Driveline losses...

[Jon 74 260Z]

An insomnia-induced question for you guys:

 

Anybody know the relative contributions of the different components to the driveline losses?

 

There's been discussion ad nauseum about total losses, particularly with respect to chassis and engine dyno numbers... and I don't wanna go there. I'm asking if we assume a 17% total driveline loss (pick a percentage if you don't like seventeen), what can reasonably be expected to be attributed to the diff, or the tranny, or the rotational losses of the driveshaft, or whatever?

 

The discussions I've seen that relate to diffs or transmissions say that if one is "better" than another, it's most often because it is more robust (ie, it "can handle the power"). But I haven't heard the comment that says "I like X-brand diffs because it transmits power with 93% efficiency versus Y-brand at 90.5% efficiency." Similarly, is there any data that quantifies what those mega-dollar CF driveshafts actually do?

 

Is this info out there and I just missed it??

 

TIA -- Jon

[MAG58]

There probably hasn't been a discussion on efficiency vs. diff simply because stuffing oddball 3rd members into cars isn't exactly a common occurrence in most car circles.

 

THAT SAID:

The highest losses should be in a transmission, provided both are equally broken in, as far as I would come to believe. Especially an automatic transmission. It has many secondary systems that draw on engine power to operate.

 

I would have to say that in relation to parasitic losses, the bigger and stronger the transmission, the higher the losses. The larger, wider gears in a bigger tranny has more total contact area on the gears, and thus more friction (given all else equal).

 

Finally, the CF d-shaft should not have any bearing on how well it transmits power (from a total efficiency standpoint) as there is no friction or moving parts on the driveshaft itself. It simply transmits power. The idea of the CF d-shaft is that there is less rotating mass so it can more rapidly change rotating speed than a heavier unit. The part of a d-shaft that would alter efficiency are the U-joints/CV's as they're responsible for changing direction of power, and unless they're 100% friction free (nothing is), there will be a loss of efficiency as the direction of torque is altered.

[sweetleaf]

Well stated. Not much point getting into too much detail on this. The one thing you CAN change that will alter the total drive train loss, is using different fluids in the transmission and diff. I.E. 10-30 over 80-90 . Etc. There is much that has to be considered, longetivity of said fluid, parts, heat dissipation etc. The only other factor maybe the difference in loss between CV's VS UJoints.

[johnc]

IMHO, just to be a dick...

 

Back when I had my race engine built at Sunbelt it was run for many hours on their engine dyno and the horsepower and torque results were consistent within some minuscule percent after all the tuning was done and the engine broken in.

 

That same engine with the same tune in the EMS (sealed and encrypted so i couldn't F if up), same intake, header, etc. gave wheel horsepower numbers from 280 to 294 after corrections. That's a variation of 4.5% compared to the original engine dyno number.

[WildBill]

Yeah, I think people put overstate drivetrain losses a bunch. Factory power ratings are fudged this way and that so don't mean a whole lot. Even wheel dynos don't tell the whole story - i.e. air resistance while driving down the road. Synthetic fluids reduce drivetrain friction losses as stated above. The main thing, is that smaller, lighter components typically use less power than larger, heavier components. But at the potential cost of reliability due to strength. For example, my El Camino has a turbo 400 transmission and 12 bolt rear end. If I ran a turbo 350 and a 10 bolt, I could put more power to the ground because the smaller, lighter parts sap less power. But the odds of the staying together behind a 502 with nitrous is not as good. As for how much power each component saps, it's going to be different with different units (four or five speed, size ring gear, etc.), fluid type and weight, temperature, etc. Hell, tires have different rolling resistance, as well as different road surfaces! Try not to lose any more sleep over all this!

[burninator]

I think that for the most part it's guess work, but for what it's worth I think 4.5% is quite low. You can't use johnc's results as a judge of drive-train loss because even different chasis or engine dyno's will report different power to the wheels by as much as a few percent. The results aren't comparable at all.

 

Another thing is what MAG58 said about light weight components (CF drive shaft) not making a difference in power output is plainly incorrect. A drive shaft has to be accelerated just like the gears in the transmission. It has mass and a heavier one will be harder to accelerate and give more drive-train power loss.

 

If you have access to a dyno (and I'm not sure it will work with all dynos) one thing you can do it run your car up and then put it in neutral and let it coast down by itself (don't apply the dyno brake) for a while before stopping the run. This should give you a negative power curve. That would give you an estimate of the amount of power your drive train leaches. This is not exact because not all of your drive train is still spinning in neutral, but it would give you an idea. If your dyno uses the rpm signal to calculate power it won't work, and if it uses rpm to calculate torque then that won't work either (most I think only use RPM for one or the other).

 

Next time I get my car on the dyno I'll try it. Won't be untill after I swap the engine though.

[woldson]

Somewhere there is thread about lightened fly wheels and potential dyno differences.

 

It talks about how much differences might be tallied base on less weight at greater circumferences, it is really a good read burninator.

 

As far as I know, no one ever tested a "spin out" on the flywheel.

