Rwkw vs Fwkw

[crackelaktor]

Interesting when you buy a car and in the case of performance cars like V8's, the power numbers always feature in the conversation. I bought a 325kw GTS and whilst dynos are tuning tools it is hard to understand how and why so much power is lost when looking at the power at the treads. I was told as a rough guide that my GTS would lose approx 28-30 percent of its power through the drivetrain.

Never been a fan of the Falcons but they seem to make more at the wheels out of the factory than the Holden's do.

So the motive behind the question is when you mod your car and make an extra 30-40rwkw, do you simply add this number to the 325kw figure (I don't think so) or is there another way to roughly calc the Fwkw numbers. Cheers.

[feistl]

Not sure exactly what your asking...

Factory cars are usually measured in flywheel power which is the amount the engine produces.

When cars are modified (or enthusiasts are talking about them) then you usually use the rwkw rating as this gives a better indication of actual performance.

However this method is still flawed as you cant compare RWKW to AWKW. This is because AWKW takes into account the rolling resistance of the front wheels, whereas rwkw doesn't.

As for the ford, well ford is actually under quoting the power output significantly. I expect the flywheel power output to be much higher than the claimed 325kw.

[lwboosted]

As a very rough guide I usually go by rwhp = fwkw.

[DukeZ]

also fpv and hsv use different dynometers to measure there power outputs and i be if they were to measure off each others dyno the power outputs would once again be significantly different.....

[Timson]

Have a play with the converters on this site.
http://locost7.info/converter.php
My GTS (307 factory) made 225rwkw with no mods, so when converted it came out to 301rwhp which is close to the factory 307fwkw claim. After mods it made 265rwkw. When converted it comes to 355rwhp. So as a rough guide I can say that my GTS has 355fwkw. So with my mods I can say that they made a 54fwkw gain.
Hope this helps.
Cheers, Tim.

[crackelaktor]

Have a play with the converters on this site.
http://locost7.info/converter.php
My GTS (307 factory) made 225rwkw with no mods, so when converted it came out to 301rwhp which is close to the factory 307fwkw claim. After mods it made 265rwkw. When converted it comes to 355rwhp. So as a rough guide I can say that my GTS has 355fwkw. So with my mods I can say that they made a 54fwkw gain.
Hope this helps.
Cheers, Tim.

Thanks Tim. Not quite what I meant. I know kw x 1.34 gives you HP but my post was more about when you make an extra 30rwkw on the dyno after mods, approximately how that translates to engine kw....I know it isn't an exact science unless you rip out the engine and place it on an engine dyno but was interested in whether it was a formula such as rwkw x 1.30 (representing 30% loss through drivetrain) or something different. Thanks.

[Steve-LS2]

Thanks Tim. Not quite what I meant. I know kw x 1.34 gives you HP but my post was more about when you make an extra 30rwkw on the dyno after mods, approximately how that translates to engine kw....I know it isn't an exact science unless you rip out the engine and place it on an engine dyno but was interested in whether it was a formula such as rwkw x 1.30 (representing 30% loss through drivetrain) or something different. Thanks.

All of the losses you experience through the driveline differ from car to car.

Vehicles with a T400 and a 9 inch will lose more power through the driveline than other vehicles with less heavy duty parts.

So there really is no formula.

Measure the RWKW after you have done mods on a dyno and forget the FWKW

Or just take it to the strip and use the Moroso slide rule to gauge power.

[QldKev]

My understanding it you still need to factor in the 27-30% loss even at the increased KW. The more toque you throw at it the more it will consume. Thats why we still use the same 27-30% rule for a V6 and a V8 even thou their power output is different.

So using the 30% your
325fwkw = 227rwkw
if you now have 257rwkw, you need to factor the 30% back in to the losses
365fwkw = 257rwkw

This is to the best of my knowledge.

[Stewge]

Thanks Tim. Not quite what I meant. I know kw x 1.34 gives you HP but my post was more about when you make an extra 30rwkw on the dyno after mods, approximately how that translates to engine kw....I know it isn't an exact science unless you rip out the engine and place it on an engine dyno but was interested in whether it was a formula such as rwkw x 1.30 (representing 30% loss through drivetrain) or something different. Thanks.

I think the point was that your rwhp is roughly the same as fwkw. So if you have a rwkw figure, convert to to HP and that's a rough number.

e.g. 300rwkw = 402rwhp > 402fwkw

I'd say it mostly comes down to the fact that the kw->hp (1.36) conversion is roughly the same figure you'd lose in the average drive train rather than actually being designed for that calculation.

[crackelaktor]

I think the point was that your rwhp is roughly the same as fwkw. So if you have a rwkw figure, convert to to HP and that's a rough number.

e.g. 300rwkw = 402rwhp > 402fwkw

I'd say it mostly comes down to the fact that the kw->hp (1.36) conversion is roughly the same figure you'd lose in the average drive train rather than actually being designed for that calculation.

Fair enough. Fine as a rough guide and not that important as the delta is what counts.

