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Can someone explain torque vs. engine speed?

hobby-man

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Hi guys, somewhat technical question but I like detail!!

Just wondering WHY it is that people say for mods i.e. bigger turbos, that they can actually be gentler on the rods because the peak torque is shifted later into the RPM range? It feels counter-intuitive to me, where with everything moving faster, greater torque is safer on the rods vs. when things are moving more slowly. I'm obviously missing something but not sure what? Cheers!



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calonzo

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(Ignore my previous edit because I didn't read your whole question.)

It does seem counter-intuitive to me, too. Torque is not based on rotation speed. It is based on force times distance. So the RPM seems irrelevant.
 
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gtman

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herox

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Your question is akin to why a car takes abuse better when the drive train is warmed up vs when the drive train is still cold. To a person that doesn't know better it shouldn't make a difference but in this scenario we know operating temperature and lubrication are important.

As to your question relating to our car, we know that applying full power in a short amount of time is more stressful mechanically than applying the same amount of power over time. It's less strain on our drive train even though we are applying the same amount of power through the drive train.
 

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Your question is akin to why a car takes abuse better when the drive train is warmed up vs when the drive train is still cold. To a person that doesn't know better it shouldn't make a difference but in this scenario we know operating temperature and lubrication are important.

As to your question relating to our car, we know that applying full power in a short amount of time is more stressful mechanically than applying the same amount of power over time. It's less strain on our drive train even though we are applying the same amount of power through the drive train.
Ok, I hear what you are saying. Just let me try to clarify something.

The original poster says "peak torque is shifted later into the RPM range? It feels counter-intuitive to me, where with everything moving faster, greater torque is safer on the rods"

'peak torque' changed to 'greater torque' in this sentence. So, if the amount of peak torque between a smaller turbo and larger turbo are the same, but at different RPM, then I understand what you are saying (I think). The torque is the same (and therefore the stress on the connecting rods is the same), but the HP generated is greater.

However, if a larger turbo has 'greater torque', even if it is at a higher RPM, then this does not hold true since the rods are definitely experiencing more force.
 
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herox

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Ok, I hear what you are saying. Just let me try to clarify something.

The original poster says "peak torque is shifted later into the RPM range? It feels counter-intuitive to me, where with everything moving faster, greater torque is safer on the rods"

'peak torque' changed to 'greater torque' in this sentence. So, if the amount of peak torque between a smaller turbo and larger turbo are the same, but at different RPM, then I understand what you are saying (I think). The torque is the same (and therefore the stress on the connecting rods is the same), but the HP generated is greater.

However, if a larger turbo has 'greater torque', even if it is at a higher RPM, then this does not hold true since the rods are definitely experiencing more force.
Materials can handle load better gradually vs suddenly. You can apply the same load to an object gradually vs suddenly but sudden load creates twice the stress on the object. This means if you were apply load gradually you can apply more load before catastrophic failure than if you were suddenly applying load.

Your assumption is that the big turbo that's gradually creating more peak HP/TQ is creating more stress on the rods than a stock turbo with less HP/TQ that's suddenly applying load on the rods. That's not necessarily true because of gradual load vs sudden load.

Since our rods are snapping at lower RPM with a lot of power then the strategy is to gradually build more power and be "gentler" on the rods. This is why people believe bigger turbos are "safer" even though you're making more peak power.
 
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Gotch

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Hydrodynamic lubrication.
 

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My guess is that if big torque is made while the rod is moving slower, more time is spent at critical load points in the rod. It is also easier to create a high torque high load peak at lower RPM with a smaller turbo than at high RPM with a bigger turbo.
 

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My guess is that if big torque is made while the rod is moving slower, more time is spent at critical load points in the rod. It is also easier to create a high torque high load peak at lower RPM with a smaller turbo than at high RPM with a bigger turbo.
This is probably the most important thing to understand. As the engine speed increases, it means that the power stroke is becoming ever more brief, and it's at this moment that the connecting rod endures the most stress.
 
