dead center. the timing can then be retarded to take advantage of this situation. less total timing is needed, for the same amount of combustion pressure and burn efficiency. the valves are then opened to reflect this, and chamber cooling is actually ENHANCED. while this may sound odd, its common practice. the rod length allows you to reduce timing and maintain the same hp, while allowing the camshaft to open the valves at more effective times in the combustion process.

"I don't beleive the stroke length has anything to do with detination."

this is where you are wrong again. detonation is when combustion pressures peak before TDC, usually from out-of-control pressures and ignition.

a long stroke does two things. it moves the piston EXTREMELY fast in the bore, yet slows it down more gradually for the reversal of direction. a long stroke motor will always outlast a short one for this very reason, given identical peak piston speeds. for all the reasons i stated above, a long stroke will help with the efficient filling and evacuation of the cylinder. the chamber is kept cooler by a more controllable flame front.

in case you had forgotten, the flame front is what cools the chamber in the first place. the air that is pushed along by the flame front travels across the face of the combustion chamber, and cools it.

all of these things contribute to octane requirement. an efficient chamber, that builds pressure, instead of heat, will be the one that can run on a lower octane.

the entire process is dynamic. stroke and rod length (i.e. dwell time) is the most important aspect of detonation prevention, short of chamber/piston design and compression ratio (dynamic or otherwise).