Originally Posted by gonzoY5
So since we're doing the question game, couldn't running a colder plug in this case work instead of adding a shim? Maybe some of the engine gurus can answer.
Having done my research I have gathered the following:
Adding NItro = Advance Timing
Adding Shim = Retard Timing
Colder Plug = Retard Timing
Since you're adding nitro the timing is advanced. You want to add the shim to bring back the timing. But wouldn't the colder plug do the same? Adding a shim, will reduce compression which should reduce power. Why would we want to do that?
Just random thoughts and questions that run through my mind, thought maybe I should throw them out there and see what others think. So rant away
GPP Model Racing
AFM
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
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. 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.
You can safely use 30% Nitro on the stock head clearance. Using a C8TGF you can go up to 40% Nitro, and If you don’t over lean it, the engine life will not suffer dramatically.
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. This is why we have always run Novarossi plugs, as they don’t do this. 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.