At PGRC 424 races, the TT 40 is NOT the only engine allowed. I believe it is the most consistently strong engine allowed. We have had a few ST 40s, but the TT 40 dominates. (I think I saw a HP 40 run last year.) I have been using old K&B 40s. And I believe any AMA legal 424 engine would be allowed. I would assume that APRA would follow that allowance as well. (Is that correct?) The problem, as I see it is two fold. First: the TT 40 is not an AMA legal 424 engine - but genie is out of the bottle . And as has pointed out, it has become the de-facto 424 engine. It is a great out of the box engine for less than $75. But since the TT 40 fails to meet AMAs definition of a legal 424 engine, we have a catch 22 situation. It would not be allowed if/when 424 is flown at the Nats (and 90+% of 424 fliers are then shut out). That brings up the second: Do you modify the rules to make the TT 40 legal? How would you do that so as to prevent speed creep (which would undermine the intent of the class )? I would like to see the 16.5k rule enforced. You would not need to check after every heat, but fast time and maybe first and second (or on challenge). Also add the $100 claim to AMA rules. (Then let the last place racer have first claim.) And you could allow any make engine. What would it take to de-tune the TT 40 to make it meet the 16.5K rule?