Originally Posted by New_Toasted
I just had a look on my NovaMega SX-12 RT M.S's head schims, and noticed that I all in all have 0.5 mm shimming. I bought this engine used from a friend who had ran it in the US, not unlikly on 30% nitro, though, I only use 16% since that's the maximum allowed percentage in Sweden.. Should I remove any shims or let it be as it is?
Thanks in advance,
This an edition of facts about head shiming done by well known tuner Dennis Richey. Hope it helps.
Engine Shim Tuning By Dennis Richey
Experience dictates that just by lowering the head (more compression) you gain more power especially in lower rpm range, idle quality can suffer, but the engine also runs cooler . Also, that a higher head (less compression) will increase top rpm speed on bigger tracks.
A decrease in head shims (an increase in compression ratio) will increase torque because as the compression ratio goes higher, the actual ignition timing occurs sooner. However there is a point of diminishing returns where detonation occurs or engine temps can soar, and if this happens a colder plug can help.
A colder plug will also increase torque, except in the instance of a colder plug the ignition is slowed until a greater point of compression build occurs.
When you increase head shims (a decrease in compression), top end is enhanced as the ignition timing is retarded and occurs later. Generally a hotter plug is needed to advance the ignition cycle so that timing does not occur to late in the cycle, but at this point you end up over leaning the engine to get it to rev properly and the engine life will suffer dramatically.
On a .12 engine we would only advise going 0.10mm over or under 0.46mm, so that is 0.36mm or 0.56mm of total head clearance at sea level.
Generally on a .12 engine 0.10mm will change the compression ratio about 3/4 to 1 point..
We should also state that the comments with regards to plugs and head shimming are when both are used together. Used alone a hotter plug will rev harder and a colder plug will make more torque.
Never assume that the head shim that is on the engine is the actual head clearance. In many cases you will find that the engine actually has 0.20mm to 0.15mm without any head shim. Novarossi based engines are 0.20mm without one, and the factory installs a 0.30mm shim, giving the engine 0.50mm stock head clearance, with which you can safely use 30% Nitro.
This has not always been the case but 99% of the time it is.
For 16% Nitro you could safely use a total head clearance of 0.40mm.
When in doubt measure the head button register and the piston to the top of the liner at top dead center and subtract, to determine proper shim. This is the only way to really know.