hp conversion from rwhp to crank

[Windsor58]

Just curious - what do you think is a good conversion factor from rwhp to at the crank hp?
One post I saw today was almost 22% increase from rwhp to crank. For some reason, I had a lower number in my head (17-18%).
What do you all think is a good conversion factor?
Thanks

[RR|Suki]

Just curious - what do you think is a good conversion factor from rwhp to at the crank hp?
One post I saw today was almost 22% increase from rwhp to crank. For some reason, I had a lower number in my head (17-18%).
What do you all think is a good conversion factor?
Thanks

the number you had in your head is prob closer to a manual tranny. 22% or so seems to be about right for the auto

[dohc324ci]

I usually use 15% for manuals and 20-22% for autos.

[RF Overlord]

Today's automatics with lock-up torque converters are not as "lossy" as the old C6s and Turbo 400s. I think Lidio told me it was like 18-20%.

[FordNut]

So is it a percentage or is it a fixed amount? We know we lose about 60 hp in a stock MM, which is 20%. If we have a 600 hp engine does it measure 480 (20% loss) or does it measure 540 (60 hp loss) at the rear wheels? And why?

[Spectragod]

It would be a percentage of loss, it's pretty hard to build in a fixed amount of loss on a production vehicle. I suspect you know this already.;)

[FordNut]

Not really, my transmission builder told me to expect a 60 hp loss, didn't matter how many hp the engine put out.

[RR|Suki]

Not really, my transmission builder told me to expect a 60 hp loss, didn't matter how many hp the engine put out.

I dunno about that, all cars running our trans don't lose 60hp out the back... my v6 96 cougar for example lost around 20 some % so yeah... I think the best way to know would be to put a motor on an engine dyno, then put it in the car and see what happens

[ctrlraven]

20-22% is usually a pretty good estimate range, I've always used 21% for the MM.

[Rockettman]

18 to 20% sounds like what I've learned also.

[Spectragod]

Below are not my words, buts someone else's, I have included a link. Everyone, my dyno guy and transmission guy both say 20%, I have never heard of anyone saying a fixed # no matter what the power level. Too many factors involved to be able to give a specific #.


Factory ratings are all well and good, but many enthusiasts modify their cars and then want to see how much of an improvement they got from their labors. The problem is that most of the time people are not interested in ripping the engine out of their car to have it tested on an engine dyno; no, they're going to be testing on a chassis dyno. The most common chassis dyno, the inertial dynamometer (popularized by DynoJet), measures the horsepower as delivered at the power wheels -- whether front or rear.
But testing rear-wheel horsepower (rwhp -- obviously, front drivers would be measuring fwhp) makes it difficult to convert from what the dyno says to what the manufacturer says. The manufacturer, remember, measures horsepower at the flywheel. All that equipment between the engine and the wheels -- the transmission, driveshaft, differential, and axles -- introduce friction and inertial losses summarized as "powertrain loss" or "parasitic losses". The efficiency of the driveline can greatly affect the amount of the powertrain loss: Ford's AOD transmission, for example, is notoriously inefficient. As a very general rule, rear-wheel horsepower on a manual-transmission car is about 15% less than SAE net, and rear-wheel horsepower on an automatic-transmission car is about 20% less than SAE net.


Linky....

http://neptune.spacebears.com/opine/horsepwr.html

[MrBluGruv]

The transmission is a conveyor of energy, not a resistor to it, it'd be absurd if you think about even simple physics to think that every engine and transmission setup loses 60 hp between the engine and the drive wheels. Every vehicle known to man would have to make at least 61 horsepower to operate then, and cars in the past haven't always been as powerful as ours obviously.

[FordNut]

Only way to tell would be to measure on an engine dyno and chassis dyno, then compare. And repeat it with different power levels.

What causes the loss? Friction/lubrication/surface area and mass of rotating parts? Aren't these factors constant?

If the experiment was done, I suspect one would find a non-linear relationship. At say 100 hp it may be 30% loss, at 300 hp 20%, at 900 hp 12%.

[MrBluGruv]

I'd seriously doubt that, I mean even what you just said contradicted itself. If the factors are constant, the percentage would be constant. How would it change?

I just can't see the system having efficiency increase exponentially like that, not in this context.

[FordNut]

I'd seriously doubt that, I mean even what you just said contradicted itself. If the factors are constant, the percentage would be constant. How would it change?

I just can't see the system having efficiency increase exponentially like that, not in this context.

Constant to me means it would take a specific amount of power to turn it. 60 hp is a constant amount of power isn't it? A percentage would be a variable amount of power wouldn't it?

Only way to know is for somebody to do the experiment. Who's gonna volunteer?

[MrBluGruv]

True, I see what you're saying there. That being the case, there would be no rule of thumb then for power loss, only ball park guesses. There would be a given amount of loss on a transmission to transmission basis given how much force it takes to rotate each one's internals respectively, plus a percentage loss extra on automatics for the torque converter slip and such.

[FordNut]

True, I see what you're saying there. That being the case, there would be no rule of thumb then for power loss, only ball park guesses. There would be a given amount of loss on a transmission to transmission basis given how much force it takes to rotate each one's internals respectively, plus a percentage loss extra on automatics for the torque converter slip and such.

BINGO

If we take the 20% rule to the extreme, we should be able to connect a 5hp go-cart engine to our transmission and have 4 rwhp. In fact it would likely be zero rwhp because the 5 hp engine wouldn't even have enough power to turn the mechanical components. So at that point it would be 100% drivetrain loss.

[dohc324ci]

This is too deep for me...LOL

Too many variables....20% is an estimate of loss through and auto. The real science behind it and why I dunno......

[MrBluGruv]

I guess the number comes from kinda sorta guessing an average HP number for the cars in production (e.g. 8-10 years ago most V6s out of Japan especially hovered around the 200hp mark, now most all V6s are between 260 and 285) and seeing the loss on average for all these cars.

Who knows, maybe when the average car with at least a V6 is above 400hp, the powertrain loss figures may go to 10%. Sad day when the average consumer automobile will have more power than most would know how to handle. :(

[Windsor58]

Thanks for all the input.
Gonna have to go crunch some numbers, now.