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Discussion Starter #1
I had a discussion last evening with some one and all it did was confuse the hell out of me.

Lets say the M5 makes 400 hp and driveline losses equal 15%. That yields about 340 hp to the rear wheels. So far so good. I undersatand that a certain amount of power gets eaten up in the driveline.

Now lets say we take that same car, do some work to the engine and it now makes 500hp. Still so far so good.

Now it gets confusing to me.

The person I talked to stated that the 15% driveline losses still applies, so now the driveline losses are no longer 60 hp, but are now 75 hp. I asked him to explain how upping the power of the engine could increase the driveline losses but he didnt have an answer.

I walked away from this discussion thinking the guy was full of crap, but I want to make sure so I'm asking those of you that might know, is that right , or are the driveline losses a fixed amount of work in hp due to the design as I believe they are?
 

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Pete said:
I had a discussion last evening with some one and all it did was confuse the hell out of me.

Lets say the M5 makes 400 hp and driveline losses equal 15%. That yields about 340 hp to the rear wheels. So far so good. I undersatand that a certain amount of power gets eaten up in the driveline.

Now lets say we take that same car, do some work to the engine and it now makes 500hp. Still so far so good.

Now it gets confusing to me.

The person I talked to stated that the 15% driveline losses still applies, so now the driveline losses are no longer 60 hp, but are now 75 hp. I asked him to explain how upping the power of the engine could increase the driveline losses but he didnt have an answer.

I walked away from this discussion thinking the guy was full of crap, but I want to make sure so I'm asking those of you that might know, is that right , or are the driveline losses a fixed amount of work in hp due to the design as I believe they are?
15% of 500 is: 500 * .15 = 75
425 hp at wheels.

If you put different components in, I would think the drivetrain loss would change, thats sort of the point of parts that reduce friction. How much friction loss the parts have, I dont know, not much, so 15% still applies well.

However, the guy is taking a general figure of 15% loss and applying it to the new hp figure. If you had even more power:

600 hp * .15 = 90
510 hp at wheels
 

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Discussion Starter #3 (Edited)
Hi de Witt,

Maybe I wasnt clear in my question.

I should have asked, " How does increasing the engine power alone also increase the driveline losses in hp?"

If the driveline takes 60 hp from an engine producing 400hp, why would it take 75 hp from an engine producing 500 hp all other things still being equal? It should still only take 60 hp.

I admit I dont work with cars, but increasing hp on industrial machinery with electric motor inputs does not increase the power required by what it drives, it just delivers more power to the driven component.

What is it about a cars driveline that caused this person to say the 500 hp engine will deliver 425 to the rear rather than the 440 I would expect?
 

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Discussion Starter #4
Thanks de Witt,

I am going to chalk this up to a year end brain drain.

I was only considering steady state operation and did not think about acceleration. DOH

Boy I cant wait until tomorrow for my new brain cell budget.
 

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Hmmmm.

Driveline Losses Steady State should equal a fixed amount independent of engine HP. This makes sense.

At acceleration, the same rotational masses should require the same power to accelerate.

Maybe I aint getting something here, but it seems that the % loss should decrease as HP increases...?????? :confused2
 

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Discussion Starter #6
hi jjc1843,

Correct, during steady state operation, the parasitic horsepower losses of the tranny are the same for the two engines at any given speed and gear. How could it take more hp to travel 75 mph with only a more powerful engine? It takes X amount of power to go 75 and any more hp the engine has is not put to use.

I thought like you did, the % should decrease with increasing hp. Basically I thought any increase at the crank should directly go to the wheels after the 60 hp loss.

If the two engines were accelerated at the same rate I still believe the tranny losses would be the same, since you are not taking advantage of the added power.

"At acceleration, the same rotational masses should require the same power to accelerate. "

That all depends on what the "rate of acceleration" is. The more powerful engine can accelerate the car faster. That quicker acceleration causes a greater temperature and friction differential with the less powerful engine, due to greater forces in the driveline. Maybe thats what gets you back to that ballpark 15% figure, and its maybe only applicable at maximum performance operating conditions.

At least thats how I am rationalizing this until I get my new brain cell infusion.:1:
 

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Now this is another great thread.

The calculations used for driveline loss in most cases is speculative at best. Certainly one hopes that the orator has some basis in fact for what they share maybe even some real time experience.

I can share that I have just enough of both to be as they say “dangerous”.

Years ago we used to build motors and then tune and dyno them out of the vehicle. Largely because chassis dynos were few and far between. Then as access to chassis dynos became easier we used to still tune and dyno the motors out of the vehicle however we would often times also run the vehicle on a chassis dyno.

We found that 15% was an average loss ratio. Generally less with manual transmission cars, in fact much less with a simple in and out box and then a bit higher for those with automatic transmissions.

Now what I find interesting is today the lubricants insure MUCH less friction, the assemblies tolerances are better therefore it stands to reason that the loss % should be no greater than days gone past, in fact less if anything yet many are soliciting drive loss percentages close to and in fact over 20%. The effort put forth by the OEM manufacturers has been exhaustive in this area because it was a no brainer way to increase MPG figures.

Now to what it take to move a given size object a given distance in a given period of time, this is a physical number that can be reproduced mathematically and remains constant. So in the case of driveline loss; it remains static….which means whatever is required to produce the results will not change based on what is available. However because the percentage of increase following a given modification is typically very small the applicable driveline loss percentage would change so little that most marketers will comfortably share the same percentage because fails within the quote “acceptable industry marketing standards” which today seem to span from 15% up to 22%......... I suppose it depends on how aggressive you want to appear because most…….in fact almost “NO ONE” and they know this will ever validate a claim, they simply accept it as Gospel.

For Example 400 HP at the crank 15% loss = 340 RWHP

450 HP at the crank less the required 60 HP = 390 RWHP or in this case a 13 -14% driveline loss percentage

By the way, it does not take 60 RWHP to maintain constant motion in these vehicles on a flat surface.......the loss is based on the amount of time you allow and energy used to go a certain distance.......not the distance you cover.

Because I no longer tune and dyno with the engines out of the vehicles I too have adopted a percentage loss that I use for conversation purposes. The percentage I use is 15% but I also make it clear that this is a seat of the pants number based on experience over time not necessarily applicable to the particular vehicle in question.

Happy New Year Folks

Shadowman
 

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Discussion Starter #8
Thanks for your input shadowman, as always , its very appreciated. I'm going to delve into this a bit deeper and see what I can come up with.
 

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Discussion Starter #9
Well now that I got my full load of 2005 brain cells engaged, I thought about this a bit more.

If accelerating at maximum power and the applied powers were different for each engine at the crank or transmission input, i.e., different acceleration rates, then the same thing applies, if the bigger one was putting 500 hp into a manual transmission at the crank, the loss would be roughly 75 hp vs. 60 hp for the 400 hp engine...but the 400 hp engine would take longer to get to the same speed, and the total energy loss, or the area under the curve, or the integration of the acceleration curve, would be identical to the total energy loss of the bigger engine, at 75 hp instantaneous loss, over a shorter period of time.

Thanks everyone for your valuable input, finally got this cleared up in my head.
 
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