Originally posted by Aaron Waldron
A larger engine head is not always the answer. Most, if not all, engines made nowadays are comprised of quality materials that can withstand normal running temperature in the mid 200's.
Larger engine heads can actually cool too well, which can make the engine load up as it cools in the infield of a track, making the tune of the engine very inconsistent. The stock heads of most race-inspired engines are a specific size for a reason.
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While, in general, I agree with you in most applications, the Revo can easily run way too hot if the needles are set too lean in an attempt to gain power. I had a friend with a stock Revo who ignored temp monitoring and wanted only to get max speed from his Revo. I pulled out my temp gauge and it was running in the 320s. The Revo is easy to get that hot mainly due to the inadequate surface area of the head vs. the high performance nature of that engine for off road use where higher speeds cannot be maintained to aid cooling. Unfortunately, it won't last very long. To deal with this dilemma on my own Revo, I cut a hole in the windshield of the driver's side of the body and used a larger cooling head which keep the temps down to between 210 and 220. The Revo will not load up at these temps if your needles are tuned correctly. If it does, your tuning is off. If you live in a 4 seasons climate with relatively cold winter temps, then switching back to the stock head during the colder weather is advised along with heating the engine with a heat gun before a cold start. After 4 months of racing and running 3 gal of Megatech 25% nitro with 14% oil content , I sold the Revo and it still had compression equal to a new engine. That wouldn't happen with the stock Revo and stock head when raced every weekend, tuned for power, and is a testament to a properly tuned engine (not too lean) coupled with good fuel.
Bottom line: If you want longevity, tune your 2.5R Traxxas engine to run in the 220-220 range max. If you're looking for max power tune it for 260-280 with the stock head then change to a larger capacity head without changing the needles. This should result in a 20-40 deg drop in temps depending on the head. DO NOT retune with the larger head back to the 260 range.