Can crank hp be calculated?

[jbradsh1]

How can you calculate actual crank hp ... when your car is modified? I know you can start with the rwhp the dyno chart shows, but say one guy has LTs and the other guy has a cat-back on his similarly cammed G8, then what?

[Blackdevil77]

you take what ever the car dynoed to the rear wheels, times that by .20 because theres about a 20 percent loss on our cars, then add that number to the rwhp and you will get crank hp. For example, most cars dyno at around 300 rwhp stock, so 300 X .20=60. then 300+60=360 so that proves that 20 percent is about right. Now you can use that to find your cank hp after modding. For example, my car made 355 to the rear wheels, so 355 X .20=71. So now 355+71=426. That's my crank hp.

Here's another one. Paul's cammed G8, now makes 452 to the rear wheels. 452 X .20=90.4. Then 90.4 +452=542. Damn Paul!

[norm8332]

I use .18 just to be safe. But it's not an exact science.

[p71]

That explanation is incorrect and shows a fundamental lack of understanding of mathematical principal.

If you are assuming a 20 percent loss then you divide your wheel horsepower by 80% (that is .8)... or 375 crank HP from the numbers posited above.

17-19 percent is a better number really so dividing by .83 to .81 will give you a crank number.

In the case of Pauls car assuming 20% loss he is putting out 565 HP (452/.8)...

Or if you believe my other argument then this is ALL a bunch of crap... but you will have to talk to Sir Charles about that... *winkz*

[norm8332]

Or if you believe my other argument then this is ALL a bunch of crap... but you will have to talk to Sir Charles about that... *winkz*

This is more like it....Pull the motor...

[chiefpontiac]

You have to make some assumptions:

1) that each and every (or at least your) G8 produces 256, 355, or 415 crank HP from the factory depending on which engine and which fuel ( the 355 of eth L76 will rise to 361 on premium, truth)

2) that you have baselin dynoed your vehicle pre-mod and have a solid figure of rwhp. and therefore a net numerical loss figure between crank and wheels

2A) there has been much discussion in the past on this issue whether for example a 400hp (crank) engine exhibiting 320 rwhp (20% loss) would show 480 whp if the crank hp was jacked up to a valid 600. When you think about it the argument can surely be made that drivetrain losses between crank and wheels on any particular vehicle with nothing else changed are an absolute number and not a %, therefore in this example a reading of 480 rwhp + 80 hp parastatic loss = 560 crank hp, not 600.

3) now using exact same dyno, with proper adjustments if necessary for atmospheric changes, under as close to same conditions you conduct test(s) post-mod you will have a new, hopefully, but not always, larger rwhp.

4) using figures from Blackdevil above, and assuming he did baseline at 300hp at the wheels for a 55hp loss, which BTW is 15.5% loss, then his new crank hp based upon a rwhp of 355+55=410, or by % 355/.845=420 (at best, but see 2A)

Another caveat; exact same vehicle on different brand dyno in different cities (even at same sea level) will return different numbers.

A true test to validate whether exact loss or % loss is most proper would be to test back to back to back 1 each of the 3 versons of G8 on the same dyno on the same day. Disregarding that the V6 only comes with a 5 speed; if losses are exact, then a GT showing 300 at wheels should result in a V6 @ 201 and a GXP @ 360. If it is %, then a GT @ 300 (15.5% loss) would be bracketed by a V6 @ 216, and a GXP @ 351.

In teh case of Paul's car, what was baseline? If it was 300, therefore a miniscule 15.5% loss he is only 535 at the crank, - or if his actual loss is 55hp then his crank is now only 507 - poor Paul, all that effort and barely over 500, tsk, tsk but wait, no one can disregard the true measure of what the car can do, which is 452 at the pavement!

[jimmytt6]

A general rule of thumb is you lose 20% power on auto's and 15% power on manuals.
This is what I've always been told. So you would multiply your dyno HP x .20 on our cars and dyno HP x .15 on say a 6sp GXP. It's not a exact science though...

