Higher rpm with less boost, or lower RPM with more boost, which is easier on the motor?

K4fxd

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I'm talking about the engine. Now it would be real cool to have an 8 second RAM 2500........

You do know people swap these engines into cars, right?

 

andrewtac

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I'm talking about the engine. Now it would be real cool to have an 8 second RAM 2500........

You do know people swap these engines into cars, right?
I do, didnt realize you meant swap. I was into the powerstroke thing for a while; but stopped when I was at the limits of the block/trans. They seem really reliable up to the point you exceed those limits. Could have been just those I was following, but they all seemed to break something at that point. it was my daily at the time and wasn't ready for the risk I perceived (and those parts are very expensive). Not sure how the cummins compares, assumed it was the same; also this was 5 years ago perhaps they have worked the bugs out.
 

K4fxd

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If you want mega power with a diesel, cummins is the way to go. They really do run 125 PSI boost in the fire trucks, with a warranty.....

Some of the military engines use more
 

Jackson1320

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Have you seen the size of the internals on a cummins. They are built to handle low rpm high boos. The coyote can not handle half the cylinder pressure a Cummins can
 

K4fxd

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Have you seen the size of the internals on a cummins. They are built to handle low rpm high boos. The coyote can not handle half the cylinder pressure a Cummins can
Yes I have. Someone earlier in the thread said to check the extreams. So that would be a Cummins and a pro-stock for comparison.
 

Angrey

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Yes I have. Someone earlier in the thread said to check the extreams. So that would be a Cummins and a pro-stock for comparison.
The extremes for a gasoline spark ignition motor. Comparing two different motors kinda nullifies the exercise.

Of course it's advantageous to run a high compression/boost diesel engine at low rpm.

I'm saying that if you take a motor that's built and designed to run 7500 rpm and you decide to compare it to the same motor rev'd to 3500 rpm but with 15 psi of boost, I think you'd find it safer to run more rpm and less boost (up to the point that you're not exceeding the capabilities of the rotating components.)

Or put more simply, if your motor can safely spin to say 8k rpm.....you currently make 800 wheel hp reving it to 7k rpm on 14 lbs of boost. To make another 200 hp you could rev it to 8k rpm or you could keep it at 7k rpm and just add 5 lbs of boost, which is safer for the motor? The answer is more rpm.

There's also a range at which the fuel's knock resistance starts to plummet (in terms of boost). So on 93, after a certain boost pressure, knock and predetonation risk goes up significantly. The same is true for E-85, just at higher boost threshold. Same for C85 or nitromethane, just higher and higher.

So if you're anywhere close to the curve where increasing boost (and heat) tends to approach the limit of the fuel's resistance, and you're not exceeding the rotational health of the motor, more rpm is better. If you're already at or near the motor's safe rpm limits, then boost is safe(r) but obviously more risk, just not as much as overspinning it.
 

Angrey

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The extremes for a gasoline spark ignition motor. Comparing two different motors kinda nullifies the exercise.

Of course it's advantageous to run a high compression/boost diesel engine at low rpm.

I'm saying that if you take a motor that's built and designed to run 7500 rpm and you decide to compare it to the same motor rev'd to 3500 rpm but with 15 psi of boost, I think you'd find it safer to run more rpm and less boost (up to the point that you're not exceeding the capabilities of the rotating components.)

Or put more simply, if your motor can safely spin to say 8k rpm.....you currently make 800 wheel hp reving it to 7k rpm on 14 lbs of boost. To make another 200 hp you could rev it to 8k rpm or you could keep it at 7k rpm and just add 5 lbs of boost, which is safer for the motor? The answer is more rpm.

There's also a range at which the fuel's knock resistance starts to plummet (in terms of boost). So on 93, after a certain boost pressure, knock and predetonation risk goes up significantly. The same is true for E-85, just at higher boost threshold. Same for C85 or nitromethane, just higher and higher.

So if you're anywhere close to the curve where increasing boost (and heat) tends to approach the limit of the fuel's resistance, and you're not exceeding the rotational health of the motor, more rpm is better. If you're already at or near the motor's safe rpm limits, then boost is safe(r) but obviously more risk, just not as much as overspinning it.
Clarification, when I'm speaking to boost saturation, it's assuming cooling capacity is fixed. (which means more boost = more IAT2). In a world with unlimited cooling or corresponding increases in cooling to keep temps the same, obviously adding more boost is much less problematic.
 

illtal

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Lets take a 12 valve cummins engine and run 8 second 1/4 miles. High boost and low RPM 3200 or there abouts

VS a 8000 RPM 8 second N/A car

Which would be more reliable?

It is a head scratcher.

I do know fire trucks running Cummins engines will run upwards of 120 psi boost. 1800 RPM governor.
well this is not so much an apples to apples suggestion as the makeup of a diesel engine is very more different than a coyote. Obviously the only factory boosted coyote is the predator. but that engine is also a lower compression and a substantially stronger block than a standard gen 3; leaving it stock would help it live many miles i'm sure.

I would lean to agree with the 12 psi car at 7500 vs the 10 psi car at 8000 rpm.
We all know that safety factors are built into things like the RPM limit and such. I have seen many gen 3 spin near 8k rpm. But under boost SC cars will have issues with the intake valves and turbo cars with the exhaust valves due to back pressure.

Leave all things the same and change only those two factors I'd say spinning it more would be more harmful in the long run.
 

 
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