Maximum safe cylinder head temperature (CHT)

Discussion in '2-Stroke Engines' started by jaguar, Jun 22, 2013.

  1. jaguar

    jaguar Well-Known Member

    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.

  2. jaguar

    jaguar Well-Known Member

    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.
  3. HeadSmess

    HeadSmess Well-Known Member

    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!
  4. jaguar

    jaguar Well-Known Member

    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.
  5. Fabian

    Fabian Well-Known Member

    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.
  6. Fabian

    Fabian Well-Known Member

    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.
  7. jaguar

    jaguar Well-Known Member

    230C = 450F which is the maximum heat you want on extended runs
  8. Skyliner70cc

    Skyliner70cc Active Member

    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.
  9. jaguar

    jaguar Well-Known Member

    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
  10. Fabian

    Fabian Well-Known Member

    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.
  11. Skyliner70cc

    Skyliner70cc Active Member

    Jetting isnt the issue, it has a airleak and it's prob a crank seal.
  12. HeadSmess

    HeadSmess Well-Known Member

    bmw's are delicate! whereas russian ladas will cart a pile of bricks and two old babushkas on the smell of a (fairly) oily rag. and HT's are an old russian design ;)

    bikes? ive never seen a yamaha R1 or ducati 999 loaded up like a honda cub gets loaded in asiatic countries!

    cheap is best :) noone wants to keyscratch a rustbucket datto. everyone has to ogle a ferrari and some get jealous...

    saying that, i cant justify buying a temperature guage that costs half of one motor!

    im scooting along above the ground at 60k. if it siezes, i might eat some gravel. so far, the only gravel is from me being stupid. thats pretty normal :D

    i am not at 10,000 feet doing 250k with my life depending on making sure things are running right!

    ive watched more guys with thermometers and guages and boxes full of gimmicks blow sieze and burn nitro engines than any of the old guys that actually listen to their engines and the only accessories they wield are starters, small screwdrivers, and some allen keys.

    most of the "safe max temp" for aluminium alloys is based on pistons that are subjected to the most abuse in any engine. a lot of the documentation is circa 1960. things have improved. maybe not in china :jester:

    where are you measuring? is it in solid aluminium with the best average temp? or on a raised (cooled) boss? are you using a flat polished surface to mount on? are you using heat sink paste? silcon or silver based? have you calibrated the unit next to a known standard? are you compensating for the changes in ambient temperature and humidity? are you listing the ambient temperatures along with the recorded temps? listeing output powers and torques at specific rpm, whilst monitoring temps? listing fuel consumption? windspeed if riding? slope or gradient? weight? rolling resistance and pressure of tyres? bearing resistance? long pants or shorts and hairy legs? shaved legs cus you actually race or sissy or both?? slipstreaming behind a truck?

    are you getting these things tested by an alternative, unbiased testing facility? getting repeated, consistent results with conclusive proof?

    being scientific? containing the variables?

    anyone, myself included, can get online and brag. most of the time were all full of :poop: we can all say that "im right, you arent".

    we can all fudge figures. we can all modify results to suit our own needs. quantum physics thrives on this concept!

    how many of us can substantiate our claims? hard conclusive evidence rather than hearsay, approximations and plain old Bull:poop:?

    so far, i cant :) i havent got a clip of a bike doing 85km yet... that was a one off :) brag brag brag :) buy my pipes cus theyre the best...

    but really, whatever. each to their own. if you want the gimmicks, if you cant operate an engine without needing an "aid" of some description, well...its your own decision :) i use what works, i fiddle around some, i discard whats awkward, fiddly, or plain waste of time.

    you know they make clear spark plugs for tuning engines? they make life easy for tuning multicylinder/carb jobs, but not many mechanics own them! why? they just arent necessary. still, an awesome tool when you own it! just.not.CRITICAL. these days an ECU scanner is.

    just like a temperature guage, its money that doesnt improve performance, its money that doesnt bolt on and make you go wow. a guage... omg. on a pushbike! i like fiddly bits but by gawd i hate working on them! KISS!!!!!!!

    and just like efi... every extra gimmick is something else to go wrong. something else to let you down. something else you relied on instead of using your head and your guts. something that you cant fix with a screwdriver, some pliers and a bit of duct tape or wire.

    nah. seems silly.

    me? if its not starting, richen it. if its not revving, lean it. if it siezes...its cheap.

    now, if id just spent $3000 dollars on the same type of engine...or if i was working a morini or something worthwhile...maybe.

    slip of paper works. charring?smouldering? too hot. white or brown? is fine. they even sell crayons for doing it. if the line changes colour, oops.

    thats mho :) KISS

    ps, auto ignition is cool :)
  13. Fabian

    Fabian Well-Known Member


    I haven't seen a video with a GPS logged 50 mph using an engine with standard internals travelling along a level road.
  14. jaguar

    jaguar Well-Known Member

    I just poured boiling water into a plastic cup and then emersed the under-the-spark-plug temperature probe of my temp gauge into it. The reading was 191F which is close enough. You know the water starts to cool by steam release and contact with the colder cup temperature so that the original temp of 212 will not be .

