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Old 02-27-2003, 08:29 PM   #1 (permalink)
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Flywheel vs Rear Wheel Horsepower

Has anyone actually done a dyno run on just the motor, then compared it with a run with the motor installed to compare rear wheel with flywheel? I've read a few different things. I've read the typical 15% loss, then I've read that you lose around 35 due to the drivetrain, period. I like the 15% number, but it does make logical sense that a higher horsepower engine should not lose more horsepower than a stocker. Just curious. Thanks!
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Old 02-28-2003, 12:55 AM   #2 (permalink)
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Flywheel vs Rear Wheel Horsepower

That's a good point. The actual HP that is required to operate the drivetrain should remain fairly constant with rpms. So the "percentage" of loss should be a little lower as the crank HP is increased. 20% loss is average for a stick shift and 30% is average for a low powered stock auto. As the engine combo gets more powerful the % of drivetrain loss will be less. But don't take my word for it, put it on a chassis dyno and see for yourself.

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<font size=-1>[ This Message was edited by: bluestreek on 4/6/03 12:08pm ]</font>
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Old 02-28-2003, 01:22 AM   #3 (permalink)
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Flywheel vs Rear Wheel Horsepower

20% to 30% is a little large. The Nova I'm working on now has a basically stock powerglide. It made about 100 less on a chassis dyno than on an engine dyno. I would figure about 8 to 10% on a stick &amp; about 10-15% on an auto.
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Old 02-28-2003, 09:54 AM   #4 (permalink)
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Flywheel vs Rear Wheel Horsepower

Are you sure? I've seen stock 5.0 Mustangs dyno at around 170-180 HP at the rear wheels. That seems like more than a 10-15% loss. Like I said in my earlier post, the percentage number should go down as the engine HP increases so your numbers may be correct for higher powered combos.
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Old 02-28-2003, 01:24 PM   #5 (permalink)
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Flywheel vs Rear Wheel Horsepower

It's hard to say. The last place we went to was a little weird. He put a strip on the damper for an accurate tach reading. His reading was 5500 max rmp/hp but the tach read 7800. I've since re-wired the car &amp; went to a different shop &amp; got something all together different. Basically in my opinion, dyno numbers are just a small part of the equation, it's still only going to do what it does. Also an engine dyno uses the shops fuel, ignition, &amp; exhaust systems so those numbers are just a ball park number for tuning. I just do mine the old fashoned way, race it like I stole it
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Old 02-28-2003, 01:35 PM   #6 (permalink)
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Flywheel vs Rear Wheel Horsepower

I guess technically all that matters is rear-wheel, because that's what sees the road. Just curious. Thanks guys.
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Old 03-06-2003, 01:13 PM   #7 (permalink)
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Flywheel vs Rear Wheel Horsepower

A constant percentage should be what happens. That percentage is determined by every moving interface that the power is transmitted through. For example, if the bevel gears in your diff are 98% effecient than no matter if you send 100 or 1000 hp through them you're only getting 98% out(98hp or 980hp in this case). What you do is multiply all the efficiencies together to get the overall net.

Trans input shaft -> output shaft (could vary depending on gear selected)

output shaft -> driveshaft (universal joint eff)

driveshaft -> diff input (universal joint eff)

diff input -> axles (bevel gear eff)

And then there are all the losses associated with seals, bearings and friction, which are also proportional. I think the bulk of it is all the gear to gear interfaces. So just using complete guesses for numbers this might be 95x99x99x95x99 etc and you get (in this made up example) 87% eff, a 13% loss. There also might be some items which are more static than proportional also.

That's what the textbooks say at least, who knows how close real life is to this.

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Old 03-06-2003, 06:00 PM   #8 (permalink)
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Flywheel vs Rear Wheel Horsepower

Really interesting. I hadn't thought about it that way, but it makes sense from a scientific standpoint.

Thanks alot for your input.
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Old 03-23-2003, 08:18 PM   #9 (permalink)
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Flywheel vs Rear Wheel Horsepower

If you say percentage then understand it only applies to *that* exact combination.
The parasitic losses are constant for a given combination.

