Question about Chassis Dyno Numbers

[Laneco]

I am wondering if I have always grossly misunderstood something (always possible...) or if someone else is getting the wool pulled over their eyes.

A friend had an engine installed in a car and the engine builder ran the car on a chassis dyno. The dyno showed 144 hp (it's NOT a 911). Anyway, the builder told him that the engine makes 216 hp.

The guy is a friend and I didn't want to embarass him. But it sure seemed like an awful lot of drivetrain loss. I've always roughly estimated 15% for transaxles and 20%for front engine/rear drive cars when I do my admittedly poor arithmetic.

But no combination of the above, even with my creative math, got anywhere near 216hp at the crank from 114 rwhp. So I emailed the guy about it. He contacted the engine builder who told him that the 216 was correct because there is a 33% loss to the wheels, sometimes even more. This is a mid-engined car with a 4 cylinder thru a VW transaxle.

Is this true? 33%? Or is someone selling an engine with a few missing hp?

angela

[RWebb]

some hp are sure missing somewhere -- 33% would be unusual -- maybe the engine "builder" mixed too much corn meal in with the gear oil

...assuming the chassis dyno is correctly adjusted and the readout is compensated for STP, etc.

[Steve W]

33% loss is a bit high but regardless, the formula to compute flywheel hp from drive train loss is:

flywheel hp = (wheel hp) / ( 1.0 - L)

where L = loss

In your example, with L = 0.333

(144) / (1 - 0.333) = 216.2 hp

[hcoles]

this may be digging a bit deep but I can't help asking..
Steve maybe you know..how are the losses divided up? It is an interesting question I think because there is a lot of energy going somewhere. E.g. 30hp is 22.3kW. If this is going into the transmission, no wonder they get hot.

- windage for the clutch asm (this may almost be the same for an engine dyno)
- transmission (gears and layshafts/etc.)
- ring/pinion
- cv joints (btw - there is a new cv joint design coming out soon, more efficient)
- tire/wheel windage
- tire to dyno roller surface loss
- dyno roller surface to dyno torque sensor loss

[84_Carrera]

There IS a significant difference in losses between a manual & automatic trans, and I've found that the differential makes a difference too.

Example, I've found that typically on my FWD open-diff SHOs, the loss is about 15% for a stick. Add a Quaife, and I usually use 17% losses for the numbers. Move to an automatic trans on those cars, and you're more in the range of 22-25%, with 25% being on the very high side. I'd usually use 22% for rough calcs. Now, that's a fuel injected post-80's car with electric fans, but still mechanical water pump, etc.

Go to an older V8 sled, with mechanical water pump & fixed-fan, you're looking at noticeably higher parasitic losses than say an electric water pump / electric fans.

I recently dyno'd my '84 911 Cab (photo attached, also here: http://www.sinclairmfg.com/fred/911_dyno_20080503.jpg)

Figuring the original numbers were at 50k, pre-Ethanol fuel, stock cat, stock muffler, but with a Weltmeister chip (blech!), peak crank ponies were likely around 240hp... not bad for a chipped / otherwise stock 20 year old '84 911. After Ethanol, I dropped about 3% power, which the BTUs of 10% Ethanol support. After making the DME fuel switch setting change last week, I'm back up north of the original numbers, and again, using 15% losses this would now be a peak of 250 crank hp.

[richde]

Peak dyno hp numbers are a distraction, maybe the dyno software wasn't set up correctly. The most important information is the shape of the hp/torque curves, that's what tells you how well the engine is running.


But that does seem like a bit much of a loss, even an AWD Subaru only loses around 25%.

[Formerly Steve Wilkinson]

Commercial dyno (as opposed to a million-dollar test cell) numbers are pretty approximate anyway. Sort of like specifying the dB of a noise by ear. Different days, different conditions, different operators = different numbers even from the very same engine. The real use of a commerical dynamometer should not be to measure the hp of one engine versus other engines (unless you dyno them all at the same time) but to assess tuning and other changes on one engine, assuming the dyno runs are being done by the same person in roughly the same ambient conditions.

[Laneco]

Thanks for the info guys! I've always considered dyno numbers to be compartives and not absolutes for all of the reasons listed above.

But a straight across the board 33% drive train loss on a two-wheel drive manual transaxle seemed REALLY high.

Most people buy engines based on HP. This guy thinks he bought 216 hp. Really, he bought about 175ish. Good honest, run-forever kinda horsepower, but only 175ish not 216.

Thanks for the info guys!

angela

[RWebb]

I agree, 33% is greater than the variance components listed by S.W. should be. It may be that the dyno is not set up properly. or.. the motor is not making what is claimed.