F/I Blowing motors facts

[whataboutbob]

I'm wondering if at lower RPMs the time duration that the engine/rods see the load is longer.

[gtengr]

Originally Posted by toast

If a machine can generate 100 lbs of force it does so regardless of how fast it is applied. You have a tougher time pushing a faster moving wheel because of timing issues and the fact that your are not a machine; if you were mechanically constrained to the rotating system and your 100 pounds of force did not require a ramp-up time like muscles do your force would always be applied at the correct time and you would generate the same amount of rotational acceleration.

The combustion process has time dependence that is somewhat comparable to a muscle ramping up in the merry-go-round example. As the combustion gas expansion drives the piston down the cylinder, the cylinder volume is also increasing via piston travel. As the rpms are decreased, the gas expansion rate might want to stay about the same, but it's contending with a slower cylinder volume expansion rate, so the result is a harder push against the piston over less degrees of crank rotation.

From that it follows that the resultant torque can be the same if combustion expansion is working over a more efficient range of leverage on the crank. For example, 1000 lb average force acting on an average crank throw of 2" would generate 2000 lb-in of torque. But if the combustion process is completed in a shorter amount of crank rotation (aka lower rpm's), you might have 1200 lb of force acting on an average throw of 1.67". Same torque in both situations, but more force at low rpm.

[toast]

Originally Posted by gtengr

The combustion process has time dependence that is somewhat comparable to a muscle ramping up in the merry-go-round example. As the combustion gas expansion drives the piston down the cylinder, the cylinder volume is also increasing via piston travel. As the rpms are decreased, the gas expansion rate might want to stay about the same, but it's contending with a slower cylinder volume expansion rate, so the result is a harder push against the piston over less degrees of crank rotation.

Yes, but the analogy for the person is almost entirely based on timing.

Ah, that makes sense. You're saying at some low rpm you are producing a higher max cylinder pressure over, say only 15 degrees of rotation, versus 30 degrees at twice the rpm. If you had a sensitive enough engine dyno you would get spikes in reported torque on every power stroke that would smooth out as rpms increased.

Thank you.

[gtengr]

Originally Posted by toast

Yes, but the analogy for the person is almost entirely based on timing.

Ah, that makes sense. You're saying at some low rpm you are producing a higher max cylinder pressure over, say only 15 degrees of rotation, versus 30 degrees at twice the rpm. If you had a sensitive enough engine dyno you would get spikes in reported torque on every power stroke that would smooth out as rpms increased.

Thank you.

Exactly. You posted while I was editing my post to emphasize that point. You're welcome!

[CSG Mike]

Originally Posted by toast

I constantly hear this convention batted around that low end torque is what breaks/bends rods or that the 'hit' of a turbo down low does it.

That makes zero sense from a physics standpoint. Every two engine revolutions any given rod will go from almost no load to full load on the power stroke. No matter how fast your turbo hits it can't cause a delta faster than that. I also don't see how having higher torque down low could be worse than having it up high.

Someone explain why I'm wrong.

Impulse.

[toast]

Yes, that is the definition of impulse. If you don't have any physics background that generally means nothing to people.

[toast]

Originally Posted by gtengr

Exactly. You posted while I was editing my post to emphasize that point. You're welcome!

Gievn modern electronic control of everything where does ignition actually occur with regard to crank angle? At low rpm is it retarded to the point that the crank is as close to perpendicular to the rod as possible? I've never looked at any of the ignition tables nor do I have a feel for how long combustion takes in an absolute sense.

[CSG Mike]

Originally Posted by toast

Gievn modern electronic control of everything where does ignition actually occur with regard to crank angle? At low rpm is it retarded to the point that the crank is as close to perpendicular to the rod as possible? I've never looked at any of the ignition tables nor do I have a feel for how long combustion takes in an absolute sense.

to a degree, flame front speed can be controlled by mixture. In this case, AFR, and the DI/PI overlap.

[GsxrMe]

I have been pushing 400hp with flexfuel that has been on average of 74% for over a year and before that running 93 for a year. It's important not to tune for to much low end with factory rods. My boost controller will not allow more than 8psi under 4500rpms for good reason. Just set your maps boost control duty cycle for rpm/duty. No knock at all. I beat the shit out of her everyday, as she's my daily driver. Minus to everything is 3k oil changes and 10k miles I need new rear tires.

I have 27k miles boosted and loving it, no problems at all.

[DarkPira7e]

Originally Posted by GsxrMe

I have been pushing 400hp with flexfuel that has been on average of 74% for over a year and before that running 93 for a year. It's important not to tune for to much low end with factory rods. My boost controller will not allow more than 8psi under 4500rpms for good reason. Just set your maps boost control duty cycle for rpm/duty. No knock at all. I beat the shit out of her everyday, as she's my daily driver. Minus to everything is 3k oil changes and 10k miles I need new rear tires.

I have 27k miles boosted and loving it, no problems at all.

What oil weight do you run? I also wonder if people are using at least 5w20 synth or if they're staying 0w20. I personally will switch to 5w20 likely.

[CSG Mike]

Originally Posted by DarkPira7e

What oil weight do you run? I also wonder if people are using at least 5w20 synth or if they're staying 0w20. I personally will switch to 5w20 likely.

that only affects cold weight, and not really hot weight. It's still 20 weight when up to temp.

[Lakitu]

Can you still blow an engine driving under 4.5K RPMs while boosted (non-spirited daily driving), or is blowing an engine entirely because people are redlining boosted cars that haven't had the bottom rebuilt?

[SPCorBUST]

Originally Posted by Lakitu

Can you still blow an engine driving under 4.5K RPMs while boosted (non-spirited daily driving), or is blowing an engine entirely because people are redlining boosted cars that haven't had the bottom rebuilt?

I think the better question is why spend the money on boost if you're never boosted? You don't even see considerable boost at those rpms so what would the point even be... Just to say that you're boosted...?

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[Teseo]

Originally Posted by SPCorBUST

I think the better question is why spend the money on boost if you're never boosted? You don't even see considerable boost at those rpms so what would the point even be... Just to say that you're boosted...?

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Why you will drive like an asshat for just to keep on boost?