I have trouble believing Dyno RWHP % factors...

I have problems with a lot of myself, but for different reason's. I havent seen anything along the lines your talking like front vs rear drive, but the loss percentages and physical HP figures and the machines being used.

There are two ways of measuring HP. One is a brake dynomometer, the other is inertial. Only the brake dyno is realistically accurate because it can hold the wheel or engine at a particular rpm through braking, and calculate HP by measuring constant torque.

The inertia dyno works completely different. By the wheels accelerating a drum of certain mass, the time to speed is calculated into HP. The problem is that there are far to many variables. Any change in mass in any of the cars components will effect the result. So if two identical cars were ran, and if both gave equal numbers, simply changing the flywheel of one to a lighter unit would show an increase in power. Yet there was no increase in power, so it shouldnt be used. Yet it is. Lighter pistons, lighter rods, valves, clutch, even wheels and tires will show a gain on an inertial dynomometer, that wouldnt be seen on a brake dyno. I see the enertia dyno as measuring the "cars" power, not the engines. I see the brake dyno completely the opposite.

As to geartrain power loss percentages, it seems to be an industry accepted practice to use a straight percentage. And without tests and trials to measure it its all we have to go on. From what ive studied it would seem more likely the drivetrain consumes energy on a curve rather than a straight line. That it starts with a fixed load that increases towards a flat line, so at a certain point in speed and power it no longer absorbs the same percentage of energy. The only way to prove it is to take an engine that was run over a wide range of rpm's and power settings, and retest it after its attached to the final drive. But if the power loss curves in the planetary geartrains of turboprop aircraft transmissions can be used as an example, it shoud prove out the same with any transmission.

 

Forget % Drivetrainlos Dismiss it totaly it isn't and wasn't anything near accurate in the past or now or in future. Just messure your drivetrainlos on the dyno doesn't matter if brake or inertia because when you messure drivetrainlos you got on both a accurate engine hp / torque reading.

 

Right because its just impossible to do its like reading tea leaves to find out the lottery numbers of next week. You have to messure the drivetrainloss and not guess it.

 

For some cars there are good correction factors documented by testing the same engine in both methods. For a US rw drive car where high gear in the trans is 1:1 locking the input to the counter shaft the number always seems to come out to 15%. For a transverse engine the number is more like 17-18% because the power is going through more gears before it gets to the wheels. Rear engine longitudinal set-ups are in the 15-17% depending on exactly how the trans is configured.

The numer is not a true straight % but it's close. Friction is directly proportional to the force, some more power means more friction and there is just no way around it.

That said the only number that matters is the rw hp and it's only really accurate for comparison of changes on the same day on the same dyno.

 

+1
Everything beyond is just an approximation, with an accepted window of error.
...but, it's fun nevertheless if not taken too seriously realizing the shotcomings.

 

You would be doing a lot of trial and error trying to build a scratch built model helicopter, or a full sized one for that matter, without making some educated assumptions on power losses in basic gear trains.

 

I agree,the only number that matters is wheel horsepower. I have an engine dyno (with over 4000 pulls on it) and I use it to extract the most horsepower available from the engine. I would like to have a chassis dyno ( sometime soon) so I can translate the losses and try to find out where the driveline could be improved. You can't just go by a percentage of losses based off of if the car is rear wheel drive, front wheel drive, etc., each and every transmission/driveline setup is different. Case inpoint: NHRA stock eliminator racers used to always use powerglide transmissions because they consume less horsepower to turn their gears and clutch packs because they were 2 speed transmissions (less parts to turn = more horsepower) vs. TH350's or TH400's. As of a few years ago they all switched to TH200's because they consume even less horsepower, but yet are a 3 speed trans like the 350's and 400's. Both are rear wheel drive transmissions, but have vastly different outputs on rear wheel horsepower. The ideal situation is to measure the engine horsepower, then install it in the chassis and measure it again. Only then can you be assured that the engine or driveline isn't killing the other with a hidden problem. Figuring engine horsepower from a chassis dyno, or figuring rear wheel horsepower from an engine dyno is nothing more than a guess at best.
And I disagree with a previous post stating that "So if two identical cars were ran, and if both gave equal numbers, simply changing the flywheel of one to a lighter unit would show an increase in power. Yet there was no increase in power, so it shouldnt be used. Yet it is. Lighter pistons, lighter rods, valves, clutch, even wheels and tires will show a gain on an inertial dynomometer, that wouldnt be seen on a brake dyno". Yes, lighter pistons, rods, etc. WILL increase the engines horsepower. Any change in an engine or driveline's rotating weight will affect the horsepower output. Race teams go to great lengths to lighten parts to increase performance.