Any publication strap this car on a dyno yet?

[serialk11r]

Well the angular velocity of the wheels/diff/driveshaft is proportional to vehicle speed (assuming no slip) so we can think of rotating mass as both part of the vehicle's mass, and adding rotational inertia. If all of the wheel's weight is on the outer rim, the rotational inertia would be "equal" in some sense to the mass of the wheel when it rolls, so we can effectively say the rotational mass actually "counts as" 2 times its actual mass. But the outer edge of the tire isn't the only weight, the brake rotors turn at the same speed as the wheel but have a smaller radius, and the driveshaft has a much smaller radius. I don't know how driveshaft/differential gears/bearings/brake rotor weight measures up against the tire weight, but the correct "correction factor" for rotating mass should be far less than 2.

When it comes to the engine though that changes things up a bit since the engine spins much faster than the wheels. The rotational inertia of a flywheel is very significant when it comes to acceleration; http://www.se-r.net/engine/light_flywheels.html this guy calculates 10lb removed from a 1 foot diameter flywheel would be like reducing 135.7lb from the rest of the car in first gear, using 1MR^2 as the moment. If we use a more likely 2/3MR^2 this reduction is more like 90lb, something to think about.

[shermanpk]

What do you guy's think the power at the wheels will be, I am very doubtful that the 200ish HP that is quoted is going to be at the wheels, so what do you think the lose due to the transmission ect is going to be?
will it be enough?

[WingsofWar]

on average 15% of drivetrain-horsepower loss for RWD is what we usually eyeball figures before we put them on a dyno.

The AS1 rated at the flywheel is officially at 200hp..but manufactures have been known to provide more or less than the official rating.

realistically we could be looking at anywhere from 160rwhp-180rwhp with a built in margin of error

[Caliban]

i say probably 150hp at the rear wheels..

[chulooz]

Originally Posted by Caliban

i say probably 150hp at the rear wheels..

I say probably you have lost it.

[serialk11r]

Originally Posted by chulooz

I say probably you have lost it.

Or he's just lowering expectations so when it shows up at say 175 on a dyno he'll be extra happy! lol.

And remember dynos (assuming they are accurately measuring the amount of energy they are absorbing from the wheel) underrate due to inertial effects and rolling resistance.

[Midnight Run]

don't forget about the curb weight also..

so 150-160 rwhp is realistic when strapped on a chassis load based dyno such as Dyno Dynamics or Mustang Dynos.

[GT3076rBRZ]

Meh, we'll see.

[Tradewind]

I will put my hat in the ring and say 150hp at wheels absolute tops at 160, wouldnt be suprised if 145hp came up on dyno. I am not going on any loss factors or anything else, just on figures from working with lots of NA Subaru 2.5s over the years, and allowing for this motor to be more powerful and with less losses.

To qualify the comment - measured on a dyno Dynamics dyno - http://www.dyno.com.au/dyno/controller

[serialk11r]

Um, there was a video where they strapped it to a chassis load dyno and got 172PS.

Wanting to know a "wheel horsepower" number in itself as an accurate measure is nonsense. That number includes "losses" that are actually sources of loss and energy storage that make more sense when independently measured. Example: lighter wheels do not add power. Example: LRR tires do not add power.

If the dyno read 172PS, then that appears to be an efficiency of 86%. However, the true mechanical efficiency is higher than this. First of all, you have to add 1-2 hp back in for the rolling resistance of the tires. The amount of energy "lost" to rotational inertia is tricky though, because the engine is rotating much faster. It's hard to eyeball how much power the wheels and stuff are sapping. However, consider this: The power from the engine goes through 2 sets of helical gears and then one set of spiral bevel gears. These gears can typically be expected to have a mechanical efficiency of around 98-99%. That alone suggests that mechanical efficiency of the drivetrain is actually well over 90%.

[Tradewind]

Dyno type is also critical

The type we use in Australia (DD brand) really sap the figures compared to USA units like dynojet which give very satisfying numbers.

[serialk11r]

Um, the whole point of my post was that the number in itself does not matter at all unless you are holding the engine speed constant.