RPM engineering Q

[Guest tagree01]

Hey guys, I was watching an F1 race a couple of weeks ago and heard that they decreased the RPM limit from 20,000 to 19,000. I think it is incredible that those engines can turn so fast when NASCAR engines only turn about 9k.

Since then a question has been nagging at me: what limits RPMs in an engine? Obviously it has to be well balanced, but what is the rate-limiting value? Is it cooling? Mass? Balance? Ignition timing? Airflow? Wear? Other?

I know that at as RPMs get higher, the engine gets out of the sweet spot of the power curve. What causes the decline in the curve?

[Young Ed]

I believe its mass that limits it. I used to run 1:10th and 1:8th scale RC cars. The 10th scale cars have 1.2 or 1.5CI engines that turn 30-35,000 RPM! Ive seen them make over 1HP. We used to knife edge the connecting rod to get more RPM out of them.

[RobertKB]

I think valves have a lot to do with it. Nascar uses a push rod engine while F1 uses compressed air to operate the valves. Used to love NASCAR and not open wheel racing but am going in the opposite direction now. Really like watching the American Lemans series also where they have four different categories racing at the same time.

[Guest jjmorrse]

A destroked engine can usually turn more rpms.

[greg g]

Formula one engines do not use camshafts to open or close the valves. If they still run cams the are used to time the valves, and the valve opeing and closing sequence.The valves are pneumatically oprated by high pressure air.

They also have a very short stroke.

[plywoody47]

Valve train is the first one-both valve float and landing speed. Second is mean piston speed (clearly linked to stroke.) Above a certain speed (combined with the reciprocating mass), it becomes difficult to maintian the bearing and wrist pin, least the piston or con-rod attempt to jump ship.

[Fireball]

Piston speed is one critical issue among the already mentioned valves. That brings us to material weight, oiling etc. So there's not an single area that prevents old fashioned V8's to run those rpms, the basic construction just doesn't allow that even with serious mods. Those F1 engines are from another planet.

[Guest tagree01]

I did some checking on the F1 engines. They are limited to 146.5 cu.in. I was also surprised to hear that they don't allow superchargers (turbos included) or nitrous or anything else. Just regular air and fuel.

The short stroke is interesting. I've heard the saying "there's no replacement for displacement". Is RPM a good replacement for displacement? Maybe it should be "there's no replacement for displacement per unit time". Some of these engines are producing 300bhp per liter (61 cu.in.). That would be in the neighborhood of 1070 bhp equivalent to my 218 cu.in. engine. Wow.

By the way, is it common for land speed racers to de-stroke their engines to run higher RPMs?