Interpreting horsepower in our cars.

[Cabbage Fumes]

When it comes to dyno numbers using a drum (DynoJet), for those that have factored the driveline losses, how much do we lose between the flywheel and road surface? I’ve heard anywhere from 12 to 20% loss. Has anyone really looked at our chassis and came up with a reality based percentage to add to the “at the wheels” number?

[TobyB]

Given how little the transmission and diff heat up, I always figured most of the losses in the racey car

were in the tires.  And that THAT number was as hard to quantify as flies on a herd of cattle in Montana in August...

 

I've never seen trans temps above water temp, and the small- case diff in good shape seems to sit around 100c 

most of the time.

 

The surfaces of the tires, however, go from 50c to 125c 5 or 7 times a lap...  so they're dumping significant amounts of heat.

 

so, no, I guess I haven't really come up with a good percentage!

 

 

heh

 

t

 

[Cabbage Fumes]

The last foray at Ed Pink was a good day. I got the wheel number, and another sheet adjusted to 12%. I think 12% sounds kind of low, though. I was thinking it was closer to 15 or 20%. I also know you can push the numbers with sorcery like raising or lowering the tire pressure, etc. I don't want to pad the graph. I just want the truth. I can handle the truth, unlike some people out there...

[SydneyTii]

Just hope it’s not the horses that they used for the seats… cos they were knackered to start with!!

[ray_]

Well...

 

Just pull the motor and measure at the crank.

 

And you'll have your answer!

 

[jimk]

As @TobyB said, convert the HP loss to KW and the numbers will make some sense.   Gearing loss isn't that great.  In the industrial world, gearing loss is a nominal 2% in power equipment.  If more than that the gearbox would get hot as hell, the lubricant in it couldn't survive.

[uai]

My dyno sheets show 78% =22% loss

[72MetallicMalaga]

I dyno'd my car on a Mustang Chassis Dynamometer a couple years, got 88 at the wheels, so 12% loss is what I use with that type of dyno, using 100 motor hp as a benchmark. Motor was refreshed and resealed a few years ago.
 

 

[jimk]

1 hour ago, uai said:

My dyno sheets show 78% =22% loss

How many KW is the loss then.  From engine dyno to chasis dyno, do you believe the absolute power numbers?

[Cabbage Fumes]

2 hours ago, uai said:

My dyno sheets show 78% =22% loss

That just seems like a lot. I assume that number was chosen by the person running the dyno, since i don't know how the dyno is able to determine that on it's own.

[uai]

Here for example: not my car but here the green graph is what in german is called Schleppleistung. All proper Modern dynos have the ability to measure Schleppleistung

Edited April 5 by uai

[Cabbage Fumes]

17 hours ago, uai said:

Here for example: not my car but here the green graph is what in german is called Schleppleistung. All proper Modern dynos have the ability to measure Schleppleistung

 

Fiddlesticks..

Edited April 6 by Furry Camel
Unnecessary response.

[uai]

Our Physics is the same as yours. Although alternative facts seem to get more used in the US...
The dyno inertia moment is known and calibrated.
When you press the clutch at a known speed / angular velocity the grade of the dyno slowing down gives you the figures. It is done twice with a little braking force on the second run. With the difference between these runs the inertia moment of the car components and the drivetrain loss can be calculated- It's integrated in the dyno software. And it even calculates the engine internal mass /inertia on the ramp up.
Just read the Maha manual. (see excerpt at bottom ) Other modern dynos can even drive your wheels and figure out the loss.
On my car it's 29.4 kW loss

 

Here is another explanation of the possible problems with dyno and calculations with more modern cars that reflect correction parameters already in engine management. If you apply the correction factors according to DIN you get wrong numbers. There is also a bit less scientific explanation of how drag is calculated.

Die Tücken der Leistungsmessung

WWW.AUTO-MOTOR-UND-SPORT.DE

---

Die korrekte Motorleistung auf einem Rollenprüfstand realistisch und reproduzierbar zu ermitteln, ist ein Spiel mit vielen Variablen. Wir erklären die Technik moderner Leistungsprüfstände und sagen, worauf man achten muss.

Edited April 6 by uai

[Dudeland]

My feeling is that unless you are testing out some magic crank case and diff sauce, the only thing that matters is what happens at the wheels. 

 

Where the rubber meets the road is where the rubber meets the road.  

[Cabbage Fumes]

7 hours ago, uai said:

Our Physics is the same as yours. Although alternative facts seem to get more used in the US...

I won’t argue with that…

I apologize for not knowing this, but in my defense, I’ve never been to a dyno that offered this detailed of information. I don’t know if it’s even offered here. I guess I should change my question to, ‘does anyone living in unsophisticated North America know of a way, without shipping a car overseas, to measure driveline losses?’ Again, apologies uai, I come off sounding incredibly ignorant after your detailed answer to my question.