[RTz]

Another thing is what MAG58 said about light weight components (CF drive shaft) not making a difference in power output is plainly incorrect. A drive shaft has to be accelerated just like the gears in the transmission. It has mass and a heavier one will be harder to accelerate and give more drive-train power loss.

 

MAG is plainly correct. Put your car on a static dyno and measure power at a specific RPM. No driveshaft material will change that number.

 

The inertia of a wheels, brake rotors, driveshaft, flywheel, etc, etc, will influence acceleration, but not power. They are not part of a drivetrain loss calculation. Friction is.

Edited October 3, 2009 by RTz

[RTz]

Here's a silly example...

 

A. Assume no tire slip.

B. Assume same size, non-vented brake rotors front and rear. And they weigh 50lbs each

C. Assume no brake pads (keeping it simple for this example)

 

 

Run the car at a dragstrip, with the front rotors removed, but the rears installed. Run the car again with the front rotors installed, but the rears removed. What do you get? The same exact timeslip.

 

Put the same car on a static dyno, same two scenarios, and the HP numbers will show the exactly same regardless of where the rotors are mounted, just like the dragstrip confirmed. Just like the real world.

 

Put the same car on an inertia dyno, same two scenarios, and the HP numbers will show higher with the rotors on the front, and lower when they are installed on the back. Funny, since the car ran the same 1/4 mile time.

 

The point is, inertia dyno's can be deceptive, especially to those young men that think installing a lighter wheel produces more power because the inertia dyno said so. They gained acceleration, not power. They reduced inertial losses, not drivetrain losses.

Edited October 3, 2009 by RTz

[burninator]

I agree with your example but disagree with your conclusion. We may just have to agree to disagree on this one. Inertial losses are a component of tototal drivetrain loss, it's not all one or the other.

 

Nobody would expect the dyno number to change by doing anything to the non-drive wheels. I think what matters is power you can put to the ground and lightening drivetrain components does produce a measurable and useful change to the power number on an inertia dyno. I understand that on a non-inertia dyno those changes wouldn't be seen, but that's just a case of needing to know what your dyno is capable of measuring. And people who use inertia dynos need to be just as aware to avoid misunderstandings.

[woldson]

You find that other thread?

[johnc]

I think that for the most part it's guess work, but for what it's worth I think 4.5% is quite low.

 

FYI... the 4.5% number I mentioned above was the variance between chassis dyno runs, not the driveline loss.

[SHO-Z]

Finally, the CF d-shaft should not have any bearing on how well it transmits power (from a total efficiency standpoint) as there is no friction or moving parts on the driveshaft itself. It simply transmits power. The idea of the CF d-shaft is that there is less rotating mass so it can more rapidly change rotating speed than a heavier unit. The part of a d-shaft that would alter efficiency are the U-joints/CV's as they're responsible for changing direction of power, and unless they're 100% friction free (nothing is), there will be a loss of efficiency as the direction of torque is altered.

 

The straighter the dive shaft the less hp loss there will be. This is also true for the haft shafts on a Z, U-Joints do take away hp. Straight cut gears in the trans have less hp loss but are not as strong as angle cut gears. In high gear on some Trans you are basically straight through but still have rotating gears in the lube oil that trough oil to lube the trans. The design of the differential also makes a difference like the trans straight cut gears in the trans have less hp loss but are not as strong as angle cut gears.

[Jon 74 260Z]

So if I've got this straight:

 

I could theoretically measure the hp difference of a lighter driveshaft on an inertia dyno to quantify the effect... but there would be a 4.5% margin of error in the results.

 

There is one opinion that says the trans is lossier than the diff (which is the opposite of what I would have guessed).

 

And there is a semantic difference about whether or not "driveline losses" includes "inertial losses."

 

That about sum it up?

[johnc]

My contribution to this thread is that trying to derive the power output of an engine by calculating driveling losses will be off (in my example) by at least 4.5% because of measurement error. That's the closest we can get it.

 

Using my example above and a guess of 15% driveline loss the engine in my car that repeatedly made 325 horsepower on one of the best engine dynos in the country now supposedly makes 330 to 346 horsepower. It didn't. The VE/BMP calcs would show its impossible to hit the 346 horsepower number given the 7,500 rpm limit.

 

But I could post on most any Internet message board that my 294 whp means the car made 350hp and most folks would believe it and say "WOW!"

[woldson]

Straight cut gears in the trans have less hp loss but are not as strong as angle cut gears.

 

Are you sure about that?

[burninator]

Straight cut gears in the trans have less hp loss but are not as strong as angle cut gears.

 

Are you sure about that?

 

I'm not sure about hp loss, but with respect to strength angle cut gears have wider teeth and engage more gently with multiple teeth engaged at once, but straight cut gears are usually made with fewer and thicker teeth and can be made stronger that way. Angle cut gears also transfer some force sideways which may add to friction and/or transfer stresses to other parts of the transmission. I've heard of split transmission cases that were blamed on the sideways load on the gears.

[burninator]

You find that other thread?

Probly, I dunno... There are lots of threads related to this subject.