I've read on several occasions on this site that the problem with 4WD cars is that you 'lose' power at the wheels compared to 2WD cars. This is of course correct, but what I don't understand is why that transmition losses are quoted as a percentage (usually about 25%).

Surely this cannot be correct, as the power loss is a 'fixed amount' and not variable, i.e. why would a 280bhp Scoob lose 70bhp at the wheels, and a 200bhp Scoob lose 50bhp? To my mind they would both lose the same, can someone answer this one?

 

Trans loss is pretty complicated once you look into it and you ARE right to a certain extent, that trans loss should be fixed. However, my understanding of the subject is that trans loss is made up of a combination of factors, some that are fixed and some that alter with power output / road speed, therefore people tend to use a rule of thumb to work out whether trans loss is acceptable or not. In general transverse-engined fwd cars suffer the least, as all you've got is a gearbox with a driveshaft sticking out each side. Your transmission losses start from the flywheel onwards and include the losses in the clutch, the gearbox, the driveshafts, wheels and tyres. A RWD car loses more than FWD in general because you've got the added complication of a propshaft having to turn its output through 90 degrees at the back end, and 4wd has the most complicated and loss-making transmission of all.

I used to have a certain webpage bookmarked for years, it was a brilliant explanation of the whole issue and made a hell of a lot of sense. Incidentally it completely wrecked the idea that you could place any faith in rolling roads as well LOL

If I can find the link again I'll post it up.

 

Basically as you are looking at peak BHP, this occurs very high in the rev range where torque has dissapated, so the c26% is about correct on a Dyno Dynamics machine. If you looked farther down the rev range the losses would be higher as you have more torque to contend with.

For example a Subaru producing 250 BHP peak power at the wheels on a DD rolling road would extrapolate to around 337 BHP flywheel.
ATW number x 0.74 or 26% The Subaru community in general wishes to see the Flywheel extrapolation for BHP whereas the Mitsubishi community prefer the At The Wheels numbers. Horses for courses

Regards

Allan

 

so whats a 245 bhp prodrived only wrx reading at the wheels...roughly...?

 

Losses increase as power increases, the percentage is fixed.

245 flywheel bhp would be roughly 186 atw.

The losses also vary model to model of course depending on gearing and transmission type.

bob

 

Bob's about right. We would expect c182 to 186 at the wheels on our dyno, as long as it actually produced 245 at the flywheel to start with.


Cheers

Allan

 

well its a 02 WRX PPP with no other mods...180ish at the wheels? hmmm...what would be the best way to get that upto 200...? mate reckons the celica GT4 i sold him is now 202...!!!

 

Fully decat the exhaust and take it to someone like Bob for a TEK3 remap.

That should do it!

NS04

 

Martin,

My old STi 5 had 261 at the flywheel on Powerstations "Cheltenham" rollers, which equated to 189 at the wheels. So the 74% figure is just about right.

Matt

 

That is until you compare it with my 192 at the wheels and 312bhp at the fly - all on Powerstation "Tewksbury" rollers.

STI05 PPP + Tek3

 

BTW there's one very simple way to cut to the chase and avoid all the pub talk surrounding rolling road figures - disregard flywheel figures and simply go on the raw power at the wheels. IIRC torque at the wheels is what's actually directly measured, power at the wheels is calculated from this using a standard simple equation. It's from here on that different rolling roads use different methods to 'calculate' trans loss and thus 'power at the flywheel'.

The simple fact is that the only way of measuring power at the flywheel accurately is to take the engine out and stick it on an engine dyno. And even then, are we talking about the engine with all its ancillaries or stripped to the bare essentials?

As long as you always rely on power at the wheels you can't go wrong.

 

Not sure "you can't go wrong", I have seen many ways it can go wrong.

I had a 12% loss on my Subaru on Dastek rollers with Subaru parts bin transmission except RCMS clutch/flywheel. I would have been very bemused if my 353 WHP gave me 477 BHP on a TD05/06-20G on straight Optimax at 1.27 bar rather than the 403 BHP which seemed more realistic.

