Engine horsepower vs wheel horsepower

[mack124]

Hello all,

I have been wondering what is the percentage of horsepower that gets to the rear wheels of a Fiat 124. I've heard that they have very efficient drivetrains and can get quite a bit more power to the road than other cars of a similar era.

Has anybody done the test? ie Dynoed an engine then dynoed the same engine in a car?
thank you guys

[Guy Croft]

Don't know any reliable percentage, sorry. I guess at 25-35 or so bhp loss in the 150-200 bhp range.

As for the 124 being an efficient drivetrain, not especially.

Loss at the wheels compared with flywheel output is an aggregate of gearbox, bearings, differential, tyres. Synchromesh gearboxes are less efficient than straight cut, but apart form that there is not much between one RWD car and another. The rolling loss in the tyres is usually the biggest loss factor.

GC

[tmvolumex]

One thing to consider concerning engine HP vs wheel hP is the transmission final drive. Front engine, rear wheel drive cars typically have a hypoid gear design for the final drive. This type of drive is quiet but has a fair amount of friction and HP loss due to the hypiod gear in the final drive. Cars with transverse engines and transaxles commonly have a spur gear set for the final drive. Spur gears are more efficient, thus for a given engine power more of it will make it to the wheels. So a car with a spur gear transaxle (front wheel drive, transverse engine or rear wheel drive tranverse engine (AKA Fiat 128, Fiat X1/9, Lancia Montecarlo, Lancia Beta) they will put more of the engine generated power to the wheels than the same engine will with a hypoid drive (AKA Fiat 124, Fiat 131)
Tom McGaffigan

[S&B125]

Hello all,
Just thought I'd weigh in on this topic, having quite some experience of using a dynamometer day to day in my work. We have dyno which (when used in Inertia Mode) measures power at the rear wheels during an acceleration run from start to max revs, at which point you declutch and the dyno measures power loss through the entire drive train in the time taken to deccelerate the mass of the rollers. It then calculates power at the flywheel by taking the measured power (at the wheels) and the measured power loss at any given point in the rev range.
I hope this is'nt confusing everyone, please look at the graphs I have posted and you will see 2 green lines at the bottom of the page which are the power-loss measurement of a 124cc (solid line) and a 125 (dot-dash).
These are our own cars, though my faith in this dyno comes from many years of vehicles producing very close figures to what the manufacturers claim (corrected to DIN).