Why different rev limits

[wizzer]

Why do engines have such different rev limits?

Honda rev to 8700rpm, Diesels redline at 4500rpm and other cars vary between these two extremes with the RX8 redlining at over 10,000 rpms.

Is it down to the engines strength or are there other factors here?

 

[crowley]

I think the valve springs are probably the single biggest limiting factor regarding rev limiters.

 

[obi_waynne]

Stroke length also plays a part. High revving engines tend to have a short stroke. It also depends how much vibration there is.

Rotary engines are totally different and can tolerate high rpm.

The red line Is set at a safe limit do that vibrations do not destroy the engine. The better balanced an engine is the higher it will rev.

F1 engines in perfect balance rev to double your average cars limit happily.

 

[HDi fun]

Shorter cranks help as well. A V6 (petrol) engine will usually rev more happily than a straight 6. The straight 6 though exhibits no second order (harmonic) vibration which means there's no need for balancer shafts etc. so the differences are often smaller than expected.

 

[Aerial Andy944]

Shorter cranks help as well. A V6 (petrol) engine will usually rev more happily than a straight 6. The straight 6 though exhibits no second order (harmonic) vibration which means there's no need for balancer shafts etc. so the differences are often smaller than expected.

That's interesting as when Nissan were developing the R32 GTR they considered a V8 and V6 before choosing a straight 6 layout because it was better balanced and could rev to, in the race versions, 9000 rpm and 8000 rpm in road form.

 

[SLEEPER]

There are many different factors because there are so many different moving parts.
Obviously the quality of the parts is very important but assuming that this ,the fuel requirement and the oil needs have been taken care of there are some physical limitations
In no particular order

Piston speed is one which is why a long stroke can't Rev like a short stroke.
The engine configuration and the cubic capacity also makes a big difference
As revs increase the valves become an issue . You can uprate the strings and valve train which will work up to a point but very high revs introduces valve bounce when the normal springs can't cope. Ducati solved this by having a cam to close the valves as well as open them .
Their desmodronic engines revved to 14000 rpm and that was at least 30 years ago

Higher revs also means reliability issues even when the ultimate materials are used.

The above is for conventional engines
Rotary engines have a different set of rules especially as there are different ways you can measure the revs.