[Tiger Blood]

My GTS (307 factory) made 225rwkw with no mods, so when converted it came out to 301rwhp which is close to the factory 307fwkw claim. After mods it made 265rwkw. When converted it comes to 355rwhp. So as a rough guide I can say that my GTS has 355fwkw. So with my mods I can say that they made a 54fwkw gain.
Hope this helps.
Cheers, Tim.

The loss is pretty big that's 26.8% loss right there.
Only thing is the percentage loss is a sliding scale the percentage you loss at this power level will be different with the same exact drivetrain and more HP. Its not a case of 26.8% loss at 225rwkw and 250rwkw the percentage changes as more HP and torque find it easier to turn your drivetrain.

But all of this doesn't really matter all that much anyway its always just a ball park figure.

[Wonky]

From things I've read on here the accepted norm seems to be about 23% for manuals, slightly more for autos. If we assume the driveline loss for an auto to be 25% that fits perfectly with the rough rule of thumb that rwhp = fwkw.

Why you may ask? Without getting too mathematical the conversion from hp to kw also involves subtracting 25% (or multiplying by 3/4 - same thing), so rwkw = 75% of rwhp and if we work on 25% drivetrain loss then rwkw = 75% of fwkw.

Equating the two, we therefore get 75% of rwhp = 75% of fwkw

i.e. fwkw = rwhp

[crackelaktor]

All of the losses you experience through the driveline differ from car to car.

Vehicles with a T400 and a 9 inch will lose more power through the driveline than other vehicles with less heavy duty parts.

So there really is no formula.

Measure the RWKW after you have done mods on a dyno and forget the FWKW

Or just take it to the strip and use the Moroso slide rule to gauge power.

Good point. Does anyone know if the drivetrain components were upgraded in the E2/E3 HSV's compared to the previous models?

[GTS PAUL]

The reason Holden and Ford show different power at the rear wheels is that Ford power is measured with all accessories fitted, like alternator and power steer and a/c.
Holden figures are engine only and not driving any accesories, so a 300 kw Holden shows less rear kilowatts than a 300 kw Ford.

The guide that is rear wheel HP is equal to engine KW is pretty well on the money. Especially for Holdens. 325 KW GTS have averaged around 325 rear wheel HP for us. Some a little more, some a little less but all within 5-6 HP.

Cheers Paul

[feistl]

The loss is pretty big that's 26.8% loss right there.
Only thing is the percentage loss is a sliding scale the percentage you loss at this power level will be different with the same exact drivetrain and more HP. Its not a case of 26.8% loss at 225rwkw and 250rwkw the percentage changes as more HP and torque find it easier to turn your drivetrain.


Actually wrong, the percentage will stay almost the same. The more power you have, the faster your spinning the drive line the more resistance there is.

So while the driveline might sap say 100kw of power in a 300kw engine, if you double the engine power to 600kw the driveline will sap 200kw.

Reason being is it takes an amount of energy to set an object in motion. The faster you try to accelerate it, the more energy is required. More powerful engine = faster acceleration = more energy required to spin up the driveline.

Hence why putting stupid big wheels on a car really hurts acceleration.

[IJ.]

Also need to factor in gear seperation/case flex as you increase power will add to the parasitic drag, too many variables to put a % number on it.

[Steve-LS2]

The reason Holden and Ford show different power at the rear wheels is that Ford power is measured with all accessories fitted, like alternator and power steer and a/c.
Holden figures are engine only and not driving any accesories, so a 300 kw Holden shows less rear kilowatts than a 300 kw Ford.

The guide that is rear wheel HP is equal to engine KW is pretty well on the money. Especially for Holdens. 325 KW GTS have averaged around 325 rear wheel HP for us. Some a little more, some a little less but all within 5-6 HP.

Cheers Paul

It's actually the other way around. Holden under GM rules measure with all accessories. Ford measure without things such as power steering and aircon.

[YMK]

It's actually the other way around. Holden under GM rules measure with all accessories. Ford measure without things such as power steering and aircon.

holden and ford both measure power with all accessories (ece)

GTS Paul, the FPV's show higher rwkw because in that gear (4th) they're not restricted to "335kW" at the fly like they are in the first few gears (this is to protect the driveline).

It also explains the anomaly of 114mph with 330rwkw for that weight.

[Steve-LS2]

holden and ford both measure power with all accessories (ece)

GTS Paul, the FPV's show higher rwkw because in that gear (4th) they're not restricted to "335kW" at the fly like they are in the first few gears (this is to protect the driveline).

It also explains the anomaly of 114mph with 330rwkw for that weight.

Well ok, when they had the 5.4 Ford used to use the SAE test with no accesories.

[YMK]

Well ok, when they had the 5.4 Ford used to use the SAE test with no accesories.

It wouldn't make sense. If they quote their figures without accessories then by right the rwkw would be lower than HSV's, not higher.

[Tiger Blood]

Actually wrong, the percentage will stay almost the same. The more power you have, the faster your spinning the drive line the more resistance there is.

So while the driveline might sap say 100kw of power in a 300kw engine, if you double the engine power to 600kw the driveline will sap 200kw.

Reason being is it takes an amount of energy to set an object in motion. The faster you try to accelerate it, the more energy is required. More powerful engine = faster acceleration = more energy required to spin up the driveline.

