Rules for 2013 - Crank power vs Dyno

[DanB]

Bruce is spot-on in differentiating between torque and work. Same units (usefully differentiated in the inch/pound/second system as ft-lb for work and lb-ft for torque). Every time I see someone refer to power as "torque over time", I cringe. It isn't! It is the instantaneous rate of doing work.

Also, a load cell is used in a dyno to calculate DYNO torque. That is NOT the same as engine torque. The dyno measures dyno drum torque and dyno drum speed and from that calculates power at the dyno drum, which *is* the same as engine power minus trans, diff, and tire losses. This is "rear wheel power".
In order to get ENGINE torque, you have to know engine rpm. Calculated as power*5252/rpm. Disconnect the spark plug wire clip from the dyno and it won't be able to give you engine torque.
What people call "rear wheel torque" is NOT the actual torque at the rear wheels, but rather the torque at the engine minus losses.
Actual torque at the rear wheels will be engine torque, minus losses, multiplied by the total gear ratio (trans ratio x diff ratio). BIG difference between that and engine torque unless you're geared 1:1 overall!

[WillM]

They are both approximations and are both approximately the same. Since 1/0.85 approximately equals 1.15.

The intent was to add or subtract 15%, so the true math is multiply by 1.15 or divide by 1/1.15 (0.8696).

assuming 15%

CP = WP * 1.15
WP = CP / 1.15

You can switch the multiplier for a divide if you have the inverse.

CP = WP / 0.869
WP = CP * 0.869

Perfect. So Les is correct and you are approximately correct. Finally...something everyone can agree on!

These are not the droids you are looking for.

[Chrispy]

Perfect. So Les is correct and you are approximately correct. Finally...something everyone can agree on!

These are not the droids you are looking for.

Technically my calculation was exact, and Nick's was approximate so therefor the comparison between the two is approximate.

"Life is an approximation in perfection" are the words I live by.

[TroyV]

Just remember to turn off all the trash compactors on the detention level.

[WillM]

Je-ziss.

Chris, I like you man, but WTF?

These quibbles and the professorial tone of responses given to questions are what is making a growing number of heads explode (figuratively).

Apparently, there cannot even be agreement on facts which are plainly laid out..!

Your calculation may be exact, however you use it to justify your incorrect statement. You stated that WP * 1.15 is the same as WP / 0.85. It is not. Or written as an equation:

WP * 1.15 = WP / 0.85

Les said the two are not the same, and he is correct. Period. End of story. You go on to 'show math' and prove that he is correct, but conclude that technically you are right because these are all approximations anyway.

Even worse is my take away from this. Nick tells Les one thing, you state another. Do the guys writing these rules agree on the math, and if so, why are they providing different answers to the same question?

[agrabau]

We're muddying the waters here. No one said that dyno torque was the same as engine torque. Interested parties are choosing a standardized method for testing and hoping to implement it.

They dyno "knows" engine RPM at a given roller speed because it is imputed into the computer when the car is set on the dyno and it's "vitals" are entered.

Bruce is spot-on in differentiating between torque and work. Same units (usefully differentiated in the inch/pound/second system as ft-lb for work and lb-ft for torque). Every time I see someone refer to power as "torque over time", I cringe. It isn't! It is the instantaneous rate of doing work.

Also, a load cell is used in a dyno to calculate DYNO torque. That is NOT the same as engine torque. The dyno measures dyno drum torque and dyno drum speed and from that calculates power at the dyno drum, which *is* the same as engine power minus trans, diff, and tire losses. This is "rear wheel power".
In order to get ENGINE torque, you have to know engine rpm. Calculated as power*5252/rpm. Disconnect the spark plug wire clip from the dyno and it won't be able to give you engine torque.
What people call "rear wheel torque" is NOT the actual torque at the rear wheels, but rather the torque at the engine minus losses.
Actual torque at the rear wheels will be engine torque, minus losses, multiplied by the total gear ratio (trans ratio x diff ratio). BIG difference between that and engine torque unless you're geared 1:1 overall!

[Chrispy]

Will, the last post was just a joke to lighten the mood, I'm sorry it came across poorly. I have a nasty cold at the moment and I don't think my sense of humor is operating correctly. My apologies for aggravating the situation.