Jim
09 Liquid Red/Premium/Sport

[Andy@Livernois]

A general rule of thumb is you lose 20% power on auto's and 15% power on manuals.
This is what I've always been told. So you would multiply your dyno HP x .20 on our cars and dyno HP x .15 on say a 6sp GXP. It's not a exact science though...

Jim
09 Liquid Red/Premium/Sport

as others have stated this is incorrect. I will use a bone stock G8 as an example.

361 crank and 309 @ the tire. that is a 14.4% loss in power so if you take 309 and multiply it by 1.144 it's only 353.5.

however if you take 309 and divide by .856 it takes you back to a hair under 361

[99-LS1-SS]

I'm going to go out on a limb and guess at my crank horsepower. I think my crank horsepower is around 1,000. :D





















This is a joke to all of you who thought I was being serious.

[99-LS1-SS]

as others have stated this is incorrect. I will use a bone stock G8 as an example.

361 crank and 309 @ the tire. that is a 14.4% loss in power so if you take 309 and multiply it by 1.144 it's only 353.5.

however if you take 309 and divide by .856 it takes you back to a hair under 361
Using this formula my car made 358 to the wheels and when that is divided by .856 it comes to 418 horsepower. I believe this formula is pretty darn accurate.

[p71]

A general rule of thumb is you lose 20% power on auto's and 15% power on manuals.
This is what I've always been told. So you would multiply your dyno HP x .20 on our cars and dyno HP x .15 on say a 6sp GXP. It's not a exact science though...

Jim
09 Liquid Red/Premium/Sport

read my post... this is wrong.

[Blackdevil77]

That explanation is incorrect and shows a fundamental lack of understanding of mathematical principal.

If you are assuming a 20 percent loss then you divide your wheel horsepower by 80% (that is .8)... or 375 crank HP from the numbers posited above.

17-19 percent is a better number really so dividing by .83 to .81 will give you a crank number.

In the case of Pauls car assuming 20% loss he is putting out 565 HP (452/.8)...

Or if you believe my other argument then this is ALL a bunch of crap... but you will have to talk to Sir Charles about that... *winkz*

How is that wrong? I'm not talking EXACT numbers. The stock wheel power is usually around 300 so I used that as an example and to get to the 361 crank hp, that's about a 20 percent loss. Your way isn't wrong either. It's another way to do it. Both ways work. Again, I'm not giving a 100% accurate way to calculate it. Just a quick calculation that will get you the approximate crank hp.

['02 ws6]

Yes that formula p71 mentioned is accurate. While on my last dyno on my TA, I talked with the dyno tech @ speedtek for sometime about this. Following my pulls, he brought me in and figured up my flywheel hp based on my pulls, and his mathematical formula matched that. BTW he was figuring in an 18% Drivetrain loss with mine being an M6.
BlackBerry8830/4.2.2 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/104

[p71]

Using this formula my car made 358 to the wheels and when that is divided by .856 it comes to 418 horsepower. I believe this formula is pretty darn accurate.

This is probably right...

[Vertical]

I'm going to go out on a limb and guess at my crank horsepower. I think my crank horsepower is around 1,000. :D





















This is a joke to all of you who thought I was being serious.


Bugatti G8?

[jimmytt6]

read my post... this is wrong.

I didn't say it was 100% correct just a estimate. Yes your formula is more accurate.:woohoo:

thanks for the new info on calculating crank hp interesting.

