Ive debated trans loss and rolling road figures more times than Id care to recall. My advice is - quarter mile it and then youll know how it compares. Rolling road figures vary from one set of rollers to another, from one day to another, from one apparently identical car to another. As an example mine made one run with a lot of wheelspin problems on the rollers yesterday, even with 4 people sitting on the bonnet to try and keep stop it spinning up. This resulted in a figure of roughly 150bhp at the wheels at 5500 rpm, about a thousand less than when I normally see peak power (above 5500 rpm it was wheelspinning). So you could add maybe 30bhp to that according to Andy, which sounds reasonable. So that would be 180bhp at the wheels. Now if you add 28% trans loss to that, it makes 230bhp which is roughly what I was expecting. On top of which, with Fred in the passenger seat we went for a cool down run on the open road and chased down an Escort Cossie fairly easily. We couldnt get past, and he couldnt get away, and there is not to my knowledge a Cossie on the roads that is running any less than standard power, most run a lot more. Now that capability on the road tells you more about the power my Clio is making that any rolling road result.
An interesting thing to note is this though: lets say that theoretically mine had made 180bhp at the wheels. Add 28% trans loss to that and you get 230bhp (180 x 1.28). However if you take the 230bhp flywheel figure as your baseline measurement, and subtract your 28% trans loss figure from it to arrive at your expected wheels figure (230 x 0.72), it comes to 166 bhp. Just working your calculations backwards rather than forwards you have lost 14bhp.
I have to say that 28% trans loss seems a huge amount for a front wheel drive car, but then thats less than Ive heard some rolling road operators claim was acceptable for fwd, e.g. 35-40% on occasions, which is clearly ridiculous. 10-15% does seem to be the rule of thumb trans loss that fwd cars should show, but then again I dont have a rolling road of my own to calibrate engine dyno results against chassis dyno results. If 28% works then all you can do is leave it at that, and as I started out saying, realise that rolling road results, wherever they are from, can only approximate the power your engine is making...its crazy to say one car is more powerful than another because of a discrepancy in flywheel figure of say 10bhp.
If you want to get down to the raw numbers, you want to disregard flywheel calculated figures entirely and just go by torque and power at the wheels. Or go quarter miling.
[Edited by Nick Read on 01 August 2004 at 5:42pm]
