How Much Is 250rwkw In Engine Kw?

[Hardman69]

Tyre pressure makes a BIG difference. Run 300kpa. Most folk in the know seem to reckon on 18-23% total loss for RWD cars. FWD lose around 15% - The Crownwheel & pinion being a BIG source of loss in RWD cars. and don't get me started on how much AWD's lose! Coastdown testing is big in the U.K, to calculate out drivetrain losses - it doesn't seem to have the same sort of following here, maybe 'coz the Dyno is seen as more of a tuning tool (and everyone's used to RearWheel figures here)

[DivHunter]

If a car makes 250RWKW legitimately at the back wheels, you're looking at probably 251RWKW at the engine.

Drivetrains don't soak up much power at all.

Having the aircon on is a 1kw loss.

[SilverECR33]

Having the aircon on is a 1kw loss.

I thought it was more like 10kw. Sure feels like 10 at least.

[Cubes]

Depends on the aircon gas your running...

I noticed a very noticable drop in performance when running R134A, the aircon compressor could also be heard inside the car when reving to 3500rpm+.

Had it charged up with R12A (not to be confused with R12) and I can't actually notice the power drop. Aircon compressor is now quiet at high rev's and its also much colder especially idling in traffic.

R12A FTW.

Our Skylines are not designed to handle the head pressures of R134A.

[2rismo]

All these percentages are nonsense. Did I imagine my previous post? Go and read it.

[Cubes]

All these percentages are nonsense. Did I imagine my previous post? Go and read it.

So what is it then? You discount the percentage yet give no indication how to determine 'roughly' what the fwhp/kw would be.

100% up it all the time. One reason why GTR's back in bathurst ran gearbox and diff coolers.

[DivHunter]

my 1rwkw is not from a skyline but was on a dyno back-to-back. I was just illustrating how rediculous the suggested 1rwkw drivetrain loss was.

[2rismo]

This has been done to death on this and other forums a million times. A search would most likely unearth my previous thoughts at greater length but put simply, any given combination of drive train components (from the flywheel/flex plate all the way back to the tyres on the road) will convert some of the kinetic energy from the crankshaft into other forms of energy (heat, sound etc) as it makes it's way to the driven wheels.

Here's the key bit so pay attention...

If my flywheel, clutch, gearbox, tailshaft uni's, diff, axles and tyres use 60 of my 200 engine horsepower then i will see approximately 140hp at the driven wheels. This is a 30% loss.

If I up the crankshaft output to 1000hp but change nothing else I will see approx 940hp at the driven wheels. This is a 6% loss.

To say that my unchanged drive train somehow magically absorbs 5 times the energy it did at 200 engine horsepower is ludicrous and surely must make plain sense to anyone who bothers to think about it.

Therefore, to talk in terms of percentages is entirely flawed in every conceivable way when comparing cars.

[Cubes]

BUT....... For the power levels we are talking its pretty much around the mark, which really is all that matters.

As I mentioned in an earlier post.

I believe its a fixed value + a percentage as with increased load = increased friction through bearings, gears etc.

What those values are I have NFI as no dyno is 100% accurate nor is an engine dyno. Its a rough figure.

Talking about accurate exactly what is accurate.. Ford or holdens method of measuring flywheel horsepower figures.

What standard does nissan use to determine flywheel horsepower figures?

[bigcarl]

i think sydney kid has posted on this topic before . he seems to think drive train loss is around a fixed 65 kws?

[2rismo]

As you increase power dramatically, you tend to have to use better drive line parts, better bearings, lower drag gear sets etc so the point that there is an increasing loss associated with higher power is moot. I would say that the supporting mods req'd for a 1000hp car as used in my example would mean a decreasing loss through the drive train. Either way, it is misleading to suggest that a percentage loss is accurate in any way which is why I strongly object to it when it is given as “advice” to someone who hasn’t thought about it enough.

[2rismo]

i think sydney kid has posted on this topic before . he seems to think drive train loss is around a fixed 65 kws?

For a 5-spd R33 GTST I tend to agree.

[ryan_o_grady]

Im with cubes here. I can't see how physically a drivetrain can absorb the same amount of power when fed different input load.

THe more load the drivetrain is put under, the less efficiently it will work hence in a 1000kw engine car, it may lose 150kw compared to the 50-60kw we agree is lost through stock engine power. There is no possible physical way that the drivetrain can absorb the same amount of power when fed different power levels. I can definately see your point though 2rismo and you make a valid point about the transfer of energy however the more load being inputted will have to result in an increased loss of energy.

[2rismo]

An increased loss of energy yes, but through the drive train...no. Certainly not to the extent you suggest which is more than double the accepted (between us) value of about 60kW. If a drive train is worked harder it could get ever so slightly hotter or ever so slightly noisier but nearly three times as hot or three times as loud?

I fail to see which component you're suggesting will be physically able to dissipate this extra energy.

[MBS206]

If you're loosing power in your drivetrain, where is going?

Energy MUST be conserved.

So, where does it?

