Engine RPMs on bikes and cars differ greatly. Why ?

Factual Questions
1

Most cars i’ve seen have engine RPMs that max out at about 7000 RPM, while bikes i’ve seen go up to 13000 RPM.

My numbers may be indicative, but the question is :

Why do most cars have RPMs only upto 7000 or so, while bikes go up to 13000 or more.

I understand that weight plays a part in the limitation of RPM, but i do not know the exact reasons for this.

Technical answers are welcome.
Also, i believe that the BMW Williams F1 car goes upto 18,000 RPM. How does this fit into the scope of things ?

*Note: I am reposting this question, as something peculiar happened. My post was submitted with only the subject line, while the body disappeared.

Moderators, you might want to be informed that on submitting that previous mysterious post i got a message that said something to the effect that you submitted a null string, or data was void or some such thing… i can’t seem to recall the exact text of the error, but NULL was part of the message… So, bibliophage, i guess the other similar posts you saw were accompanied with this message being displayed to the poster. Also, either my connection or the board or both was running at a very slow pace at the time. Before reposting i checked the GQ first page, because many times my post does not appear to go through but when i refresh GQ it is there… since i did see it on the refresh i didn’t repost… until i saw my post being closed by a mod. Not since my first post has that happened, so i was a bit surprised, until i realized that it was closed on a technicality.* :slight_smile:

2

i shall take the liberty to reproduce Padeye’s post here for reasons of continuity:

Padeye said:

"

Curious, does your post have no body text?

For the most part it’s a matter of scale. A bigger displacement engine has more recipiocating mass putting practical limits on RPM. While a bigger engine can be built for high RPM the vibration and noise is a distinct drawback in a car but not as big an issue in MCs. Engine designers can trade low RPM torque for high RPM peak horsepower as appropriate.

"

3

Well you have part of the answer, mass. Recipocating mass is the enemy of high RPM. Considering the biggest motorcycle engines are about the displacement of the smallest car engines and that’s a start. That’s only a generalization as there are exceptions such as low RPM MC engines like Harlies and high RPM car engines on F1 cars.

On typical street vechicles other things enter into it. High RPM means lots of vibration and noise, unacceptable in most passenger cars but tolerable in a high performance cycle. In a car it’s better to have a wide torque curve at low RPM to minize shifting but in a cycle with 6 or more gears a high RPM peak horsepower may be better.

Some engine designs are better than others. Inline sixes are exceptionally smooth at high RPM but rare because such a long engine is hard to fit into most cars. Inline ours are the most common passenger car engine for compactness but the worst for secondary imbalance vibration.

4

Power is a function of torque and RPM.

Generally you get more torque with larger displacement, or at least there is greater potential.

Cars also have large heavy flywheels, which is not very practical on bikes.

Those flywheels have the effect of smoothing out the engine at low revs, so one way to get around this problem for bikes is to use higher revs.

Bike engines tend to have far greater power to weight ratios than those of cars, a 1000cc bike engine can develop over 150BHP in a machine whose all up weight it less than 400pounds.

The only way to suqeeze that power out of that engine relatively easily is to increase the number of RPM, turbos have been tried on bikes but they are expensive for the gains they produce and the lag is the last thing one wants on a bike whose whole purpose in life is to accelarate.

5

You should also keep in mind what is the planned life of the engine. The engines in Formula One vehicles are treated about like we treat paperclips, meaning disposable, while in road vehicles they need to last for years and years.

I must say that the sound of a Formula One car at high RPM is very sweet though. Another engine that sounds sweet is that of a P51 Mustang or the Spitfire but I do not think they turn more than 10,000 rpms. Probably a lot less.

6

The more massive something is, and the faster it is traveling, the more momentum it has. So a big, heavy engine turning at very high RPM is experiencing a great deal of stress on the internal components.

There is also inertia to consider. Some engines aren’t limited by the strength of the components, but by how fast the parts can be moved. One problem that some engines have at very high RPM is that the valves ‘float’, they don’t move through their entire cycle fast enough. They end up hovering in a half-open position when they should be closed. Lightweight valves help with this problem, but that means either expensive materials or a bunch of tiny valves. Either way, it’s not practical for a production car.

Those F1 car engines are built with lightweight (expensive materials), and use some advanced technology to get those incredibly high max. RPM. I think they all use pneumatic valve systems these days, which eliminates a lot of moving parts. So, those engines can reach over 10,000 RPM because they are a ‘cost no object’ endeavour. Also, as NoSubsititute pointed out, they don’t need to survive any longer than the race.

The Merlin engines in a P51 Mustang or Spitfire are another good example. They are 27 liter v-12 engines that produce around 1500 horsepower (there are lots of different versions, so there is quite a range on the power). They have a maximum RPM limit of around 3500, for short periods only. Maximum sustained RPM is around 2700. So, they are huge compared to a typical car engine, and they have an even lower rev limit.

7

One point people have only sorta touched on: higher revving engines tend to have wider bore and shorter stroke. Since the limiting factor is how fast you can turn that reciprocating mass around(without, shorter stroke=higher redline. This results in lower torque but higher horsepower.

The opposite is true of big truck engines. Longer stroke results in more torque(think about the length of the “lever” turning the crank), lower redline.

8

There are several things that may be done to increase the power output from an engine but each in turn has its pros and cons.

Increasing displacement generally increases torque but the RPM is limited by the mass of the reciprocating parts, especially the piston itself.

One can introduce a greater fuel/air mixture by using forced induction such as a supercharger or a turbo.

The supercharger inceases torque and power at all revs whereas the turbo tends to cut in at a definate engine speed.

The use of compressors increases the temperature of the air so an air cooler is often introduced, which help to increase the density of the air, more air means more oxygen which means you can intruce more fuel and hence get a bigger bang.

Induction compressors indroduce a layer of complexity, bulk and cost which makes them generally unsuited for bikes, but very useful for other applications such as diesel engines.

You can increase the compression ratio, squash the fuel/air mix harder and you get a bigger bang, but in doing this the temperature of the mixture increases to such a point where it may spontaneously combust, which is fine for diesel engines but not in gas engines where is is called piknking or knocking and can lead to serious engine damage.
It is possible to get around this problem to an extent by using differant fuels or additives to that fuel but whatever happens there is always a limit.

So increasing the torque is possible but can be expensive and/or heavy.

If the torque is not changed one can still get more power by increasing the RPM and this is what happens on bikes.

Some road bikes go right up to 18k!!

When one looks at the whole package there is a price to be paid for extracting power in this manner.

As you increase the revs you increase friction loss virtually linearly and this manifests itself as heat.

High revving bikes neat a lot of cooling.

High revs means high rates of engine and valve wear, the result is that only a few bikes are completely reliable to 100k miles whereas the higher torque - lower revving car engine can easily run to two, three of even more times that, and often without major work.

High mileage per year on a bike is usually taken to be anything in excess of 10k per annum, and yet cars are good for double that and more without too much effect on their resale value.

High revs, high losses makes for lousy fuel consumtion figures. Bikes often have figures of around half that of the most economical cars, and if you wind the throttle round and get a bit naughty this figure will plummet.

Bikes are often regarded as leisure vehicles and the economics that apply to primary transport such as cars do not apply.