Model Engine Company of America


Compression Ratio is the amount of volume the piston compresses the fuel air mixture to.
So an 8 to 1 compression ratio would compress a volume of 8 into a volume of 1.
Plainly said, it is like taking the volume of 8 bottles and squeezing them into one.


The compression ratio on model engines actually controls the ignition timing which is also effected by the nitro content of the fuel.

A glow engine is similar to a diesel engine. The compression ratio in model engine can be from as low as 7.5 to 1 to a high of 9 to 1. The compression ratio varies with the type of fuel, altitude, port timing, and exhaust configuration. Higher performance engines hold very close tolerances to assure the proper combustion point.

The basic rules are as follows:

If you engine is overheating...

It could be due to the compression ratio being too low for the fuel you are using. Lower nitro content require higher compression ratio. If the compression is too low the firing of the combustion charge will be too late or retarded creating poor burn and thus allowing a burning fuel charge to be released out the exhaust. This elevates the temperature of the engine and lowers the performance.

If you increase the nitro content the combustion will be advanced and can produce a more powerful and cooler running engine.


You can increase the compression ratio by removing a head shim (see also How to set squish band).

NOTE: No all engines use or require head shims. Many sport engines only have a metal to metal (head to cylinder) fit without head shims because they are usually run on less than 10-15% nitro fuel which is more forgiving than higher nitro fuels.

On the other hand if your compression ratio is too high the engine will detonate. This the same as the ignition being too advanced in you car or if you run too low of an octane rating. Like when you car engine pings with regular gas.

If you want to run higher nitro fuel to get more power you may have to lower the compression ratio. You may need to add head shims. Detonation is basically the fuel charge exploding before the piston reaches the top of the stroke so the piston forces the explosion to be contained without expanding. It has no place to go because the inertia of the crankshaft is forcing the the piston to move thru its proper course. Thus you get a ping which is the sound of all the clearances in the connection rod being driven together squishing the oil film out of the bearing. If you have detonation you can destroy your piston as the crown (or top) of the piston is subjected to extreme heat and pressure because of the trapped exploding gasses.

Engines are designed to have the combustion expansion drive the piston down the cylinder, not have it trapped.

Note the higher the nitro content is not like a higher octane gasoline rating. Higher octane gasoline requires higher compression ratio as it actually burns slower than regular gas and requires more compression to ignite properly. Nitro actually lowers the compression ignition point of glow fuels.

The above information is provided as a guide. Since MECOA/K&B has no way of determining the ability of the individual using and understanding this information, we assume absolutely NO RESPONSIBILITY for any damage to person or property from the use of this information.

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