Thus -- you make more FWHP you get more RWHP. End of story.

I have engine dyno'd and rw dyno'd multiple combinations. The numbers are not percent but fixed.
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Old 04-01-2003, 06:21 PM   #10 (permalink)
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Flywheel vs Rear Wheel Horsepower

Thats the Joey!!!!
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Old 04-03-2003, 03:45 PM   #11 (permalink)
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Flywheel vs Rear Wheel Horsepower

Hmmm...is it so that it takes a certain/same amount of power to rotate moving parts and to overcome friction? I mean it takes a certain amount of power to rotate, say a 9" rearend at 6000 rpm.But, the needed power is only certain/same without a load.When the load increases so does the friction.And the load increases with power.The harder the car accelerates the more friction is created in the rearend and thus more power is wasted. Without the load the rearend does not care whether the power comes from a 100 hp 4 cylinder or a 400 hp V8. If it takes 40 horses, that is 40 % from the 4 cyl output( 60 hp still available from rear wheels) but only 10 % from the V8 output( still 360 from rear wheels).With a load more power means more friction/heat buildup between the gears and in bearings more power loss/ heat build up.
The simple(?) conclusion is that power loss through the drivetrain is a combination of stationary loss of the rearend / trans + proportional to the power the input shaft of the trans rearend will see.
This is my humble theory...does it match with real world experiences from dynos?
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Old 04-03-2003, 04:04 PM   #12 (permalink)
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Flywheel vs Rear Wheel Horsepower

..perhaps my theory will explain the different values quoted here: anything from 8% to 30+ % .
Cannot imagine it would take 300 hp to drive a C6 behind a 600 stroker ( 1000 flywheel hp and 700 rearwheel).That would be 30 % loss.
But, if it was physically possible to bolt a C6 behind a 100 hp 4 cylinder, it would be easy to imagine it would take 30 hp ( =30 %) to drive the C6.In that case 30% loss would sound reasonable.
Reverse that example : Lets imagine it would be possible to run the gearbox from the 100 hp 4 cylinder behind the 1000 hp bigblock.
If that case the loss would be very small, perhaps something like 1-2 %.
...and if you bolted that 4 cylinder to a Sherman tank the drivetrain loss would be 100 % since that motor could not move the tank at all
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Old 04-06-2003, 01:05 AM   #13 (permalink)
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Flywheel vs Rear Wheel Horsepower

Good examples!!
Makes much better sense now.
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Old 04-09-2003, 06:12 PM   #14 (permalink)
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Flywheel vs Rear Wheel Horsepower

Quote:
On 2003-04-03 14:45, ville wrote:
Hmmm...is it so that it takes a certain/same amount of power to rotate moving parts and to overcome friction? I mean it takes a certain amount of power to rotate, say a 9" rearend at 6000 rpm.But, the needed power is only certain/same without a load.When the load increases so does the friction.And the load increases with power.The harder the car accelerates the more friction is created in the rearend and thus more power is wasted. Without the load the rearend does not care whether the power comes from a 100 hp 4 cylinder or a 400 hp V8. If it takes 40 horses, that is 40 % from the 4 cyl output( 60 hp still available from rear wheels) but only 10 % from the V8 output( still 360 from rear wheels).With a load more power means more friction/heat buildup between the gears and in bearings more power loss/ heat build up.
The simple(?) conclusion is that power loss through the drivetrain is a combination of stationary loss of the rearend / trans + proportional to the power the input shaft of the trans rearend will see.
This is my humble theory...does it match with real world experiences from dynos?
Sounds logical to me. Very well thought out. I guess unless someone has actually done a before/after comparison, it is pretty much a best-guess...
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Old 04-14-2003, 01:58 PM   #15 (permalink)
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Flywheel vs Rear Wheel Horsepower

Some parts are constants (like seals, ring gear turning in oil etc.) and most are proportional to the load (gears etc).
If you where to have car with 500 flywheel/400 rear wheel hp, then it wouldn't be possible for a 100 hp 2 liter engine to move it an inch if it were a constant. A 302 from the seventies would have only 30 hp left at the wheels.... That is of course not the case.
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