VW Vortex - Volkswagen Forum banner
1 - 20 of 26 Posts

·
Registered
Joined
·
2,542 Posts
Discussion Starter · #1 ·
I'd like to hear an argument as to why some people think it's a fixed number.
The argument for percentage: Friction and Heat increase as power increases.
The argument for fixed number: Using same hardware, drivetrain loss stays constant.
 

·
Registered
Joined
·
563 Posts
Re: Drivetrain Loss: Percentage or fixed value? (lojasmo)

Quote, originally posted by lojasmo »
Clearly a percentage. Drivetrain loss in a veyron will be about half the engine output in my mighty 1.8T

I agree with you, but I think the OP is asking about whether it's a fixed number or percentage on the same car, with the same engine, when said car/engine is modified.
i.e., take a 2.0T, with 200HP and 207TQ, chip it. It now has 272HP, 254TQ. Is the difference at the wheels the same as the difference at the crank? Or do you only gain a fixed percentage of your crank gains as you increase power?



Modified by acuraudi at 12:16 PM 4/27/2010
 

·
Registered
Joined
·
2,820 Posts
Re: Drivetrain Loss: Percentage or fixed value? (Lepsis3942)

Good topic! Lots of guys on corvetteforum think the loss is pretty fixed, even though engineering textbooks advertise the losses as a percentage. Dynos can be very misleading, but I've always wondered if the percentage loss is correct only in a certain band of operational range . . .
Regardless, more power definitely means more heat, which implies more frictional losses.
 

·
Registered
Joined
·
122 Posts
Re: Drivetrain Loss: Percentage or fixed value? (Lepsis3942)

Percentage, the faster you accelerate (your car or your drivetrain components) it takes exponentially more power/force to accomplish faster and faster acceleration. Friction should also gradually increase with increased speed (in most applicable circumstances).
So to answer your question, to make x amount of mass go x amount of speed, it will require the same amount of force, however we're not talking force, we're talking power.
So its actually a semi-fixed percentage that should gradually increase in parasitic loses as power/acceleration goes up http://****************.com/smile/emthup.gif
 

·
Banned
Joined
·
16,878 Posts
Re: Drivetrain Loss: Percentage or fixed value? (uberR32)

I'm not mechanical engineer, but it seems to me that drivetrain loss is best represented as a percentage, as it is certainly not a linear figure from 1000 RPMs to 8000 RPMs. I think this can only be used accurately for stock cars, though.
For example - take an FD RX7. Stock with 255 hp, factor in 18% drivetrain loss. That comes out to 46 hp used to turn the drivetrain. Add on a GT35R, fuel mods, etc., and no you're at 430 hp. The same 18% rule would state that it now takes 77 hp to turn the exact same, untouched drivetrain. Obviously heat and friction will come into play, but 30 more horsepower to turn the same components? Call me skeptical.
I would think the best representation for loss on a modded car would be to take 20-25% of your stock horsepower and factor that into your modified numbers. Like I said though, I'm no engineer.


Modified by djsheijkdfj at 12:55 PM 4-27-2010
 

·
Registered
90 VW Corrado, 96 BMW 318ti, 03 VW GTI 20th, 21 Jeep Gladiator, 21 Jeep Wrangler
Joined
·
7,858 Posts
Re: Drivetrain Loss: Percentage or fixed value? (Grey Fox)

Quote, originally posted by Grey Fox »

So its actually a semi-fixed percentage that should gradually increase in parasitic loses as power/acceleration goes up http://****************.com/smile/emthup.gif

I would have to go with this as well. Since it's a constantly changing set of variables you can't really have a constant percentage. I'm thinking that you may have to get into some calculus to really figure it out for certain (been years since I've used that).
I may just be talking out of my ass but it sounds good to me.
 

·
Registered
Joined
·
122 Posts
Re: Drivetrain Loss: Percentage or fixed value? (djsheijkdfj)

Quote, originally posted by djsheijkdfj »

For example - take an FD RX7. Stock with 255 hp, factor in 18% drivetrain loss. That comes out to 46 hp used to turn the drivetrain. Add on a GT35R, fuel mods, etc., and no you're at 430 hp. The same 18% rule would state that it now takes 77 hp to turn the exact same, untouched drivetrain. Obviously heat and friction will come into play, but 30 more horsepower to turn the same components? Call me skeptical.
Modified by djsheijkdfj at 12:55 PM 4-27-2010

The reason it would take another 77hp "to turn the exact same untouched drivetrain" is because you're trying to accelerate that same drivetrain faster. Faster as in faster accelerating not faster in terminal speed.
It will always take the same amount of force to get a fixed amount of mass to a fixed speed however it takes exponentially more power to accomplish increased acceleration.
Rate of acceleration/power is the crucial thing here that makes it percentage rather then a fixed amount.
 

·
Registered
Joined
·
745 Posts
FV-QR

So you're telling me no one has ever put an engine on a dyno to get the flywheel HP, then drop it in a car and dyno to get the WHP, then mod the engine and do the same song and dance all over again to see what the difference was?
 

