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Discussion Starter · #1 · (Edited)
An Official Crank HP Calculation

A Little history first. Some people use a 1.35 multiplier to calculate the horse power at the crank. The 1.35 multiplier is found by taking the quoted crank horse power of the WRX published by SOA which is 227 and divide this by 168 (stock wheel hp number dynoed by TurboXS).

Some people do not believe you can use a fixed multiplier like 1.35 to calculate the crank hp. These people believe the hp is more of a fixed number and they typically use 59 hp as this number. You can arrive at this number by rounding the difference of 227 minus 168.

Others probably believe the crank horse power is someplace between the two formulas above. So I propose to average the two together. As an example if you had a TurboXS Stage 2 package that is putting out 222 hp at the wheels.

The first group would claim 299.7 at the crank (222 * 1.35)

The second group would claim 281 at the crank (227-168+222) or (59+222)

What I propose is the average of these two number represented with the following formula:

crank hp = ((1.35*whp)+(59+whp))/2

Using the 222 whp above this would be:
((1.35*222)+(59+222))/2 or ((299.7)+(281))/2 or 580.7/2 or 290.35 rounded to 290 at the crank

Note: The above forumula is easy to do on any standard calculator by doing: 1.35 * whp + whp + 59 / 2

Hey, lets work the numbers and come up with an official ClubWRX calculation to guestimate crank horsepower.

What does everyone think?

Carlo
 

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Well I'm in the school of using the Multiplier to calculate this, but I would agree that it is nothing more than a guess.

It is pretty definite that the ACTUAL HP would be *somewhere* between the two calculations you gave, this is only obvious. Averaging the two 'may' be more accurate, but even if it's not, it might help please both parties when quoting HP figures.
 

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I like it - there are going to be fixed and variable HP losses (ie neither camp is 100% right) - whether it is an equal weighted average or skewed one way or the other is always up for debate, but hell equal works for me in the absence of any more detailled empirical analysis!!

Andrew
 

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Discussion Starter · #5 ·
Strng1dah said:
Well, if its official then I guess we dont get any choice. :(

;)
Hehe, it's not official, that why I put it up for debate. Maybe I should have used a slightly better title for the thread :)
 

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ALSO, if you take atm pressure and temp (on most time slips and ALL dynos) you can bring it to SAE standard AWHP, which is the ulitimate leveler. We also talked about atm. pressure due to altitude on the RSX BS thread too.

-Jim
 

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I'll play the game! Here's my submission.

A 40HP fixed overhead loss plus a 10% variable loss. In the other thread I got this the wrong way 'round. The overhead loss should be applied to the crank number first, or the wheel number last. Not vice versa. So applying this to the 227HP stock WRX crank number we get:

227 - 40 * 0.9 = 168HP

Some examples calculating for crank horsepower instead:

Godspeed's former white wagon which was dynoed at 311 to the wheels, which Dan figured to be around 390 at the crank.

311 / 0.9 + 40 = 386 horsepower at the crank.

My car when spraying Nitrous, let's say 330 at the wheels (a conservative calculation based upon my trap speeds).

330 / 0.9 + 40 = 407 horsepower at the crank.


Enjoy!

-Pace
 

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Re: An Official Crank HP Calculation

Carlo said:
What does everyone think?

Carlo
Sorry Carlo, the system is flawed, it doesn't take into account the differences in dynos. You formula is fine, if everyone uses it, but only if you have a different correction factor for each type of dyno (dynojet, dynopack, etc). Instead, people should just scan or post their dyno charts with some relevent info (altitude, temperature, dyno type, etc).

jb
 

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Discussion Starter · #10 ·
Re: Re: An Official Crank HP Calculation

jimb said:


Sorry Carlo, the system is flawed, it doesn't take into account the differences in dynos. You formula is fine, if everyone uses it, but only if you have a different correction factor for each type of dyno (dynojet, dynopack, etc). Instead, people should just scan or post their dyno charts with some relevent info (altitude, temperature, dyno type, etc).

jb
Your comparing apples to oranges here. This is not the topic of this conversation. It doesn't matter if you get your wheel hp from a dynojet, dynopack, hp calculator from 1/4 traps or any other means that is generally used to determine whp. BTW most dynos are fairly close in there numbers. It's kind of easy to compare them as "rough" baseline.

