Overheating, Excessive Idle Time, ETC.
on your BMW Airhead Motorcycle
© Copyright, 2012, R. Fleischer
There is a relatively common misconception that even moderate idle time will cause
overheating on our BMW air-cooled bikes. Not so. There are, of course, limits,
even to our well designed & well-finned BMW Airhead motorcycles. Truly excessive
idle time can certainly overheat the top end & cause problems from warping. It is
even possible to loosen valve seats, besides the usually spoken-about frozen or
Our Airheads have a LARGE amount of cooling fins which total a considerable surface area. Cooling is quite good, even when not moving. The cylinders are outwards & have good convection cooling. A few minutes of idle time at a stop light, even in the hottest weather, is not going to do any harm. This assumes you will ride off a bit after that & not be idling for several minutes at every block in a city, for miles. Being stuck on the freeway in a traffic jam for 10 minutes and longer in hot weather is NOT a good place to continue to idle the engine. It is especially not a good idea for a low RPM idle constantly, because as the oil thins from heating, the oil pressure in the system drops at low RPM, and there is less and less, perhaps NONE, of oil coming out the pressure relief valve located above the crankshaft sprocket. Thus, the timing chest parts are not getting enough oil....for lubrication....and cooling.
But, for brief stops, there is perceived over-heating. Human beings know what outdoor temperatures are comfortable to THEM, & may unconsciously transfer that to our engines.
Engine's are generally worn more by the first minute or so of start-up in cold weather, where engine clearances are decreased from cold shrinkage, & wear is therefore high. Even in quite mild weather, initial start-up is where a large amount of any wear tends to occurs.
Idle & low power levels produce vastly less engine heat.....than cruising down the road at highway speeds. The faster you go, the more heat is produced by the combustion process because you are using more power.
The heat is gotten rid of primarily by the exhaust, and the turbulence effects of oncoming air as it passes very irregularly around the fins. Some cooling comes from air flowing over the motor in other areas. Yes, there are radiation and other cooling effects too. IMO, BMW has designed out Airhead engines to have more cooling ability than needed, for any power output level of the stock motor.
Airhead owners who have the BMW oil-cooler radiator (with thermostat version) occasionally think something is wrong with the thermostat if the oil-cooler radiator did not feel hot to their hand after they stopped. The thermostat will SOMETIMES open for the additional oil cooling, allowing a warm/hot oil cooler radiator, when driving slowly in town, or moderate stop and go traffic. This is because the oil is hot enough to open the thermostat. The GS models don't have a thermostat, but a bypass hole, so they MIGHT feel warmish much of the time. But, it takes a fair amount of oil heating to cause the thermostat models to pass oil through the radiator.
I personally think that "ten minutes as being OK for idle", as has been cited in BMW's literature, CAN be excessive. I think 3 or 4 minutes is OK, but nearing the SAFE limits, especially if there is not a mile or or so of riding between multiple stops. In that instance I'd pull off the road, turn off the engine, and let it cool down. I am speaking here of both heat effects on expanding metal & lubrication concerns. Petroleum-based oils, IF THEY GET HOT ENOUGH, will deteriorate MUCH faster than part synthetic or full synthetic oils. The deterioration usually is not reflected in lubrication qualities, until extreme. By deterioration, here, I mean not only the base oil itself, but the additives (which MAY ALSO evaporate at high enough temperatures).
While excessive idling time can be worrisome for overheating of various parts, especially the top end, I would also be concerned about inadequate lubrication to the timing chain, sprockets, etc...... This is particularly so if your engine is worn moderately or more, and/or your idle is below 9500 rpm, which is a BAD idea. As the oil heats up, it thins, oil pressure DROPS, and thin oil passes through bearings & other places much faster, which can be good, as it carries away heat,... but... leaving less oil for the overflow from the pressure regulator, which is HOW the timing chest area parts are lubricated.
