Calculating flywheel power from rear wheel power (adjusting for drivetrain loss)

TURBOTAXI · 01-10-2006, 11:10 AM

Hi Folks,

As the title says, does anyone have any tips for deducing an approximate value for flywheel power from rear wheel power?
My car has made 162rwkw on a dyno. If I allow for the automatic trans, the differential and any other sources of friction, drag, heat etc..... What would an approximate value be for flywheel kilowatts?

XR Martin · 01-10-2006, 11:32 AM

About 210-220kw. All dynos are different and so are the operators so theres never an accurate figure that can be agreed apon.
But a generally accepted figure is to multiply the rwkw by 1.3 or so to get you flywheel power.

A better guide is actually a 1/4mile, in particular the mph.

ratter · 01-10-2006, 11:44 AM

I have been told by a guy that owns a engine dyno as well as a chassis dyno, the difference on a late model car is about 22%

macca_779 · 01-10-2006, 09:11 PM

it tends to work out that rear wheel HP equars Flywheel KW. Not gospel but a good guide.

Steffo · 01-10-2006, 11:03 PM

It isn't possible to calculate that accurately. Or anything approaching accurate.

The most general guide would be 18-20% loss manual, 20-22% loss auto, RWD.

However, DynoDynamics Dynamometers (and in general, dynos & operators in this country) don't follow this, as they have silly software that adjusts the readings to what it believes (or is programmed to believe) is correct. That's a pretty poor system, there should never be any correction factor, especially attempting to "adjust" for variables like humidity, air temp, traction.. yadda yadda yadda.

PS- I'm well aware people will attempt to flame me for the above post, and I'm well prepared for it.

Steffo · 01-10-2006, 11:11 PM

Quote:

Originally Posted by XR6 Martin

A better guide is actually a 1/4mile, in particular the mph.

Oh, and I can't agree with this more.

Full Spectrum · 02-10-2006, 12:25 AM

I just go 25%

Steve · 02-10-2006, 02:27 AM

This has been discussed heaps of times before. You CAN'T calculate drive train loss across the board using a % figure. Each drivetrain will have a fixed kw loss regardless of what engine is powering it. A 25% loss of a 180fwkw engine is 45kw through the drivetrain. Turbocharge that same engine to get a 300fwkw figure and apply the same 25% loss theory and you have a 75kw loss through the very same drivetrain. Impossible.

XR Martin · 02-10-2006, 03:25 AM

Quote:

Originally Posted by Steve

This has been discussed heaps of times before. You CAN'T calculate drive train loss across the board using a % figure. Each drivetrain will have a fixed kw loss regardless of what engine is powering it. A 25% loss of a 180fwkw engine is 45kw through the drivetrain. Turbocharge that same engine to get a 300fwkw figure and apply the same 25% loss theory and you have a 75kw loss through the very same drivetrain. Impossible.

Friction is not a constant

Full Spectrum · 02-10-2006, 05:03 AM

Theres a fraction to much friction

HSVETAR · 02-10-2006, 06:02 PM

Quote:

Originally Posted by Steve

This has been discussed heaps of times before. You CAN'T calculate drive train loss across the board using a % figure. Each drivetrain will have a fixed kw loss regardless of what engine is powering it. A 25% loss of a 180fwkw engine is 45kw through the drivetrain. Turbocharge that same engine to get a 300fwkw figure and apply the same 25% loss theory and you have a 75kw loss through the very same drivetrain. Impossible.

This is very true. But from what i know and have read, its normally 25% for autos and 20% for manuals.

Steve · 02-10-2006, 08:50 PM

Quote:

Originally Posted by SlickHolden

Theres a fraction to much friction

You know I actually heard that song today and thanks to your post I had it freaken stuck in my head for ages.

out:

nugget378 · 03-10-2006, 07:32 AM

Quote:

Originally Posted by Steffo

It isn't possible to calculate that accurately. Or anything approaching accurate.

The most general guide would be 18-20% loss manual, 20-22% loss auto, RWD.

However, DynoDynamics Dynamometers (and in general, dynos & operators in this country) don't follow this, as they have silly software that adjusts the readings to what it believes (or is programmed to believe) is correct. That's a pretty poor system, there should never be any correction factor, especially attempting to "adjust" for variables like humidity, air temp, traction.. yadda yadda yadda.

PS- I'm well aware people will attempt to flame me for the above post, and I'm well prepared for it.

Were I go to tune,we dont use correction or inertia and dont even use shootout,same operator,same dyno.
We still get varying numbers from one time to another,the biggest difference highest to lowest was over 60hp.
I dont put much faith in em,just tune the car and thats it.
Although they can give a rough guide..

Steffo · 03-10-2006, 10:24 AM

Quote:

Originally Posted by nugget378

Were I go to tune,we dont use correction or inertia and dont even use shootout,same operator,same dyno.
We still get varying numbers from one time to another,the biggest difference highest to lowest was over 60hp.
I dont put much faith in em,just tune the car and thats it.
Although they can give a rough guide..

Tuning makes sense, yeah, best if done at the same dyno time and time. But as you've said yourself, the variance they give on figures is huge. And that can happen on the same dyno in 5 minute intervals, let alone two different ones on two different days.

The best guide to know how well your car's performing is to take it to the track and give it a run.

If you really want to know how much power your engine has (and can afford it), take it out and send it to a proper engine dyno. Power figures from chassis dyno's are close to useless. Ask any 1000rwhp Supra owner from the US who runs 12.5 @ 135mph about it....

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