I've done a bunch of work on the HPI .15FE engine in my HPI RS4 2 -- replacing the single-needle carb with a two-needle carb, cleaning out obstructions in the transfer channels, smoothing airflow through the transfer ports, rebalancing the crankshaft, raising the compression by lowering the shim stack from 0.5mm to 0.3mm, etc. I've managed to wring-out a significant increase in performance from all these mods. I tried a 0.2mm shim-stack and the top speed decreased a little, so that tells me 0.3mm is optimal.

After all this work, the engine now produces enough torque on takeoff that it's actually a little difficult to control, yet the top-end still isn't quite as fast as I'd hoped for. One thing I haven't done yet is raising the sleeve by inserting a shim underneath the lip of the sleeve. As I understand it, this will increase breathing at high RPM and decrease torque at low RPM, which is pretty much exactly what I'm looking for. Is my understanding correct? Should I raise the sleeve by only 0.1mm, or should I go crazy and raise it by 0.2mm? Obviously I can try one then the other, but at-present I only have one spare shim of each size, and I'd rather not cut-up both of them without having some reasonable expectation that there would be a noticeable benefit.

Thoughts?