Originally Posted by freestyles
How many head shims are you guys running. It came with a .1mm and a .2mm shim. In the manual it says when breaking in use both but when using low nitro content take the .2mm one out.
What's considered "low nitro content"?
I'm using 16% fuel with both .1mm and .2mm head shims and it seems to run ok but i want to experiment and try to get the best out of it.
Thanks, any help would be much appreciated
On a .12 engine I would only advise going 0.10mm over or under 0.46mm, so that is 0.36mm or 0.56mm, of total head clearance.
Generally on a .12 engine 0.10mm will change the compression ratio about 3/4to 1 point..
Also, never assume that the head shim(s) 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's are 0.20mm without one, and the factory installs a 0.30mm shim, giving the engine 0.50mm stock head clearance. This has not always been the case but 99% of the time it is.
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.
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.
I 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.
For the OS TZ use .1mm shim with 16% nitro, .2mm shim with 20% nitro, and both .3mm shim with 30% nitro......now following the guidelines above, you can fine tune your engine for torque or high speed using more or less shims and changing glow plugs......with your 16% nitro, right now and with both shims, you should have plenty of top end.
Hope it hepls