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Old 11-06-2006, 04:35 PM
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Peak torque estimate from peak HP??

I'm looking for the formula that predicts peak torque @ rpm from a peak HP @ rpm.

For example, an engine that makes 300hp at 5000 rpm will have more peak torque than an engine that makes 300hp at 8000 rpm. I know the formula to make the simple conversion from hp to torque but that only works at the same rpm, and peak torque happens at a rpm less than peak hp.

So if you know your 1/4 mile trap speed and car weight, then you can calculate hp. And by estimating the rpm at the engines peak hp, one should be able to figure out peak torque @rpm.

Years ago, I had a program that would allow the user to enter peak hp @ rpm and the program would estimate peak torque @ rpm.

I know with new engine tech this is hard to estimate but for old school engines it should be fairly simple, right?

Any Ideas?

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Old 11-06-2006, 05:21 PM
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http://www.offroaders.com/info/tech-...wer-torque.htm
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Old 11-07-2006, 08:06 AM
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Sorry, I didn't see the information I was looking for in your link. Could you point me to it?
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Old 11-07-2006, 08:15 AM
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Quote:
Originally Posted by 454C10
Sorry, I didn't see the information I was looking for in your link. Could you point me to it?
Sure. It gives the formula for calculating torque when horsepower and RPM are known.
Torque = 5,252 x hp / RPM

Bob
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Old 11-07-2006, 08:16 AM
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Quote:
Originally Posted by 454C10
Sorry, I didn't see the information I was looking for in your link. Could you point me to it?
Directly from the link provided:

Hp = rpm x torque/5,252

Conversely, to calculate torque the equation is:

Torque = 5,252 x hp/Rpm


If you know either the hp or the tq, you can calculate the other.
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Old 11-07-2006, 08:29 AM
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I guess I didn't explain very well what I was looking for.


I don't want to know what the torque is at the peak hp rpm. I want to know what the peak torque @ rpm estimated from the peak HP @ rpm data.


For example, a 454 making 300hp at 4500 rpm makes about 450 ftlb of torque at 2500 rpm.

Or a 283 making 300hp at 6000 rpm makes about 300ftlb of torque at 4000 rpms.

I know there is some kind of "peak torque @ rpm" estimate that can be made from the peak HP @ rpm. Years ago I had a program that did that.
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Old 11-07-2006, 08:37 AM
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Maybe this will help you:

www.rbracing-rsr.com/runnertorquecalc.html

There are many variables involved in caculating what you desire, but you may be able to get an appx. estimate.
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Old 11-07-2006, 08:40 AM
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[QUOTE=454C10]I guess I didn't explain very well what I was looking for.

Seems to me that the formula works for all variables of horsepower and RPM whether "peak" or not.
If you know the horsepower at any RPM, you can find the torque with that formula.
Graphing the torque curve would then be an exercise in deriving the torque from several known values of power and RPM and plotting them, no?

Bob
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Old 11-07-2006, 08:54 AM
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Yes, that formula would work great if I knew the hp at the peak torque. But, all I know is peak hp @ rpm.

Knowing the peak HP @ rpm, it is possible to estimate a hp curve, and from there estimate a torque curve.
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Old 11-07-2006, 09:07 AM
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[QUOTE=bobscogin]
Quote:
Originally Posted by 454C10
I guess I didn't explain very well what I was looking for.

Seems to me that the formula works for all variables of horsepower and RPM whether "peak" or not.
If you know the horsepower at any RPM, you can find the torque with that formula.
Graphing the torque curve would then be an exercise in deriving the torque from several known values of power and RPM and plotting them, no?

Bob
If I needed to know what he wants to and didn't have a program to do the work for me, this is exactly what I would do...pick an rpm increment scale and graph multiple caculations to show the curve. Time consuming but it works.
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Old 11-07-2006, 12:56 PM
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Quote:
Originally Posted by 454C10
Yes, that formula would work great if I knew the hp at the peak torque. But, all I know is peak hp @ rpm.

Knowing the peak HP @ rpm, it is possible to estimate a hp curve, and from there estimate a torque curve.
Nope, it would be impossible. It's also impossible to calculate peak torque from peak RPM.
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Old 11-07-2006, 01:08 PM
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Quote:
Originally Posted by Rick WI
Nope, it would be impossible. It's also impossible to calculate peak torque from peak RPM.
It would be impossible to calculate peak tq from peak rpm (not enough data), but getting appx. values using the afformentioned formulas and graphing them would not be...the problem would lie in trying to figure out how much error to factor in as we all know that the curves can be moved up or down in the power band by changing components.

The program that he mentioned earlier most likely figured in part selections and calculated the answer based on the inputs.

On the other hand, it seems that there was a program called "Engine Analyzer" or something like that which was availabe for free download. I think it was on a thread here a ways back...maybe if he could find that it would help.
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Old 11-07-2006, 01:25 PM
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To many variables. Dependent on the combination and the camshaft as to how broad the powerband is going to be. Some engines are designed with a very narrow powerband (Pro Stock) some engines are designed for a very broad powerband, (marine engines). You peak torque rpm will very.
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Old 11-07-2006, 01:32 PM
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If you have only one known data point it's impossible to solve for another data point when only that one variable is know. If we know peak HP is 300 at 5000 RPM all we really know is peak torque is going to occur at a lower RPM, as it always does. Where is another matter. Unless you know the HP at the peak torque RPM point you can't calculate peak torque.

Engine analyzer calculates a torque curve based on engine modeling then assigns a HP number to the torque curve via mathamatical calculations as was shown earlier. In other words it's an engine simulation program. It's quite accurate if all the data is input correctly.

I am sure you can write an equation which might get you in the ballpark but whether it's in the left field of the ballpark or the right field I have no idea.

I guess surrounded by a Cam Doctor machine, two flow benchs and a real live dyno week to week I'm not used to swagging things or put much stock in the results.
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Old 11-07-2006, 02:29 PM
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Quote:
Originally Posted by Rick WI
I am sure you can write an equation which might get you in the ballpark but whether it's in the left field of the ballpark or the right field I have no idea.

I guess surrounded by a Cam Doctor machine, two flow benchs and a real live dyno week to week I'm not used to swagging things or put much stock in the results.
That is what I meant by the "error factor", your calculations would have to be written in based upon your current knowledge leaving a confidence interval for which you could be 80 or 90 percent positive your results would fall into.

Obviously real world testing is a far better way to go about this, but it still can be done...just too much work for me.
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