The operator runs the car up to whatever rev limit you select, or a figure he knows will allow a max. bhp reading, usually in 4th gear. The way the driven wheels overcome the resistance of the rollers produces a power curve. He then immediatly de-clutches allowing the resistance of the transmission to be measured by the dyno as it slows. The computer then adds this trace to the original "at the wheels" curve to give the "at the flywheel" curve. I assume the computer could produce a read out of the transmission loss if required.
At the end of the day, it's the power transmitted to the road that really counts. It would be interesting to see how quick a two wheel drive Scoob would be!
J.

[Edited by JohnD - 6/2/2002 4:07:39 PM]

Yep I agree RR are great for diagnosing faults (eg. fueling and boost issues across the rev range), showing before and after figures for various mods on the same day with same operator but the only really way to show output is an engine dyno ;). If you treat them as a diagnostic tool and how well you engine is running then they are cheap and effective piece of kit.

[Edited by chrisp - 6/2/2002 4:09:12 PM]

I have always wanted to know how they calculate the bhp lost through the drivetrain??
Does 170kw @ wheels equal the same KW @ the flywheel on different dyno's or will it be 240kw on one and 230 on another???

Reason I ask is I have had a dyno run, but only told that the figure @ wheels is the only reliable one to go by. They told me some tuners will escalate the figure they tell you for the flywheel...

I notice most figures quoted on the BB are in flywheel figures so hopefully some of you will have some insights...

Cheers
Bruiser

Not sure but dont they use the rundown on the rollers to calculate the transmission loss and add that to the figure? When the power run is finished I think they let the car freewheel to a stop?

MB

Does the rundown measure the resistance in the drivetrain??
How do they put a figure on this though???

The power at wheels changes as you get more heat into the transmission. It is not usually corrected for temperature. It is dependent on tyre pressure. It seems to vary a lot between rolling roads - so do the flywheel figures. Perhaps the only valid measurement is a relative difference on the same day on the same car and same rollers with different mods, but then even back to back runs have their differences with heatsoak, ECU learning etc. A fairly imprecise pseudoscience overall with multiple pitfalls.

Why do so many members on here quote flywheel figures when the figure they should really quote is @ wheels?? Surely then its a level playing field, takes the guess work out of transferring the figure to flywheel....

Simple because the power at flywheel is a lot more due to transmission losses. If I said my car has 200bhp @wheels or 280bhp @ flywheel which sounds better ;).

Also its the same as cars which get 280bhp on one RR and 245bhp on another RR so the 245bhp figure is bound to be wrong ;).

Crisp- Thats exactly my point..how do they justify 80bhp loss through the drivetrain?? What if it was only 60bhp loss....
Are you confident that 280 is the true figure??
Or is it a standard to add a certain % of your @ wheels figure??

I know of several "standard" STi's, all meant to have 280bhp...but some have achieved 155kw @ wheels, some 175kw @ wheels....Does it mean the first is less powerful or just that the different dyno's produce a differnet amount of drag??

The reason I asked is most magazines now featuring cars in this part of the world are quoting all the figures @ the wheels....but I have never heard a formula for transferring it into what has always been "pure power" at the flywheel.

They just throw a line in the article like, " putting out over 200kw @ wheels so is good for easily 350bhp" but not stating how they come to this conclusion...


Some tech experts need to give me a long winded and factual explanation, and I can sleep at night!!!

Engine dynos also give different hp readings. In 1993 I built an approximately 560 hp V8 engine for my Ford V8 Falcon GT back in OZ. I took it to one engine dyno and managed 585 hp and then took it to another and got 521 hp. This was to prove to a few guys back there that engine dynos are also inacurate. Others had gone to these two dynos with their engines and the one dyno always gave quite a bit more hp readings than the other.

Cheers,

Wrexy.

Now Im getting really confused....even Engine Dyno's can;t be relied on!! WHat next??

Congrats on the V8 GT...I had a Fairmont Coupe and then a XU-1...but had to trade up to a Subaru once the comforts of the old girls got a little hard to bear...

Wish I hadnt now of course!!! Should of kept them all and started a stable...

Wrexy so what you are saying is that my scooby has 500bhp and no-one can prove otherwise :cool:

I bet that falcon was fun and you must have got through some tyres

It how it feels on the road and driveability that really matters.

Hard to brag about how good it feels!!!!
Gotta have the mine is bigger element!!!!!

Doesn't the loss through transmission also depend on the vehicle. There was a post on the board a while ago stating the new STi V7 lost less bhp from flywheel to driven wheel than other scoobs.

Seems like a lot of the bhp figures are more 'bragging' than actual fact, the Lb/s foot torque to me is more important.

hawkeye

I think you all need to stop worrying about it cause the power you have available varies with a number of factors. The VTS will usually have about 130bhp at the fly but on a hot muggy day I'm sure this would drop to 120 or under as the car has felt really bogged down. Other times on say a nice chrisp cold day when the engine has just warmed up she flys and I'd imagine is putting out a heck of a lot more power than the muggy day. The bottom line is both RR and manufacture quotes should be used as a guide.

I wasnt worried about the issue, just has always made me wonder.
I know that conditions are never perfect on a dyno, but was looking for some insight into the process used to make the calculations.

Bruiser.STi,

The runs were done on Baker brothers dyno in Sydney. This was the one that read low and the other was done at Sam's performance in Sydney. They were different model dynos and I must say that Bakers make their own dynos and they sell their engine dyno's to workshops.

The car ran a 10.9 sec pass at Eastern Creek raceway in 1996. That Christmas the car got written off. It died after 12 years ownership. Purchased in 1984. I then bought myself a 1988 HSV Group A Commodore (Walkinshaw), totally standard, and sold it before I moved to Greece. When I moved here I bought the MY00 turbo.

Chrisp,

I don't think you could get away with saying your scoob has 500 hp It's just a little bit overdoing it.:D The less hp to start with, the less the diference in readings between dynos, like a car that would have read say 250hp on Bakers dyno would read around 270hp on Sams performance one. A car like mine with approximately 560 hp would read 520 on bakers and 580 on Sams. The differnce grows with higher hp. I just took a hp figure between the two dynos roughly and quote that.

As for the way the Ford felt, it would spin like it was on ice when travelling at 120kph in top gear, (car had a 2 speed Powerglide auto tranny) and pushing the accelerator halfway to the floor. Yes it went through lots of tyres.

Here's a pic of it back in 1993.

http://upload.turbosport.co.uk/galle...3337211437.jpg


Cheers,

Wrexy.