Austarion

To all the automotive engineers and generally smart people who frequent this forum..

What determines the maximum rpm that a motor can safely reach? What causes some motors to be limited to a 6k redline, while others can happily get to 9k? I'm not talking about rotaries, just piston engines.

I know that different cams allow a motor to breathe better at different rpms.. but is this the limiting factor? I would assume there's a lot more to the equation than one single component..

Is there some sort of mathematical formula or method that manufacturers use to determine max rpm?

One of the main contributing factors is the rebound on your valve springs (or how stiff they are) because valve bounce is often what you will hit that will stop your engine revving any higher.

Valve bounce is when the engine is rotating so fast that the valve springs do not have the rebound time to fully close the valves before they begin getting pushed open again. At this point you lose power.
Putting a bigger CAM on will affect this too and often require a valve spring upgrade to handle the further distance or longer dwell on the valve from a larger/lumpier cam.

As for forumulas etc to work that out, I personally don't know details. Only method I know is to push it and see where it won't go any higher (very briefly only), but that's probly not such a professional method, so I don't recommend it.

Another limiting factor is also the speed and quality of spark that you can provide, so upgraded ingition systems will give you higher RPM too. Coils require a certain off time before they are on again, some work better than others, same with the switching circuitry that drives them.

Hope that helps, will be curious to see if there's some deeper technical advice people have?
:)

Oh and I know I probly didn't have to explain valve bounce to you thrash :D but just thought I'd put it in here for others reading this thread.
:beer

As far as Mitsis are concerned, for the petrol engines they seem to have a redline of minimum 6000rpm, or approx 500rpm above the peak power figure, and then for the ECI-Multi engines the imposed rev cut out limit is often 250-500rpm above that.

Piston-speed and/or peripherial(?) crank-speed...

There are many limiting factors....

1) Camshaft

2) Rod/Stroke ratio

3) Valve Springs

4) Rotating Mass

5) Volumetric efficiency

6) Engine Harmonics

7) Oil Supply

8) Metal Fracture points due to fault points

get the picture?

and conrods is a big one..

Come on fella's.. Primarily an engines ability to rev is directly related it's bore size and stroke length and to a lesser extent, air flow (DOHC vs SOHC). Engines can be known as square, under square or over square.

square engine 86mm bore/ 86mm stroke eg (SR20DE Nissan)
Good revs and torque

over square engine 100mm bore/ 80mm stroke eg (EJ20 Subaru)
Higher revs, lower torque

under square engine 90mm bore/ 120mm stroke eg (BP14 Mazda)
Lower revs, higher torque

vr4gone wrote:Come on fella's.. Primarily an engines ability to rev is directly related it's bore size and stroke length and to a lesser extent, air flow (DOHC vs SOHC). Engines can be known as square, under square or over square.

square engine 86mm bore/ 86mm stroke eg (SR20DE Nissan)
Good revs and torque

over square engine 100mm bore/ 80mm stroke eg (EJ20 Subaru)
Higher revs, lower torque

under square engine 90mm bore/ 120mm stroke eg (BP14 Mazda)
Lower revs, higher torque

Its only one factor of many, look at the 1JZ it has a bore/stroke of 86/71.5 while the F20C has a bore/stroke ratio of 87/84. The 1JZ has a more oversquare design but has a redline lower than the F20C.

vr4gone wrote:Come on fella's.. Primarily an engines ability to rev is directly related it's bore size and stroke length and to a lesser extent, air flow (DOHC vs SOHC). Engines can be known as square, under square or over square.

square engine 86mm bore/ 86mm stroke eg (SR20DE Nissan)
Good revs and torque

over square engine 100mm bore/ 80mm stroke eg (EJ20 Subaru)
Higher revs, lower torque

under square engine 90mm bore/ 120mm stroke eg (BP14 Mazda)
Lower revs, higher torque

an engine's ability to rev has fk all to do with where the redline gets set, ability of the engine to handle the forces generated by high rpm is what sets the marker...

ie shitty ZZZ gemini's have a redline of 6500rpm, good luck getting a stock one (82/75 bore/stroke) to rev that high while in a gear! however you can turbo a stock one and it will hold together fine at and above that rpm..

manufacturers set a redline as a point that which over that point engine operation becomes risky to the life of the engine.. ie stock mum and pop version holden red motors tend to lose the tops of pistons over 4500rpm..

an engine's ability to rev has fk all to do with where the redline gets set, ability of the engine to handle the forces generated by high rpm is what sets the marker...

Seriously?.. Alright, time for " stick into hornets nest" Now I admit there are many factors that effect the rev range of an engine and, s&*t, you can set a red line limit any where you want with only two results, either a reliable running engine or catastrophic engine failure. You cannot take a Haltec managed SR20, for example and set the rev limit to 14,000RPM and seriously expect it to rev that high making an engines ability to rev having everything to do with where the red line is set. Don't forget, The first question asks,

"What determines the maximum rpm that a motor can safely reach?"

