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Torque Management

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Why is high torque at low rpm more rod-snapping that high torque at higher rpm? General rule of thumb I've always heard is low rpm + high torque = snapped rods. What about RPM and making more power at higher rpm potentially adds to engine longevity?

Perhaps it's because the torque results in a longer sustained period of high load on the rod due to the reduced engine speed. The slower the engine speed, the closer to a static load and the ultimate compressive strength of the rod.

Snapping or bending? Have you got anyone who has actually got test or practical data to back this up, or is it more an urban legend?

Snapping is generally the result of high rpm, bending of too high a cylinder pressure - which can lead to breakage.

I would expect compressive failure to occur at higher rpm, might be quite wrong, because of the additional bending stresses (loads) being applied to the 'rod.

There are (at least) two things that may be clouding the debate - bearing failure and bottom end seizures that can break rods, and piston failures where the rod may be broken as it flails around in the engine?

For various reasons, many motorsport engines are deliberately built to have their power produced at lower rpm, which means more torque and cylinder pressure, and I'm not aware of this being a problem - although they are expressly built for it.

Just realized a very similar post to this one was made a few days ago:

https://www.hpacademy.com/forum/general-tuning-discussion/show/does-low-end-torque-hurt-a-petrol-engine

Sorry for the duplicate thread, but this one seems to have some good explanations.

Gord, no concrete proof from my end. General consensus in the stock block Evo community (largely what I tune in) has been to limit torque at low RPM and let it eat at high RPM on big turbo stock motor set ups for max longevity. This has been said all the way back to the 90's with DSMs. It is also mentioned in many articles, an example here: https://low-offset.com/mitsubishi/best-turbo-for-evo-x/

Hydraulic piston pumps in high duty applications and intake restrictor turbo racing classes would seem to be the counterpoint to the urban legends. I'm pretty sure it's nearly all down to incorrect ignition timing, thermal management, fuel quality vs build spec or poor bearing clearance spec.

Alejandro, the Mitsi' post just mentions wear, without any further comment. It could be bearing wear from poor maintenance or, more likely, ring wear which is a normal aspect of higher combustion pressures and may be expected to worsen with rpm.

In both instances, I suspect the underlying issues would be poor tuning and overfuelling - both were very common back in the day.

Forgot to mention, many production engines are also being designed specifically around high boost and low(er) rpm, and surely if there was an intrinsic problem with that philosophy they wouldn't be using it?

Gord,

I agree. I do see the way the industry is going. However, this "strategy" if you will is largely used for stock motor small displacement engines when making "big" power 500-600whp+. It's said to be a way to lengthen the longevity of the engine when pushing it very far past it's rated power capacity. I don't think this applies well when talking about purpose built engines designed to handle these stresses from the factory. That article I linked was only to show the spirit of the saying, high torque, low rpm = part wear and eventual failure. I have been doing some more research on this, here are a few forum posts that mention this more. Still overall, the premise seems to be that when pushing stock rods to the limit, delaying torque hit for higher rpm is better.

https://www.k20a.org/threads/more-boost-on-the-stock-k20-697whp-is-close-enough-right.113265/

See also this comment by TSCOMP

"A stock block internal engine that's smoother and comes in later in the RPM will last a lot longer then something that comes in early and peaks tq sooner. (rods are happier)"

https://www.evolutionm.net/forums/evo-dyno-tuning-results/721621-600whp-stock-block-93-pump.html

I think its just the missunderstanding of a simple concept. cylinder pressure, and in turn torque, is often the limiting factor for a lot of engine internals. A lot of engines with stock internals that survive reasonably high power for a period of time have moved their torque production higher up. They are quite often making similar torque numbers to a stock / highflow turbo, but they are making it at higher engine speeds.

When you keep in mind that horsepower = torque x rpm / 5252.

Torque is largely a product of airflow, so to make torque higher in the rev range, you need a lot of airflow. As the time to get air into the engine decreases as engine speed increases, it becomes harder to make torque at higher engine speeds, whereas a smaller turbo that can build peak boost at low engine speed can make torque easily as it generates airflow, and has the engine cycle time to get good cylinder fill.

If a rod can only safely handle 500nm, you will eventually bend it making 600nm, whether you make 600nm at 3000rpm or 6500rpm. You will just find the car that can make 600nm at 6500rpm will make a lot more top end power than the engine that makes its 600nm peak torque at 3000rpm (due to above formula)