Hence why putting stupid big wheels on a car really hurts acceleration.

so what if you had a drive train loss of 100kw and then for shits and giggles swapped the engine out for a 100fwkw engine? then it will make 0kw at the wheels....it doesn't work like this.

[YMK]

so what if you had a drive train loss of 100kw and then for shits and giggles swapped the engine out for a 100fwkw engine? then it will make 0kw at the wheels....it doesn't work like this.he's not saying that at all.
going off what he's saying, and to use your example, a 100kw motor would show a LOWER PERCENTAGE LOSS of rwkw than it would if it was a 300kw motor.
for example,
100kw = 80rwkw (20% loss)
300kw = 225rwkw (25% loss)

[SV300]

Have a play with the converters on this site.
http://locost7.info/converter.php
My GTS (307 factory) made 225rwkw with no mods, so when converted it came out to 301rwhp which is close to the factory 307fwkw claim. After mods it made 265rwkw. When converted it comes to 355rwhp. So as a rough guide I can say that my GTS has 355fwkw. So with my mods I can say that they made a 54fwkw gain.
Hope this helps.
Cheers, Tim.

My GTS is as above, but I never had a before figure..

Yes rwhp is basically engine kw.

Oh auto's will read about 10-15kw lower at the wheels for the same manual due to loses described already.

Good info..!




Sent from my iPhone using Tapatalk

[Tiger Blood]

he's not saying that at all.
going off what he's saying, and to use your example, a 100kw motor would show a LOWER PERCENTAGE LOSS of rwkw than it would if it was a 300kw motor.
for example,
100kw = 80rwkw (20% loss)
300kw = 225rwkw (25% loss)

Ok so lets just be stupid and ignore what i said previously about percentages changing based on power output and torque and blow these figures right out of the water.

10,000fwkw @ 25% loss is a loss of 2500fwkw does this thread and question still look pretty stupid now.

There is no way in hell a 10,000fwkw engine is going to have a loss of 2500fwkw by processing that energy through what 120kg of rotational parts that build momentum.....ever hopped of the gas at 110km/h put it into neutral and coasted.....momentum.

The answer is to the OP that the percentage can vary dependent on HP and Torque. The more HP and more specifically torque that you generate the more ease the engine will have to rotate the driveline of the vehicle.

Not that any of this really ever matters as the car should always be tuned in real world environments and never tuned just purely to produce a top end HP figure.

[1BEAST2NV]

http://www.messengerfreak.com/emoticons/funny/HomerInsane.gif

another measuring thread :slap:

[Evman]

FFS. Power loss is a variable. Get Googling and get reading, it's simple physics.

If power loss was fixed and we lost 100hp in our driveline, fitting anything up to a 100hp motor would not be able to rotate it. We know this isn't the case because we can turn it over by hand.

Try this link (http://www.modified.com/tech/modp-1005-drivetrain-power-loss/viewall.html) for the non-believers. It was on the first page after searching "horsepower loss through driveline".

[Tiger Blood]

FFS. Power loss is a variable. Get Googling and get reading, it's simple physics.

If power loss was fixed and we lost 100hp in our driveline, fitting anything up to a 100hp motor would not be able to rotate it. We know this isn't the case because we can turn it over by hand.

Try this for the non-believers. It was on the first page after searching "horsepower loss through driveline".

exactly what i have been saying thank you Evman.

[Martin_D]

Take it out and put it on an engine dyno then you know.....but what does it actually tell you....just something else to laminate and put in the glovebox :bow:

[spiv]

Take it out and put it on an engine dyno then you know.....but what does it actually tell you....just something else to laminate and put in the glovebox :bow:

Sam actually did this for one of my engines years ago, 455 FWHP, 447 Ft LB of torque on his dyno log engine dyno and 367 RWHP on his dyno log chassis dyno, so there you go so approx 19%.....All through a T56 manual and restrictive 2 1/2" exhaust with triple flow mufflers and not straight through ones. This should settle a few arguments:hmmm:

[Martin_D]

Sam was the guy I was going to quote on this. He always reckoned 17%-19% on his dynos :)

[feistl]

so what if you had a drive train loss of 100kw and then for shits and giggles swapped the engine out for a 100fwkw engine? then it will make 0kw at the wheels....it doesn't work like this.

As mentioned, thats not what i said. What i said was the more power you have the faster your try to accelerate the driveline the more energy it takes to spin and the more power is lost.

So a 100kw engine might lose 25kw through the driveline, but a 200kw engine in the same car would lose 50kw.


10,000fwkw @ 25% loss is a loss of 2500fwkw does this thread and question still look pretty stupid now.

Assuming the driveline could handle it and you had traction, that would be fairly close. The problem is with 10,000fwkw your going to try and accelerate so fast that the resistance to that sort of acceleration is going to take 2500fwkw to overcome.

Think of it like this... If you try to spin up a bike tyre the faster you try to accelerate the harder it is. Same principal.

[Martin_D]

does this thread and question still look pretty stupid now.

Not just now...from the beginning :)

[HYMEY]

some chassis dynos rwkw is fwkw seen that before. some have a big loss.