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Flywheel & Rear wheel kW difference

16K views 33 replies 15 participants last post by  JC-XR8 
#1 ·
I wanted to know if there is a formula or somewhat near that I can use to calculate the difference between the two.

Depending on what car the test is on will depend on the difference because of the mechanics of everything between the motor and the wheels

Factory figures come from the flywheel but figures from shops and many dynos are from the wheels.

Can anyone help?

Peter
 
#2 ·
I certainly don't know of an exact formula, but I think (someone correct me pls), that you generally reckon on losing 25% or so between the motor and wheels, usually a little more in an auto.

Like you said, every car / model is different, and I can't really see how any formula could claim to have good accuracy ( better than the above guesstimate anyways ).

Beyond this, even on the same car, different dyno's will produce wildly varying results anyway.

What are you trying to get from the formula ( I'd guess you want to know flywheel KW's after a mod ? ). Generally I'd say ( I think most ppl here would agree ), forget about the factory flywheel figure, and go with the dyno results, factory figures are good for impressing mates at the pub, but rwkw's will be what smokes that hsv at the lights :)
 
#3 ·
I did want to find out the flywheel kW after a mod because then you can brag about that :flip: . But around the know-it-alls you talk about the actually figures, the end product is, the raw wattage. :evil:
 
#5 ·
There is no real accurate way other than putting the motor on an engine dyno .
Some work off 30% loss but this is bullshit , If you had a 600 hp motor that means you would lose 200 hp through the drivetrain which = 149000 watts of energy , The drivetrain would melt with this amount of energy being absorbed into it .

But a simple one to use that some work off is RWHP = FWKW
 
#6 ·
Correct. RWHP = FWKW (approx) .The conversion is roughly the formula posted previously (RWKW * 1.33).

For a manual, you could work on around 20-25% loss as a rough figure. For an auto, it's more like 30-35%. Both of these are roughly the same as RWHP = RWKW.
 
#7 ·
JCV8 said:
Correct. RWHP = FWKW (approx) .The conversion is roughly the formula posted previously (RWKW * 1.33).

For a manual, you could work on around 20-25% loss as a rough figure. For an auto, it's more like 30-35%. Both of these are roughly the same as RWHP = RWKW.
Well going by that theory I come to 190KW(front) for mine,you sure about that.and that's after the mod's.
 
#8 ·
What power readout do u actually have?
 
#12 ·
Hmm . . . I guess it depends on whether you've got the 200 or 220 XR8 . . .

If you work off a 30% loss for the auto, you multiply by by around 1.42, soo . . 1.42 * 142 is around the 200kw mark . . .

I guess you'd prolly expect more given some exhaust and chip work, but the key is to remember that the calculated figure is only a really rough estimate . . .

Also, remember dyno's can read differently in different places, and depending on the ambient conditions on the day. Some ppl on this forum have reported 20rwkw differences between dyno's . . .

At the end of the day, I think most ppl here would agree that dyno figures are only useful when figures come from the same dyno, same day, same operator ( i.e. usually before and after a mod ). There are waaay to many variables for any kind of accuracy otherwise . . .

If I were you, I'd hook up with one of the XR clubs on your state and attend one of their dyno days, as well as a cheaper dyno, you should get a more accurate comparision between your car and others with slightly different mods . . .
 
#13 ·
galaxy xr8 said:
Are you asking me,if so I have 142rwkw.
I have had 2 dynos done in the last 2 weeks. 142rwkw and 140rwkw. And yes, 142rwkw equates back to approx 202kw at the engine for 30% loss (142/0.7), or 218kw for 35% loss. I think 35% is too much, but 30% is reasonably close.

Mine was originally rated at 175kw at the engine (and was dynoed at 119rwkw), which means it is actually 32% loss. Based on this, my engine kw are 209, from 142rwkw.

Mine is an AU2 Forte V8 Auto with 70mm TB, Pacemakres and full 3" exhaust. Assuming the driveline losses are the same for your car, yours should also equate to around 209kw at the engine.
 
