bigman

iTrader: (1)

Forget all the mathematics and take the C5 crank horsepower and wheel horsepower and measure the percentage loss. Then do the same for the C5 Z06 which has the same drivetrain, and see the percentage loss. If both manual c5 and z06 lose similar percentage from rated power to wheel power, then you know that it's a percentage.

dynocar

Amen, we have been chassis dyno testing everything from the lowest HP factory stock VW to the highest HP Vipers for many years and we have always come very close with "a percentage" of estimating FWHP and comparing it to the manufactures SAE net ratings. I would estimate this to be within 5% and contrary to popular believe, much of that is from the differences in HP from similar car to car. "In theory, there is no difference between theory and practice. But in practice there is." (quote from unkown author)

J-Rod

According to Harold Bettes of Superflow , you know, the Dyno people. Its a static loss, and not a percentage.

As for A to B testing here is what we've found down under. See, down there, many of the shops have Engine dynos and Chassis dynos. The dynos are made by the same company, Dyno Dynamics.

The Dyno Dynamics engine dyno is nice as it is fully OBDII compliant so you can monitor your PCM while you dyno.

Anyhow, in speaking with Peter, they have done A to B testing up to around 730 RWHP. The loss through their cars is right at 110 HP regardless of HP. This isn't with 1 or 2 pulls we are talking about. I think Peter has had 8 or more different engines including H/C and 422 strokers. N/A, S/C, and single turbo setups. In one marathon push they did 8 or more different sets of heads and 10 or 12 different sets of headers. This ammounted to over 300 dyno pulls on one engine. After which it was installed into a 3900lb Ute and went 11.4 @ 120 MPH in 2002-2003.

Tony Mamo has tested his 346 and 383's, and then swapped them into a C5. Loss froma Superflow to a Dynojet was ~50HP. Even when Hp went up, loss remained constant.

Adrenaline_Z

There is some awesome reading here, but this topic got really complex for
such a simple answer.

If nothing changes in the driveline and you toss another motor in front of
the transmission, the efficiency of the driveline does not change.

No offense to anyone that provided details (because I like that sort of thing), just keeping it simple.

J-Rod

Correct, but some folks question how you figure that efficiency, as a percentage of total crank HP, or as a fixed loss in the driveline.

dynocar

This discussion could go on forever without a "simple answer" because there are other variables that are not even being considered which would even make it more "complex". Variables such as, what test proceedures were used on A to B testing, RPM forward or reverse sweep tests and at what acceleration rates, or RPM step tests that are commonly used on engine dynos that could not be compared to RPM sweep tests commonly used on chassis dynos. Another example, it cannot be a fixed HP loss on an inertia dyno because a low Vs high HP engine would accelerate at different rates and we would have different engine/drivetrain inertial losses. When possible, all of our HP tests are performed at a constant 300 RPM per second sweep rate which helps to minimize this inertia difference from test to test on our load control chassis dyno. An inertia dyno cannot control this variable which would affect our ability to give a "simple answer".

J-Rod

You bring up good point on an interia dyno.

My comments were based on observations of an Eddy Current load dyno which can load and hold or sweep RPM ranges. Again, it comes back to a constant there.

Adrenaline_Z

I think someone needs to develop a "Spintron" device for the driveline.

Something that connects to the input of the transmission and can be
tested in the car to account for chassis flex and suspension geometry.

Wouldn't that be cool...

Sparetire

This ought to be stickied or archived somehow. best discussion on it that I have read. ncie job keeping it in laymans terms.

In the end I think the best answer for the average guy is to figure on a constant power loss combined with a small percentage increase in power loss as loads go up.

vetred

DAMIFINO,........But,.....seems to me, that there you are,... after that hart transplant, and you got 552rwhp. No what I mean? you need to get to work insulting all those "imports!

Zeus

As much as what J-Rod says makes sense, as a mechanical engineer in training (read: no actual experience, but just talking out of my ***), it seems as though the loss would be a percentage.

Even if you have a perfectly frictionless drivetrain, you're still going to be losing HP through it. The reason is that you have a bunch of components such as wheels, gears, rings, pinions, shafts, etc that all have mass and all need to be rotated along with the engine.

Going back to one of the most basic equations in all of physics: F=ma, you have a fixed mass. You want the acceleration to be as great as possible, therefore more force is "wasted" accelerating these components. This is why the heavier wheels lose up to 20HP on the dynos. It has much more to do with overcoming inertia than any sort of friction.

It's more complicated than that as you need to take into account things such as the moment of inertia, but the basic idea is the same.