    I really dislike the attitude that "it doesn't matter because these are cheap engines". Like money is everything.
    How you treat your engine is a reflection of your personality. And by changing the quality of your actions you can affect a change in your personality, for the better if you like. The motto I like is "anything worth doing is worth doing right".
  15. KCvale

    KCvale Motorized Bicycle Vendor

    One important thing that has not even been mentioned is the heat rating in the spark plug number and how toasting a head beats toasting a piston and cylinder.

    Read this.

    "NGK Spark Plugs Heat Rating

    The spark plug heat range has no relationship to the electrical energy transferred through the spark plug. The heat range of a spark plug is the range in which the plug works well thermally. The heat rating of each NGK spark plug is indicated by a number; lower numbers indicate a hotter type, higher numbers indicate a colder type. "


    Take my favorite plug for these 2 stoke motors, the NGK BPR?HIX where the ? is a number from 5 to 8, the higher the number the cooler the plug meaning it dissipates more heat out of the combustion chamber to the head.

    Here in the desert with 110+F summer temps I run 7's and ya that may make the head hot but not frying hot and I'm not toasting engines.
  16. jaguar

    jaguar Well-Known Member

    the range of spark plug dictates the temperature of the tip, not of the head. A hot plug has a longer distance for heat to travel from the tip to the cooler cylinder head, thus allowing the tip to become hotter.
  17. HeadSmess

    HeadSmess Well-Known Member

    i agree with that. the temp range of a plug is how hot the insulator nose gets (not just the tip, the whole nose) but then again, when the tip of a plug is cooler, all that heat has to go somewhere...where? into the head.

    not just the distance the heat has to travel. but the volume of material the heat has to travel through. only so much energy can travel through a conduit of a given dimension! this is why heatsinks use thick bases (when theyre any good). this is why high amperage cables are so thick. and why fuses are so thin.

    a heatsink or head doesnt care about how long the fins are. what it does care about is surface area. (the stuff up im more than happy to admit with my first trial head :jester:. though i said that was due to raw material supply constraints... which it was.)

    they also like tapered fins, as the base is heavy, thick, allowing max heat transfer to the fin. the tips are cooling, dont have to conduct so much heat. also mechanical strength. flat parellel surfaces tend to resonate more easily. resonance is vibration and excessive vibration destroys aluminum. why do they make studios out of square? to stop resonances.

    plus it makes for easy mould release during manufacturing.

    the insulator doesnt stick out any further on a 4 than on a 11. the metal part, the thread, is the same as well. the internal nose length is different. the 13 has a much shorter nose, and also much thicker. theres more insulator material to assist the heat in getting out and into the head. high end exotic aero plugs use different ceramics to affect heat transfers as reducing the thickness of an insulator reduces its strength and therefore, reliability.

    compare it to the fuse....use too low a rating, and the thing will melt! its trying to conduct too much heat for its dimensions!

    and the analogy holds because it is on cross sectional area TIMES length. not just length of conductor alone..

    youre both correct. why cant people just think a bit more? bridge the gaps between ideas?

    lastly. at sea level, water boils at 212 f. you can fill a paper cup, place it on a gas burner, even use an oxy torch (dont overlocalise the heat) and until the water has boiled away and the paper is dry, it will not burn. it will sit at the same temperature as the water.

    so, at sea level, you should get 212f with the thermocouple in the boiling water.

    you, being at whatever altitude you say it is, 5000 or summink...lets see now... 202 f?


    frozen into an icecube, as it melts, you should get 32f, altitude irrelevant as freezing temp of water is fairly constant despite reduced pressure.

    either result showing a discrepancy means measurements are inaccurate.

    i once had a lamborghini i used to thrash out in the paddock :) why? it was made for it. but i loved that rustbucket of a tractor :)

    ive seen planes fly using a victa 160cc lawnmower engine, with mods. anythings possible.

    the wright brothers built their engine.

    my point? cost is irrelevant. needs and necessity are. you could waste your life polishing feacal matter, or you could waste it polishing diamonds. they make the latter from the former these days anyway!

    i work to the quality the JOB DEMANDS. so do all manufacturing plants.

    you do not grind shafts to a tolerance of 0.01mm just because you can, if you want to make a profit.

    you grind them to within 0.1mm because that is cost effective and all the job requires, nay, what is specified on the drawing!! and both finishes will work just as well in service. so why waste the extra time and money on the high finish when its pointless?

    i do agree that china grinds to within 1mm when the job requires something nicer. so in this regard, sure...theres plenty of room for improvement. fix these and forget about gimmicks that do nothing :)

    a friend found out last night why i still use a brick phone, after her smart phone screen cracked and become completely inoperable. bit of snipping at an old sim card to make an adaptor, and her old trusty bricks back in service too.

    just because it cost a lot doesnt mean it was worth it! you dont always get what you paid for...

    and for this reason i wouldnt buy a morini anyway because thats an outlay i really could not justify!

    (though i spent much the same on a 5cc wankel... :jester:)
  18. jaguar

    jaguar Well-Known Member

    yeah but only the exposed end of the plug tip is conducting heat to the head. The insulator doesn't. So maybe only 10% of that inner circle area is conducting heat to the head, therefore its effect on head temperature is relatively unimportant. Figure out the percent area that the tip has in relation to the complete dome then you'll see what I mean.
  19. KCvale

    KCvale Motorized Bicycle Vendor

    With the countless expensive parts and tweaks we do to get the most out of our little Chinese 2-stroke motors it is easy to overlook the spark plug even though nothing you do will help with a crappy spark.

    There are 3 important parts of a spark plug, the sparking tip, the ground electrode, and the insulator.

    Until recently, Platinum was considered the best material to use on the top of an electrode because of its durability. However, Iridium is 6 times harder, 8 times stronger, and has a melting point 1200 degrees higher than platinum.

    Put that into a harsh environment such as a 2-stroke engine piston chamber, and you have a spark plug that can resist wear much better than platinum. Additionally it is so durable it allowed engineers to produce the world?s smallest center electrode (.4mm) which reduces the voltage requirements, concentrating its sparking power.

    For 2 strokes that means even a weak CDI should have a good spark and a good CDI with everything the magneto has via better wiring and you not only get a whopper of a spark, the Iridium can take it.

    Also, its smaller size, combined with the tapered ground electrode, allows more room for the flame kernel to develop and produce a more efficient combustion.

    The flat ground electrode design of ordinary spark plugs crush the spark, inhibiting its growth; this effect is known as a 'quenching effect.' The simple, but efficient, U-Groove or tapered ground electrode results in better fuel efficiency, improved throttle response, and greater firing energy.

    Besides NGK, Champion and Bosh also make Iridium plugs but the one I know that works in most of our 2-stroke China engines is the NGK BPR?HIX Iridium IX.

    The ? in the part number is the heat range, which equates to how much insulator material the plug has and refereed to as how Hot or Cold the plug runs.


    Part of your spark plug's responsibilities, in addition to firing a spark, is to remove heat from the combustion chamber. This is accomplished by channeling the heat through the insulator material and metal housing. From there, the heat is transferred to the cylinder head where the engine cooling system can go to work.

    A spark plug's heat range is its ability to dissipate heat. The 'colder' (higher number) the plug, the more heat it can channel out of the combustion chamber. In a performance application, colder heat ranges may be necessary to handle the extreme temperatures brought on by higher compression ratios, forced induction, and high RPM?s.

    While 'Colder' plugs may seem to be the way to go, please remember that the spark plug must achieve its 'self-cleaning' temperature where it can burn off fuel and carbon deposits. Otherwise, the plug could 'foul out' where it is prone to misfiring and poor acceleration.
    A plug that is too 'hot' on the other hand can overheat, also causing power loss, detonation, pre-ignition, and possible engine damage.

    Here in Arizona I put BPR7HIX's in all my new builds, I can feel the difference and never had to replace a plug yet.

    They are hard to find but SBP has them for $9.

    One more note, do NOT attempt to gap them, you will snap the tiny Iridium tip off and they come pre-gaped with a cardboard tube around the end to protect it in shipping.

    I have been saying I could feel the difference for years, now we all know why ;-}
  20. jaguar

    jaguar Well-Known Member

    good info except for the part that the ceramic insulator channels heat.
    Ceramic is what they coat piston tops and head domes with to lessen the heat transfer from combustion to metal so that more heat is available for expanding the gas and moving the piston.