Specific Torque

[MadBill]

<EDIT> OK, so now I see the 'Similar Topics' threads I should have checked for first, but the dubious data in my last paragraph still invites comment.

HP per cubic inch in well-developed race engines can vary widely due to many factors, but specific torque is much more constrained. 1.6 lb-ft/c.i. seems to be pretty close to the limit for well-documented tests on NA pushrod V-8 gasoline engines.

I'd like to see engine details of any credible results that exceed this number. (I'm looking at a chassis dyno sheet for a 427" LS that shows 675 lb-ft. "corrected"* or 1.58 lb-ft./c.i., which I suppose could mean estimated crankshaft numbers, but even so...)

[Momus]

No graphs but.

Australian V8 Supercars- 5 litre 304 CI 10:1 engines Ford, Chev, Nissan, Mercedes, Volvo on E85 with individual throttles and a lot of restrictions re heads, camshafts, valve sizes component weights, and rev limited to 7500 were regularly making 650 hp at around 7200 rpm.

650 hp @ 7200 rpm requires 474 lb/ft.

This gives about 1.56 lb/ft per CI at peak HP.

If we allow 10% more twist at peak torque which is typical then the 1.6 figure is easily exceeded.

[maxracesoftware]

<EDIT> OK, so now I see the 'Similar Topics' threads I should have checked for first, but the dubious data in my last paragraph still invites comment.

HP per cubic inch in well-developed race engines can vary widely due to many factors, but specific torque is much more constrained. 1.6 lb-ft/c.i. seems to be pretty close to the limit for well-documented tests on NA pushrod V-8 gasoline engines.

I'd like to see engine details of any credible results that exceed this number. (I'm looking at a chassis dyno sheet for a 427" LS that shows 675 lb-ft. "corrected"* or 1.58 lb-ft./c.i., which I suppose could mean estimated crankshaft numbers, but even so...)

NHRA ProStock 1.683+

[David Redszus]

A calculation of Specific Torque, while useful, does not consider the effect of
engine speed.

The method used to compare the state of tune of various engines is to normalize
for engine size and engine speed. The calculated value is called Brake Mean
Effective Pressure (BMEP). It reflects the average cylinder pressure that an
engine must achieve, for a complete cycle, to produce a specified or measured
power or torque value. The units are pressure, either psi or atmospheres (BAR).

It is no simple matter to compare the states of tune, or stages of development, of
engines that are dissimilar in size and operating speed. Often a smaller engine, while
producing less horsepower, is actually much more fully developed than a larger
engine making more power.

The engine with the higher BMEP will be the more highly developed engine. High
BMEP values indicate an engine with a very high state of tune, regardless of size
or operating speed. Engines with high BMEP values are very difficult to improve;
low BMEP engines are usually much easier to improve.

Two BMEP values are calculated, one for peak torque and one for peak power.

The BMEP value for peak torque is always higher than for peak power. As engine
speed is increased, BMEP (and torque) will normally fall, but since the engine is
running faster, the power will increase. When engine BMEP (and torque) falls faster
than engine speed increases, the power peak of the engine has been exceeded.

If the BMEP falls steeply as speed increases, it indicates that the engine is air flow
choked. If the BMEP increases as speed increases, it indicates that the BMEP
torque peak has not been reached and the engine can run even faster.

[gmrocket]

On our 2019 REC entry peak TQ was 1.54 on a bad tune,,we think 1.56 will be it when right on

[maxracesoftware]

A calculation of Specific Torque, while useful, does not consider the effect of
engine speed.

The method used to compare the state of tune of various engines is to normalize
for engine size and engine speed. The calculated value is called Brake Mean
Effective Pressure (BMEP). It reflects the average cylinder pressure that an
engine must achieve, for a complete cycle, to produce a specified or measured
power or torque value. The units are pressure, either psi or atmospheres (BAR).

It is no simple matter to compare the states of tune, or stages of development, of
engines that are dissimilar in size and operating speed. Often a smaller engine, while
producing less horsepower, is actually much more fully developed than a larger
engine making more power.

The engine with the higher BMEP will be the more highly developed engine. High
BMEP values indicate an engine with a very high state of tune, regardless of size
or operating speed. Engines with high BMEP values are very difficult to improve;
low BMEP engines are usually much easier to improve.

Two BMEP values are calculated, one for peak torque and one for peak power.

The BMEP value for peak torque is always higher than for peak power. As engine
speed is increased, BMEP (and torque) will normally fall, but since the engine is
running faster, the power will increase. When engine BMEP (and torque) falls faster
than engine speed increases, the power peak of the engine has been exceeded.

If the BMEP falls steeply as speed increases, it indicates that the engine is air flow
choked. If the BMEP increases as speed increases, it indicates that the BMEP
torque peak has not been reached and the engine can run even faster.