Jim
09 Liquid Red/Premium/Sport

[jbradsh1]

Well, to be fair, I actually asked a ringer of a question. I think that no matter how your figure it, you're going to arrive at an estimate, in which case, I like the first response because it has the virtue of being simple and easy to estimate with. Because that is all you will actually ever get is a ballpark estimate. I'll tell you why. Say you have Paul's car (before the ported intake and TB) and compare it to mine. We had essentially the same engine (with a slight variation on the original stock hp figure) with the Livernois stage 1 cam, right. Well his car made 26 rhwp more than mine did after the cam install but we had the same engine. Well I had a different exhaust system which is where the difference lie. So if you do the 1.2X or the .18 divided into or do whatever you will come up with way different crank hp figures for Paul's car vs my car, but if you took them out of the car and strapped them to an engine dyno you would get essentially the same crank hp numbers, well actually, mine would probably put out just a little more than Paul's.

[p71]

the problem with the other way of calculating horsepower is it is inaccurate and gets progressively more inaccurate as HP goes up.

so let is say you have 300 whp with a motor rated to 360 hp that is a 16 2/3% loss in HP (fairly accurate for our car). You now have your conversion factor... that is 83.3333% of the hp put out at the crank makes it to your wheels. 60 HP is lost.

Using the incorrect methodology and a rwhp of 300 and a 16 2/3 percent loss you get an engine loss of 50 HP and a crank hp of 350... at which point you say.... hmmm my car dynos way lower than it should... and you sue GM and get laughed at in front of many people...

The numbers get worse and worse as HP goes up. The delta gets bigger as the numbers go up.

Basically you cannot switch numerators and denominators willy-nilly. You should have learned this in junior high.

On the law suit thing... correct use of the calculation of crank HP from whp got owners of mustang cobras new exhausts (and chips and payouts I believe) from ford.

Oh and I suck at typing.

No offense intended... I can by kinda brusque with this sort of thing.

[p71]

Well, to be fair, I actually asked a ringer of a question. I think that no matter how your figure it, you're going to arrive at an estimate, in which case, I like the first response because it has the virtue of being simple and easy to estimate with. Because that is all you will actually ever get is a ballpark estimate. I'll tell you why. Say you have Paul's car (before the ported intake and TB) and compare it to mine. We had essentially the same engine (with a slight variation on the original stock hp figure) with the Livernois stage 1 cam, right. Well his car made 26 rhwp more than mine did after the cam install but we had the same engine. Well I had a different exhaust system which is where the difference lie. So if you do the 1.2X or the .18 divided into or do whatever you will come up with way different crank hp figures for Paul's car vs my car, but if you took them out of the car and strapped them to an engine dyno you would get essentially the same crank hp numbers, well actually, mine would probably put out just a little more than Paul's.

That point is moot as current FWHP standards assume use of full exhaust, intake and accesories...

[jbradsh1]

That point is moot as current FWHP standards assume use of full exhaust, intake and accesories...

So you're saying that to get crank hp you would take my engine out along with the intake & full exhaust system that it has and then test it that way? You must be joking?!!!

[p71]

How is that wrong? I'm not talking EXACT numbers. The stock wheel power is usually around 300 so I used that as an example and to get to the 361 crank hp, that's about a 20 percent loss. Your way isn't wrong either. It's another way to do it. Both ways work. Again, I'm not giving a 100% accurate way to calculate it. Just a quick calculation that will get you the approximate crank hp.

Well if you just want approximate HP why bother measuring anything? I mean just add up the manufacturers HP gains and you will be in the ballpark... not!

I understand that there is inaccuracy inherent in the system. However the desire should always be to remove as many variables and fluctuations as possible... if you want a WAG say you want a WAG... The OP was a question on how to CALCULATE crank HP... not how to guess it, so the most accurate method is called for... which I provided.

Again sorry to be snotty, but this kind of thing gets under my skin... as y'all have probably worked out.

[jbradsh1]

No, not in the least. So with 416 rwhp, that means I have 486 crank hp!!! My god, no wonder the car runs like a scalded cat!

[p71]

So you're saying that to get crank hp you would take my engine out along with the intake & full exhaust system that it has and then test it that way? You must be joking?!!!

