That seems counter intuitive when you look at the progression of the KTM head. I'm not saying its wrong, it just doesn't follow what KTM did as far as the quench area. Though it does kind of follow what Roostius did with his head mod, maybe he can chime in. Working within the constraints of the stock casting there is probably only so much you can do without weakening the chamber, so that may be the way to go.
Ok let me take off my IMAX glasses and we’ll go deeper.
Let’s be perfectly clear, we are talking about CR 500’s and the first rule of thumb, when it comes to quench type combustion control, is that the larger the bore diameter the more critical all this becomes. With that said we are also dealing with an over square engine, as delivered by the factory. Meaning the bore diameter is larger than the length of the stroke. 89mm (3.503”) X 84.5mm (3.326”) bore and stroke. A 300 KTM on the other hand in stock configuration is 72mm X 72mm (2.834”).
Adam knows, as do most quality engine builders, that once you get over a 3” bore diameter a lot of things change dramatically and likely the reason he has spent a great deal of time working with these big bores. We also know that combustion technology, fuels, fuel delivery systems and a host of other items have changed since 2001 when the last CR500s were built. In the most conventional sense it’s long been widely embraced that increases in compression ratio, which can be accomplished simply by milling a few thousandths from the cylinderhead can work minor miracles with a 2 stroke engine. The down side of course is if the compression ratio is high enough, heat input into the piston may raise the piston crown temp to the point where detonation and pre-ignition occur. And where this gets really crazy with a 2 stroke is that our static (cranking) compression numbers are vastly different than actual running compression numbers with an effective pipe because of supercharging. And here is where most run into trouble.
Engineers work within a theoretical level of efficiency in terms of converting heat into work, referred to as “air standard efficiency”. In this theoretical level it is assumed that the cylinder is only filled with dry air, and then heat added, which completely ignores the fact that in practice this air contains some moisture and a considerable percentage of hydrocarbon fuel. What is important in working with air standard efficiency (ASE) is that it does provide a way to gauge the way actual efficiency can be measured and tells us a lot about the effects of power output relative to compression ratio. For example, at a compression ratio of 5:1, ASE is 47.5%, while if we bump that double at 10:1, ASE is now 60.2%. Which, of course is a large gain and the results measured at the output end of the crankshaft are the reason many tuners are fixated on raising the compression.
One way to work around this is to raise the compression by removing several thou from the bottom of the head and then open up the combustion pocket to balance it out. And this is normally what most do and all well and good until you start getting into 3.5+” cylinder bores and then we have to start taking into consideration flame speed (Which is what we were talking about when this conversation started, in regards to highly atomized fuel burning faster than heavy wet fuel droplets). This is furthermore where fuel selection and octane rating become very important, and in the case of our discussion here, adequate quench combustion control for large bore engines that display certain anomalous characteristics.
Flame speed as defined by Wikipedia is “the measured rate of expansion of the flame front in a combustion reaction". Of note also is that fuel and explosives are the same measured relationship, however combustion engines in general and 2 strokes in particular must differentiate between laminar flame speed (explosives) and turbulent flame speed (squish) combustion. And as one last thing the flame speed is also dependent upon stoichiometry or ideal air/fuel ratios.
By now you are either completely confused or you can see where I am going with this. A large bore engine that has a long path of flame travel the design of the combustion chamber becomes more and more critical to avoid detonation issues, while achieving good usable power.
Bore diameter isn’t the only difference between the new KTM heads and older CR 500 and modified heads. Understand KTM is getting ready to release a DFI engine, someday anyway, and to inject a plume of fuel in through the top of the cylinder head requires feature changes to provide a more even distribution, and if you look closely at Ski-Doo's E-TEC cylinderheads you will see that they use a very large curving combustion chamber cylinder with no sudden step changes. Same thing KTM appears to be leading up to.
In short, there is a threshold of bore size that leads to an increase in quench area to accommodate the longish burn time that occurs in larger engines. What exactly that is for a modified CR 500 I do not know, and it appears that for stock Honda was a little unsure as well. Also remember these are loop scavenged engines and during the exhaust phase, the shape of the cylinder head is also required to provide a gently curving surface to help whisk the exhaust residuals out of the combustion chamber while the fresh charge is coming up the back wall of the cylinder and pushing it out. This is partly the reason we used to polish the combustion chambers to a mirror shine to eliminate boundary layer stiction and provide a smooth surface for the gases to travel along.
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