(TQ / CID) Ratio is just Torque divided by Cubic Inches

BMEP is just the (TQ / CID) Ratio times the Constant = 150.8

BMEP = (TQ * 150.8 ) / CID

BMEP is just scaled up value of the Torque/Cubic Inches ,
or sort of like an "Orders of Magnitude value"

still BMEP value seems easier to grasp + remember

Also , BMEP or TQ/CID Ratio changes
with changes in Dyno test RPM/SECOND rates .
So you need to account for that effect !

some additional equations :

5252.113122 = 16500 / Pi
150.7964474 = 792000 / 5252.113122
150.8 rounded-off

BMEP = (TQ * 150.8 ) / CID

BMEP = (HP * 792000) / (CID * RPM)

HP = TQ * RPM * 0.000190400

 

[MadBill]

David's assessment is absolutely correct, but lb-ft/c.i. has the advantage of being just a single division operation away from the most readily available raw data. There are other B.M.E.P. factors however, such as the difficulty of attaining a given value at say 15,000 RPM vs. 5,000 and the effect of scale, which explains why an ant can lift 200 x its own weight but an elephant can't jump...

[gmrocket]

David's assessment is absolutely correct, but lb-ft/c.i. has the advantage of being just a single division operation away from the most readily available raw data. There are other B.M.E.P. factors however, such as the difficulty of attaining a given value at say 15,000 RPM vs. 5,000 and the effect of scale, which explains why an ant can lift 200 x its own weight but an elephant can't jump...

And small cubic inch motors are over achievers

[dannobee]

(I'm looking at a chassis dyno sheet for a 427" LS that shows 675 lb-ft. "corrected"* or 1.58 lb-ft./c.i., which I suppose could mean estimated crankshaft numbers, but even so...)

What's the correction factor on the dyno? I've seen some bogus torque numbers too. Then you look at their correction factor and you can plainly what's going on. Compare it to the uncorrected number.

The 1.6 lb ft/cubic inch is pretty much the limit across damned near all normally aspirated engines, from what I've personally seen. 1.4 seems to be the limit for streetable engines. Sure, there might be some engines that very slightly exceed that, but it's not going to be a few tenths above that.

[maxracesoftware]

David's assessment is absolutely correct, but lb-ft/c.i. has the advantage of being just a single division operation away from the most readily available raw data. There are other B.M.E.P. factors however, such as the difficulty of attaining a given value at say 15,000 RPM vs. 5,000 and the effect of scale, which explains why an ant can lift 200 x its own weight but an elephant can't jump...

then you can use this equation :
BMEP = (HP * 792000) / (CID * RPM)

=================================

more related info on various methods of judging Engine efficiencies :

NHRA 499-500cid
for example makes 1500 Peak HP and 845 Peak TQ

845TQ / 499cid = 1.693386774
BMEP = 255.363

BMEP @ Peak HP at 10000 RPM = 238.076

1500 HP / 8 Cylinders = 187.5 HP/CID Ratio
a Range of 2 examples :
Flow CFM = 600 CFM
187.5 HP / 600 CFM = 0.31250

Flow CFM = 640 CFM
187.5 HP / 640 CFM = 0.292968750

================================================

Sonny's 1005.84cid 2150HP and 1550TQ

1550TQ / 1005.84cid = 1.541000557
BMEP = 232.383

BMEP @ Peak HP at 8000 RPM = 211.614

2150 HP / 8 Cylinders = 268.75 HP/CID Ratio
Flow CFM = 740 CFM
268.75 / 740 = 0.36318

=================================================

If a ProStock 500cid had the Flow CFM efficiency of Sonny's 1005.84cid
it would equal a potential of = range of 1743.3 to 1859.5 Peak HP
potential gains of = 243.3 to 359.5 Peak HP

so Sonny's 1005.84cid is way more efficient at turning CFM into HP than a ProStocker !
however, a ProStocker , likewise is way more efficient turning CFM into HP/CID
mostly because of difference in RPM points of Peak HP .
 

[MadBill]

.

What's the correction factor on the dyno? I've seen some bogus torque numbers too. Then you look at their correction factor and you can plainly what's going on. Compare it to the uncorrected number.

Unfortunately, CF was not provided.

[swampbuggy]

I remember the tiny Cox .049 two cycle engine which i guess was 49/100ths of a C. I. . It was an over achiever for sure. It would take 616 of them to make 302 C. I. 's . I wonder how much power 616 of those little stinkers would produce ? Mark H.

[Momus]

http://www.epi-eng.com/piston_engine_te ... dstick.htm

[swampbuggy]

Nice read Momus, thanks. Mark H.