I need your help understanding drivetrain-induced losses. My recolection is that for a C5 with the 6 speed, drivetrain-induced losses have typically been found to be 15%. Is that just for a stock car, or is the loss always 15%? It doesn't seem that the loss should go up as the horsepower increases. So, if mods are done that yield a 50% increase in hp at the flywheel, would the net drivetrain-induced losses then be reduced to 10%?

For example, if the flywheel hp starts at 345hp, then the drivetrain-induced losses are 345 * .15 = 52hp. A 50% increase in the flywheel hp gives 517hp, and a 52hp drivetrain loss then give a 10% loss. So, should we quote drivetrain loss in hp, rather than %? Just curious. Thanks, John

 

I don't think so, it takes 15% to move the gears in the trans, rear end, etc. & 20% if its an automatic. Those are not hard #'s but their close. :cheers:

 

The easiest way to describe drivetrain loss is like this... If the engine's flywheel was bolted to the rear tire, then you would have no loss. Since the engine has to spin a driveline, transmission gears, ring and pinions and axles and everything runs on bearings or roller bearings with races. All of that mechanincal motion takes ENERGY from the engine to turn above and beyond just the rear tire that we first discussed.

The Drivetrain loss remains consistant 15% (for manual trans) across the board regardless if you are using a 345 hp motor or a 500 hp motor. Some arguments have been made here and there that once we get to a certain HP that we should just pick a HP number for a drivetrain loss and stick with it. So far I have not seen any "Official" information saying that is what it is that we should do.

vetterdstr :cheers:



[Modified by vetterdstr, 6:44 AM 4/6/2003]

 

Thanks, guys! John

 

Lets take a look at how gears get hot:

Assume that we have an engine producing 350 lbft of TQ and gears of 4" in diameter (rolling radius of the gear mesh). Each gear tooth sustains a load of 2100 lb of force (only one tooth is engaged at a time, 4" gear has a 2" lever frm the shaft). Even if the gear is made of high quality materials, 2100 lb of force will distort the tooth surface and actually bend the tooth by 0.001" or so.

If you ever took a coat hanger and bend it one way then the other until it breaks from fatigue, you know that the breakage point get hot. This is the kind of heat we are talking about here.

Adding TQ adds to the tooth pressure and adds to the distortion and adds to the heat generated.

Now let us look at the tooth angle. Gear teath are inclinde to the axis of revolution to make for quiet gears. However, this inclination causes a portion of the TQ transmitted across the gear tooth to show up as side loads on the bearings carrying the shafts. These side loads also increase with TQ.

Notice that I did not mention the oil holding the two gear faces apart! The oil is there to prevent even more losses, and to cool the losses it cannot prevent.

And likewise down the whole drive line, U-joints/CV-joints, bearings all take their tool. In fact, its down right hard to put more than 99% energy through any particular device! There are plenty of sources for loss from the flywheel, across the clutch, down the drive shaft, through the transmission input shaft, across a gear face, out another transmission shaft, into the differential, out the differential, across the half shafts, and through the hub bearings. Lots of individual parts each taking a little HP and TQ--but it all adds up.

But the important thing to notice is that most of these losses are directly related to the TQ being transmitted. Only a few of these losses are related to the RPM at which it is transmitted. So, what we have found, is that a well designed driveline will loose on the order of 15% from the flywheel to the rear tire. And there is little one can do about this.

However, it is not quite a constant, and if you have an engine dyno graph and a subsequent chassis dyno graph, you can measure the loss with respect to RPM.

 

Wheew! thanks for the great explanation

 

And he didn't even mention centrifugal losses. As acceleration increases, centrifugal losses increase. That is one of the reasons you normally dyno in a higher gear.

Dave

 

I just love it when someone says it plain and simple. :yesnod: :cheers:

 

The 20%/15% rule doesn't apply to the C5. If it did, then why do A4s keep dynoing with only a 12% loss?

 

I'm confused here. Where did the 12% loss on an A4 come from? Halltech assumes a 22% loss on the A4 and most of us generally use 20% as the accepted A4 loss. I'm not trying to flame you here. I just want to understand this topic a little better.