In this months addition of Hot Rod Magazine they rate various methods to calculate RWHP (something we like to talk about a lot on this forum). The article rated accuracy on a 1-5 scale. SOTP gets a (1) for example, a chassis dyno 3-4.5 (depending on dyno) which is why I use a dyno for indicative power and tuning only.

The number one rating? Quarter Mile top speed, at 4.5. As many of us have said on this forum, trap speed is the best indicator of your RWHP.

The formula is RWHP = (mph/234)3 X weight (mph divided by 234, cube that number and then multiply by weight).

Our cars weigh roughly 3100 pounds, so if the driver weighs 200 pounds it goes like this:

115 MPH divided by 234 = .4914529
.4914529 cubed = .1186986
.1186986 X 3300 = 391.7

The article does allow that shifting ability on a manual has a big affect, and frankly I believe drag coefficient does as well. That said if you want to use the formula to see how much more HP it takes to push you car say 115 vs 119 MPH it is pretty accurate.

In my case I ran 115 MPH stock and now run 119+ with my mods, the above example is mine actually, so in theory I am making 434 RWHP on the top end (a bit high I believe) but the 43 additional ponies needed to run 119 vs 115 is about right with my mods, and not very different from the dyno.

Les

 

One thing you should consider is that these are *uncorrected* RWHP numbers from those calculations. That is, when you get your graph and there's a 0.95 correction factor (and you pull 350 for example), the *uncorrected* number would be 368. I guess you could use one of those trap speed correction calculations first and then get you HP number and it should be more accurate.....interesting though.

So what I'm saying is that 1/4 mile trap speed means alot, but it also means that's the power you were making under those conditions.

I ran 116.24 in the 1/4 but my car dynoed at only 354 yesterday. I was so dissapointed that when I got home I took the intake system apart and found out that my airbridge wasn't properly connected to the throttle body and was sucking air in at that point. I had been throwing lean codes since I put it on, and had to clear one right before the dyno. So my computer was pulling timing to compensate for the lean condition.

also, congrats on the Falcons big win last night, that was some impressive!

 

Hey Z06 Quinn, did you know your sig is over 1 MEG in size!!! :eek: :eek:
It's nice but it is waaaaaaaaaaaaaaaay to big!! :nono: :cheers:

 

Does this apply equally to cars that are naturally aspirated and using various forms of forced induction? It seems that in a turbo charged car, the RWHP number would be artificially high?

I tried calculating for my last time to the track . . . I trapped 124.5 (stock turbos without Nitrous), but my highest dyno. on that setup was 503 RWHP. Using the formula (my car with me in it was 3,765 lbs.) . . . I get over 567 RWHP. I think because of the nature of turbochargers, the formula would come up artificially high?

I think with Superchargers and Nitrous Oxide, the formula would be more accurate 'cause of the nature of those methods.

And, I believe you're right about Coefficient of Drag having a big effect. Imagine a vehicle with a .08 Cd . . . it would of course have an easier time generating the speed at the traps.

 

That is an empirical formula that's been around for some time, not exact science. It seems to be fairly accurate, but will loose accuracy if the car has an unusual shaped power curve or unusual gear spacing or unusual aero drag.

I haven't read the article, but a dynomometer in good repair and calibration with accurate weather data to compute accurate correction factors is about as good as it gets.

Duke

 

The trap speed HP formula is only estimating one peak HP number. It does not attempt to show you what your power curve looks like over the entire RPM range. I cannot believe that magazine said that trap speed is more accurate than a dyno run.

 

Howdy Folks!

Below are two formulas - the first, is the one already being discussed, the second one however adds something interesting. I originally read these in a Vette magazine, where they were testing a few C4 tuner cars (LPE, Rippie, etc) and they were kind of using them to validate the HP claims (vs. the actual ET/MPH).

The article indicated the Trap/MPH calculation would estimate OVERALL power the car was putting down. Then (and this is kinda interesting) the second formula would estimate HOW MUCH of that power was being utilized by the car (of course it doesn't take into account dozens of things).

It is interesting though to see a car generate 614HP from the MPH, but only derive 520 from the ET, meaning it has lots of "HP Loss" from traction, etc. (assuming, again, lots of other variables)

The Trap-speed method
This method uses the weight of the car and the speed at which the car completed the quarter-mile run.

The formula is: hp = weight * (speed / 234)3


The ET method
This method uses the weight of the car and the time it took that car to travel 1330 feet (¼ mile).

The formula is: hp = weight / (ET / 5.825)3

Here's a great source for the formulas (plus some other good stuff):
http://www.rpmoutlet.com/formula.htm

And the F-Body site has a bunch of these automated with [/img] Report this to a moderator
http://www.fbody.com/calc.htm

Take care -

 

Well, since a dyno. run is a direct measure of RWHP . . . I believe that the dyno. run would be the control in this experiment . . . scoring a 5. The others are methods of estimating this direct measure. That's my guess . . .

 

This formula is interesting and seems to come close when you read some of the guys dyno numbers and compare it to their 1/4 speeds. I tried a 135mph at a total 3300lb weight and the formula indicates 634rwhp would be required. From the guys threads with this much power I wonder if they have the traction to get up to this speed in the 1/4 even though the power is deff there to do it.

 

It sounds pretty close. The 73 dynoed at 268 (Dynojet) at 2750 ft and runs 102 at 2750 ft. The formula calcs at 265.

 

Dunno about this calculator, it says I am pushing 421rwhp and I only dyno at 372rwhp.... i wish I really had that much..hehe

 

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