Why can motorcycle engines rev much higher than car engines?

Question

Does it have anything to do with capacity, engine configuration, materials, or all of these?

Answer 1

Max rpm is limited by the max piston speed. Exceed the max speed of the piston, and you'll have cooling problems.

Short-stroke engines, therefore, can rev higher than long-stroke engines of the same displacement.

Also, at very high rpm, you'll have limits based on the valve train. A conventionally configured valve train will have the valves opened by the camshaft and closed by the spring. At very high speeds, the spring isn't able to close the valve all the way before it opens again. This can lead to the valve colliding with the piston crown. This is bad. There are methods to get around this, like desmodromic valve actuation. Some MotoGP teams have also experimented with pneumatic valve actuation to get around this problem.

Some Formula 1 car engines run up to 22,000 or 23,000 rpm, higher than any bike I'm aware of. These are typically million-dollar engines that have high-zoot valve trains and a service life of hours. MotoGP bikes, the two-wheeled equivalent, rev up to about 19,000 or 19,500.

Answer 2

It isn't always the case - it depends on the car and the bike. For example the Acura RSX type-S and the Mazda RX-8 have relatively high redlines. Many Harleys have lower redlines in the 5000rpm range. Engines with long stroke produce a lot of low end power but can't rev as high. High revving engines often have less low end power....it applies to cars, motorcycles, boats, etc.

Answer 3

depending on the motorcycle engine you are talking about. engines with a short stroke (length the piston travels within the cylinder) will wind out to a much higher rpm. harley's have about the same stroke as the average car engine, so they usually rev to about 5500 rpm. japanese bikes generally have a short stroke and rev to over 10,000 rpm. now formula 1 race care also have a short stroke, they can rev to over 15,000 rpm. high rpm is for horsepower, the long stroke engines almost always have much more torque.

Answer 4

Primarily because small pistons, shorter piston arms, and the fact that there are usually 2 or 4 cylinders compared to car motors with large block, long arms, so many lobes on the cam, etc. timing gear ratios are much smaller also.
That's about the only reasoning.

Answer 5

Overhead cam. Valve in an overhead cam have less prats and can move the valves faster.

Shorter stroke. Piston has less distance to travel during each stroke.

Less reciprocating mass. Less weight in the moving parts of the engine allows the engine to turn faster bedoer flying apart.

Answer 6

The length of stroke and rpm are the variables, but pretty much the real limiting factor is the piston speed, which needs to be kept below 4,000 feet per minute for any kind of piston ring longevity. Faster than that and you end up with broken rings. That might be an engine with a 6 inch stroke turning 4,000 rpm or a 3 inch stroke turning 8,000 rpm. The connecting rod length also affects the maximum rpm, the shorter the rod the faster the piston accellerates and decellerates at each end of the stroke. The longer you can make the rod, the more gradual the speed increase or decrease and the easier it is to overcome the inertia of getting the piston stopped and started. For example, take your kitchen refrigerator. If you want to move it, you start pushing slowly and it starts moving. If you backed up and ran into it, you'd knock yourself silly.

Piston, connecting rod, crankshaft and valve train weight also affects top speed. The heavier anything is that moves back and forth, the slower the speed can be.

Valve spring tension also affects the top engine speed. The stiffer the spring, the less change of valve float and damage. But, the heavier the spring the more energy used and stress involved in compressing it. The valve size and flow characteristics may not limit the physical limits of sped, but certainly determines how high an engine will rev.

Plain bearings, especially on the crank, can limit speed. The pin drags the outer bearing surface along with it and as a result, it isn't centered. Very high speeds can elongate the bearing clearance area so much that the oil film is sheared.

In short, the smaller and lighter the engine parts are and the better an engine breaths, the higher the potential rpm.

Answer 7

Thumprr, you are very close to being right on. In more or less normal production engines, 4000 ft/min is considered redline for racing purposes. For normal(highway) use, 300 ft/min is considered max. Of course, an engine can exceed this - for a very short period of time. So, taking stroke, and rpm, you can figure the max usable RPM for any engine. Note, Bore size has nothing to do with it. Bore size will influence other design factors, such as inlet ports, valve size, carb/FI size, etc. A short stroke, big bore engine makes more power than a small bore. longer stroke of the same size, and at a much higher rpm. At low speed, the longer stroke engine will perform better. An old 90 cu in Sportster I once ran (3 3/8 X 5 In bore/stroke) ran low 11's , 125 mph in the quarter, turned all of 5200rpm. Replaced it with a Kaw, 1200cc, 76mm bore, 66mm stroke, turned 10000rpm, ran mid 9's at aprox 140mph. 300 cc less, wieghed a bit more..hmmmm. Rebuilt the Sportster 2x a year at least, did the Kaw every two-three years. Same Kaw ran 157+ record at Bonneville in 1975, first run qualified 5 mph over the old HD record. (same class in those days.) You can compare the Sportster engine to a car engine.

Tomcotexas

Answer 8

The crank is half as long as auto engines.

Answer 9

the pistons are much smaller is one reason plus the ones you wrote down

Answer 10

they have a much shorter stroke.