Maximum safe cylinder head temperature (CHT)

[Deleted member 12676]

The temperature it take to ignite gasoline is 495F. 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. What are factors contributing to a higher head temperature?
1. the stock Grubee CDI. It has way 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).
2. a high compression cylinder head. The higher the compression, the hotter the engine. I never recommend more than 150psi 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. I measured a 50 degree difference when I tested heads back to back.

People who have had cracked cylinder heads probably didnt even know their engine was overheating. They just went and bought another head to replace it.

 

[Deleted member 12676]

JNMotors sells 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.

 

[HeadSmess]

wheres the evidence to say that that bicycle caught on fire due to excess head temperature? petrol does things like that even at 0!

petrol has a flash point of -60C. that means any temp above that, its producing flammable vapor.

any heat source that reaches a temperature of over 232C in the presence of this fuel vapor and air, will cause ignition.

plenty of people have lived to tell the tale of extuinguishing cigarettes in petrol. the liquid doesnt burn, the vapor does. and i believe smouldering paper is just below the ignition temp of petrol. theres a book with it as a title...cant think. (yep, farenheit 451)

so liquid petrol will extuinguish paper coruscations!

plenty of people have been killed/maimed/injured by thinking that just cus its cold, it wont ignite as readily.


i do get what youre trying to say, but seriously?


i can lean a HT out to the limit, and keep it WOT, without any thermally induced issues with a standard head. ie, it wont run before it can be so lean it siezes.

even with half the fins, it still wont overheat.

whereas, once you DO start raising compression ratios, then CHT does become a concern. ive proved it with a post or two on here, its been proven before elsewhere, aye aye aye, point proven

i think STOCK cylinder heads cracking is more a metallurgy problem. or the lack of metallurgy in china, to be precise!

 

[Deleted member 12676]

I appreciate your viewpoint but really I think you need to have a temperature gauge on your engine to really know what is going on with it.

 

[Fabian]

I have sent a TTO cylinder head temperature gauge (sold by JN Motors) off the scale and it maxes out at 275 degree Celsius, so i wouldn't mind betting that the cylinder head temperature exceeded 300 degrees Celsius using the standard cylinder head.

The max exhaust temperature at the time was 550 degrees Celsius.
It was one of those heart breaking hill climbs on a 40 degree (Celsius) day with virtually zero airflow over the engine as i was climbing at around 4 mph with the tacho reaching into the 5,000 rpm zone.

At 20:1 oil fuel ratio the engine still ran like a champ.

 

[Fabian]

That said the CR Machine Manufacturing billet cylinder head never sees the engine reaching over 230 degrees Celsius, due to it's significantly large cooling surface area compared to the standard cylinder head and other billet cylinder heads that i have tried.

 

[Deleted member 12676]

230C = 450F which is the maximum heat you want on extended runs

 

[Skyliner70cc]

Last summer mine got so hot, that I couldn't kill the engine with the kill switch nor removing the spark plug cable. It was running like a diesel-auto ignition.

 

[Deleted member 12676]

I just tested a 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 stock head w/squish band (which causes higher head temps and lower piston temps). how? Jaguar CDI

 

[Fabian]

Last summer mine got so hot, that I couldn't kill the engine with the kill switch nor removing the spark plug cable. It was running like a diesel-auto ignition.

Considering that these Chinese bicycle engines are dirt cheap, it's amazing to see just how much abuse they will put up with.
Having said that, if the engine gets so hot that it auto ignites, there is room to increase the jet size which should enable the engine to make more power as well as reducing engine temperature.