Drive Train Loss?

[RedTwo]

What's a reasonable estimate for drivetrain loss on our cars?
The general internet consensus is 20-25% loss for a 4WD car.
I assume a dogbox or similar and pretty heavy clutches, super tight diffs and the like would lower this loss somewhat? Would they stay relatively constant until you hit the big 7, 8, 9, 1000 hp ranges?

Some hard numbers:
Pot dynoed his (otherwise) stock VR4 at 174 WHP (13.5 PSI), where as your stock cars were 195 BHP (7 psi?) at the factory. These numbers can't be compared directly but what if we bump up the stock figure? If we assume a very generous 10BHP per PSI we would be looking at 174WHP vs 255 BHP and 32% drive train loss, while 174 WHP with 25% loss is 232 BHP...

The Bonneville land speed VR4 claims around 15% - which leads me to believe it could be a 2WD Galant or just a highly polished speed machine?
I've read a stock 3000GT has about 25%. The Nissan GT-R has been measured at 17% drivetrain drag AND published with understated power figures. An 1100 mile Evo X has about 16%, Evo IX around 20%, Evo VIII around 20-25% (depending on year and assuming constant power level).

Food for thought:
(the first two cars that turned up in the search)
Garfield Wright put down 361 Front Wheel HP, while Nartanian put down 301 All Wheel HP. In theory Garfield only has 10-15% drivetrain loss and Nartanian has around 20-25% (or whatever value we come up with).
This gives us 401-425 BHP vs 376-401 BHP

 

[belize1334]

My car ran 11psi factory so that would be 40bph more by your estimate.

Also, a big part of drive train loss has to do with the torque necessary to accelerate the rotating parts of the drivetrain. So as a land-speed car approaches the speed where it is drag-limited, the drivetrain loss falls to a MUCH lower value consistent with the simple frictional losses. Now, those in turn grow as things spin faster but, I suspect, they never get as big as the mechanical issues during acceleration.

 

[Barnes]

Belize1334 pretty much nailed it. /ubbthreads/images/graemlins/rock.gif

 

[4thStroke]

There are two schools of thought on this one (maybe more, but just go with it).

Some insist it is a percentage, like mentioned.

I know some people who insist that for the Mitsubishis and Subarus, it's a loss of 50-60hp from flywheel to tires for the common manual street car.

 

[cheekychimp]

Quoting RedTwo:
What's a reasonable estimate for drivetrain loss on our cars?



I think irrespective of what your opinion is of what causes it, you are going to have to accept that this is going to be exceptionally difficult to calculate and even harder to generalize.

In a perfect world you'd throw the engine on a 'crank dyno' and get the power output of the engine then put it in a car and strap that to a dyno and calculate the loss. But is that loss accurate? Three different dynos will give three different power readings. As we know for tuning purposes, that's irrelevant as long as you do all your tuning on the same dyno, but for calculating drivetrain loss the 'actual' output at the wheels as you have mentioned is critical.

Where were all these cars dynoed? Was altitude taken into account, in other words is that 'corrected' horsepower?

Then you have the cars themselves. We tend to look at drivetrain loss as primarily loss through the transmission but wheel size, tyre size, condition of wheel bearings and all manner of things are going to create issues here if (as is being suggested here) we are talking about overcoming inertia and gaining momentum.

And what is inside the transmission? You mentioned a GTR. As I recall the GTR AWD system is somewhat unique in that the car is essentially a RWD car until it encounters loss of grip whereupon it diverts power forward making it AWD. Some guys in the UK actually adapted a GTR transmission to run in AWD from launch at the drag strip and return to RWD once the car had hooked up in order to take advantage of the lower parasitic loss of 2WD throughout the remainder of the quarter mile. So was the GTR drivetrain loss measured in RWD or AWD?

Do differentials have an impact on this? I'd say they have to have. The whole point of a differential is to prevent a wheel from spinning, so in effect they save more power than they use but whether they work through gears, clutch plates or liquid viscosity (in the form of a viscous coupler) they must take some energy. Is an electronically controlled differential more efficient than a mechanical one? When a mechanical differential has not engaged does it still create any parasitic loss?

There are just so many different variables involved here I just cannot see how you could come up with a figure, unless you are talking about a completely stock car where you can limit the variables. But even then the information or figure you eventually come up with is rendered meaningless if you subsequently fit a Cusco centre diff, wear out a bearing, change your rims, fit wider tyres etc etc etc.

I think I'm coming across as negative and condescending here and I don't mean to be. I'm just wondering where you are going with this Rangi? Just seeing if our cars need to make more or less power to be on a par with an EVO IX or something else?

 

[RedTwo]

I don't know if the Nissan GT-R still uses the switchable hydraulic differential system (HICASS or ATTESSA maybe?) of the older Skyline GT-Rs so I can't answer that.

I have several reasons for determining this, chiefly I've got to compare any dyno figures with a pair of Skylines and a MR2. It would also be useful if the 'VR4 Preservation Society' knew everything there was to be known about these cars /ubbthreads/images/graemlins/laugh.gif

that.... and most E/T 'calculators' require flywheel HP /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/tongue.gif

 

[cheekychimp]

OK, I've got you now. So you want to work backwards from current dyno figures and establish how much power you are making at the crank with your current mods?

 

[slugsgomoo]

assuming it's a percentage is kind of silly.

to pull numbers from my ass, at 195 crank horsepower, 20% is 39. at 1000 crank horsepower it's 200. Does it really take an extra 161HP to turn the same drivetrain that it did with 39 horsepower before?

I'll accept that losses go up, but I don't think that they're linear losses.