Hi all,

I was reading the post on converting a 4WD drive Scooby to a RWD. It was interesting.

If the pros had to guess what would the percentage loss be for a JDM STI-7 or other wrx / stis, flywheel to wheel power ?

So if my car is making 330 bhp x .75 (for 25% loss from flywheel to wheel) = 248 WHP ? Some say it may be less, closer to 20% ? Can you guys post what you think it is and reasons why please.

What is the normal power loss for a FWD or RWD car ?


Thanks,
Ray

 

bttt

 

Ray,

I don't think it's so much a percentage thing, more like a reasonably fixed cost. It also depends on which gear you are running in and and it also differs between the old 5 speed 'box and the new 6 speed one. IIRC, roughly 100bhp is lost on the old cars (or WRXs with the 5 speed) run in 4th gear, and about 80 in the new cars with the 6 speed (STi's) run in 4th.

Unless someone can give us their Engine dyno vs Chassis Dyno figures (and then there is the argument about whether or not the chassis dynos create more losses than on the road due to there being 2 contact patches), I doubt we will know the definitive answer but some of the clever chaps on here might be able to give you a near (calculated) guess.....

Matt

 

Thanks for the reply matt.....so you are saying that its more of a fixed amount and doesn't go up proportionally ? i.e.

Before mods you had 210 whp in 4th gear road dyno delta dash = 290 bhp (new model - 80 hp drivetrain losses)

After mods you have 270 whp in 4th gear road dyno delta dash
= 350 bhp (new model - same 80 hp drivetrain losses)

Is the above roughly what you mean ?

Experts please post opinions and explanations if possible

Cheers,
Ray

 

not really an expert opinion at all, but i was wondering that maybe you could compare RWD and AWD cars with similar power at the wheels to work out whether transmissions losses are really that high.

I mean with, say, a 100bhp loss, a standard impreza will be 115bhp (!!) atw - put this against a similar powered car and look at the differeces, say, in 50-80 times where traction is not an issue? of course you've gotta take things like gearing, CD etc into account, but it'll give a rough idea. maybe?

 

Ray,

Transmission losses (gears, diffs, bearings, tyres etc) are a % of input power, not fixed.

For example when flywheel output is 50hp you would lose around 12hp. If losses were fixed at say 100hp then you would not get very far, iyswim

15% fwd, 17% rwd and around 22% for 4wd seems like a good guide.

 

Yea makes sense I was thinking more long the lines of fixed once you get up to a certain hp...obvioulsy not going to be 100 hp if you only have 50 bhp

Cheers,
Ray