[schnellmann]

Brian,

I won't pretend to be more of an expert than Charles, but I'll explain the issue slightly differently.

What you say is true regarding compression, but taller deck heights leave larger "dark corners" in which conbustible mixtures will lurk. Optimal deck heights, and their affect on compression ratios and combution are also determined in part by piston dome shape and the location of the spark plug.

And plug position in Porsche heads really factors into this deck height equation. Imagine a piston coming up to TDC and the plug waaaaaay off to the side sparking to start combustion, say ~25+ degrees before TDC. Now as the piston continues to move up the cylinder, the flame front starts to travel across the cylinder. But what happens if the mixture all the way on the opposite side of the cylinder ignites due to compression heat before the flame front reaches it? Detonation.

By the way this problem is the whole reason for twin plugs - they effectively halve the distance the flame front has to travel and start from both sides of the cylinder.

Larger deck heights leave larger pockets of combustible mixture to ignite before its time, particularly in larger bore motors with way offset plug locations such as ours. Sometimes the flame can't get all the way across the cylinder before the mixture reaches ignition temperature. Every ignition stroke is literally a race across the cylinder.

So shallower deck heights are really a risk-mitigation tactic. Less fuel to detonate in the dark corners of the deck height void means less potential for damage if it happens.

Along with the problems I mentioned above, Porsche motors are air-cooled and that leads to wildly varying combustion chamber temps that are heavily influenced by ambient temperatures - thus further increasing detonation risk as the air temp of both mixture air AND the cooling air increases. Basically all engines are water cooled these day simply because that method offers much better combustion temp control (and emissions reduction as a result).

Anyway, I hope this makes sense. It took me a lot of reading on the subject to comprehend this interrrelationship between engine/piston design, deck height, plug position and so forth.