LS_RX-7

While risking sounding rude, I must say that I strongly disagree, Zeus.

dynocar

If you believe in the "fixed" or "constant" HP loss thru a drivetrain, such as 110 HP, then I physically must have more then that because I have often pushed such cars so my buddys could pop their clutches to get their engines started.

Adrenaline_Z

It doesn't take 110 HP to turn a driveline at 2 RPM.

You could actually calculate your power if you knew the distance you pushed
the car, the time it took to move that distance and the weight of the vehicle.

As for driveline loss, it's a static/fixed/or constant amount to maintain a certain
RPM.

The other discussion about interia, methods of measurement are purely valid
but a overshadowing the intial question.

If you connected an electric motor to the driveline, you could measure the
wattage needed to maintain a certian RPM. That value doesn't change just
because you have a different motor under the hood.

OKcruising

That's a nifty method. There is a constant HP to Kilowatt ratio, 1:1.4ish something. Why not hook a dynamo with specific efficiencies upto to the wheel hub ? Then you could compare that location's output to the engine crank power generation via a dynamo.
Or is this how they build some of the machines?

dynocar

Previously stated, "If you connected an electric motor to the driveline, you could measure the
wattage needed to maintain a certian RPM. That value doesn't change just
because you have a different motor under the hood."

Respectfully disagree, every powertrain engineer that I've ever talked to or related articles read has stated something to the affect that, "if you increase the pressure (power) at a given RPM, you increase the friction of every moving part dramatically results in higher losses, less mechanical efficiancy." However, I trust more what I observe then what I hear or read and my observation from 15 years of full time chassis dyno experiance has indicated to me that if I can accurately predict the FWHP of an engine, low or high HP, from the wheels with a standard percentage factor for various types of drivetrains, it probabably isn't close to a fixed loss.

Adrenaline_Z

Not sure about the Dynamo, haven't read much about that.

I would imagine the power required to maintain a constant RPM would be quite
low. The required power to turn an unloaded driveline after reaching the desired RPM is extremely low.

It's the power required to reach the target that is substantial. Add vehicle
mass, wind resistance, and acceleration to the picture and that's were these
guys are shining in their replies.

Adrenaline_Z

I will respectfully disagree back.

The motor will only produce enough power to overcome the load.

So let's say the driveline (in what ever gear combination selected) is turning
at 1000 RPM.

Once the driveline has reached and stabilized at 1000 RPM, the power required
to sustain that RPM will not change.

Have a read here, maybe we're not on the same page.

http://www.glenbrook.k12.il.us/gbssc...les/u6l1e.html

If you think about this another way, grab a torque wrench and set it to 500 ft.lbs.

Now apply that wrench to a finger tight bolt and nut. The force required to turn
the nut will be far below 500 ft./lbs.

Just as the more powerful motor will not use extra energy to turn the same
driveline at the same RPM.

dynocar

Quote from Adrenaline_Z, "If you think about this another way, grab a torque wrench and set it to 500 ft.lbs.

Now apply that wrench to a finger tight bolt and nut. The force required to turn
the nut will be far below 500 ft./lbs."


The original question ralated to RWHP, drivetrain losses Vs different engine HP levels that is normally measured on a chassis dyno, I'm trying to stay on that page the best I can. I understand the formulas that I was directed to but they are not considering all of the elements of a drivetrain, such as friction. Using your example of a torq wrench, if we use a wrench with a scale, not a click style, if the bolt is only finger tight, the pointer will not deflect much, but if we tighten the fingers (more friction due to more HP) the pointer will deflect more and the fingers (drivetrain) will be absorbing more energy. If a gear (rear wheels) was attached to the other end of the bolt, less energy would be transferred to it. Equating this to a vehicle's drivetrain, a higher HP engine will cause more friction on every part in contact with another part thus increasing the amount of drivetrain losses, it's not a "fixed" amount of loss at a given RPM with different HP levels as previously state by some, that is if I understand them correctly. Do you see where I'm coming from?

Adrenaline_Z

The original question was based on an assumption that all drivelines lose
15% of crankshaft power. I do not agree with that statement, nor does
the gentleman asking the question.

My position concerning driveline losses has been stated - it is a fixed loss
to maintain a constant RPM.

Whether the higher output motor creates more pressure and friction is not
what I'm debating. I'm soley discussing the total loss while maintaining a
constant RPM.

Another arguement is that chassis dynos are probably not the best tool to
measure driveline losses because of the variables and methods used to acquire
the data (IE: different types of dynos - which is better...which is correct, if any?).

This is where I believe the confusion comes into play.