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calonzo

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This is probably the most important thing to understand. As the engine speed increases, it means that the power stroke is becoming ever more brief, and it's at this moment that the connecting rod endures the most stress.
More brief, but more often.
 

Myx

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Imagine hitting a ball that is not moving with a certain amount of force.
Then imagine hitting that same ball, with the same force, while it slowly moves away from you.
Then imagine again hitting that same ball, with the same force, while it is moving away from you quickly.

Again...as a boxer.....
Imagine someone punching your jaw while you are not moving.
Imagine someone punching your jaw while you move away from them slowly.
Imagine again someone punching your jaw while you are moving away from them quickly.
All with the same punching force.

Without all the technical jargin, physics and calculations, most people will understand that the impact of the force hitting the object (the ball, your face) 'feels' less when the object is moving away quickly vs moving away slowly vs not moving at all.

This is the same as torque at low rpms vs higher rpms. :)
 
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calonzo

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Imagine hitting a ball that is not moving with a certain amount of force.
Then imagine hitting that same ball, with the same force, while it slowly moves away from you.
Then imagine again hitting that same ball, with the same force, while it is moving away from you quickly.

Again...as a boxer.....
Imagine someone punching you in your jaw while you are not moving.
Imagine someone punching you in your jaw while you move away from them slowly.
Imagine again someone punching you in you jaw while you are moving away from them quickly.
All with the same punching force.

Without all the technical jargin, physics and calculations, most people will understand that the impact of the force hitting the object (the ball, your face) 'feels' less when the object is moving away quickly vs moving away slowly vs not moving at all.

This is the same as torque at low rpms vs higher rpms. :)
I am not saying you are wrong, but without the physics and calculations, it is meaningless.
 
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IronFusion

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Imagine hitting a ball that is not moving with a certain amount of force.
Then imagine hitting that same ball, with the same force, while it slowly moves away from you.
Then imagine again hitting that same ball, with the same force, while it is moving away from you quickly.

Again...as a boxer.....
Imagine someone punching your jaw while you are not moving.
Imagine someone punching your jaw while you move away from them slowly.
Imagine again someone punching your jaw while you are moving away from them quickly.
All with the same punching force.

Without all the technical jargin, physics and calculations, most people will understand that the impact of the force hitting the object (the ball, your face) 'feels' less when the object is moving away quickly vs moving away slowly vs not moving at all.

This is the same as torque at low rpms vs higher rpms. :)
Yeah- gotta hit that heavy bag as it moves away
 
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Benster

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as Myx was saying, let's say that at 2500rpm, the rod takes 300ft-lbs or torque over 0.1s(making up numbers here) between when it's at the top of its course to where it's at the bottom and the exhaust valve starts to open(power stroke). The rod material is able to withstand 300 ft-lbs max(let's say).

Now, at 5000 rpm, the rod spends 0.05s in its power stroke, same 300 ft-lbs. It will take twice as much torque to give the rod the same amount of force over the power stroke to make it fail since it stays in that load for half the time.

The same goes for nitrous, if you hit nitrous too early, it creates too much torque due to the increased fuel and air entering the engine and you will grenade your engine because the rods spend too much time in the power stroke and the pressure on them bends and eventually breaks them. If you spend less time in that power stroke, it's less time for the pressure to bend the rod. You very rarely snap a rod clean, it'll almost always bend before it breaks.

Not sure what calculations you're expecting from a car forum, hopefully we have a motor engineer here that can chime in but this is basically how torque at low rpm kills engines. Too much stress for too long.

Another way to look at it is to look at diesel engines. They operate between 1400-3000 rpms for a medium-sized inline 6 in a pick up. Pull a rod from that engine, it's at least twice as big as an engine the same volume but gas powered. Why? Diesels make power at very low engine speeds, so you need a lot more meat to withstand the torque it makes.

Yes faster engine speeds over 1 min(for example) will mean the power stroke time will be the same, but in this case over and over again is not what breaks rods but how much time it's under that pressure.

I'm repeating myself a lot here and I hope it's understandable enough for you.
 

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