I did not say anything of the sort

I am saying you car makes X HP when fully dressed... as in how it sits in the car... and if you measure that between the engine and transmission you get that HP. That is a description of SAE net HP... (net because it has all those accesories and exhaust on there subtracting HP from the gross number you would have on an engine, not chassis, dyno when the engine is not always fully dressed)

In other words crank HP is Crank HP.... if you get a higher HP because of bolt ons or engine mods it does not matter... it is still crank hp...

But to play devils advocate... if you do not like my formula and you do not want to rip everything out of the car just drop the tranny and measure HP at the end of your crankshaft....

[p71]

No, not in the least. So with 416 rwhp, that means I have 486 crank hp!!! My god, no wonder the car runs like a scalded cat!

Actually assuming 16.6666 loss you are actually at just about 500 (499.2) FWHP... see why my formula is better

Math...

416/.833333=499.2

[Blackdevil77]

Actually assuming 16.6666 loss you are actually at just about 500 (499.2) FWHP... see why my formula is better

Math...

416/.833333=499.2

psssh.....smart people... ;)

[zosoboogie]

I going to use the 20% rule I have 496.5 at the wheel and
496.5 x .20 (99.30) = 595.8 hp WOW

what is the torque percentage loss or gain at crank numbers?

[p71]

I going to use the 20% rule I have 496.5 at the wheel and
496.5 x .20 (99.30) = 595.8 hp WOW

what is the torque percentage loss or gain at crank numbers?

Ironically you got the answer write even though your working was wrong...

partial credit

as to your other question...

Given that HP= TQxRPM/K, where k=5252... exactly the same amount is lost %wise.

HP is a scam anyway... but that is another argument altogether.

[norm8332]

I'll take the 528.75 number...

[zosoboogie]

so correct me then!?!

[jbradsh1]

Actually assuming 16.6666 loss you are actually at just about 500 (499.2) FWHP... see why my formula is better

Math...

416/.833333=499.2

Thanks for explaining further ... I get it now. But I do like your last rendition ... 500 hp at the crank! But you know, based on my 1/4 mile time & speed, I think it's realistically more like 450 hp (12.8 @112 mph). This time was posted with only 2 runs (70+ degrees & high humidity) and I need to have some adjustments done with it and probably my launch technique as well; I had it dialed in pretty well before the cam, but the cam changed things quite a bit as you would imagine. And I'm considering going with LTs now so that will change things again.

[p71]

so correct me then!?!

Let me just throw this up

y=x*(1-k)

where:
y is whp
x is fwhp
and k is percentage of lost expressed as a decimal.

now solve for x

basically you just divide both sides by (1-k)
so

y/(1-k)=x or x=y/(1-k)

The solution put fourth in the first response later referred to as the 20% rule is expressed as:

x=y(1+k)

Plug your numbers in and have fun.

[jbradsh1]

Ironically you got the answer write even though your working was wrong...

partial credit

as to your other question...

Given that HP= TQxRPM/K, where k=5252... exactly the same amount is lost %wise.

HP is a scam anyway... but that is another argument altogether.

See, this confirms Blackdevil77's hypothesis of just using the 20% figure, simple - easy - accurate.

[p71]

Thanks for explaining further ... I get it now. But I do like your last rendition ... 500 hp at the crank! But you know, based on my 1/4 mile time & speed, I think it's realistically more like 450 hp (12.8 @112 mph). This time was posted with only 2 runs (70+ degrees & high humidity) and I need to have some adjustments done with it and probably my launch technique as well; I had it dialed in pretty well before the cam, but the cam changed things quite a bit as you would imagine. And I'm considering going with LTs now so that will change things again.

What may be happening here is a problem of standards...

If Livernois is using SAE gross instead of net HP it will inflate the claimed wheel HP... does not matter if you are comparing before to after numbers... but it makes it impossible to work out what % of the horsepower is being sucked up by the drivetrain...

That trap speed justifies a very high 400s number IMHO...

Course I know way more about math than drag racing...