 

The flywheel HP figure is the sum of the HP "@ the wheels" and the drag measured during coastdown. The Coastdown drag curve is exponential therefore the drag is NOT a fixed percentage. The wheels figure is meaningless without some form of drag measurement. All these factors affect the power @ the wheels. Tyre type/size/pressure/temperature, gear/diff ratio, gear oil viscosity and gear selected. The gear ratios and gear selected have the most dramatic affect because, simply put, the greater the speed the more the drag hence the lower the wheels HP figure.



The greater the speed the higher the drag figure and the lower the "@ the wheels" figure.



It should be noted that the drag figure measured on a rolling road is far greater than you would experience on the road as on the dyno the car sits on 8 small circumference rollers which deform the tyre. The drag measured is true though, you would have a lower drag and a higher "at the wheels" figure on the public highway.

I hope the explains some of the variations seen between different cars.



Rich
www.powerstation.org.uk

 

I'm sure I've seen a pic of a 'rolling road' that was not rolling.
ie, the cars hubs bolted directly onto a form of carrier. Hey presto no rolling losses to worry about.

dipster

 

Another idea, if a 4wd drive system works each diff less because the drive is shared. Then would the losses per diff be less, but combined about the same as a harder worked 2wd taking all the drive.

Just and idea, it's late

dipster

 

Apparently Thor Racing use a dyno where the hubs bolt into the dyno (ie wheels off).

It seems good enough for prodrive (they do some PPP evaluation there or so I've been told). Then again that means it probably overeads by 15%..!

 

Hub Dyno's have been around for a while. Unfortunately losses will still be apparant as power still has to travel from the crank to the hubs. Some say that bench dyno's (engine dyno's) are the only true way of getting a reading (no transmission losses and fixed repeatable conditions).... unfortunately these are very expensive to perform (anyone fancy adding the cost of taking an engine out and putting back in to the car on top of a RR cost lol) and may be open to the similar questions on calculation (even though for different reasons).

One tuner used to go by a G meter... that way if the car pulled more g's after mod's it was more powerful.... who cares by how much BHP (I do lol)!

Regards,
Shaun.

 

Lots of interesting stuff here, and further up the thread. My fundamental point is that the less complication between the engine and the power reading, the more reliable it is. You can spend all day calculating coastdown loss and tyre drag, but the bottom line is that the only totally accurate way of rating an engine is to take it out and engine dyno it.

A couple of posts there implying that somehow power at the wheels is NOT accurate. But what I'm saying is: I believe the ATW figure (however you get it on whatever brand of dyno) is going to be much closer to the true ATW power, than the calculated ATF figure would be to the genuine power the engine is putting out. There are less ways for an ATW figure to go wrong. There are dozens of variables in calculating an ATF power reading.

On top of this, the important thing is surely the fundamental question of how fast does your Scoob go? Which do you think is the faster - Scoob A with 500bhp calculated at the fly and 350 at the wheels? Or Scoob B with 480bhp calculated at the fly and 400 at the wheels? It's a no-brainer. Everyone can (or should be able to) instinctively understand that power at the wheels is the real thing.

Let's put it another way. The argument is that all these variables (tyre pressure, drag, number of rolllers, coastdown loss, gearbox variations etc etc) are justification for the variations in the calculated power at the flywheel. Think how ridiculous that sounds...you're trying to tell me that an engine which makes say x amount of horsepower on the open road, gets to the rollers and suddenly it makes a completely different amount because it's on a rolling road?!?!?! Nonsense. It's making the same or perhaps very slightly less due to the lack of cold airflow, but any significant differences are down to errors in the rolling road's calculations - the engine is identical and so is the power it makes!

IMHO of course. :-)

 

Double contact patch and the vehicle being pulled down onto the rollers inflate the losses dramtically on some rollers. Tyre pressure/type/wheel size/gearing all make a difference. AWD vehicles are getting the drag losses through a second pair of tyres that the 2WD vehicle is not. Just some reasons why I like the info from a coastdown run, flawed as it is in some respects.

If you want to know how fast it goes measure it on a bit of tarmac with timing gear/accelerometers.