Originally Posted by blanks596
Could someone explain how shimming, nitro percentage, and glow plug heat affects timing.
My understanding is that higher nitro percentage advances timing, while lower nitro percentage retards timing. Also shimming an engine retards the timing by increasing head clearance while reducing head clearance advances timing. So, the overall affect would be: advanced timing increases acceleration, while retarding timing decreases it. Also, I heard that using higher nitro percentage will increase acceleration at the expense of top end speed while lower nitro percentage would lessen acceleration while increasing top end speed.
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. The colder plug will also increase torque, but 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.
One last comment. never assume that the head shim that is on the engine is the actual head clearance. 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.
You can safely use 30% Nitro with 0.56mm of real head clearance, and If you don’t over lean it, the engine life will not suffer dramatically. For less Nitro % you can safely use 0.46mm of real head clearance.
Someone asked about plug fatigue the other day, plug fatigue can occur under high sustained RPM's, and this is where the plug stays shiny and the wire just fractures for no reason. When this happens it is usually not a carb. tuning or head clearance problem. If you see the wire start to pull out of the hole this means that the engine is scavenging really hard and when this occurs the engine is making serious power.
There you have it.
AFM