[zosoboogie]

Let me just throw this up

y=x*(1-k)

where:
y is whp
x is fwhp
and k is percentage of lost expressed as a percentage.

now solve for x

basically you just divide both sides by (1-k)
so

y/(1-k)=x or x=y/(1-k)

The solution put fourth in the first response later referred to as the 20% rule is expressed as:

x=y(1+k)

Plug your numbers in and have fun.

thanks but I like my equation better

[p71]

See, this confirms Blackdevil77's hypothesis of just using the 20% figure, simple - easy - accurate.

Simple easy accurate... as long as you assume that your car looses 16 2/3% hp.

Which it may... but probably will not. it it looses 15, or 18 or 20% that method does not work.

[jbradsh1]

If Livernois is using SAE gross instead of net HP it will inflate the claimed wheel HP...

Well, I think Livernois is reading what it says on their Dynojet Dyno.

but it makes it impossible to work out what % of the horsepower is being sucked up by the drivetrain...

Exactly my first and basic point!

That trap speed justifies a very high 400s number IMHO...

Thanks, you're my friend again.

Course I know way more about math than drag racing...

That's for sure, but say, how did you come up with that formula anyway?



OK

[Blackdevil77]

See, this confirms Blackdevil77's hypothesis of just using the 20% figure, simple - easy - accurate.

THANK YOU!! lol. That's all im trying for. Simple-easy-accurate




Simple easy accurate... as long as you assume that your car looses 16 2/3% hp.

Which it may... but probably will not. it it looses 15, or 18 or 20% that method does not work.

That's why you find your car's BONE STOCK RWHP from the dyno, than use that and calculate percent loss using the advertised 361 crank. Than use that percent loss when you get new dyno numbers after mods to figure out what your putting to the crank.

[p71]

That's why you find your car's BONE STOCK RWHP from the dyno, than use that and calculate percent loss using the advertised 361 crank. Than use that percent loss when you get new dyno numbers after mods to figure out what your putting to the crank.

smiles... and if you do that it will not work when you use your method...

which is my original point... if you are guessing 16 2/3 loss then you need to multiply by 1.2

now what is the correction factor for 18.5.. or 15 or 20 for that matter...

[p71]

OK

I came up with the formula by... observing a bunch of data and a little logic.

[p71]

Also apparently I am a post whore.

[jbradsh1]

Very good and a very entertaining & lively discussion. Now that we all have a handle on this subject, and I assume that none of us has changed his way of thinking even the slightest amount about "how to calculate crank hp", I will quote a wise old sage who said "can we now move on to the next subject of discussion"!

[r33pwrd]

so is the HP loss linear (percentage) no matter what your power?

[p71]

so is the HP loss linear (percentage) no matter what your power?

ummm that is a completely different kettle of fish... some say it is a percentage, some says it is a percentage that decreases as HP increases and others say it is a fixed amount... so...

[R.Penguin]

I'm going to go out on a limb and guess at my crank horsepower. I think my crank horsepower is around 1,000. :D





















This is a joke to all of you who thought I was being serious.
It would be if you had Pacific Slate Metallic! :nah:

[99-LS1-SS]

It would be if you had Pacific Slate Metallic! :nah:

With the $66 exhaust mod too!:nah:

[wyldman]

you take what ever the car dynoed to the rear wheels, times that by .20 because theres about a 20 percent loss on our cars, then add that number to the rwhp and you will get crank hp. For example, most cars dyno at around 300 rwhp stock, so 300 X .20=60. then 300+60=360 so that proves that 20 percent is about right. Now you can use that to find your cank hp after modding. For example, my car made 355 to the rear wheels, so 355 X .20=71. So now 355+71=426. That's my crank hp.

Here's another one. Paul's cammed G8, now makes 452 to the rear wheels. 452 X .20=90.4. Then 90.4 +452=542. Damn Paul!

So your telling me Blackdevel that a CAI, LT's, a tune and an HSRK and I can have 426hp at the crank?