This is a clear example of the direct relation between bore/stroke ratio and an engines ability to rev.

The power a Formula One engine produces is generated by operating at a very high rotational speed, up to 18,000 revolutions per minute (RPM).[citation needed] This contrasts with road car engines of a similar size which operate safely at typically less than 7,000 rpm. However, the torque (turning force at a given speed) of a Formula One engine is not much higher than a conventional petrol engine. For example, the 2006 2.4 litre Toyota RVX-06 V8 engine produces 552 kW (740 bhp, 751 PS) at 19,000 rpm and outputs 274 N·m (202 lb·ft) of torque giving the engine a 14.3 bar (1.43 MPa) mean effective pressure.

The bore is the diameter of the cylinder hole in the engine block for the piston and the stroke is the distance the piston travels from Top Dead Center(TDC) to Bottom Dead Center(BDC) inside the cylinder. A shorter stroke enables the engine to produce a higher rotating speed at a constant mean piston speed but also increases the speed at which the piston must travel in each revolution. Shortening the stroke however requires enlarging the bore to produce a Formula One engine's 2.4 litre displacement resulting in a less efficient combustion chamber.[citation needed] The stroke of a Formula One engine is approximately 39.7 mm (1.563 in), less than half as long as the bore is wide (98.0 mm) producing an "over-square" configuration.
This info is taken from Wikipedia

Piston speed which is determined by the stroke is one of the main things to consider. But in reality, there is not much point in revving to much higher than the speed of maximum power. The faster an engine, the higher the friction, the faster the wear.

vr4gone wrote:

an engine's ability to rev has fk all to do with where the redline gets set, ability of the engine to handle the forces generated by high rpm is what sets the marker...

Seriously?.. Alright, time for " stick into hornets nest" Now I admit there are many factors that effect the rev range of an engine and, s&*t, you can set a red line limit any where you want with only two results, either a reliable running engine or catastrophic engine failure. You cannot take a Haltec managed SR20, for example and set the rev limit to 14,000RPM and seriously expect it to rev that high making an engines ability to rev having everything to do with where the red line is set. Don't forget, The first question asks,

"What determines the maximum rpm that a motor can safely reach?"

This is a clear example of the direct relation between bore/stroke ratio and an engines ability to rev.

The power a Formula One engine produces is generated by operating at a very high rotational speed, up to 18,000 revolutions per minute (RPM).[citation needed] This contrasts with road car engines of a similar size which operate safely at typically less than 7,000 rpm. However, the torque (turning force at a given speed) of a Formula One engine is not much higher than a conventional petrol engine. For example, the 2006 2.4 litre Toyota RVX-06 V8 engine produces 552 kW (740 bhp, 751 PS) at 19,000 rpm and outputs 274 N·m (202 lb·ft) of torque giving the engine a 14.3 bar (1.43 MPa) mean effective pressure.

The bore is the diameter of the cylinder hole in the engine block for the piston and the stroke is the distance the piston travels from Top Dead Center(TDC) to Bottom Dead Center(BDC) inside the cylinder. A shorter stroke enables the engine to produce a higher rotating speed at a constant mean piston speed but also increases the speed at which the piston must travel in each revolution. Shortening the stroke however requires enlarging the bore to produce a Formula One engine's 2.4 litre displacement resulting in a less efficient combustion chamber.[citation needed] The stroke of a Formula One engine is approximately 39.7 mm (1.563 in), less than half as long as the bore is wide (98.0 mm) producing an "over-square" configuration.
This info is taken from Wikipedia

make a F1 piston and rod combo out of shitty cheap materials then try and rev it to 14000rpm, and see what happens. all the bore/stroke ratio in the world wont save it from shitting itself.

thrash wrote: Is there some sort of mathematical formula or method that manufacturers use to determine max rpm?

Well of course there is. Engineers would calculate the stresses imposed on every single component before even one part was cast.

More importantly manufacturers aren't just thinking about how hard an engine they are designing can rev. They are thinking about:

1/ what sort of work are we designing this engine for.
2/ How long do we want it to last.
3/how efficient can we make it.
4/ How much will it cost to manufacture.

Also Thrash, maybe you might want to clarify your question a little further ie what exactly are we talking about here, racing such as F1/MotoGP or just everyday production cars?

the balancing of an engine affects its big time.... a friend had a stock engine that wouldn't rev past 5500 no matter what (no fuel cut, was carby fed).. he got it balanced and all of sudden it could go to 7500.. needless to say it ended up with a hole in the block as the rest of the motor wasn't up to it.