V-Twin Tech: Engine Warm-Up

Warming-up an engine and how it is accomplished is generally not particularly critical. If it were, there would be one or two ways to do it and engine makers would surely draw our attention to the matter more strongly than they do. However, engine warm-up is important and it is a good idea that we all know something about the matter.
There are reasons why one should indulge in coddling an engine until it has reached operating temperature, reasons why one should not, and at least a couple why it doesn’t matter very much.
Safety is the most important reason to warm an engine before riding. Most stock engines tend to lack power and deliver uncertain throttle response until they have run for at least a few minutes. In fact, some engines will stop running altogether if the throttle is quickly and fully opened immediately after they are started. If that were to happen in front or someone running a stop sign with their SUV, well…. This kind of scenario, by the way, is the main reason manufacturers often recommend long pre-ride warm-ups.
Many of the motor parts inside your Harley engine do not fit and work together well until they are at or near normal operating temperatures. I think the best example is the cylinder-and-head stack. Evo, Sportster and Twin Cam engines have aluminum cylinders, heads and rocker boxes. However, the studs that hold those parts together are steel. The difference in the temperature-related expansion rate between aluminum and steel is about 10 to one. Thus, the head gaskets are squeezed much tighter after the motor is hot. Early Evos would often blow head gaskets if they were given full throttle before the cylinders and heads were warm. The earlier Shovelheads and Iron Sportsters did not have this problem and, at least in this one respect, were superior designs.
I once asked Erik Buell if he had considered using the very stable Axtel iron cylinders on his racing Buells. Erik replied that they had tested the cylinders but could not use them because road racing motorcycles must endure full throttle and high rpm at the drop of the flag; the iron cylinders, needing more time to come to temperature, wouldn’t grow fast enough and the pistons would seize. I have seen this happen to water-cooled two-stroke road racing engines when they were not fully warmed before the flag dropped. These engines must do their best work at high temperatures and the most dangerous time for them is the first lap.
On the other hand, drag race motors never get very hot. The heads do, but nothing else does. Drag racing engine tuners know this and set the motors up accordingly. Byron Hines told me that a Pro-Stock Suzuki or Kawasaki’s engine oil would be barely warm to the touch after a race. The wonderfully powerful engines he built would make 225-plus horsepower for six or seven seconds, the time it took them to get through the quarter mile. They might not have finished a half-mile in one piece.
So much for the extremes. Lets look at our street Harley motors and how we use them. Most of the time we are asking them to produce less than 15 Horsepower when traveling at steady speeds. About 10 horsepower are needed to push a Sportster down a flat, windless road at 60 mph. An FXD or bare Softail needs 11 or so and a full touring rig about 13. This isn’t much of a load for an engine capable of producing 60 peak horsepower.
I can assure you that Harley-Davidson has spent many hours and miles refining the piston, ring, valve guide and other dimensions to serve this standard load. They have also had to allow for the occasional extremes which include “seeing what she’ll do” and the stresses of abusive warm-up.
The extra ring end-gap needed to ensure there is no binding during long full throttle runs allows blow-by when the gap is wider, as it is when the engine is cold. The ring end-gap must be large enough for a hot engine and is likely too large in a cold engine. There are other such compromises like piston skirt-to-cylinder clearance and cylinder-head torque. If you were to make sure your Harley motor were completely warm before using full throttle to get up to 100 mph, you could run the piston clearance within a half-thousanths of an inch of the cylinder walls instead of the standard two-thou (or so). Harley must certainly assume that you might not do that and fits pistons accordingly.
So, what is a person to do to properly warm-up the engine? Basically, give the motor a chance swell up and get its clearances act together. The bit of extra piston/cylinder clearance or piston ring end gap won’t matter if you keep the loads and rpm moderate during warm-up. The lower clamping pressure holding the heads in close proximity to the cylinders aren’t so important at half-throttle.
Why not just let the engine idle until it is warm? This is important. A cold engine generally needs the choke to run at all. Chokes are relatively crude devices and dump too much fuel into the engine, more than it needs. This extra fuel washes oil off cylinder walls, finds its way past loose pistons (and ring gaps) and down into the oil. Other byproducts of combustion find their way into the crankcase and engine oil as well. These include: water, acids and carbon particles.
The best way to minimize all this contamination is to warm the engine as quickly as is reasonably possible. “Reasonably possible” includes placing a load on the engine by actually riding the bike. One or two horsepower are needed to idle an engine, but it takes 10 or so to go down the road. More horsepower develops more heat, hence, the heat from the 10 warms the motor faster, reducing corrosive oil and engine contamination.
If you keep revs below half-way to redline and the throttle below one-half, you’ll have no trouble with blow-by or overstressing cold and still loose parts. Long ago, before emissions standards and too many lawyers with time on their hands, General Motors learned that engines lasted longer if they were warmed by driving the car. I have ridden with that recommendation in mind for 40-plus years and preached it to thousands. It makes sense and seems to work.
But please, never “drop the hammer” on your cold Harley motor and never red-line it until fully warm. By “fully warm” I mean that the oil, all of it, engine, gearbox and primary are up to operating temperature. This usually requires about 12 miles at 65 mph. Warm oil is a reliable sign that all the parts are well lubed and that all clearances are within the tolerances of what Harley-Davidson intended.
So, I recommend that you ride your bike moderately just as soon as it is safe to do so. If the engine warms enough to reliably respond to the throttle, ride.