Why do I have a woody?

[jbradsh1]

So your telling me Blackdevel that a CAI, LT's, a tune and an HSRK and I can have 426hp at the crank?


Yep, and that is only about 65 hp over what you get stock, how about that.

[Blackdevil77]

So your telling me Blackdevel that a CAI, LT's, a tune and an HSRK and I can have 426hp at the crank?

Why do I have a woody?

Because those are awsome gains!!!

[jbradsh1]

Because those are awsome gains!!!

Boy, you're not kidding. I wouldn't have believed that 65 hp could make that much of a difference, but it does. Now try about 135 hp with a cam! :devil:

[johnh]

Multiplying the whp by .20 and adding it doesn't make a 20 % loss.

crank hp * 80% = whp (with 20%) loss.

361 * 80% = 289 ;
or if you prefer 15%
361 * 85% = 307 ; (seems closer)

BUT
289 + 289 * 20% = 346 -- Not 361
307 + 307 * 15% = 353 -- Not 361
These show a 20% gain from whp, not a 20% loss from Crank.

You divide the wheel hp by (1 - loss).

Or simply as Andy put it divide by .85 (a 15% loss).

So,

355 whp / .85 = 417 hp
400 whp/ .85 = 470 hp
452 whp/.85 = 531 hp

that 65 whp = +76 at the crank, you'll notice that.
that 135 whp = 158 at the crank...ooh baby.

Bottom line all this is making me feel like my car doesn't have enough power.

[Blackdevil77]

Boy, you're not kidding. I wouldn't have believed that 65 hp could make that much of a difference, but it does. Now try about 135 hp with a cam! :devil:

oh yes that is coming this June hopefully.

[GeorgeInNePa]

I'm going to go out on a limb and guess at my crank horsepower. I think my crank horsepower is around 1,000. :D





















This is a joke to all of you who thought I was being serious.

Mine does, didn't you see my dyno sheet?

http://img.photobucket.com/albums/v320/GeorgeInNePa/fakedyno.jpg

[p71]

Multiplying the whp by .20 and adding it doesn't make a 20 % loss.

crank hp * 80% = whp (with 20%) loss.

361 * 80% = 289 ;
or if you prefer 15%
361 * 85% = 307 ; (seems closer)

BUT
289 + 289 * 20% = 346 -- Not 361
307 + 307 * 15% = 353 -- Not 361
These show a 20% gain from whp, not a 20% loss from Crank.

You divide the wheel hp by (1 - loss).

Or simply as Andy put it divide by .85 (a 15% loss).

So,

355 whp / .85 = 417 hp
400 whp/ .85 = 470 hp
452 whp/.85 = 531 hp

that 65 whp = +76 at the crank, you'll notice that.
that 135 whp = 158 at the crank...ooh baby.

Bottom line all this is making me feel like my car doesn't have enough power.

Somone was paying attention in 8th grade math...

I said this about 8 times....

[wyldman]

Boy, you're not kidding. I wouldn't have believed that 65 hp could make that much of a difference, but it does. Now try about 135 hp with a cam! :devil:

Hows your drivability? I would love to do this one day. Gotta start saving my lunch money I think.
Have you thought about a converter or are you afraid it will mess with around town driving?

[jbradsh1]

Hows your drivability? I would love to do this one day. Gotta start saving my lunch money I think.
Have you thought about a converter or are you afraid it will mess with around town driving?

Well, the driveability is good. Has a nice easy lope, you can tell it's cammed but not much lope actually. Idles at 750, smoothly. Drives like a normal G8 GT. You can hear the lifters when you get out of the car some but not inside the car on mine. From idle to about 3,000 it feels like the stock motor but from there on up it's a whole different car. And when you want some power, man it's there!

There is no way I would get a converter. You just don't need one because there isn't a hint of softness on the bottom end, not a hint. With a converter and headers, I think someone on this forum put it best when he said "your tire dealer will become your new best friend".