Rear wheel BHP

[msd1107]

Rohan makes a good point that the rolling road introduces additional losses.

The 15% figure is based on a car on a level road.

For a rolling road in 2nd/3rd gear, you have 2 gear mesh points for at least 4-5% additional loss, additional bearing losses, and an indeterminate and variable additional loss at the tire to roller interface. For proper testing and repeatability, the rolling road should have measured tie down force, so the minimum force neccessary is applied, and can be repeated.

For a given car, a certain horsepower will produce a certain acceleration, no matter what the engine rpm. The drivetrain gearing converts the engine torque/rpm to the rear wheel rpm. This means that high torque at low rpm produces the same acceleration force at the tire interface as lower torque at higher rpm, assuming the product of torque and rpm is the same. There are secondary effects that modify this general statement.

David
1968 36/7988

[rgh0]

As always, Rohan, an excellent summary. I learned a lot from a few words about interpreting my own results, an S2 Europa dynoed at 88 RWHP, so between 101 and 110 FWHP (also third gear), which is about right for a healthy Hermes. (Some argue that with one less driveshaft, the Europa loses less in the translation and might instead spec me as low as 96.5 FWHP).
...

A Europa has an all indirect gear box I believe so in 3rd gear, only 1 gear set operating rather than 2 as in an Elan so lower losses. The diff is also a spiral bevel gear rather hypoid gear as in the Elan so again lower losses. I would guess around 3 kw losses per thousand rpm versus 4 to 4.5kw per 1000 rpm in an Elan so up to around 110 hp at flywheel sounds about right

cheers
Rohan