Mechanical influences on maximum rev limit

Hello,

I was wondering what parameters are neccesary to define or calculate the maximum rev limit an engine can handle. I've searched in the forum and the different course curricula but couldn't find some information.

I know that some aftermarket piston and rod manufacturers tell you what their parts can handle, but what if I don't have these informations or I want to raise the limiter on OEM parts. There are some guidelines with piston speed and rod length to stroke ratio, but that can't be working for all parts the same, or does it?

My estimations on the possible influences besides piston speed and stroke to rod length ratio are:

- Piston and rod assembly weight (and therefore oscillating and rotating mass and forces)

- Piston and rod yield strength

- Camshaft timing and valve lift

- Valve mass and valve spring force or material

- Hydraulic vs mechanic lifters

- Maybe belt vs chain driven cams

- Flow of intake and exhaust

Does some of you know how to calculate or define the possible rev limit to start testing with? Or is there just an experimental approach if the power still rises and vibrations are low to rise the rev limit step by step?

Short information about me, I am from Germany and working on Jag AJ30 N/A, Nissan SR20DET and Nissan/Datsun L28 N/A engines right now.

Thank you and best,

Jonas

It is possible to calculate a lot of the forces, and hence stresses, on most components, but it is VERY complex and if you don't know what the components are made from, any heat or other treatments that may be used, etc, if may not be totally relevant.

Some engines have a lot of excess strength built into them, and 10%, or more, is 'safe'. However, some engines may have a single weak point that has to be addressed but are otherwise fairly bulletproof (the GM/Cosworth 20XE/KBA is good for 8k except for weak big end bolts that have been known to fail at the OEM limiter of 6k8 - they MUST be replaced, but 'rod, forged pistons and crank are fine).

I'm not sure what you mean by "vibrations", as failure normally occurs without any warning. Just to confuse things, running close to the mechanical limit of a component can have a drastic affect on the fatigue life of the part, you may be able to use 8k without a problem but extended running at 7k8 may mean the next time you use 8k fatigue cracking has weakened tha part and it fails.

All those engines you list have been used in motorsport, so I would start by looking to see what you can find from the associated on-line clubs/forums, and what aftermarket parts are available to counter the weaknesses that have been found by others breaking parts.

Hi. Most of the times the mean piston speed is your limiting factor. For instance in NASCAR and Formula 1 it does not exceed 25 m per second. On stock engine components it is not recommend to go above 23 m/sec, whilst on forged components you can go up to 25-26 m/sec. If you are using the strongest parts avalable you can probably get away with 27-28 m/sec but most likely you will face the issue of flame propagation speed - your piston will be moving faster than the flame so you'll be losing torque and power making high reving meaningless...

Going to depend on the engine as the bogger the combustion area the greater the distance to cover. However, that's less of a concern with high swirl and other design considerations - the last of the NA, non-rpm limited F1 engines were running close to 21k with bores close to 4", and some of the smaller class motorcycle engines were running over 25k.

I recall people claiming, not so long ago, that even smaller diesels would never make power over 4k, because the flame front was too slow - larger race diesels are now turning up to 8k!

Something related to the valve train considerations already noted is the problem with the diameter of direct acting followers limiting the camshaft profiles and accelerations possible. That's why even F1 turned from those to roller rocker designs that gave the advantages of better geometry at the camshaft to roller follower, greatly increased reliability as the 'scuffing' action was removed, the 'lever'action of the rockers to multiply the lift at the valve compared to at the camshaft. True, there was a slight increase on mass, but air springs assisted in overcoming that and the reduction in the cylinder head 'height'gave an overall reduction in CoG.