Calculating Drivetrain Loss

[Jay95R33]

Just a quick general question.

I've read a few posts saying that drivetrain loss is a %, while other posts say it's fixed (one I read said about 80ps).

It makes sense to me that it should be expressed as a % cause as hp increases so does friction which makes heat.. blah blah blah...

So which is it?

Soooo (yepp, you knew it was coming ), if a stock GTS-t does arooooound 130rwkwish, and factory specs are around 180kw at the flywheel (take some for a worn motor) so that's around a 28% drivetrain loss.

Am I thinking right??

I know the question is a bit like 'how long is a piece of string', but let's see what people think.

J

[surreal]

I don't know about actual calculations to work this out for a specific car.

However, as a rule of thumb, 30% seems to be the number generally allowed for loss in a drivetrain.

[red17]

Percentages are a silly way of calculating it, For instance:

1000hp GTR Skyline would by your example have 300hp of drivetrain loss? Do you know how much heat that your claiming the gearbox is absorbing? The gearbox would melt, as would parts of the car.

So the more i modify my car, the more i lower my gearbox efficency?

Drivetrain losses are generally fairly constant. Sure they would vary according to certain factors like heat, but not by huge amounts. Drivetrain loss for an R32 GTSt ive seen listed as 38hp.

It will vary in every car, so its difficult to determine exactly without pulling the motor out and engine dyno'ing it.

Red17

[surreal]

As a 'rule of thumb' there are not too many 1000hp skylines around.

The 30% general number is one that is easy to work out and possibly should be seen as a minimum number for loss in a drivetrain (because there is always going to some loss) - not as an extreme maximum number that is exponential or without limit ( in obvisiously extreme or high horse power applications).

[rev210]

several hundred HP of drivetrain loss is normal for engines of 1000HP capacity. YES they do melt a standard gearbox. As power increases at the motor the power loss through heat,noise and movement increases also.

It's difficult to work out without having a motor reading and the rearwheel reading. Trying to guess the motor from rearwheel readings is quite pointless, and is just that A GUESS.

[dojobi]

Also, I don't think it's just drivetrain loss you're seeing when you pull 130rwkw from a 184kw car on a dyno. Maybe the dyno itself is inefficient at reading it or there was tyre slip.

[Duncan]

As I understand it, rwkw from a roller style dyno also include dyno loss, not just drivetrain loss.

[rev210]

like i said if you don't know how much water was in the bucket to start with how can you tell how much is missing.

take the engine out, dyno it.

put it back in and dyno it.

the difference is drivetrain loss.

ultimately it doesn't matter. What matters is how fast you go.

Then again if you were ging to use you car as a bore water pump then rwkw might help.

[whatsisname]

Originally posted by Duncan

 

As I understand it, rwkw from a roller style dyno also include dyno loss, not just drivetrain loss.

:werd: Just look at the difference between hub & roller dyno figures.

[SmoothLine]

Ive always been told its around 25% drivetrain loss... Seems about right...

Take for example, stock R33 factory quoted 187kw (correct me if im wrong) and on the dyno these usually make about 140-50 rwkw...

[Jay95R33]

Sooooo.... The question still stands.. Is it expressed as a % or a fixed amount??

Maybe the figure isn't linear, as power increases the % of power loss is less maybe??? The overall amount would be more, but the % would be less.

I'm not posting this question up to try and find out what my ful sik car would have at the motor , it's just a general question about drivetrain loss.

I know the long winded story about whether to trust dynos etc, and all that matters is how your car performs blah blah...

I'm just interested on how the losses are effected as power increases.

J

[SmoothLine]

Its always a percentage...

If it was fixed, it wouldnt make sense...

25% is about right.

[Doctor]

It not a blind guess. I think the research they have done to make these a genuine tuning tool has been a little more educated than you think. Yes I believe they may have to make an educated guess to get it down to the final 5rwkw.

[Xeron]

A 1000kw skyline will loose a fair bit of power in the gearbox and diff, cause ull need a big ****ing gearbox and diff.

generally 25% is a good figure, it makes it easier cause thats pretty close to the conversion rate for kw to hp, 300rwhp gives u aprox 300kw at the crank.

[giz01]

1.I know of a young bloke around here who's 351 made 430hp on engine dyno & then only made 250hp@wheels & this was at the same performance shop as they dynoed it to run it in? He has std Borg Warner diff & FMX auto! I have heard that a 9" is around 30 or 40hp on average for drivetrain loss & turbo 400 & toploader is about the same, where as a powerglide is around 20hp & trimatic is about 22-25hp?

2.On gf'sx std car has 105rwkw's for 147kw's @ engine? & mates auto TT300zx had 150rwkw's std for 206kw's @ engine? so guess?

[GraemeWi]

Although it may easy to think of loss as a fixed percentage, IMHO it is a fallacy to do so. Think through the example further up the thread where 1000hp engines have a supposed 30% loss. My god the gearbox would be a puddle on the ground. I've yet to go to a drag meet and see a gearbox melt (break yes, but melt never) Any engine hp level derived from a chassis dyno can only be treated as pure speculation until the engine itself is put on an engine dyno.

G