Drivetrain losses (ad nauseam...)

[THE ASH]

wouldn't the %age rise with power/torque? I mean compare wheelslip on the rollers between a 120Y and an LS1 commode - both front engine, rear drive - you can't assume they'd both get the same amount of loss (%age wiese) on the rollers can you?

[Datto-Zed]

Has anyone mentioned that drive train loss is also going to be a function of drivetrain speed? Seemed pretty f'n obvious to me.

[BigMuz]

load has a far greater impact than speed.

Yep- and part b of that is that pulling the tie downs a little tighter will get rid of another 5% of your power....

Muz

[260BC]

How about the difference between using say sticky R tyres and hard elcheapo's? Has that been compared. Also, the more power the more tyre/roller slip.

Putting universially applicable firm figures or percentages on all these variables is simply rubbish, a chassis dyno is a tuning tool, nothing more.

A real life example. The Zed was being dyno'd to tune the new setup on the FJ20ET.
Maximum HP read on the dyno was IIRC ~340, a momentary figure with no real life relevance, realistically the max constant HP was ~230, on a conservatively accurate dyno.

[matlennon]

GTSboy wrote some good stuff on this a while ago..my simple version is that basically most of the drivetrain loss is in the tyre - roller interface...

Basically,the 'drivetrain loss' say of 47 kW for example, is a fuckload of energy- eg.. that's 47000 watts. A light bulb puts out 100, and think about the heat that generates.. then think about pouring 470 times that into a gearbox, and that's just instantaneous, basically you'd just keep on adding lots of heat... we all know that gearbox's etc don't cool down that quickly when you stop the car, so in the end you'd end up heating the gears red hot, and then BANG!

Rowds

but that 100watts is going through a very small metal element. This 47000 watts has bearing material, oil, gears, casing etc etc to soak this heat energy up. Whats more i have absolutely no idea on sound energy and if there are any or how much losses would be in regards to that.

[BigMuz]

How about the difference between using say sticky R tyres and hard elcheapo's? Has that been compared. Also, the more power the more tyre/roller slip..

yep- up to the point that slip becomes an issue, a harder, skinnier tyre will show a fair bit more power than a wide, fat one. A few extra psi in the tyres will do that too.

When we ran a openwheel car on the rollers with hillclimb tyres on, they melted and the power died. We put Formula Ford tyres on and it made more power than it had ever made before - we thought we were pretty clever at the time. But on track it was identical :D

Can't remember the numbers, maybe 150 hp with the slicks, and 160 or 165 with the ff tyres. But that is probably pretty extreme, when the tyre actually melts. Evidence the guys running horsepower heroes who now use huge diameter wheels with very low profile tyres.

Muz

[Uwish]

Driften, thanks for posting!

Well the car in Q is a 05 Rex.
Stock they are said to make 168kw.
Most people who have dynoed their cars on DD dynos
get around 120kw to 125ke ( Average. )

Now say my car made 121kw atw stock thats a 47kw loss.

Now I have re-dynoed my car after mods at 192kw atw.

So I would say it is making roughly 239/240kw at the fly.

I have not changed a box or diffs. So why would I lose anymore power
that the original 47kw?

[Belly_up]

but that 100watts is going through a very small metal element. This 47000 watts has bearing material, oil, gears, casing etc etc to soak this heat energy up. Whats more i have absolutely no idea on sound energy and if there are any or how much losses would be in regards to that.

Read my post where i worked out a rough temp increase for the mass of a gearbox. There can't be much sound energy being emitted, otherwise we'd all go deaf from gearbox noise.

Now say my car made 121kw atw stock thats a 47kw loss.

Now I have re-dynoed my car after mods at 192kw atw.

So I would say it is making roughly 239/240kw at the fly.

I have not changed a box or diffs. So why would I lose anymore power
that the original 47kw?

the important point is that initially you lost 47/168 = 28%. it's not the figure thats important, it's the percentage. Frictional losses are proportional to applied force. Put more force through the gearbox, lose more to friction. The percentage should stay the same though (within reason).

So whats more likely is that 192kw atw you have now represents 72% of your current flywheel figure, which should be about 192/72*100 = 266kw.

This does assume that all of that power is being lost in the drivetrain, i don't know how the curve for roller losses vs power would look, it's still frictional but there's slip and other stuff involved, and the other posts above seem to indicate that thats where lots of the losses are.

[IMOA]

One way to put it would be if a standard S15 200sx runs up on the rollers it tends to make about the 110-115rwkw losing ~ 35kw or 24% from the factory claimed figure

Put a 300kw HSV GTO on the dyno and it tends to make about 225rwkw losing 75kw or 25%.

Modify the 200sx so that it also puts out 225rwkw on the dyno and keeping in mind that it has the same basic driveline setup as the GTO is it likely to be making ~300kw or 255 - 260kw at the engine. And if it's the latter wtf is wrong with the GTO's driveline/tyres that it loses twice the power as the 200sx driveline.

fwiw, that same 200sx will make around 135rwkw when using a hub dyno so the difference between this and what it makes on the dyno dynamics (ie, most of the losses) can certainly be attributed to the tyre/rollar thing and have nothing to do with the driveline whatsoever

[Belly_up]

from those figure then IMOA the sx is loosing about 10% in the drivetrain correct?

[The Pupat]

It will be a percent up until tyre slip starts becoming a large percentage.

Gears used in cars have an inherent loss since they are helical (angle) cut gears. This is why you have thrust bearings in a gearbox. If it were Spur (straight) cut gears you would not need thrust bearings. These thrust bearings and helical cut gears pull power related to a percent of the force they are transfering. You also have the drag of the gears through the gearbox/diff oil. You then also have the clutch interface and the tyre interface.

Though 30 percent might be a bit high I can't see how an average car wouldn't lose 20+ percent of power through the drivetrain.

[IMOA]

Something in that ballpark, that would be what is converted to heat and noise in the gearbox diff which going by the calculations in this thread seems pretty reasonable

[Jim]

are we talking about power change through the transmission, or actual drivetrain efficiency? They are two different things.

Drivetrain loss is the amount of power absorbed by the transmission in order to rotate it. This is, as mentioned, uaually about 15%ish on late model cars, and is all to do with friction in the bearings and between the gears, the changes in direction, and the gears being coated in oil etc etc.

Power loss is due to passing the power through a ratio change, you have a decrease of power and an increase of torque. These go hand in hand and the total energy expended is exactly the same (disregarding frictions in the drivetrain).

[The Pupat]

Ummm you can't lose power through a "theoretical gear ratio interface secoh . The whole torque x Speed = Power bit

[Jim]

hey you, stay out of my happy place :p