#14 ·
JCV8 said:
I have had 2 dynos done in the last 2 weeks. 142rwkw and 140rwkw. And yes, 142rwkw equates back to approx 202kw at the engine for 30% loss (142/0.7), or 218kw for 35% loss. I think 35% is too much, but 30% is reasonably close.

Mine was originally rated at 175kw at the engine (and was dynoed at 119rwkw), which means it is actually 32% loss. Based on this, my engine kw are 209, from 142rwkw.

Mine is an AU2 Forte V8 Auto with 70mm TB, Pacemakres and full 3" exhaust. Assuming the driveline losses are the same for your car, yours should also equate to around 209kw at the engine.
When I had the Dyno run it was in Summer and the car had only done 6000klms,it is a 200kw or so they say,I have heard people say that ford have over calculated the power of these motors and they are more like 192kw like the ED sprint's,which I beleive to be close,now If I calculate 192-37% it comes to 120 rwkw which is what the Dyno read and 192 add 35kw for the mod's (20kw,exhaust,12 chip,3,air filter,rough guesses)this comes to 227kw,so 227 take 37%=143rwkw,which is what the dyno read again,I'm told Auto's loose 37% and all the calculations add up,go fiqure,I'm confused.Blo@#y Dynos.
 
#17 ·
that x 1.33 figure cant be right my au futura dynoed at 120 rwkw with cam extractors exhaust and cold air intake and that would calculate it to only be 159 fwkw which means only a 2 fwkw icrease from those mods when the dyno indicated a 22 rwkw difference from stock to that figure
 
#19 ·
galaxy xr8 said:
......so 227 take 37%=143rwkw,which is what the dyno read
Hey, I'll take that. even at the lower of my 2 figures (140rwkw), that makes it 222. Or multiplying by 1.5 = 210. Either way, it's more than 175 stock.

The thing with dynos is that they are good for measuring increases based on before and after. Trying to convert a chassis dyno figure back to an engine dyno figure is just not possible. You can get close, but it's not accurate to the kilowatt, let alone fractions of one!

And Ford's quoted figures used to be based on a 'short' engine (no a/c, alternator) etc. These days, I think they are based on an engine with all ancilliaries attached.

Anyway, if 142rwkw feels fast - it is fast. If it's not fast enough, then you need more mods!
 
#21 ·
JCV8 said:
Hey, I'll take that. even at the lower of my 2 figures (140rwkw), that makes it 222. Or multiplying by 1.5 = 210. Either way, it's more than 175 stock.

The thing with dynos is that they are good for measuring increases based on before and after. Trying to convert a chassis dyno figure back to an engine dyno figure is just not possible. You can get close, but it's not accurate to the kilowatt, let alone fractions of one!

And Ford's quoted figures used to be based on a 'short' engine (no a/c, alternator) etc. These days, I think they are based on an engine with all ancilliaries attached.

Anyway, if 142rwkw feels fast - it is fast. If it's not fast enough, then you need more mods!
Yes your right,it's just heart renching when you hear story's of cars that have what I have at the wheels they have the same or more in stock trim and to make matters worse their 6's (no offense to the six guy's) and there seems to be alot of people around with similar results than what I have just makes you wonder and then you hear about those Gen III's and some claim 190+ stock,this car is my first v8 and to me it feels fast but so would any v8 I was just a bit surprised at my RWKW in stock trim compared to others off similar car's I know Dynos will allways read difrently from cars to cars but 20rwkw difrence's means about 30 or so at the flywheel does this mean that mine was 170kw when Ford claim 200kw
and I'd love to get more mod's but that folding stuff dosn't come as easy as it does to others.
 
#23 ·
XR8 Lamah said:
Check out this thread Galaxy:

http://www.fordforums.com/showthread.php?s=&threadid=33928

Some of the posts will make you glad . . . and some sad ( 275 rwkw - EGADS ! )

Haven't had mine dynoed yet . . . cause I know I'll end up in the "sad" category ! :doh:
Yes your right, (i'm crying)no hang there I'm glad again,oh well I'll take it as it is.
 
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