Er, no, incorrect.

Upon reaching the peak, the RR operator dips the clutch, leaving the inertia of the entire drivetrain driving the RR (all but flywheel and clutch cover). The RR consists of 4 Eddy current brakes, which can pivot & act on load-cells at a known/calibrated offset generating a torque (+ve on drive, -ve on coast). Measuring the coastdown torque or rate of decelleration the drivetrain losses ARE estimated.

Paul

 

Correct Pavlo, although I wouldn't like to say how accurate this is. Different gear ratios have NO effect on power at the wheels, except for the differing transmission losses, gearing is a torque multiplier not a power multiplier.

Nice torque curve on the dyno plot.

 

So, Pedalo me old mate, from what you're saying, a 200 bhp car will have the same coastdown losses as a 400bhp car and that the 'real' transmission losses will be identical despite twice the load (when under power) on bearings/gears etc ???

Doh..I don't think so ?

 

different gear ratios will affect power losses in a couple of ways.

Running 1:1 means 2 equally sized gears, running in say 1st, would mean one very small gear, which may reduce the gear efficiency, but no more than a knat's chuff.

Main difference on the Rolling Road (only) is that longer gearing will lead to higher wheel speeds, and this will definitely increase losses due to tyre deformation.

One thing I can't see how they "calculate" is the drivetrain innertia, I think they must assume this. There is no use of the curve shape to split out a paticular element of the loss, it's calculated straight away during the coast down. SO anyway, there will be differences in inertia from wheel/tyre and brake choices (let alone different drivetrain configurations) which will affect the calculations. That said, from one identically geared scoob to another, the readings are probably quite reliable.

Paul

 

The drivetrain inertia may well effect the PAW figures but the flywheel power will be unchanged, this is due to the inertia load on run up equalling, and hence cancelling out, the inertia on rundown (assuming the same acceleration/deceleration rates are applied)

I agree with the tyre/roller losses, they appear to increase almost exponentially with speed.

The only question mark for me is how the lighter loading on the drivetrain accounted for in the rundown calcs

 

Good point Andy, although without sitting down and wrking it out, I don't know how the innertia differences would tally to actual torque.

Obviously if you have lead wheels, then this would lead to some of the torque on acceleration going into accelerating the extra rotational mass. Upon the coast down, the wheels could exact a higher torque on the rollers for the same deceleration, giving higher apparent losses. I suppose this may add to the apparent difference in drivetrain losses between differently geared cars.

It is possible to hold a certain torque or speed on a rolling road, which would give you a better guide to power at the wheels. The downside is you have to specifically test at various speed points to get a (lower resolution) graph, and coast down testing would be a bit off too.

Anyway, think this has been done to death now

Paul

 

Well done on the power outputs. Come on be serious, Saab 9000 Vs Scoob? Legend versus anonymous Swedish box? We don't have to defend the Scoob or promote it. It's place in history is cemented.

I have driven powerfull Saabs and although I do like their quirky interiors, they handle like most other cars of that type; very
twitchy round corners and forget pushing it in the wet. Saabs may be cheap but they are not a patch on similar Beemers.

Just my opinion. Good luck with the car. Glad to see Scoobies are still the yard stick.

 

311 bhp for another £ 800 squid - sounds very cool.

Does that equate to a remap or chip or what?

FWD or not I imagine the smile rating is also very high!!!!

ENJOY !

 

Scoobee,

No I don't think so, but we are not comparing 400 vs 200hp cars, rather 290ish to 313. However, the losses will still be similar.

Edited to add: The above is not what I'm 'saying', it's a fact. It's the method the PE rolling road (Maha twin axle, Eddy Current brake, rolling road) actually uses, and the point is, that it's NOT a perfect way of doing it.

Driveline losses are composed of three things, where mentioned, "losses" refer to losses in effective torque. "Speed" generally relates to vehicle speed, as many of the losses are dependant on it directly rather than engine speed.

1. Tyre losses, since the tyre runs with a flat contact patch on the road, since you are constantly flexing the tyre, you are doing work, hence it costs you power. This will generally the same on 2 cars with similar tyres, despite large power differences. Tyre losses are generally non-linear due to hysterisis in the rubber, but no massively so.

2. Frictional losses, since there are sliding parts involved, with a variety of plain bearings, roller bearings, meshing gears, some seals etc. The losses should be proportional to the load on a given car, so double the torque, double the loss, at any given speed.

3. Viscous losses, since you are moving parts through oil and grease (gears and plain bearings in oil, grease packed wheel bearings, some seals etc), there will be another set of losses. Viscous losses for normal fluids (ie not thixotropic gel) will generally be proportional to the RPM squared.

You could split the viscous and friction losses into engine speed and vehicle speed too if so inclined. Of the 3 losses, the viscous losses will make up a significant chunk, especially due to the non-linear relationship with speed.

So if you do go from 200-400hp on the same car, your driveline losses will not double speed for speed at WOT. You can probably expect them to increase 30%, but it will depend on specific application obviously.

Innertia is not a driveline loss, and is treated seperately. It's commonly called "equivalent mass", and for the most part ignores innertia on the engine side, as this can be seen on a bench dyno. If you know the various speeds, polar moments and masses for all your rotating parts (which a driveline engineer probably will) then you can calculate it anyway.

As it happens most mass produced cars performance figures are computer calculated anyway, top speed is easy, and so is in-gear acceleration (not so easy on turbo cars though).

Pedalo? Surely you can do better than that? Either way you'll find I rarely take the bait.

Paul

[Edited by Pavlo - 11/24/2002 9:25:08 PM]

 

Very interested to hear about this result - was the £800 a result of bargain secondhand bits or just the going rate for the work done?

This sounds like an excellent candidate for a second car too me, 30+mpg when cruising looks good to me and 300bhp for the occasional overtake...

Is an aero the best 'base' car for these mods or does it make no difference?

Cheers,

Jon (needing to stop doing 3k/month in the skyline!)

 

£800 gets you a remapped ECU and a 3" straight-through turbo-back exhaust system. All brand new, including shipping from Sweden.
I actually got mine from a different tuner and reckon mine's better than Alan's as the torque is significantly higher up to 4500rpm

In addition to that, I also spent another £800 or so on suspension mods and along with S-03s all round (still on the standard 16" rims), I get zero torque steer, no straight-line wheelspin in 2nd gear in the dry and the car handles very well - a lot better than you'd expect for a big, powerful FWD car.

The Aero (manual version only) has a larger turbo that supports well over 300hp. All other versions have a smaller turbo and this is probably feeling the strain above about 240hp. I'm told some of the very last 9000s also came with the bigger turbo, but opinions differ as to which ones.

Incidentally, 300bhp isn't the whole story. The enormous rush of torque from about 2000RPM makes overtaking a breeze. Having had a look at the curves from dr_jones' STi, it's seriously lacking in "area under the torque curve" compared to the big fat curve of the Aeros. I've no doubt it can overtake well, but I imagine it would be a seriously noisy affair

[Edited by BillJ - 11/25/2002 12:34:09 AM]