Sound?

Heat?

Is your drivetrain that noisy? I know mine isn't

Do you boil your gearbox oil within a minute or so? I sure as hell don't.

So then, where'd the energy go? To the back wheels.

There's a nice little article out there, that you don't actually loose the power in your drivetrain, the issue is known as SLIP. Which occurs between the rollers and your tyre. And then the energy that is also required to spin up the fans on certain types of dynos.

The energy isn't lost in your driveline, it's lost at the actual dyno.

Just remember, conservation of energy, you don't just "loose" power, it turns into an "unuseable" energy, so sound or heat in our case, problem, the drivetrain ain't that noisy, nor do you boil your gearbox oil. So therefore, it isn't disappearing, we just can't physically measure it.

[DivHunter]

If you're loosing power in your drivetrain, where is going?

Energy MUST be conserved.

So, where does it?

Sound?

Heat?

Is your drivetrain that noisy? I know mine isn't

Do you boil your gearbox oil within a minute or so? I sure as hell don't.

So then, where'd the energy go? To the back wheels.

There's a nice little article out there, that you don't actually loose the power in your drivetrain, the issue is known as SLIP. Which occurs between the rollers and your tyre. And then the energy that is also required to spin up the fans on certain types of dynos.

The energy isn't lost in your driveline, it's lost at the actual dyno.

Just remember, conservation of energy, you don't just "loose" power, it turns into an "unuseable" energy, so sound or heat in our case, problem, the drivetrain ain't that noisy, nor do you boil your gearbox oil. So therefore, it isn't disappearing, we just can't physically measure it.

Hub dynos experience losses too. I'm pretty sure your gearbox doesn't stay at ambient temperature all the time either. Did you check how much energy is required to heat your transmition/dif fluid? How quickly it can be dissipated? Did you calulate the moment of inertia and angular acceleration of drivetrain moving parts? The associated drag?

Your 1rwkw post was stupid at best. Losses can be measured just no one cares or has enough money to bother.

[MBS206]

Hub dynos experience losses too. I'm pretty sure your gearbox doesn't stay at ambient temperature all the time either. Did you check how much energy is required to heat your transmition/dif fluid? How quickly it can be dissipated? Did you calulate the moment of inertia and angular acceleration of drivetrain moving parts? The associated drag?

Your 1rwkw post was stupid at best. Losses can be measured just no one cares or has enough money to bother.

The 1RWKW is an exxageration.

And read up on how much energy it takes to raise 1ml of oil 1degree celsius.

In the region of around 2.6J from memory (Or that might be per 100mls, can't remember, haven't played with fluids in a while)

And think how much J is in your 60kw.

And how does it dissipate this heat? Through the gearbox? I've never felt a gearbox get THAT hot.

Metal doesn't dissipate heat as quickly as people think it does.

I'll attempt to dig out the article that I was reading on slip issues with roller dynos. Unfortunately, I don't think I'll have much luck, as I can't even remember the people who did it.

And the hub dyno, you will notice still looses energy, but not as much. Why? Well the factor that there isn't slip. But there are still the fans being turnt etc inside of the dyno.

[sinistaGTR]

depends how many phat exedy, orc or extreem clutch stickers you have. do you know you can actually gain power the more of these stickers you have......bout the same as 1rwkw statement.....

more seriously agree with 2rismo here...

[Cubes]

I still stand by a fixed drivetrain loss + a percentage.

Its obvious there's not a fixed loss.

Street cars can with no problems cruise around and not require diff and manual trans coolers.

Then look at the old Bathurst R32 GTR and you see diff and manual trans coolers hanging off them.

Increased power = increased friction within the driveline and motor its self; all of of which requires coolers to keep them reliable.

IF there is no added friction at WOT we would require trans and diff coolers when simply cruising around the streets. Simply not the case. WHICH suggests there is indeed a percentage value some where in the equation.

On the dyno; cruise with partial throttle. There's absolutely no way the driveline at this point is sucking 65kw.

There must be a fixed loss + a percentage.

Its IMPOSSIBLE for there to be a simple fixed value loss for all power levels. Its silly to think so.

I make reference to a car at cruise as it is comparable to one making 200rwkw and another 500rwkw. The 500rwkw may be at cruise when making 200rwkw.

Hope that makes sense.

[govich]

And think how much J is in your 60kw.

LOL. Chuckled at this for some reason.

60kW = 60kJ/s.

I think you'll find a decent estimate of the specific heat of oils will be around 2 kJ/kgK. Meaning 2kJ of energy is required increase the temperature of 1kg of our theoretical oil up one degree (K/C). So at full load, assuming 60kW of energy is transferred to the oil as heat, and that we have 1kg of oil, there is potential for raising its temperature 30 degrees every second. Simple analysis suggests we need somewhere else to put all this energy..

Anyway, a dyno will read lower because of inefficiency everywhere. 250rwkW in a R33 GTS-T will correspond to 300-320 kW at the flywheel. /arguing

Edited May 21, 2007 by govich