What we are talking about here is once you ALREADY know your wheel hp, how do you determine crank horse power for general conversation.

Carlo
 

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Re: Re: Re: An Official Crank HP Calculation

Carlo said:


Your comparing apples to oranges here. This is not the topic of this conversation. It doesn't matter if you get your wheel hp from a dynojet, dynopack, hp calculator from 1/4 traps or any other means that is generally used to determine whp. BTW most dynos are fairly close in there numbers. It's kind of easy to compare them as "rough" baseline.

What we are talking about here is once you ALREADY know your wheel hp, how do you determine crank horse power for general conversation.

Carlo
Sorry but the same car run on different dyno types will yield (sometimes vastly) different wheel hp and torque numbers. (what about correction factors on that particular dyno, or the lack thereof). This is all a moot point really, because the peak hp number really doesn't tell the whole story anyway (look at the whole curve baby). You can't do this comparison, it is totally invalid.

jb
 

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Re: Re: Re: Re: An Official Crank HP Calculation

jimb said:


Sorry but the same car run on different dyno types will yield (sometimes vastly) different wheel hp and torque numbers. (what about correction factors on that particular dyno, or the lack thereof). This is all a moot point really, because the peak hp number really doesn't tell the whole story anyway (look at the whole curve baby). You can't do this comparison, it is totally invalid.

jb
jimb,

Good point and trust me, Carlo, [email protected], Myself, etc. TOTALLY AGREE, but people like quoting peak figures and we are trying to accomodate them. see? FYI, the dynapack at TXS is hands down the most accurate way to measure torque, rpm, and calculate the rest.

-Jim
 

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Re: Re: Re: Re: Re: An Official Crank HP Calculation

PlatinumWRX said:


jimb,

Good point and trust me, Carlo, [email protected], Myself, etc. TOTALLY AGREE, but people like quoting peak figures and we are trying to accomodate them. see? FYI, the dynapack at TXS is hands down the most accurate way to measure torque, rpm, and calculate the rest.

-Jim
If you realize that these numbers are not accurate, why bend over to accomodate people wanting to make peak claims that are totally unfounded? At this point, you might as well let anyone claim whatever numbers they want. The complex formula is totally unnecessary, garbage in, garbage out.

Lets not even talk about the fact that drivetrain losses probably vary based on the RPM, so if peak hp or torque vary at the rpm of the baseline graph, the conversion factor is wrong.

This kind of thing has been discussed ad nauseum on message boards, mailing lists, magazine articles, books, car clubs, etc for years. This isn't anything new. The concensus is that if you want to talk about crank HP, you need to run on an engine dyno. I don't see what the problem is talking about awhp and simply stating the dyno you ran at and the basic environmentals.

jb

edit: Don't take this as an attack or criticism. It's just really slow at work today, I'm bored, and I've always liked to argue.. :)
 

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I say who cares? Wheel hp and weight is all that matters.

my $.02
 

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Discussion Starter · #15 ·
pace said:
I'll play the game! Here's my submission.

A 40HP fixed overhead loss plus a 10% variable loss. In the other thread I got this the wrong way 'round. The overhead loss should be applied to the crank number first, or the wheel number last. Not vice versa. So applying this to the 227HP stock WRX crank number we get:

227 - 40 * 0.9 = 168HP

Some examples calculating for crank horsepower instead:

Godspeed's former white wagon which was dynoed at 311 to the wheels, which Dan figured to be around 390 at the crank.

311 / 0.9 + 40 = 386 horsepower at the crank.

My car when spraying Nitrous, let's say 330 at the wheels (a conservative calculation based upon my trap speeds).

330 / 0.9 + 40 = 407 horsepower at the crank.


Enjoy!

-Pace
So using the 222 (TurboXS Stage 2) whp number I used in the first example and came up with:

((222*1.35)+(59+222))/2 = 290 crank hp.

