They are not really secrets, just tricks.
If the rules allow they will run huge compression ratio's, just like a nascar restrictor plate motor.
They will run dyno proven (not flow bench proven) ported heads. If you are determined enough, you can optimise for port velocity, swirl and quench. Some heads are even blueprinted to a different volume for each cylinder to take compression out of the cylinders most likely to detonate.
They will polish the inside of the block and the top of the heads to make oil drain back to the pan better.
They will run a deep sump pan and fill it two quarts low.
They will use hydraulic rollers with plungers that can move .005" so they are essentially solid rollers.
They will grind the crank just so they can use the largest undersize bearings for the least friction.
They will install oil restrictors for the cam and top end, and run the lightest weight oil that will make it down the track a few times without spining a bearing.
They will run cams that don't make any power below 3,000 rpm and idle at 1800.
They will rebuild the engine many times per season and use many many stock cranks and rods in search of parts that are out of spec in a good way.
There are lots of little things that can make power at the expense of reliability and drivability. For a race car these things do not matter, especially in a super competitive class with contingency money at stake.
It's like any top level motorsport, you build your engine to run the race, then you throw it away after the race.
