what head shim too run .20 .30 or.40

[madmechanic]

i was just wondering who has changed their head shim to .20 and what type of performance gains and issues that come with the .20?

[Oomp]

I think the .30 shim is the stock shim on the 3.3.

By going to a .20 thick shim, you risk pre-detonation, unless you are planning on also going down in nitro %. Worst case scenario, pre-detonation will lead to a broken connecting rod. Remember that our engines are really just modified air pumps. The combustion process works more along the lines of a diesel engine. By going with a thinner shim, you are causing the point of maximum compression to occur sooner in the up-stroke of the piston (pre-detonation). By pushing it too far, pre-detonation will cause the engine to want to reverse rotation upon combustion, leading to the rod breaking. You can hear pre-detonation if you have a keen ear. It will kinda sound like a pinging noise.

To add to the topic, if you are looking for more overall power out of your engine, run as cold of a glow plug as you can tune. Glow plugs can be used to fine tune your engine's performance, by adjusting the timing in smaller increments than head shims do. A hotter plug has the effect of advancing the timing (more low end power, better idle). A colder plug retards the timing (more top end power, choppy idle).

[Nitronaught]

.30 Use the stock shim... I run 30% nitro and have no issues as well as most everyone else at the tracks I race at....