head compression vs fuel octane

[russhill]

It's not really "flame speed", but flame proagation and, in theory, slower flame propagation could/should provide some power down the stroke. That's why various additives could provide more power.

Inhibiting flame propagation by engine design (piston baffles) does nto serve any positive function.

[John (Taylor) Gabrowski]

Years of misunderstanding what ignition was really all about for outboard racing engines for me was finally set straight by a number of people running a variety of engines talking about their experiences and then reading about and seeing an engine with a quartz combustion chamber window was all about. It just was not a matter of setting timing to meet the compression ratio of the engine, it was a combination of so many factors many posters have identified here time and time again all relevant. What everyone wants to end up with is an engine that in the end does not fight with its own power producing rotation no matter how the engine is configured which includes its own engineering and metalurgy involved. Its like selecting the coldest possible sparkplug that takes away and transfers the heat it absorbs to the water jacket without cold fouling. It follows that you need an ignition system that can fire real cold, dense, airfuel mixes of a leaned out 2 stroke engine running the coldest sparkplugs possible. Similarly, ignition timing must be set not to fight the engine but flow with it harmonically with regard to everything built into the engine and the metals its made from. Sounds kind of simplistic approach to the conclusion and it is. That is the hard part to get by at times and that is not to take a complex approach to a very simple two stroke engine which all have wonderful attributes and just as many limitations to take best advantage of.

[Fast Fred]

at the lake, with a 3 banger, choose your flavor, we know this { at 150psi or better, on pump gas 93-94 , you have to 1, over fatten the main jet or 2, pull the timin down to 15-16deg max. with the fat main jet you can keep some timin in it, but eather way less power is made than could be. it's just a fix to keep it from eating pistons.}

with Real Gas, we can lean on the jets and the timin, make some real Power

[John (Taylor) Gabrowski]

Not lost on all this is the roles temperature and humidity have on these engines. The higher the temperature the leaner you go on mixture control. When you couple temperature to humidity of the surrounding air even though with all that water around for the race course, humidity over 80% or higher coupled with the higher temperature of the air even with air/fuel ratio jetting adjustments for temperature and humidity can still cause double digit power losses that can only be partially offset by keeping the fuel supply cool if not cold.

Watching some Merc Alky Deflector Ds and Fs back in the mid 1960s, perfect setups for 1:00 PM racing day starts became dogs by 3:30PM when the temperature moved up 15 degrees hotter. Seen some reversals too where temperatures became cooler, then these engines piston life spans could be measured in minutes when there was nothing like them burning up the track just some hour earlier.