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

Andy13186

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I am wondering if its better to have a higher RPM shift point with lower boost vs a lower RPM shift point with more boost.


Higher rev example : 8000 rpm shift point with 10 psi
Higher boost example : 7500 rpm shift point with 12 psi

I have revved to 8k+ hundreds of times with no problem but I am just wondering if it would be better for the motor to pulley down and not rev it out as far. I guess I think the higher rev setup would be more satisfying to drive

 

Jackson1320

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High boost low in the rpm’s is harder on a engine. But with those samples the lower rpm’s and more boost is easier
 

engineermike

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Man what an interesting question.

I think the answer could go either way, but spark timing is probably more of a controlling factor regarding engine stress. Thermodynamic modeling reveals that spark timing affects peak cylinder pressure and, thus, component stress probably more than anything else.

That said, I think lower rpm/higher boost is probably lower engine stress as long as the timing is conservative. I can think of a few examples of factory forced induction engines running 15-20+ psi boost and 200k mile reliability (on 87 octane no less), but the max rpm is low and spark timing is pitifully low….like in the sub-5 degree range.
 

engineermike

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I’ve been contemplating the exact same issue but more from a knock standpoint when octane limited. Intuitively, you would think if octane-limited then you would be cylinder pressure-limited so extracting the most power means rev’ing higher on limited boost. However, as you rev higher you run out of time for direct injection to take place, which hurts knock suppression. There’s also some weird dynamic taking place in the 5000 rpm range that allows more spark timing without knock than the higher rpms. I can’t explain it but it’s repeatable.
 
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Andy13186

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I think lower RPM with more boost may be slightly easier on the fuel system, but I really dont know for sure about that.
 


engineermike

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I think lower RPM with more boost may be slightly easier on the fuel system, but I really dont know for sure about that.
Fuel is a function of power and efficiency. If it’s making the same power then the fuel flow will be close to the same. However, on the gdi side there is a big advantage to lowering max rpm. The gdi injectors flow a rate just like port injectors, and that rate is based on the time domain (e.g. lb/min as opposed to lb/deg crank angle). However, as the engine spins faster, the time each cycle takes is less and less so the time available for injection is less and less. I have the xdi pump and I’ve raised the gdi pressure from 2900 to 3300 psi and I can still only maintain 90% gdi blend up to around 6200 rpm. Beyond that, the blend starts falling because I run out of time to get the desired amount of fuel directly injected. So you might say ok, lower the rpm and raise the boost, but then you start trading off spark timing.

It’s a big bag of compromises for sure.
 

engineermike

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On a side note, think for a minute about what failures we’ve seen posted on the gen3 here. I don’t Facebook so you might have to help me out with what’s reported there.

I can count on one had the number of rpm-related failures I’ve seen posted, all of which were opg’s and even that can be chalked up to hitting the rev limiter or an improperly set up rev limiter. On the other hand I’ve heard of more piston failures, but those are due to either knock to too much cylinder pressure, both being from running too much timing for the compression ratio and boost.
 

Angrey

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Whenever you have a trend question, take it to the extremes and it helps flush out information.

Would it be better to rev to redline (8k) with no boost or rev to 4k rpm with 14 psi?

I think the answer as Mike pointed out is clouded with other factors (like timing) but consider if you're NOT overspinning the motor, it's better to make the same power with less boost (but at redline that the motor can handle reliably).

Boost increases the demands on the fuel's resistance (and limiting timing advance), so all things being equal, rev it to what it should make and run less boost (rather than shorting it and throwing more boost).

Additionally, although not an engine health issue, shorting the RPM's significantly reduces performance because it gives less range for the motor to drop when shifting. Having a wider RPM band means you keep the car more in the meat of the power/torque curve when shifting. So even if you made the same power in each scenario, an A10 car would do better with the higher redline than the lower version just because it stays higher in the rpm range for each shift.
 

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I am wondering if its better to have a higher RPM shift point with lower boost vs a lower RPM shift point with more boost.


Higher rev example : 8000 rpm shift point with 10 psi
Higher boost example : 7500 rpm shift point with 12 psi
Unfortunately it's not a straight answer, I think it depends on the various individual components. Some components would be more stressed with more RPM (like the valvetrain), while others would be more stressed with more boost (like the pistons, cylinder walls).

Out of those choices, I would probably go with choice #2 since that stays within the RPM limits of the engine as designed while the other doesn't.
 

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This IS interesting.

Generally whatever makes more torque (the only realistic way to infer cylinder pressure) is harder on the bottom end. And most coyote failures I've seen are bottom end related.

I can't speculate which scenario is going to have more are under the torque curve, but that would be my answer for which is harder on the coyote.
 

engineermike

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This whole subject got me thinking....

OEMs, including Ford, tune the engines for a maximum cylinder pressure because they know the strength of the components intimately. Some engines, like the stock coyote, rarely hit the cylinder pressure limit. It has its own timing table used specifically to cap cylinder pressure.

I'll find out soon, but I believe that the GT500 runs against it's cylinder pressure limit at all times WOT.

If you look at the cylinder pressure limit spark timing tables, you can see that at the same speed and load, the GT500 is allowed about 8 deg more timing than the Coyote. From these two tables, you can extrapolate the Coyote table up to the higher forced-induction load range and see the timing would be capped around 16.5 deg and dropping with more boost.

I would venture to say that if you used the Coyote and GT500 cylinder pressure limit spark tables to create a halfway accurate boosted Coyote table, and kept the stock rev limiter in place, then you could run as much boost as you want an never hurt anything.

You'd sacrifice a good bit of power though, probably on the order of 50 hp guessing.
 
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K4fxd

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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.
 

andrewtac

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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.
You'd have to throw 3000 lbs or so of ballast in the na car to make it even, assuming same time constraints in the 1/4.
 

K4fxd

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You'd have to throw 3000 lbs or so of ballast in the na car to make it even, assuming same time constraints in the 1/4.
Quick google says the cummins engine weighs 1000 Lbs
Coyote about 500. So 500 lbs ballast would make the work even.
 

andrewtac

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Quick google says the cummins engine weighs 1000 Lbs
Coyote about 500. So 500 lbs ballast would make t
Quick google says the cummins engine weighs 1000 Lbs
Coyote about 500. So 500 lbs ballast would make the work even.
My Google showed cummins 2500 at about 7000 lbs.
 

 
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