The conversion formula's used as according to the current proposal are 15% for RWD and 20% for AWD so use appropriately. The guys are looking into the viability of a dyno brand specific formula for v1.1, so we'll see how that goes.

[brucesallen]

By the way ft-lb is a measure of WORK. Torque is measured in lb-ft (or in newton-meters in metric)

Torque is a force. Work is force times distance.

Torque is force times radius. With a force or a torque there may be no work performed.

[brucesallen]

Bruce do you know what a load cell is?

Also your statement above is incorrect.

Alex

ALex,
I have designed products with load cells as a process control engineer.
I Think my equation is correct. Here is the answer from a Dynapack engineer. Dynapack is a friction chassis dyno like the one you use. It is the answer I was looking for. The displayed torgue is simply the wheel torque divided by the ratio of the wheel rpm to the shaft rpm. Thus is shares the gear train losses of the wheel horsepower figure.

Hi Bruce,

The Dynapack dyno measures wheel torque, as you have described, but because we can work out the ratio between engine speed and wheel speed (engine rpm divided by wheel rpm) we can divide the wheel torque by the same ratio to produce an approximation of engine torque. It is not a direct measurement and does not take into account losses in the transmission and other parasitic losses. There is an option in the F11 screen to provide a Total Correction factor (TC or TCF) to offset some of these losses and provide parity with engine dyno data.

Horsepower is a multiplication of speed and torque (along with a constant factor), and if no TC factor is used (i.e. TC = 1.00) the engine HP will match the wheel HP. In the dyno software Engine HP = Wheel HP * TC

I hope this answers your questions. Please let me know if you’d like more info.

Thanks,

Richard Hicks
Dynapack Support

[agrabau]

Earlier Dan B wrote that a dyno does not measure wheel torque as you described earlier. This engineer replied to say that their machine does measure wheel torque. (which you have been denying from what I can understand) I'm sure something's getting lost in people's concept of wheel torque.

Dynapack requires the removal of the wheels and therefore maybe measures "hub torque?" Not sure.

Our dyno isn't a friction dyno either it's an eddy current retarder, magnetic.

The result of this arguing is that there is effectively no way to agree on measuring power to create rules. It's pretty clear that no one will agree.

[wizzman]

They are both approximations and are both approximately the same. Since 1/0.85 approximately equals 1.15.

The intent was to add or subtract 15%, so the true math is multiply by 1.15 or divide by 1/1.15 (0.8696).

assuming 15%

CP = WP * 1.15
WP = CP / 1.15

You can switch the multiplier for a divide if you have the inverse.

CP = WP / 0.869
WP = CP * 0.869

This confused me as well when I first was working through the dyno calculations to generate new crank hp & tq numbers to replace the engine point section. You cannot calculate this both ways however. Rather than write a lengthy explanation. Lets use an example:

Say my car dyno'ed at 85 whp. I want to figure out what my crank hp number is. Here is the math:

85 whp/0.85=100 crank hp

This calculates the 85% drive train loss from 100 crank hp. You cannot multiply 85 whp by 1.15 because 115% of 85 is a completely different calculation and is not the same thing (97.75). You must always divide dyno numbers by 0.85 (or whatever the drive train loss factor is determined to be) to determine the crank numbers. The Nickparlands (see what I did there) have already built the correct calculation outlined above into all of their tools.

[Chrispy]

Dave I'll agree. The confusion was that the rules doc stated assume "15% drivetrain loss" but it didn't state the relationship formula.

It is clear now that the intention was to express that you have 100%-loss at the wheels, i.e. 85% or 80% of crank. My mistake, too much cold medicine.

I don't make mistakes often, but when I do I drink Dos Equis.

[blindsidefive0]

Dave I'll agree. The confusion was that the rules doc stated assume "15% drivetrain loss" but it didn't state the relationship formula.

It is clear now that the intention was to express that you have 100%-loss at the wheels, i.e. 85% or 80% of crank. My mistake, too much cold medicine.

I don't make mistakes often, but when I do I drink Dos Equis.

[Mark Swinehart]

Nick -

Please reconsider terminating Chris. Can't you keep him on?

Maybe he could sweep up in the garage or chase parts for you?

[paultg]

I don't make mistakes often, but when I do I drink Dos Equis.

*chuckle*

First we have an earthquake this month, and now Chris makes a mistake on a public forum.

Yikes. I'm afraid of what's next!!