Anybody got any input on my previous question? What sort of compression ratio increase can I expect with what amount of decking if nothing else is altered in the stock combustion chamber?
Well...you might be looking in the wrong direction, or possibly asking the wrong question? The "dead" info is: the stock heads ranged from 22.0cc (K0 sandcast) to 23.8cc (F2/3), with 22.6cc being the 'nominal' value during the K1-K6 series.
But: the cylinders in mid-year K3 (thru K6) became 1mm taller (it was part of improving MPG and the U.S. gas crisis of the time) while the chambers tended to come in around 22.2-22.4cc. In the K4/5, the valves tended to be recessed into the heads, and not evenly, causing shrouding and uneven breathing, with attendant loss of power. And, in any given head, the chambers can range from 22.2cc to 22.8cc, very common variations until the F1 engines appeared.
The real issue might be: how much flow improvement can be provided with a given head and piston combination? The domed pistons cause a loss of flow as compared to the flat-topped pistons, especially in the all-important 4000-6000 RPM range. The higher domed pistons make this worse: Wiseco tries to offset the loss of low-end torque from their domes with their higher 10.25:1 compression ratio, as one example of a way to work around the difference. The once-popular 811cc pistons (square bore at 63x63mm) came in both the flat-topped types and the 11:1 types from Powroll: the former were far more 'even' in power on the street, while the latter worked up more HP at the high RPMs, once the extra-light valve keepers, stronger springs, trimmed and polished valves, and extra flycut pistons, with the HD cylinder studs, were ALL also added to the engine.
Once the compression squish reaches above 10:1 on these engines, the piston-valve clearance must be carefully controlled, and $$ add up quickly from it. And, the biggest power losses these engines suffered were from poorly made ports and large variants in the cylinder fins (flash leftovers) that caused heat buildup in uneven ways.
Generally, using larger pistons (836cc) means the throttle becomes the intake valve. So, anything that can be done to improve on this restriction will improve the flow, too. I find the deshrouding, quench band addition, port shape and smoothing, valve polishing, guide narrowing, and head detailing and chamber balancing can make within a few HP of the larger inlet valves: to go to the max, MRieck can add the latter for you, too.
