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Does low end torque hurt a petrol engine?

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Hi.

One comon think I hear all day is that “low end torque does hurt the engine“, “as long the boost comes after xxxx RPM, you will be fine“ etc.

Is there actually any truth to that? In my head 600nm on 2000rpm or 6000rpm, is still 600nm. Or is there something I don’t understand?

Bit unsure if this is the right section to ask.

Yeah - this is something i'd like to know as well - subbing to see replies

Could less oil pressure at lower rpm be a reason?

i believe its due to cylinder pressures, to make low end torque you need high cylinder pressures.

pressure = stress.

Any claim that is is safe/not safe around a specific rpm should be taken with a large bucket of salt.

While "low end" may mean different things to different people, I suspect any risk may be down to the effectiveness of oil film/hydro-dynamic wedge at the big end bearing which may not develop fully at "low" rpm. If so, this may be countered by closer bearing clearances and/or higher viscosity oil.

Josh, while torque is generally proportional to cylinder pressure, that relationship is generally independent of rpm - heck, considering frictional losses, cylinder pressure may be higher.

If the engine is correctly built, and tuned, I can't see it being much of a concern. Heck, it's common practice in many series, especially where fuel limitations may apply, to try and lower the rpm the power is produced to increase engine efficiency by reducing frictional losses.

If some more experienced people would care to comment?

I cannot fathom how cylinder pressure will be higher at lower rpm for the same delivered crank torque, friction and windage losses will be lower and less spark advance will be required due to flame speed/crank angle relationship, so there will be less negative pumping and "chasing" the piston down the bore. Less chemical energy and peak and average pressure should be required for the same crank torque at lower rpm, that exactly why shopping trolley models are tending to cam/manifold designs that limit operational rpm range even though we have fancy dual VVT and switcge multi-runner lengths etc these days, better for fuel efficiency and emissions to make the power at lower rpm.

The only caveat would seem to be LSPI experienced by some of the modern DI turbo motors. If you are talking older style port injection, especially with ethanol mix or water injection that should not be an issue at all.

I suspect the urban legends are as a result of either use of fixed timed distributors or distributors without boost retard or people not understanding timing advance requirements and/ or reeeeally loosely build engines combined with the above.

I hear it a lot relating to these "stock engine k20 / k20 + turbo" builds that youtube loves

where people are able to make like 600hp on the stock unopened na engine for the k series for example - but have it all top end and have deliberate laggy turbo response programmed in - and if they make it too responsive and spool too fast for the power - then the engine will get religious and throw a rod to god

i curious as to why they would die on say 350-400hp on a super fast spooling and lag free turbo compared to "living under hard thrashing for years" at say 700hp on what seems to be the usual a gt3582rs type turbo that is slow spooling on that engine.

I think there have possibly been some wires crossed in the quotes here. The general comments like this are usually in context to "too much power" at a low RPM rather than torque. For example with an Evo7 with the stock thin rods, you can bend the rods at about 380HP with the stock sized turbo (~450ft.lb @ 4500RPM). Whereas they will often survive ~500Hp with a larger turbo if you ramp the torque in so it is roughly only the same peak value, just peaking 1500RPM higher in the rev range.

It is the torque/cylinder pressure that is the limiting factor in many cases - but you can make higher power by limiting torque and increasing RPM instead. RPM will obviously also become a limiting factor at some point too...

There is good explanation of that thing that was given by Jay from Real Street Performance. Torque is important but the torque application time is also important. Having the same torque at 2000 and 6000 RPM makes different period of time when that torque is applied during each engine cycle thus puts different level of stress on the parts. It is just like power lifting - let's say you can lift 5 times 220 lbs of weight quickly. But try to lift the same weight very slowly and you'll find it very hard to make the same 5 times. Of course higher engine speed brings other types of stress such as moment of inertia but the time of stress is still much less than at low RPM... So that high stress caused by combination of torque and application time forces conrods and bearing to fail sometimes whilst the same torque at higher RPM doesn't cause as much stress because of less impact time.

Not sure the exact answer but I know my tuner intentionally tuned my boost control to bring it in boost slower. This is a stock sleeve b18c with forged piston/rods. From what I understand is the higher torque/cylinder pressure at lower rpm can lead to cracked sleeves. (My tuner has been tuning these cars for decades and many of his own setups were similar to mine)

I didn’t get too into details with him but my thought is maybe the lower frequency of combustion events along with the higher cylinder pressure at lower rpm could be part of the issue? I’m really just guessing at this point.