Calculating Crank HP from RWHP

[bradz2125]

OK this may be a stupid topic but I'm curious anyways. Ive searched around and it seems like the best approximate estimation of drivetrain loss is about 15%-16%. For the sake of arguement lets say it is 15%. What would you guys feel be a more accurate way of estimating the HP produced at the crank? Either multiplying by 1.15 or dividing by 0.85? For example you see in my sig i'm making 468 RWHP. If I multiply that by 1.15 I'm adding 15% of 468 to come to a total of 538.2 HP. But if I instead divide by 0.85 I'm finding out 85% of what number = 468. so 468/0.85= 550.58. So whats more accurate to say my crank HP is. 550 or 538? I would think the latter way is more accurate but I'd like the opinions of the other mathematicians on the forums. Thanks.

 

[stang99]

You divide by .85.

 

[SVT4ME]

Cool i needed to know that too

 

[03CobraFreak]

Originally posted by bradz2125
OK this may be a stupid topic but I'm curious anyways. Ive searched around and it seems like the best approximate estimation of drivetrain loss is about 15%-16%. For the sake of arguement lets say it is 15%. What would you guys feel be a more accurate way of estimating the HP produced at the crank? Either multiplying by 1.15 or dividing by 0.85? For example you see in my sig i'm making 468 RWHP. If I multiply that by 1.15 I'm adding 15% of 468 to come to a total of 538.2 HP. But if I instead divide by 0.85 I'm finding out 85% of what number = 468. so 468/0.85= 550.58. So whats more accurate to say my crank HP is. 550 or 538? I would think the latter way is more accurate but I'd like the opinions of the other mathematicians on the forums. Thanks.

I used to multiply by 1.15 but I did some digging on the Kenne Bell website and they say to divide by .85 so who knows. :shrug:

 

[99cobrajms]

Divide by .85

 

[satx]

add 40 hp to the rw number:idea:

 

[ygohome]

Satx,

this is because 15% doesn't figure properly for engines that produce more than 250 hp?

I've heard about this also. but no one really knows for sure until they do an engine dyno on a modded 03 cobra engine

 

[j card]

The problem is that the drivetrain loss is neither a fixed number or a fixed percentage. None of us have the equipment to measure it exactly. Ford has the equipment to do it - its a ten million dollar test room ( I saw it).

Whichever method you choose, they're both still a guess. Pick your favorite and go with it until we get better data. I'll try a couple of people I know at Ford and see if I can get a more definite answer.

 

[Cobra10thaniv]

I called SVT and asked what is the # to use cause i used 15 and herd others say more. And yes J card is right they know and have tested it.
I was told to use 18% for what its worth. That came right from SVT

 

[CobraBob]

And I've heard numerous times recently to use 17%. So we have a range of 15-18% if we go by these posts. I say split the difference and use 16.5%.
:lol1:

 

[Got Incon?]

I would divide by .88 as I personally feel that 18% is way to high unless you have an auto.

 

[Black2003Cobra]

j card - that would be great info! I would think that it wouldn't be a fixed loss though, since the coefficient of friction between surfaces usually changes w/ speed. So I would expect it to vary with, say, engine speed. What do you say, Got Icon? Does that make sense?

And has anyone thought about how much power that is?! Say it was around 15%...on my car, that would be about 70 hp. That's 52.2 kW!!! That's alot of heat!

 

[j card]

Originally posted by Black2003Cobra
j card - that would be great info! I would think that it wouldn't be a fixed loss though, since the coefficient of friction between surfaces usually changes w/ speed. So I would expect it to vary with, say, engine speed. What do you say, Got Icon? Does that make sense?

And has anyone thought about how much power that is?! Say it was around 15%...on my car, that would be about 70 hp. That's 52.2 kW!!! That's alot of heat!

I put in the request for the info with my contact who has access to that kind of stuff - so we'll see. I told him that if it is some type of confidential data that he could get in trouble for sending out, then don't worry about it. Keep your fingers crossed.

 

[slow lane]

There's a quote in one of the Mustang magazines from last year regarding the SAE RWHP numbers the first run of '03 Cobras were making (around 370 on average stock). John Colletti made the comment, "they have very efficient drivetrains".

1 - 370 / 390 (rated) would equate to a 5% drivetrain loss.

Paul of Paul's High Performance, who dynoed the engine pre-production, was quoted in the same article saying that with the rollerized T-56 and lightweight flywheel the drivetrain loss had been reduced from 15% to about 13%. This would lead to the stock SAE BHP rating being about 425 which seems about right.

The 13% figure fits with what the Viper and Z06 guys tend to use.

At this level of efficiency, adding heavier wheels and tires or brakes adds enough rotational innertia to the driveline to bump that figure back up a point or so.

 

[j card]

The "very efficient drivetrain" that Coletti made was a joke in order to elude the question. The Magazine guys were trying to get him to admit the real vs. rated power - based on what they were seeing on a chassis dyno. There's no way the loss is as low as 5%.