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  1. #1
    RC poster
    Join Date
    Nov 2012
    Location
    nitro heaven
    Posts
    12

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

    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?

  2. #2
    RC Champion
    Join Date
    Jul 2006
    Posts
    2,220
    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).
    Believe 1/2 of what you see,and 0 of what you hear

  3. #3
    RC Turnbuckle Jr. Nitronaught's Avatar
    Join Date
    Oct 2005
    Location
    South Florida
    Posts
    18,588
    .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....
    Founder of H.U.A.
    Hop Ups Anonymous

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