HP: Crank vs. Wheels vs. BHP?
Thread Starter
Pro
Joined: May 2011
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From: Chicago Suburb
St. Jude Donor '13
HP: Crank vs. Wheels vs. BHP?
Another Noob question:
I understand that there a parasitic power loss between crank and wheels - is 20% about right? And how do those numbers relate to BHP?
I understand that there a parasitic power loss between crank and wheels - is 20% about right? And how do those numbers relate to BHP?
Team Owner
Joined: May 2003
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From: East Meadow NY
St. Jude Donor '12
Racer
Joined: Apr 2009
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From: Myrtle Beach SC
IMO, the best way to determine your power "loss" through your drivetrain is comparing an engine dyno run vs. a chassis dyno run. 
If you aren't going to pull your engine anytime soon, then that's not feasible.
In my own personal experience, I am putting down 86% of my HP from the engine, or a 14% loss through the drivetrain. I think 15% is typical for a manual, and 20% for an automatic.
Someone can correct me if I got the auto figure wrong...
Hope this helps...
If you aren't going to pull your engine anytime soon, then that's not feasible.In my own personal experience, I am putting down 86% of my HP from the engine, or a 14% loss through the drivetrain. I think 15% is typical for a manual, and 20% for an automatic.
Someone can correct me if I got the auto figure wrong...
Hope this helps...
Team Owner
Joined: May 2003
Posts: 23,480
Likes: 9
From: East Meadow NY
St. Jude Donor '12
IMO, the best way to determine your power "loss" through your drivetrain is comparing an engine dyno run vs. a chassis dyno run. If you aren't going to pull your engine anytime soon, then that's not feasible.
In my own personal experience, I am putting down 86% of my HP from the engine, or a 14% loss through the drivetrain. I think 15% is typical for a manual, and 20% for an automatic.
Someone can correct me if I got the auto figure wrong...
Hope this helps...
If you aren't going to pull your engine anytime soon, then that's not feasible.In my own personal experience, I am putting down 86% of my HP from the engine, or a 14% loss through the drivetrain. I think 15% is typical for a manual, and 20% for an automatic.
Someone can correct me if I got the auto figure wrong...
Hope this helps...
Race Director
Joined: Apr 2007
Posts: 11,152
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From: South Western Ontario
The powertrain has losses due to accelerating the inertia loads and losses due to friction in the components. So, the HP that appears at the rear wheels does have a dependence on the acceleration speed. The wheel dyno typically lets the drivetrain accelerate quicker than the car really would on the street so the losses due to accelerating the drivetrain inertia will be artificially inflated (at least for the 1:1 gear runs typically done on a dyno).
Brake HP typically refers to the power available at the crank without accessories and with a fixed speed load so there are no losses due to accelerating the engine inertia. Of course, there are different details in the way this can be defined or measured.
Brake HP typically refers to the power available at the crank without accessories and with a fixed speed load so there are no losses due to accelerating the engine inertia. Of course, there are different details in the way this can be defined or measured.
Melting Slicks
Joined: Apr 2004
Posts: 2,301
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From: Valencia CA
Using % as a loss or gain through the drive train is incorrect.
The parasitic loss of the drive train is a fixed amount for any given rpm.
For example, a stock vehicle making 300bhp at 5000 rpm, is run on a chassis dyno and found to make 255 rwhp. A difference of 45 hp or a 15% loss as was stated in an earlier post.
Adding heads, cam, long tube headers, intake, throttle body and tuning to this very same vehicle, testing it again at 5000rpm results in a signifigant increase in hp. Let's say 500hp at the crank. Is the new rwhp 45 less than 500 or 15% (75hp) less than 500?
The mass of the rotating components has not changed, so the parasitic loss is the same in the second test as it was in the first part of the example.
The parasitic loss of the drive train is a fixed amount for any given rpm.
For example, a stock vehicle making 300bhp at 5000 rpm, is run on a chassis dyno and found to make 255 rwhp. A difference of 45 hp or a 15% loss as was stated in an earlier post.
Adding heads, cam, long tube headers, intake, throttle body and tuning to this very same vehicle, testing it again at 5000rpm results in a signifigant increase in hp. Let's say 500hp at the crank. Is the new rwhp 45 less than 500 or 15% (75hp) less than 500?
The mass of the rotating components has not changed, so the parasitic loss is the same in the second test as it was in the first part of the example.
Melting Slicks
Joined: Apr 2004
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From: Valencia CA
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Race Director
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From: South Western Ontario
Hmmm, I guess I shouldn't point out that the rotating mass is not a power loss. The rotating mass in the powertrain will store energy when it's accelerated and then it will release that energy when it's decelerated. This is quite different then the friction losses. The more power you try to put through a set of gears the more friction losses in those gears and the power lost to making heat is not recovered when you decelerate.
The drivetrain losses are not simple, so expressing it as a fixed loss or as a % loss are both wrong and trying to claim it's simply one or the other is not helping anyone understand what is really going on.
The best you can do for a basic calculation is just throw out a wild estimation of some percentage. The percentage will really mean little though.
The drivetrain losses are not simple, so expressing it as a fixed loss or as a % loss are both wrong and trying to claim it's simply one or the other is not helping anyone understand what is really going on.
The best you can do for a basic calculation is just throw out a wild estimation of some percentage. The percentage will really mean little though.
Thread Starter
Pro
Joined: May 2011
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From: Chicago Suburb
St. Jude Donor '13
Hmmm, I guess I shouldn't point out that the rotating mass is not a power loss. The rotating mass in the powertrain will store energy when it's accelerated and then it will release that energy when it's decelerated. This is quite different then the friction losses. The more power you try to put through a set of gears the more friction losses in those gears and the power lost to making heat is not recovered when you decelerate.
The drivetrain losses are not simple, so expressing it as a fixed loss or as a % loss are both wrong and trying to claim it's simply one or the other is not helping anyone understand what is really going on.
The best you can do for a basic calculation is just throw out a wild estimation of some percentage. The percentage will really mean little though.
The drivetrain losses are not simple, so expressing it as a fixed loss or as a % loss are both wrong and trying to claim it's simply one or the other is not helping anyone understand what is really going on.
The best you can do for a basic calculation is just throw out a wild estimation of some percentage. The percentage will really mean little though.
Safety Car
Joined: Apr 2007
Posts: 3,582
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From: SoCal CA
Hmmm, I guess I shouldn't point out that the rotating mass is not a power loss. The rotating mass in the powertrain will store energy when it's accelerated and then it will release that energy when it's decelerated. This is quite different then the friction losses. The more power you try to put through a set of gears the more friction losses in those gears and the power lost to making heat is not recovered when you decelerate.
The drivetrain losses are not simple, so expressing it as a fixed loss or as a % loss are both wrong and trying to claim it's simply one or the other is not helping anyone understand what is really going on.
The best you can do for a basic calculation is just throw out a wild estimation of some percentage. The percentage will really mean little though.
The drivetrain losses are not simple, so expressing it as a fixed loss or as a % loss are both wrong and trying to claim it's simply one or the other is not helping anyone understand what is really going on.
The best you can do for a basic calculation is just throw out a wild estimation of some percentage. The percentage will really mean little though.
Race Director
Joined: Sep 2005
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From: Pasco WA
Not to mention; dyno differences. Their numbers can be all over the place. Also, it's kinda apples/oranges when you compare chassis dynos, and engine dynos. A set percentage is just an estimate, not unlike the numbers your neighborhood tuner will give you for a few bucks.
Melting Slicks
Joined: Apr 2004
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From: Valencia CA
[QUOTE=lionelhutz;1583529405.
The drivetrain losses are not simple, so expressing it as a fixed loss or as a % loss are both wrong and trying to claim it's simply one or the other is not helping anyone understand what is really going on.
****************
No-one is forcing you to actually learn something new....
[/QUOTE]
I find complicating the subject matter causes the student's eyes to roll back in their head and they lose interest very quickly. Start with some basic concepts and build upon that........
Making it simple and even fun, allows learning to take place without realizing learning is occurring. No forcing necessary.
Most engineers don't understand this, for them the world is all absolutes.
The drivetrain losses are not simple, so expressing it as a fixed loss or as a % loss are both wrong and trying to claim it's simply one or the other is not helping anyone understand what is really going on.
****************
No-one is forcing you to actually learn something new....
[/QUOTE]
I find complicating the subject matter causes the student's eyes to roll back in their head and they lose interest very quickly. Start with some basic concepts and build upon that........
Making it simple and even fun, allows learning to take place without realizing learning is occurring. No forcing necessary.
Most engineers don't understand this, for them the world is all absolutes.
Race Director
Joined: Apr 2007
Posts: 11,152
Likes: 890
From: South Western Ontario
I find complicating the subject matter causes the student's eyes to roll back in their head and they lose interest very quickly. Start with some basic concepts and build upon that........
Making it simple and even fun, allows learning to take place without realizing learning is occurring. No forcing necessary.
Most engineers don't understand this, for them the world is all absolutes.
Making it simple and even fun, allows learning to take place without realizing learning is occurring. No forcing necessary.
Most engineers don't understand this, for them the world is all absolutes.
This is a tech forum. If you don't want to deal with the tech then go to another forum. I also don't get paid enough to care about coddling anyone with baby steps.
The first line you attempted to quote is how it is. The losses are not fixed or a percentage. They are also not constant at any particular rpm. The losses vary depending on the gear and acceleration rate.
Last edited by lionelhutz; Apr 2, 2013 at 09:25 PM.
Melting Slicks
Joined: Apr 2004
Posts: 2,301
Likes: 199
From: Valencia CA
It is a tech forum, but a noob asked a simple question looking for a simple answer.
Your position is that everything is a variable therefore nothing can be stated with any certainty. I don't think that really helps the OP understand anything.
Therefore your contribution to this tech forum is to confuse the noobs rather than help them understand?
Your position is that everything is a variable therefore nothing can be stated with any certainty. I don't think that really helps the OP understand anything.
Therefore your contribution to this tech forum is to confuse the noobs rather than help them understand?
Race Director
Joined: Apr 2007
Posts: 11,152
Likes: 890
From: South Western Ontario
It is a tech forum, but a noob asked a simple question looking for a simple answer.
Your position is that everything is a variable therefore nothing can be stated with any certainty. I don't think that really helps the OP understand anything.
Therefore your contribution to this tech forum is to confuse the noobs rather than help them understand?
Your position is that everything is a variable therefore nothing can be stated with any certainty. I don't think that really helps the OP understand anything.
Therefore your contribution to this tech forum is to confuse the noobs rather than help them understand?
Saying it is exactly 58hp is the correct answer?
No......
Even if you measured the engine on an engine dyno and then put the engine into the car and measured the car on a wheel dyno you don't get any real useful HP:RWHP data. When you calculate the power ratio to get the percent difference, that percent difference only applies for that transmission gear and only when the drivetrain is accelerating at the rate it was accelerating on the wheel dyno. Change anything and the ratio will change. Take the car off the wheel dyno and run it on the street in the same gear so it accelerates at a different rate and the ratio of engine hp to wheel hp is not the same.
Like many things in life, it's not that simple and my stance is that the exact ratio doesn't matter because it's never a fixed ratio and the number you calculate means jack squat.
So, since the ratio is always varying don't run around using some fixed number or percent number as gospel. It's nothing more than a rough guess. Just pick whatever number you feel like and use it if you insist on calculating one from the other. I suggest using 25% if you're calculating engine HP from RWHP and 10% if you're calculating RWHP from engine HP. Why? The bigger numbers will make you feel all warm and fuzzy.
Last edited by lionelhutz; Apr 2, 2013 at 10:44 PM.
Melting Slicks
Joined: Apr 2004
Posts: 2,301
Likes: 199
From: Valencia CA
Saying it's exactly 15% is the correct answer?
Saying it is exactly 58hp is the correct answer?
No......
Even if you measured the engine on an engine dyno and then put the engine into the car and measured the car on a wheel dyno you don't get any real useful HP:RWHP data. When you calculate the power ratio to get the percent difference, that percent difference only applies for that transmission gear and only when the drivetrain is accelerating at the rate it was accelerating on the wheel dyno. Change anything and the ratio will change. Take the car off the wheel dyno and run it on the street in the same gear so it accelerates at a different rate and the ratio of engine hp to wheel hp is not the same.
Saying it is exactly 58hp is the correct answer?
No......
Even if you measured the engine on an engine dyno and then put the engine into the car and measured the car on a wheel dyno you don't get any real useful HP:RWHP data. When you calculate the power ratio to get the percent difference, that percent difference only applies for that transmission gear and only when the drivetrain is accelerating at the rate it was accelerating on the wheel dyno. Change anything and the ratio will change. Take the car off the wheel dyno and run it on the street in the same gear so it accelerates at a different rate and the ratio of engine hp to wheel hp is not the same.
We could calclulate the difference at any rpm increment, this would assign a know value defining the delta at that specific point in the rpm range under those exact conditions.
Most people would choose to look at the rpm that peak HP occurred at, because it makes them feel "warm and fuzzy".
Now make a change to the engine that increases the output of the engine and run it under the exact same conditions as before. Same acceleration rate etc. As close as possible to duplicate conditions. Allowing for differences in the thermodynamics of the drivetrain, the delta remains near the same.
Now wasn't that fun?
I've actually had the luxury of being able to perform this experiment a few times over my 30 year career, and the results are quite interesting.
I'll try and dig up the papers I wrote, over the weekend.
