Horsepower Calculation

[Smokie]

I was reading an article in Hot Rod magazine today that discussed 10 ways to calculate HP at rear wheels and which method is most accurate on a 1-5 scale, 1 being least accurate and 5 most accurate.
To my surprise it mentioned dyno's and gave them an accuracy rating that varied from 3 thru 5, the 5 was for the current-eddy (oppossing electromagnets, with computer software for SAE correction) dyno. Anyway what I found quite surprising is that it gave a 4.5 accuracy rating to calculating rwhp from car weight (with driver) and trap time.
I have tried several sites that have calculators based on this method and my results are so high that I will not repeat them because... well they seem too damm high.
I used for total vehicle weight 4400 lbs (half a tank of gas and my 220 lbs) and my trap speed of 97.51 mph. The speed is accurate the weight is an estimate. The lowest reading I got on one of those calculators was about 285 rwhp using a total weight of 4000 lbs. Does anyone know a good formula that will yield a realistic rwhp estimate for my car? Thanks, Javier.

PS: A dyno run out of the question for now.

[TripleTransAm]

I think the problem is that you're expecting rwhp numbers from those equations. I've always found that they seem to produce numbers that are more like net crank hp numbers rather than rwhp.

I haven't dyno'ed my WS6 with the somewhat-uncorked exhaust configuration (the extra unmuffled exhaust tip), but I do have numbers from when it was in totally stock exhaust configuration. 310 rwhp, which if you take into account a supposed 12-15 % loss (manual 6 speed), results in 352-364 hp. So, given the supposed efficiencies of modern drivetrains, I usually stick with an even 350 hp number.

Unfortunately, I have not had my WS6 weighed, but using a 3800 lb race weight (me included) gives me a figure of about 360 hp out of one particular equation. 3700 lbs would result in about 350 hp coming out of that equation. Seems pretty close...

[TripleTransAm]

The equation I used in the above numbers was
hp = weight * (trap speed / 225) ^ 3.1447

Don't ask me how it takes into account driveline losses resulting in a certain trap speed. Must be using some sort of generalization factor in there, to make it end up in the ballpark.

I believe I got that out of a Car Craft or Hot Rod in 1986.

[Smokie]

TTA Thanks for info, here is the place I went: http://www.speedworldmotorplex.com/calc.htm, it claims to calculate rwhp with an estimated 18% drivetrain loss, when I plug in my numbers the results are very high. I guess Hot Rod magazine's accuracy rating for this method is not even close to the truth. :)

[stumpy]

Try this site SMOKEmUp (http://www.smokemup.com/auto_math/hp_mph.php) , it prompts you for the drive train loss and I found it to be very close to what I dyno'd.

[Smokie]

stumpy thanks a lot :) I tried the calculators with trap speed for the 1/8 mile and 1/4 mile and the results are very consistent and seem far more accurate than the other sites. This is the info I entered:
Car weight w/driver: 4400 lbs. ( I weigh 220 lbs, removed spare, 1/2 tank of gas)
1/4 mile trap 97.51 mph
Drive train loss: 20%
You Entered:
Vehicle Weight - 4400 (lbs)
Trap Speed - 97.51 (mph)
Drivetrain Loss - 20 (%)

Results:
Rear Wheel HP - 277
Crankshaft HP - 346.3
Power to Weight (rwhp) - 15.9 (lower is better)
Power to Weight (crank hp) - 12.7 (lower is better)

PS: If anyone feels any info I entered is incorrect please comment, I am trying to get the most accurate result possible:D

[TripleTransAm]

Just my gut feeling, but it seems kind of high. I was thinking you were somewhere along the lines of 320 crank hp...

I'll try running my WS6's numbers through there, but I have a feeling they're a tad high...

[Smokie]

Triple, I agree my numbers seem too high, however if you like to have a little fun, go to the web site mentioned by stumpy and plug in the numbers provided by our own member profiles and you will see some interesting results. By the way when you plug-in Disney's numbers the rwhp he states is identical to the calculator. The calculator uses a math formula, if the results are wrong too high, they should all be too high; however that is not the case....boy I love math.:lol:

[stumpy]

If the no.s seem to high then there are two possibilities. Drivetrain loss is less or the trap speed is off. Trap speed can vary from track to track and mods can change drive train loss.

[1 BAD 03 MM]

Try this one. (Using trap time) not MPH

http://www.fordmuscle.com/calculators/horsepower.shtml

[stumpy]

Calculations based upon ET are VERY VERY inaccurate! They're off by so much that it's not taken seriously.

[1 BAD 03 MM]

I don't know much about this, but, the numbers I get through this calculation sure seem alot closer to dyno numbers, than calculating using MPH.

[Smokie]

I done some reading on the subject and what stumpy said about HP estimates based on ET being inaccurate is the consensus, having said that; I tried the calculator that 1 BAD 03 MM suggested and the results are closer to what I think my car's HP is: results 258 rwhp at 14.494 ET, with stumpy's calculator: 277 rwhp at 97.51 mph.... so here is what I'm doing, I am splitting the difference and hereby declare that my car has ......267.5 rwhp This is my official number and I am sticking to it until proven wrong..... and if you are planning on proving me wrong, you better be willing to buy the beer. :argue:

[TripleTransAm]

Jeez... that would mean that my bone stock as-it-left-the-factory '03 Marauder was putting out some 325 hp at the crank if I was to use 18% drivetrain losses, what with my 97.2 mph trap speed. :up:

[Smokie]

TripleTransAm I'll drink to that:beer: Real dyno numbers are all over the place anyway... sooooo I am sticking with math, it is much cheaper !!!!:banana2: :banana: