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Oil temps, how hot are you running?

Discussion in 'The Garage' started by weekendrider, Apr 4, 2012.

  1. GreasyC

    GreasyC XS650 Addict

    Of any temp measurement it's head temperature that tells you most directly if the engine is in trouble or not, measured with a thermocouple on the washer under the spark plug usually. I'm sure there's a relationship between oil temp and head temp but it would be hard to know what it is without having both measured on the same bike.

    The lean=hot thing is a bit of a myth, or at least not completely understood by most. IF your bike was already running on the rich side of stoichiometric and you make it a step less rich then the temperature of the head and exhaust gases will increase. This is telling you that you are still on the rich side of stoichiometric NOT the lean side. As you keep making it less rich the exhaust gases will keep getting hotter until you reach a peak temperature at exactly a stoichiometric mix. If you continue past that point into the lean side of stoichiometric the exhaust gas temperatures will actually start to go down again and they go down faster on this side than the rich side so lean running can actually be cool running. If you think about it this makes sense: a stoichiometric mix has the perfect ratio so all of the air and all of the fuel burns. If you make it lean you have less gas in the cylinder to burn and less of the oxygen gets burned, so less heat is generated by a truly lean burn than a stoich one. If you make it richer than stoich you can burn all the air but not all the gas and the extra gas spraying around is evaporating and cooling things again reducing temp vs. a true stoichiometric burn.

    I think the confusion comes from the fact that we are probably always running quite rich relative to stoichiometic. Peak horsepower occurs well on the rich side of the scale and detonation resistance is much better when rich as well. Even modern fuel injected cars that are computer controlled run rich when you stomp on the gas as a protective measure. Very high cylinder pressures occur when the throttle blade is wide open and running quite rich during this condition keeps you away from the range where detonation sets in. The difference is the modern car can drop back to stoichiometric when you lift off the gas and cruise there. With our primitive carbs this is a tough trick to do and I think we always end up compromising well on the rich side of stoich. From that point of view where we are always well to the rich side of the curve, yeah, going less rich = hotter.

    If you want to know more about head & exhaust temps and how it relates to fuel/air mix and detonation there was a great series of articles by a pilot a few years back on the subject. He was testing in a fully instrumented engine with cylinder by cylinder control over air/fuel mix and head temp sensors on every spark plug. Sadly the author had a stroke and was unable to keep flying or writing but the articles he did complete are really good. This is a chart of the basic relationship and a link to his writing:


    EGT = exhaust gas temp
    CHT = cylinder head temp
    ICP = internal cylinder pressure

  2. DogBunny

    DogBunny XS650 ThermometerDipstick

    GreasyC, I have done comparisons of temps measured at the sump with a ThermoDipStick to temps at the spark plug using an infrared temperature sensor. Those results are documented in post #24 of the thread that I referred to earlier:
    BTW, "Bike 3" in that test was wxmjx's.
    I posted some other head temp results in post #13 of that thread. In that test, I measured 225 at the sump, 280 on the left cylinder, and 315 on the right cylinder.
  3. GreasyC

    GreasyC XS650 Addict

    Cool, nice to have an idea of how they relate. All the reported head temperatures are actually quite reasonable compared to the larger world of piston engines where head temps in the upper 300s aren't uncommon but the XS is an antique iron head with a hemi shaped chamber which puts it at much higher risk of detonation than a flat piston in an aluminum head so I expect the "safe" number for the XS to be very much specific to this engine and probably quite a bit lower than modern ones.
  4. With Red Line products it is safe to run up to 300 degrees F. The oil itself can operate at much higher temps before the additives begin acting strangely and the lubrication qualities begin degrading.
    However, the problem is that engine seals begin softening around 325 and hard facing and other metal treatments can begin degrading between 325 and 350. Obviously, these are triggered once the temperature limit is reached no matter what oil is in the engine.
    So, assuming you are using high quality motorcycle specific street or race oils you should have no problems so long as you are below about 290.
    I'd worry much more about the top end having temp problems resulting from excessive spark lead, leanness, etc. leading to destructive detonation-related problems.
  5. TwoManyXS1Bs

    TwoManyXS1Bs Kablatta, kablatta...

    The important thing, to me, was in a lubricant industry report showing life expectancy vs temp for various engine oils. For mineral oils, the "knee" in the curve was at 240°F, life expectancy 350 hrs, followed by a rapid drop, 300°F - couple hours. The synthetics were stable out to 500°F.
  6. DogBunny

    DogBunny XS650 ThermometerDipstick

    About 18 months ago I did a (very) little research into the limits of oil temperatures, which I posted on another forum. Below is a copy and paste of what I found:

    The following really simplistic answer says that motor oil begins breaking down at temperatures above 260 F:
    On the other hand, Wikipedia says that in petrol gasoline engines, the top piston ring can expose the motor oil to temperatures of 320 °F:
  7. Greasy C, that is a very good write, I enjoyed reading that! Thank you.
  8. XSLeo

    XSLeo XS650 Guru

    I agree with head temps being critical. There is a very direct relation ship between oil temps and head temps. I'm not sure of the exact percentage on an air cooled engine but on a water cooled engine to oil removes about 30% of the engine heat. I would assume an air cooled engine it might be more.
    So cooiling the oil cools the whole engine. In my testing I found that adding a cooler dropped the head temps about the same as the oil temps. 45-50 degrees. From around 290 to around 240 degrees as measured with a non contact thermometer. In my testing, it was before I had the tempstick. I plumbed in a car temp gauge just where the oil returns to the engine. It was around 240 before the cooler, add the cooler it was around 170-180 degrees.
  9. FLEA

    FLEA XS650 Junkie

    old thread but found worth rereading as I'm running in a new motor ,went on a 60 kl ride temp 30 degs cel mountain rd lots of hills /corners sitting 3-4000 rpm when I stopped oil temp of 260 degs (750 kit,pamco,etc) on old healthy 650 motor (different motor) 200-220 degs still not sure if should be concerned ,(did have that oil leak from cooler/filter) but the difference seems fairly large
  10. DogBunny

    DogBunny XS650 ThermometerDipstick

    I assume measured with a ThermoDipStick.
    30 Celsius equals 86 Fahrenheit.
    The 200-220 degree motor sounds exactly right. I have zero feedback or experience on what temp 750 motors run, but if the 750 is the one with the oil cooler then 260 sounds high. All I know to try is bigger main jets -- this assumes you are hot because you are lean -- or changing timing. I forget which way to go, but timing affects engine heat.
  11. TwoManyXS1Bs

    TwoManyXS1Bs Kablatta, kablatta...

    Same here. I'm finding my oil temps (max) hit ambient plus 130°F.
    On a 70°F day, I got 200°F
    On a 100°F day, I got 230°F
  12. FLEA

    FLEA XS650 Junkie

    that's intresting 86 degf +130 deg = 216 which would be about right for the old unphased 650 motor (few weeks before changing motors over was about same air temp) anybody got any readings on their rephrased 750 motors...do they run hotter? when I said running at 3-400rpm that's with hi ratio primary gears ,not that I think it would make any difference ,except the primary gears are spining 20% faster but motor definitely not being worked hard
  13. Stoop22kid

    Stoop22kid XS650 Addict

    I've got a rephased 750 that I'm putting back together now....ill get one of these dipsticks and let y'all know when I find out.
  14. FLEA

    FLEA XS650 Junkie

    no worrys,they are definitely worth getting,good way to keep a eye on your motor
  15. FLEA

    FLEA XS650 Junkie

    heres a up date on my motor running hot ,had a large tool roll sitting under the head light ,removed it and now running 20 degs f cooler ,.knew it would make a difference but this much was surprising
  16. Bob Kelly III

    Bob Kelly III Ranch Kid from way back,.... that got Old !

    Old thread but a good one !
    thanks to 2M I now know that synthetics can handle 500deg F.. before breaking down
    and knowing on a Hot day temps can be in the high 230deg F and normal oils brake down at around 300deg F
    so parts inside the engine are burning normal oil on a hot day.... that's a given so switching to synthetic for wet clutches seams like an excellent idea !
    thanks 2M !
    someone mentioned a oil cooler on his side plate... any idea what that oil cooler was or where it came from ? and How would you hook one up when the engine has no fittings for an external oil cooler ?
  17. TwoManyXS1Bs

    TwoManyXS1Bs Kablatta, kablatta...

    Yeah, good stuff, Bob. The one spot that *can* exceed 500°F is the exhaust valve stem, in the exhaust port area where it enters the guide, especially on a worn, high-time engine with loose valve/guide fits. Poor heat transfer. That was a bit of a concern 30-20 years ago.

    Post #4.

    And here.
  18. Bob Kelly III

    Bob Kelly III Ranch Kid from way back,.... that got Old !

    Hi 2M ! , the oil cooler I was referring to wasn't the one in post #10 but SXLeo's post #14
    .... ( I had to look it back up ) it sounds like it's a different animal completely.... because it worked ! LOL
    I've not seen a good one for the XS650 yet just the Hughes and I don't care for it as I want allot of voluum/cooling area like maybe a Power steering pump oil cooler off a car ! or perhaps a automatic trans cooler ! ....
    ...Yes the exhaust valves are usually the hottest thing in the engine hotter even than the piston top and having good cooling on that part of the head is a necessary thing ! ....
    .... I got to looking at my bike the other day and noticed the Horn was sticking down into the air flow if the head.... I thought that's kind'a stupid ! so I took off the horn and it's bracket and flipped the bracket over added a 1" extension to the part that holds the horn on and re welded it, and put it back on so in essence I just moved the horn up and forward a bit and Now instead of blocking the air it scoops even more air in to the head ! ...<GRIN> the horn now sets just under the lower fork tree up snug ...and sense it was wiggled it even beeps louder too ! HA !
    .... I am contemplating adding scoops to the down tubes to scoop more air in over the cylinders
    so how would you hook up a oil cooler to this beast anyway ? take it from the center oil tube ? then go through the cooler and then back into the head where it was going ? that should work but that is only a small portion of the oil I think maybe a plate would have to be made with 2 hose fittings to plumb it with then put it where the oil pump/filter resides...eh ?
    peanut likes this.
  19. I have a thermodipstick Lucille ('76 stock) and I was reading about 200F around town and about 230-250F on our 60-70 mph blast up I75 to the Meet-Up on Saturday. The weather was a bit cool - perhaps 68-72F at the time.


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