keekster

Is it a percentage say like 15% so the more you make the more it takes. Or is you drivetrain take a certain amout of power to move say like 35 HP and everything after that is put to the wheels.

vader86

More of a percentage type thing, run a search, this was covered just recently.

CorvetteZ51Racer

Actually, what was discussed in the recent thread is that it's most definitely NOT a percentage. It's a very difficult relationship to nail down, and nearly impossible to do if the car's an automatic.

Darkness

Well 15% is the rule of thumb. Of course that an automatic will take more to spin but I've seen it from 10% to 20% loss in the drivetrain. :thumbs:

vader86

Nevertheless, its still a good rule of thumb to use 15% for our manuals and 17-18% for the 700r4.

Further information is for the real tech nuts like me who can actually see that each transmission is different and the frictional losses can vary widely depending on the multitude of variables involved, this stuff just confuses newbies.

CorvetteZ51Racer

Yeah, it's a decent rule of thumb to use, but I would only use it at or around stock power levels. Once you start getting a fair bit higher than stock power levels, the rule of thumb doesn't apply anymore.

95AquaC4

You simply CANNOT use a percentage. If my A4 is putting 400 hp at the crank, and your A4 is 300 at the crank, and you subtract say 18% for drivetrain loss, which in each of our cars is identical, you'd be saying I lose 72 HP, and you only lose 54 HP. percentage simply does not work in this case. :nonod: :nonod:


[Modified by 95AquaC4, 8:08 PM 2/10/2003]

red-y

[QUOTE]You simply CANNOT use a percentage. If my A4 is putting 400 hp at the crank, and your A4 is 300 at the crank, and you subtract say 18% for drivetrain loss, which in each of our cars is identical, you'd be saying I lose 72 HP, and you only lose 54 HP. percentage simply does not work in this case. :nonod: :nonod:


:iagree:

GusBustamanteJr

Cant one of our mathematics wizzes out there come up with a formula for this???

If we get a stock car as the benchmark, we know it has 250 FWHP, then we can get the RWHP from a Dyno. Then someone can come up with an is/of statement or something mathematical and ingenuous. This will give us the actual # of HP lost due to the tranny. Lets say its 50 HP. From that point on, we only add 50 HP to our RWHP dyno results instead of using a percentage. So if I dynoed stock at 200 RWHP, and we can assume that GM figures 250 FWHP, the tranny is accountable for 50 HP. Now if I add mods and dyno at 250 RWHP, cant we assume 300 FWHP?

Seems simplistic, I must be missing something there, but thats why I sucked at math!!!

Or we can all just start reporting only our RWHP (but that wouldnt be any fun!!)

CorvetteZ51Racer

At or near stock power levels, there will be an increase in power loss through the driveline with increased power, but it is most definitely not linear (a straight percentage). If you read my comments above, I mention that "at or near stock power levels". Well, if your motor is 300 crank HP stock, and you're now looking at 400 crank HP, that's not "at or near stock power".

I would say that if you're not terribly concerned with the accuracy, you could use the percentage for about the first 50 or so HP over stock, but I wouldn't personally use it beyond that.

If you can use the percentage all of the time, and you can use a 15% loss for Winston Cup cars, then when they chassis dyno them after a race and they're putting ~750 HP to the wheels, that would insinuate ~862 crank HP at the end of a 500 mile race. They don't make quite that much when they're fresh (although it's close).

I will say, however, that automatics are worse about their non-linearities than manuals are, due to the torque converter.

P.S. - JackdaRoofer, if you're reading this, notice I didn't put my sig down here, since I've already posted on this thread!! ;)

CorvetteZ51Racer

Gus, unfortunately there is an increasing trend in parasitic loss from transmissions and differentials as power from the engine increases, but it's not linear...it's far from it. It could be done, but it would take graduate-level statistical analysis (something I'm doing right now, as a matter of fact) with a LOT of data points to come up with a reliable relationship. And the only way to reliably correlate the FW to RWHP would be to dyno the engine in the car, then dyno it on an engine dyno, etc, etc.

Long and short, too many variables (oil temp, oil viscosity, age of oil, ambient temp and pressure, humidity, age of bearings, etc, etc) and not enough time.

GusBustamanteJr

After thinking about it more and what you guys have said, it would be a differential equation. Someone would have to come up with several equations for varying trannys, rear-ends, and HP ranges.

I would dare to say this has to have been done before, and thats where our 18 % average guestimate came from. So for arguments sake and simplicity, perhaps we should come to an agreed to % !!!!

95AquaC4

The percentage theory would work for comparable cars. You simply could not compare cars with great horsepower differences. I agree that mathematically there must be a solution, but it would be difficult to find, and different for each vehicle. The percentage theory could have come from someone who knew their crank HP and their rwhp and simply figured out the loss. You bring up things to think about, but it's getting late and I have to be awake at 5am!! :D :crazy: :crazy:

vader86

Its been done before, but probably not comprehensively across different brands and nameplates. I could find old Masters' Thesis and Dissertations from the Automotive engineering people here at Clemson, if i felt like doing the research. But its damn time consuming.

merrick

Just to add to the confusion, car dynos have a range of error and varies from brand to design. :confused:


[Modified by merrick, 1:12 AM 2/11/2003]

CorvetteZ51Racer

The problem with developing an equation to express the relationship is that it's as simple as finding some differential equation. The problem is that you have to set up and solve a Response Surface system and optimize it for minimum variance. It's a pain in the butt to do for a small number of factors, but to find it for drivetrain loss would be a thesis in and of itself.

No Go

It is closest to a percentage, but there is far to many variables for anyone to quote a number. Any number is just a WAG...:)

If you want to know flywheel HP for your car...then dyno the engine on a engine dyno!

:)

MSR

it's as much a function of speed as anything else. dynojets have the ability to measure absorption of power in coast, and that's almost a linear function of speed. (25hp @ ~125mph, almost 0hp @ ~10mph.) this coast-down loss is not the only loss when measuring power, but it is part of it, which proves all by itself that power loss simply can't be some percentage (as this would fail to integrate a known phenomenon into the equation: hp loss vs speed).

further compounded by the fact that differing finish speeds results from different gearing, which in turns affects the rate of rpm increase, which, of course, affects measured power due to inertial energy changes over time. (in fact, the inertial losses are probably more significant than speed changes, since cars with shorter gearing, all else equal, tend to measure less power on an inertial dyno.)

the more you delve... i mean, i've only touched on a few of the factors, hopefully some of you will get a sense for how flawed the notion of some set percentage of loss is. but, as i've said before, it's amazing how close the standard 12-17% can be for close to stock automobiles.

SouthernSon

Maybe I am missing something here. I have never really been that interested in anything but RWHP. Does anything else really matter?

swoosh

Oh my GOD!! How did i stumble into this post. I feel like i just walked into a conversation between ALBERT EINSTEIN AND STEPHEN HAWKINGS talking about the theory of relativity! :crazy: :skep: :confused:
Here's my 2 cents that isn't worth a penny.
Get 4 vettes 2 stock; 2 modified/ both pairs are composed of A4 and M6 DYNO the engine and rear wheels of all cars. NOTE: make sure dynos are calibrated and so on. Then have a team of ENGINEERS analyze the data and come up with a formula. LETS ALL SEND THIS IDEA to VETTE MAGAZINE since they have the resources to conduct this experiment. :grouphug: