RE: Evening the field
As others have said, some folks may just fly better. If you are not seeing a real big difference from one airframe to the next then there may not be a whole lot you can do about it. I read your other post and as long as you are tech'ing and inspecting the planes and as long as the props are the same and appear to be un modified and as long as you are sure there is nothing sneaky going on with the fuel, it is hard to say what can be done....
However, hear me out on this and don't jump the gun on less than civil replies. We have been doing our own versions of club racing for the last 4 years, not the 1/2A stuff but more of the .40 size. We started out with a spec class with the intent of it all being for fun and we supplied the fuel and required a specific prop. However, we ran into the same problem and I can say that alot of it was due to some of the pilots just being a better flyer but there appeared to be a few of the engines that for one reason or another were just a little better than the rest. No real explanation as to why and nothing apparent upon inspection although we never ended up tearing one down. What did end up happening is that we bagan to lose some of our entries because some gained the mindset that if they were going to have to fly against someone who was known to have a faster plane, there was no reason to try.
A couple of years ago we developed another class and took a page from the Warbird racing using a declared breakout time. There were many reasons for this but the biggest were that we weren't worried about what fuel or prop was being used nor were we concerned what the weight was and it made the average Sunday flyer more comfortable using his basic everyday plane even if he was going up against the guy with unlimited funds and time.
We inspect the planes for airworthiness and safety but other than that, the only real determining factor is that if the plane flew faster than the breakout time it gave a zero score for the heat. This has turned into a good class for the local contingents and we fly it along with our spec class on race day with very good results.
Just my $0.02