http://rcboat.com/past.htm
Some interesting stuff here, especially the Exhaust Timing, Combustion chambers and Tuned Pipe articles.

A few years ago I took a box stock (but broken in) TT40 pro and did some mods to it, I would need to go back and look at my notes but basically went from about 15.8k to 18.8k rpm on 15% fuel with an APC 8x8 prop. I also added a MACS tuned pipe and header. I did change out the back bearing, those were a problem on these engines. I also tried 20 and 25% fuel which gave a hundred or so rpm increase but didn't think it was worth the extra cost of the fuel. Basically I wanted to see how far I could take it before it blew up, it never did, so I built a plane for it, pictured, and those that saw it fly estimated it was doing between 130 and 140 mph.
All mods were done one at a time with several bench runs. I used two tachs, one hand held and one with a magnetic pickup attached to the engine and battery operated, they were always within 100 rpm of each other.
Some of the mods:
Raised exhaust port (more duration)
Chamfered the boost and intake ports
More clearance between crank pin and rod bushing
Set head clearance at .014 at first but kept blowing plugs at 18k rpm, increased it to .017 and that problem went away.
Drilled out the fuel nipple on the carb, the engine became hard to needle until I did this.
Set the tuned pipe length, ended up shortening it by an inch or so

Some observations:
As you go up on nitro and rpm, 3000 in this case, you need more head clearance. The engine will detonate (pre-ignition) if you don't do this.
If you can not get a steady tach reading, the engine is detonating, the rpm jumps around plus or minus 100 rpm. This shows up as pitting either on the top of the piston or on the combustion chamber.
The engines fuel consumption doubled, at least! A 4 oz tank lasted about 1 1/2 minutes at full throttle on the bench.

In the end it was an interesting experiment.