Torque Equation

[AL427SBF]

Anyone have a theoretical equation for Torque that doesn't use HP or VE? I have all engine specs but no actual measurement data on the engine. (But if I needed to plug in say Cam specs, I could).


I found one for HP that uses Air Pressure, VE, CID, Comp Ratio and RPM. That gives a theoretical plot of increasing HP with RPM. Actual numbers will be different once dyno'd, but for a ballpark prediction good enough.


If I use the HP values from above in calculating Torque, the result is the same over all RPMs - no surprise, VE is fixed which is not the case in the real-world.


So I'm looking for a theoretical Torque equation that doesn't use HP or VE, but other engine specs that I can plug in to get there. The other option would be to find a theoretical VE curve over RPM (carbureted motor version) and use that to calculate the torque. I have googled extensively but with no luck, all I've seen is T equations that use HP somewhere.

[FWB]

first calculate the hp, then use it to get the torque
Calculate HP From Bore and Stroke

[AL427SBF]

I already have HP, and over the RPM range.
Using that calculator doesn't help as T is being derived from HP.


I'm looking for a Torque equation that doesn't use HP or VE.

[olddog]

Do you understand what Torque is?

If you attach a lever to a pivot point and apply a weight to the end of the lever, it will apply a rotational force to the pivot point.

Say the lever is 1 foot long and the weight is 100 lb.
The resulting torque is 1 ft * 100 lb = 1 ft-lb

If the lever is extended to 4 feet long: 4 ft * 100 lb = 400 ft-lb of torque

If we use the original 1 foot lever and increase the weight to 400 lb:
1 ft * 400 lb = 400 ft-lb

So a 4 ft lever with 100 lb weight and a 1 ft lever with 400 lb weight results in exactly the same about of torque.

There is no formula to convert cubic inches to torque. There are complex calculations that must be done that include bore, stroke, rod length, and cylinder pressure. The cylinder pressure is a huge set of equations that include compression ratio, amount of air, amount of fuel, type of fuel. You have to integrate these equations.

I think what you are wanting is a computer program like desk top dyno. They have done all this for you.


If you want my personal rule of thumb here it is. 1 lb-ft of torque per cubic inch was achievable with OHV mass produced engines in the 1950'S. This is easy to get. Any decent performance engine should make 1.25 lb-ft per cid. Normally aspirated 1.33 lb-ft per cid or more requires tuning ports and cams to supercharge the cylinder which requires expertise beyond the average hot rod guys.

In short shoot for 1.25 or so lb-ft of torque per cid. The head, intake, and cam combination will dictate what rpm range the engine can make that torque.

[Gaz64]

From an early book of mine as an apprentice:

Auto-mechanics, second edition, Herbert E. Ellinger, printed 1977.

Indicated horsepower = PLANK/33000.

P= Average indicated pressure
L= Length of stroke in feet
A= Area of cylinder cross section (square inches)
N= Number of power strokes per minute
K= Number of cylinders

Typical V8: 150 x .3125 x 9.62 x 2500 x 8 / 33000 = 273.29 hp


1 "horsepower" is determined from a horse could move 33000 lbs one foot in one minute.

[Jerry Clayton]

but a 4 foot lever will hit the ground as the crank turns!!!!!!!!!!!!!

[AL427SBF]

Originally Posted by olddog

Do you understand what Torque is?

If you attach a lever to a pivot point and apply a weight to the end of the lever, it will apply a rotational force to the pivot point.

Say the lever is 1 foot long and the weight is 100 lb.
The resulting torque is 1 ft * 100 lb = 1 ft-lb

If the lever is extended to 4 feet long: 4 ft * 100 lb = 400 ft-lb of torque

If we use the original 1 foot lever and increase the weight to 400 lb:
1 ft * 400 lb = 400 ft-lb

So a 4 ft lever with 100 lb weight and a 1 ft lever with 400 lb weight results in exactly the same about of torque.

There is no formula to convert cubic inches to torque. There are complex calculations that must be done that include bore, stroke, rod length, and cylinder pressure. The cylinder pressure is a huge set of equations that include compression ratio, amount of air, amount of fuel, type of fuel. You have to integrate these equations.

I think what you are wanting is a computer program like desk top dyno. They have done all this for you.


If you want my personal rule of thumb here it is. 1 lb-ft of torque per cubic inch was achievable with OHV mass produced engines in the 1950'S. This is easy to get. Any decent performance engine should make 1.25 lb-ft per cid. Normally aspirated 1.33 lb-ft per cid or more requires tuning ports and cams to supercharge the cylinder which requires expertise beyond the average hot rod guys.

In short shoot for 1.25 or so lb-ft of torque per cid. The head, intake, and cam combination will dictate what rpm range the engine can make that torque.

The more complex equation is what I would need, Gaz64 is on the right track with PLANK, I'll look at that.

You are correct, I need desk top dyno. For now, I'll have to settle for a spreadsheet crack at it.

[olddog]

Originally Posted by Jerry Clayton

but a 4 foot lever will hit the ground as the crank turns!!!!!!!!!!!!!

Not in this engine. It has an 8 foot 2 inch stroke.
Be warned this is engine geek porn!
New 100,000-hp Marine Diesel Engine | BoatTEST.com

[DAVID GAGNARD]

Originally Posted by olddog

Not in this engine. It has an 8 foot 2 inch stroke.
Be warned this is engine geek porn!
New 100,000-hp Marine Diesel Engine | BoatTEST.com

I've got a cousin that has worked on the smaller 8 and 10 cylinder versions of these engines!!!!!
he said you actually open a big door and walk into the engine's crankcase to do maintinence on them........

The had a smaller 8 cylinder engine being used offshore on a rig to pump natural gas to shore, they shut it down for servicing and when they re-fired it,it back fired and bent the crankshaft,they found a good used one for 1.2 million bucks!!!!!!!!!!!!!

David