Safest Maximum Cylinder Head Temperature

The hottest a two stroke cylinder head should get is 450 degrees continuous running, 500 for short periods. (see graph below) The picture below is of a motorized bicycle that caught on fire after the owner parked it after a ride and some gasoline dripped onto the cylinder head. (gas ignites at 495F) What are factors contributing to a higher head temperature?
1. too much ignition advance at high rpm which causes engine overheating by combusting the mixture too early (and so causes more peak temperature and pressure). The Grubee CDI has too much ignition advance at high rpm which is corrected by the Jaguar CDI.
2. a high compression cylinder head. The higher the compression, the hotter the engine. I never recommend more than 140psi for these engine, but some after market cylinder heads can easily give 180psi.
3. too lean an air/fuel ratio. Try different main jet sizes and use the size just bigger than the one that gives the highest top speed. The excess gasoline serves as coolant.
4. a cylinder head with a squish band. The band lessens the piston temperature (which lessens liklihood of seizing it) but raises the head temperature in the center. I measured a 50 degree difference when I tested heads back to back.
5. a spark plug with too hot a rating (such as a #4 with NGK or a #87 with Champion). My testing showed an average 45 degree head temperature increase from using a Champion 82 to 87 (which is equal to an NGK #7 to #4) which is 15 degrees per NGK heat rating. A Grubee engine with a standard CDI should have a cold #7 as well as with a Jaguar CDI. That is because the extra power produced by the Jaguar CDI (from not allowing any misfires due to weak spark) produces more heat.

People who have had cracked cylinder heads probably didn't even know their engine was overheating. They just went and bought another head to replace it, thinking the head was faulty.

Here's a good digital head temperature gauge for those of you who want to be sure you are running safe temperatures. Here is a graph showing that as you jet richer the temperature goes down and the power goes up, to a point beyond which it goes down.

I just tested my high rpm piston port intake 55cc Grubee engine and I redlined it for a mile at 9100 rpm and it only got up to 425 degrees farenheit. That is with high compression (155 psi) and a head w/squish band (which causes higher head temps and lower piston temps). how? the Jaguar CDI


Nowhere have I ever read (from a spark plug manufacturer) that hotter spark plugs increase cylinder head temp. I thought that they couldn't because ceramic (the insulator) is a very poor thermal conductor. If I was right about the ceramic then the small area of the center electrode wouldn't be enough to absorb a lot of the heat. But I was wrong. The ceramic is not a typical kind of ceramic and does conduct a lot of the heat to the body of the plug and to the head. Here are some tests I just did:

55cc ported for 8300 rpm. 18mm Mikuni with reed valve. non-squish-band slant plug head with cranking pressure of 140psi and spark plug screwing in at the rearward intake side of the combustion dome. Jaguar torque pipe. Fiber head gasket insulating the head from the cylinder. Fiber washer insulating the temperature gauge thermocouple from the cooler cylinder studs/nut. Thermocouple connected under a forward head stud/nut leading to digital readout at handlebars.

NGK     max temp
B5HS    299F
B6HS    268F
Second test:
Same engine but with squish band head with 165psi. Metal head gasket. Champion plugs tested which spanned 3 NGK heat ranges. The #87 had an extended tip (protruding more into the combustion area).

Champion L82C = NGK BP7HS
Champion L87YC = NGK BR4HS

Thermocouple at spark plug base:
Champion    max temp
L87YC        448F
L82C           418F

Thermocouple at front head nut/stud:
Champion    max temp
L87YC        399F
L82C           333F

So basically there was a 10 degree change per heat rating at the spark plug, and a 20 degree change per heat range at a forward stud.