Use Pace's formula you would have:

222/0.9+40=286.6 rounded to 287.

Either way it works for me. To close to call at this point :)

Carlo
 

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Well I guess I WAS wrong, it STILL won't make both camps happy will it? :(

To answer the question about WHY crank numbers anyway: First of all, I care about WHP like the rest of you guys, but it's the noobs with the Camaro SS that quote 320HP from the car brocure and ask you what you have. Replying back with 285WHP makes them think they have 35 HP on you, when really, you are making MORE than them. They just don't know the difference and have never been on a dyno and like to magazine race instead. At least if you convert it, you could come back with "somewhere around 365HP" or so. :D

-Jason-
 

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Discussion Starter · #17 ·
Re: Re: Re: Re: Re: Re: An Official Crank HP Calculation

jimb said:


If you realize that these numbers are not accurate, why bend over to accomodate people wanting to make peak claims that are totally unfounded? At this point, you might as well let anyone claim whatever numbers they want. The complex formula is totally unnecessary, garbage in, garbage out.


Umm, who said that these (wheel hp) numbers are not accurate? They are very accurate and are scientificly proved by putting the car on a dyno meter.

It is general knowledge that the Dynapack dyno meter is the most accurate chassis dynamometer ("dyno") measuring power at the wheels since wheel diameter, tire pressure, and tire bounce off of a roller plays no factor in the measurement of power at the wheels. This however isn't the topic. You can put the same car on the dyno time and time again and if nothing has changed in the car you should be within 2% of your other reading. That's the beauty of a dyno. It's a scientific measurement.

Again this isn't the topic of the conversation so please stay focussed on the topic. :cool:

Carlo
 

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I guess in the end - if your car puts a smile on ya dial then all is good regardless of what the dyno says.

Using my HIGHLY accurate and calibrated 'ARSE Mk1' fitted in the geosynchronous 'favourite khakis', my WRX pulls 290 Shcummers (Sc = 1 woot per hour) and 276 yipyahs (Yy = 9 scphincter tighenings per minute)......both measured at the bunghole!
:D :D :D :D

Andrew
aussieinstlouis
 

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Re: Re: Re: Re: Re: Re: Re: An Official Crank HP Calculation

Originally posted by Carlo Umm, who said that these (wheel hp) numbers are not accurate? They are very accurate and are scientificly proved by putting the car on a dyno meter.
I'm talking about the crank horsepower estimation, not dyno'd wheel horsepower.

Originally posted by Carlo It is general knowledge that the Dynapack dyno meter is the most accurate chassis dynamometer ("dyno") measuring power at the wheels since wheel diameter, tire pressure, and tire bounce off of a roller plays no factor in the measurement of power at the wheels. This however isn't the topic. You can put the same car on the dyno time and time again and if nothing has changed in the car you should be within 2% of your other reading. That's the beauty of a dyno. It's a scientific measurement.
Great, Dyno A is better than Dyno B because a scientist proved it. That still doesn't mean you can compare Dyno A to Dyno B at all.

Originally posted by Carlo
Again this isn't the topic of the conversation so please stay focussed on the topic. :cool:

Carlo
It most certainly is, your trying to figure out how to come up with a generic formula that when given an wheel horsepower rating will give you a generically comparable number that accurately represents crank horsepower. All I am saying is it is futile, given any input number, you can do whatever math you want, it doesn't mean it is correct. The reading a dyno will give you is inextricably linked from how it takes the measurement, and any correction factors it uses to give you a corrected (or uncorrected) estimation of wheel horsepower.

If you want to do what you are doing, at the minimum you need to do something like:

If you dynoed on a dynopack, use X+Y = Z

If you dynoed on a dynojet, use X+F=B

If you dynoed on a mustang, use A + B = C

Unfortunately, you are still working off of Subaru's estimation that the stock engine even puts out 227 horsepower, which could possibly be another inaccurate number (manuf. are notorious for inflating estimations). So, basically, your using an estimation to compare to another estimation put through a crude formula to yield yet another estimation so far removed from the truth it is entirely useless.

So, my point, if your going to brag, you better be able to post up your dyno graphs.

jb
 
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