The "flame front" has a much farther distance to travel across the top of the piston on the 500 vs the 250 in the same period of time. Therefore, the bigger cylinder cannot tolerate as high of a compression ratio before detonation as a 250 cylinder can. A stock 500 has 120 - 125 psi of compression while a stock LT250R has 175 psi of compression. I realize that 175 psi sounds way too high believe me it is true (please check the facts before you tell me I am wrong).

Halflifter, what do you mean when you say the stock head flows coolant better? Are you suggesting that the stock head cools the engine better than a Cool Head? I assume you know that there is more to the design of a head than simply providing an easy way for coolant to flow over the combustion chamber. The flow needs to be "metered" so that heat excess heat has time (and enough surface area) to travel away from the engine. Providing an easy flow path is not the characterist sought after in the design of a good two-stroke head. You seem to know what you are talking about so I assume you know these things but you may want to elaborate on this issue. Thanks

 

Triple brings up an important parameter; flame front. With a big bore it takes longer for the spark in the middle to reach the outside. So to help that we put in a wider squish band. That puts the gas closer to the plug. Now that brings other problems mentioned above. Always a comprimise. Now you know what the aftermarket has to deal with. Anyway, thinking along those lines, the 500 would have a wider squish band than the 250. With a wider sqishband you need less compression to reduce detonation. 500 at 125 psi; 250 at 175 psi. Something to keep in mind also is the 500 revs less than the 250. That gives the fuel more time to burn so now the squish can get a little narrower and a little more safe compression. Another point, since we have been talking psi, psi readings are greatly affected by exhaust port height. You can use the same head and have 125 psi on one motor and 175 on another just by having different exhaust heights. This helps a little because a lower exhaust port will rev less and give a higher psi reading. So that works out. Thats why I like to talk compression ratio. When someone asks what psi is safe its hard to answer because of all the parameters.

About water flow. Again, Triple you brought up important issues. The water on the stock head is metered using the restricter holes in the gasket. Those look like they rout the water o.k. The reason I like the water flow on the stock head is because the outside (blue part) is shaped close to the inside. That means the water is flowing over the chamber. The cool head is just a big square volume. That could mean standing water. Nothing is forcing the water to flow over the hot spots. Think of this really exaggerated example. Put a garden hose in the middle of both ends of a full 55 gallon drum. There is alot of coolant but the water flowing thru is not really "rubbing" the sides (combustion chamber surface).
I hope I've explaned myself better. If not let me know. Or if you want me to shut up thats ok too. Later