·
Registered
Joined
·
122 Posts
Re: FV-QR (Beltaine)

Quote, originally posted by Beltaine »
So you're telling me no one has ever put an engine on a dyno to get the flywheel HP, then drop it in a car and dyno to get the WHP, then mod the engine and do the same song and dance all over again to see what the difference was?

I'm sure it has been done, but its not entirely necessary, the answer is covered in here.



Modified by Grey Fox at 1:34 PM 4-27-2010
 

·
Registered
Joined
·
5,284 Posts
Re: Drivetrain Loss: Percentage or fixed value? (djsheijkdfj)

Quote, originally posted by djsheijkdfj »
I'm not mechanical engineer, but it seems to me that drivetrain loss is best represented as a percentage, as it is certainly not a linear figure from 1000 RPMs to 8000 RPMs. I think this can only be used accurately for stock cars, though.
For example - take an FD RX7. Stock with 255 hp, factor in 18% drivetrain loss. That comes out to 46 hp used to turn the drivetrain. Add on a GT35R, fuel mods, etc., and no you're at 430 hp. The same 18% rule would state that it now takes 77 hp to turn the exact same, untouched drivetrain. Obviously heat and friction will come into play, but 30 more horsepower to turn the same components? Call me skeptical.
I would think the best representation for loss on a modded car would be to take 20-25% of your stock horsepower and factor that into your modified numbers. Like I said though, I'm no engineer.

Modified by djsheijkdfj at 12:55 PM 4-27-2010


I used to think like you did a few years back. What finally made sense to me that it takes 46 hp to spin the drivetrain because that's what 18% of 255 is. Not that it takes 46 hp period. Make sense? You have your stock loss, but its not like OK the crank/dyno hp was 300/250 so it takes 50 hp, it's that it loses 16% of its power. You assume 50 hp, but that's the answer to the equation at that exact power level. You can't assume it's a fixed #.
Also the accelerate the same components faster thing if you are more physics inclined.
 

·
Registered
Joined
·
79 Posts
As has been stated, it is NOT a fixed number but rather a sliding scale of diminishing returns. If you have 200hp at the crank and say, 150whp that is a 25% loss or an absolute value of 50hp. If you double the engines power to 400 crank hp your whp will not be 300whp (a 25% loss) NOR will it be 350whp (a 50hp loss) but somewhere inbetween. Probably 320-330whp-ish depending on the drivetrain weight, friction, temperature, oil viscocity etc. As you make more power it WILL take more power to accelerate the same components in terms of absolute value but less in terms of overall percentage. Make sense?
 

·
Registered
Joined
·
122 Posts
Re: (Mothball)

Quote, originally posted by Mothball »
As has been stated, it is NOT a fixed number but rather a sliding scale of diminishing returns. If you have 200hp at the crank and say, 150whp that is a 25% loss or an absolute value of 50hp. If you double the engines power to 400 crank hp your whp will not be 300whp (a 25% loss) NOR will it be 350whp (a 50hp loss) but somewhere inbetween. Probably 320-330whp-ish depending on the drivetrain weight, friction, temperature, oil viscocity etc. As you make more power it WILL take more power to accelerate the same components in terms of absolute value but less in terms of overall percentage. Make sense?


Reading on this subject its also really hard to scientifically analyze it as its extremely hard to eliminate all the variables (just ambient temperature and barometric pressure can make a huge difference)
There's also changes in how cars respond to load.
If your load on your engine dyno isnt exactly the same as in the vehicle itself it can result in a change in horsepower numbers from what it should be based upon your parasitic loss estimate. This would show your parasitic loss estimate as wrong... even though it might actually be accurate.
 

·
Banned
Joined
·
1,395 Posts
Re: (Grey Fox)

Fixed at cruise, percentage under acceleration, IMO.
 

·
Registered
Joined
·
5,275 Posts
Re: (Shomegrown)

Quote, originally posted by Shomegrown »
Argue all you want, but Thermodynamics guarantees it cannot be fixed. It's not open for debate. And a percentage is not a bad estimation. Not perfect, but way better than a fixed number in most cases.

This.
 

·
Registered
Joined
·
10,092 Posts
FV-QR

Quote, originally posted by Shomegrown »
Argue all you want, but Thermodynamics guarantees it cannot be fixed. It's not open for debate. And a percentage is not a bad estimation. Not perfect, but way better than a fixed number in most cases.

Well I won't be satisfied until Discovery Channel and its busters of myth destroy a car or two to demonstrate this crazy theory of "Thermo-Dynamics"
 

·
Registered
Joined
·
24,098 Posts
FV-QR

Quote, originally posted by Shomegrown »
Argue all you want, but Thermodynamics guarantees it cannot be fixed. It's not open for debate. And a percentage is not a bad estimation. Not perfect, but way better than a fixed number in most cases.

I thought it was percentage-based largely because in stock vehicles, the energy required to drive the moving components scaled upwards with the power of the engine due to manufacturing standards.
That is... nobody would sell you a 400hp motor with drivetrain components that were designed for a 100hp motor (i.e. - more lightweight, smaller, etc.). I'm sure that's partially a factor, no? At least as far as why most vehicles with the same engine location/drive set up (FR/RR/etc) have similar loss percentages.
 
1 - 20 of 26 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top