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Parasitic loss
Does anyone out there know what percent of parasitic loss the drivetrain has on horsepower. I put my car on the dyno at DVSF and ended up with 360 rwhp and 398 rwtq. I've always heard the drivetrain loss wound up around 25 to 30 percent. Anyone have an answer to that question.
Just Curious, Jim Downard |
Jim,
That's a tough one. I don't think a definitive answer can be given without alot more information, like transmission type, rear-end type, gear oil you are using in both, wheel bearing preload, etc. Also, the horsepower loss should be related to speed (not linearly either), so a "percentage" isn't real meaningful. Probably better to think of it in terms of 40 hp drivetrain loss at 50mph , or something like that. I think 25 to 30% sounds too high for your setup. I wouldn't think that you have 100 hp or more in parasitic losses (at any speed). I know that your Tremec transmission recommends automatic transmission fluid to help reduce those parasitic losses. |
Jim,
I don't know the actual loss but what I have read and heard the loss seems to be mostly in the 15% to 25% range, so yours does sound high. I am sure some of the more knowledgeable guys will be able to really tell you what to look for and what you should have. I can't say this for absolute certain but if I remember correctly mine was around 18% to 19% several years ago. But I have made changes since then that may have raised it. Ron |
I read once that Ford anticipates 18% loss through their drive-trains. But, as previously stated, that varies.
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The guy on www.427sc.com dyno'd his FFR with a FMS 351 crate engine rated at 385 flywheel hp. When he rear-wheel dyno'd his car, it had 365 at the tires. So he lost 20 hp. That's about 6% loss...he was using a 3550 Tremec and a solid axle 8.8".
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I don't see how losses would be a percentage. The ought to be a quantitative number, irrespective of the HP the motor makes. Rotating the tranny, driveshaft and rear end is a specific amount of work(Mass x Accel x Dist) which is unaffected by whether the motor makes 150 hp or 650 hp. Having an automatic tranny eats up HP because you have to turn the fluid pump. Maybe on a factory subcompact, the average may be 20%, but they have so little power that using up 30hp to turn the transaxle-slushbox is 20%. I think correction factors on dynos are BS, they are dubius lies generated to protect the ego of the car owner that thought he ought to have umpteen million HP.
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..and you thought your rear end was a "drag"...
"The supercharger on a top fueler takes more power to drive then a stock hemi makes. It will achieve over 300 mph before you finish reading this."
I can't figure out why SOMEBODY hasn't posted SOMEWHERE the approximate "numbers" for typical "parts". Like a 9" Ford rear end at "X" rpm will require "X" horse power. The "X" should in fact be a "known" number (within a small percentage + or -). Same for trannys! Whats up with that? You can BET those top fuelers know exactly how much H.P. it takes to turn any given part on their "rail". Ernie |
Excellent point Excaliber! Those figures have to be known by the dragstrip guys and the manufacturers. Do those computerized dragstrip programs take parisitic drag into account? They would have to, I think! If so, then perhaps they have the data in some database within the program.
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It would be easy to determine, just spin the drivetrain parts with an electric motor, measure the amp draw. Spin the same motor at the same speed without being hooked up to the drivetrain parts, record amp draw.
(Difference in amps) x volts = watts used. 746 watts is one HP. This is theoretical, I haven't ever done it. |
Fixit....I don't know how accurate that would be because of the increasing drag factor with rpm. If it was "linear" (1 hp at 1000 rpm, 2 hp at 2000 rpm, etc.) then the math would hold true. I suspect it is not linear, but I could be wrong.
Ernie |
Not a single RPM measurement, but a spin-up from 0-6000. Recording amp draw and RPM increase. You could then graph the amp/rpm and calculate the HP it took to spin the tranny, driveshaft and rear end from 0-6000 rpm, knowing the volts, amps, and seconds it took.
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Oh I got you! I'll just drop by the local Ace Hardware store and pick up one of those 6000 rpm electric motors and.........:JEKYLHYDE
The idea is right on. But how would you actually rig that baby up? Ernie |
I have a friend who works at an electric motor rebuilding house that can dyno their electric motors, up to 1000 HP. (those are big motors) I only play with little electric motors, I have a 45,000 rpm spindle motor sitting on my desk I need to rebuild this morning.
I agre it wouldn't be a standard NEMA housing electric motor that you could buy from Grainger or McMaster-Carr, but it wouldn't be a too difficult proposition to undertake, given a few resources. |
That dyno......
....... they call that dyno the 'bubble buster' here in central Ohio:LOL:
If you estimate 425 at the flywheel then 360 sounds about right. That what, about a 16% reduction. There is a huge range out there when you are looking at it from a percentage standpoint. I wonder if their dyno had the conversion factors built in for tempature, humidity, altitude and barometroc pressure? Rick |
Gears don't just lose power based on RPM and independent of force.
The rate at which power is lost in friction is proportional to the product of the tangentially transmitted force and the velocity of tooth engagement. If EITHER the force (HP) or the velocity (RPM)increases, the product, which is proportional to frictional losses, also increases. I think. Scott |
Re: That dyno......
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:D414 RWHP & 409 RWTQ:D out of my lil' ol' home-built 306. Of course, the belt-driven deer whistle helps too. :cool: :cool: |
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I think the losses would increase, the quicker you attempt to spin-up the drivetrain. But probubly not too greatly. Just theorizing here. |
Way too many variables to put this on paper. I can tell you from a drag racers point of view we have taken a motor off the dyno that made 20HP more than the motor in the car. We put the new motor in the car and it did not run as well on the track as the weaker motor did. I have a set of carbs that are a STUD on the dyno but a DUD on the track.
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I like the idea of measuring it on a bench with an electric motor and graphing the results. Very cool idea Mr. Fixit. You are a natural engineer! Some error from lack of loading on the drivetrain of course, but I don't think the gear contact surface friction (and distortion) is the biggest driver here. I would guess the only BIG variable left out of the bench test is the tire distortion, and that's not a huge effect on Cobras. Tires could lead to big losses through deformation if the tire loading is heavy enough. . . but I can't get much tire deflection on my Cobra with wide tires. . . not enough tire distortion to make a huge loss there.
No I believe the biggest loss is in fluid friction. It can absorb lots of energy trying to turn around all that thick gear oil, hence the switch to lighter oils these days. That kind of loss, like the tire deflection loss, is definitely not linear, but could be measured on the bench. We are getting a pretty involved test though. . . much quicker to just load it on a dyno somewhere and measure it! |
Wade..
.. the owner of the dyno himself told me that name. They biggest number I heard was 440 from the big block chevy powered car so 411 is pretty darn good!
Chris in his small block was just a pull or two away from BEATING the 440 mark. The dyno owners words, not mine nor Chris. At least tying it. No bottle, no blower, SMALL BLOCK. BRB, going to get some popcorn! Rick |
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