Explaining a bit differently: The timing chain is oiled by the output of the high oil pressure release valve (plunger type) located just above the crankshaft sprocket in the timing chest. It is set for ~75 psi actuation by an internal spring. Oils thin out as the oil temperature rises, & some oils have a poorer viscosity index than others so are worse at this, & although thinner oils get through the system faster, and probably can carry heat better due to the velocity, there WILL BE much lower oil pressure, particularly on an higher mileage Airhead, where bearing clearances are increased due to wear. Thus, there may be very little or no oil at low idle RPM to the timing chain & sprockets, etc. It is the overflow from that pressure relief valve that provides all the oiling for the timing chain, chain guides, and timing sprockets. Enough experience & tests have shown that idling rpm should not be too low, certainly NOT lower than 900 rpm, for just this reason (let alone others, such as carburetor function at low rpm as you transition to the off-idle mode), and, chain system jerkiness...which adds extra wear.
I think the worst situation for overheating would be either long waits in stop & go driving in the city.....or; worse....an excessively long stop right after quite a few miles at warp speeds. Perhaps worse yet with a very low idle rpm at that particular stop. Blipping the throttle now & then at a stop will help with oiling, some anyway. One of the worries is excessive cylinder head heating could cause head warping. We do see R100 engines warped now and then....usually when someone complains a regular valve cover gasket is not sealing well, or there are leaks at the head gasket, or a dual-plugging job is being done and the head is being checked; or, checks done during a top end job..
If you KNOW you will be stopped in traffic for quite a few minutes, & perhaps there is
intense stop & go with long stops, you should probably consider turning off the engine. The cooling is much more rapid than you may think (from such as the rising heat).
Oil, particularly some petroleum-based oils, deteriorate fast, on a very steep deterioration with temperature curve, above very roughly 250°F or so. Some might say the accelerated deterioration starts around 230°F. Most part-synthetics are better, and good quality part-synthetics, such as Golden Spectro 4, 20W50, are quite good. The hottest area for oil is not inside the main engine case, but around the valve stems in the heads. Thus, oil is heated quite high, briefly, as it passes those areas.....but the bulk of the oil is hardly going to reach such high temperatures. Dino oil that reaches towards 300°F is deteriorating rapidly.
Full synthetic oils, such as Spectro's, are particularly good with hot engines with quite hot oils.
These things are why oils do not need to be changed too often....but do need to be
changed, and WHEN, depends on how one uses and rides.
Just when to change oil is something that has been talked about, and argued about,
forever. Changing oil TOO often is also not the best thing, and FEW know anything about
that ....the information, from SAE no less, is in my oil articles: http://bmwmotorcycletech.info/Oil.htm. If you wish to learn about how oil REALLY works & REALLY lubricates, read this:
There is a tendency, perhaps carried over from the /2 days, for people to set the idle rpm on the Airheads too low. In the /2 era, the engine oiling system was quite different; the flywheels were very heavy, helping to keep the engines quite smooth. In that era, it was sometimes considered a matter of pride... or ego of some sort... that the engines COULD be made to idle very slowly/smoothly with a very careful carburetor adjustment. This was actually foolish. Rather too many carry over these ideas to the Airheads.
Excessively slow idle rpm on Airheads makes for far less oiling to the timing chain/sprockets/guides. The chain and sprockets will also jerk about more at very low rpm, which is harder yet on these items, as well as other areas of the engine, including the camshaft, and even various bearing areas. Because of these things, I have long recommended that the ideal rpm for idle is likely ~ 1025, or even somewhat more. An added advantage is that the Bing CV carburetors work better at that idle RPM. Not only can idle itself be smoother, which is better for the timing cover innards, BUT, the transition period (that point you JUST BARELY open the throttle from idle resting) is much smoother, & the carburetors synchronize and adjust much better. Those with dual-plugged engines & Bing CV carbs have further reasons for having a 1025+ rpm idle, as the carburetor adjustments are more critical (as the butterflies are closed more on the dual-plugged bikes).
Copyright, 2012, R. Fleischer
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Last check/edit: Thursday, June 16, 2016