Pick the gear you're interested in. You do a run from low RPM to high RPM, just like on the dyno. Power will vary from gear to gear for the same conditions (from changes in rate of acceleration of parasitic inertias), but what you measure will be a pretty good indicator of the power you put down during that run.

That's up for debate. Clearly variations in temperature, flatness of the road, wind, etc affect the measurement. You can try to minimize this (do runs when there is no wind on flat ground), and record the air temperature and know your altitude. From this you can get the density of the air to account for drag.

There is a local weather station recording wind direction and speed, but they're not doing it just for me.

Assuming you don't account for wind speed in the drag calculations, a tail wind would increase your calculated power, and head wind would decrease it.

Lower tire pressures would increase rolling resistance. However for a given vehicle setup, I do a decelleration in neutral from high speed to get a good estimate of drag and rolling resistance values. So assuming you keep the same tire pressures, this would be accounted for.

The greater the parasitic inertia, the greater the power lost to accelerating these inertias. Doing runs in higher gears minimizes these losses by minimizing the acceleration rate.

Correct. You will actually measure slightly higher power numbers in this case (similar to doing a run in a higher gear) because your rate of acceleration is decreased, so less power is lost to accelerating parasitic inertias. All you care about is having an accurate vehicle speed, accelerated mass, drag, and rolling resistance parameters. If you put aftermarket wheels on that make your vehicle speed log off by 50%, you need to account for that.

Sure, I understand your point. But the difference here is that what I measure with the log, assuming the road is reasonably flat and there aren't any weird conditions like a large tailwind for which I'm not accounting, is truly the amount of power that my car is putting down on the street. It is what is accelerating my car. It is a real-world load, so the numbers are meaningful.

I'm sure that if the thread-starter road-dyno'd his car in 3rd gear he'd be seeing about 215 real-world horsepower to the wheels.

IMO, dyno numbers should be used for relative calculations only, and they should be scaled to the crank numbers for the car from a stock dyno before mods (this accounts for the dyno being off by a proportional factor).

And IMO, if you want to know what your car is "really" putting out at the wheels on the street (at least for those exact conditions at that instant), there is only one way to do it: measure it on the street.

 

I don't think we're at odds here, but I do think we're interested in different goals. The only reason I would ever want to determine how much power I'm putting out is to measure the effectiveness of a mod(that is, if I have a baseline and if the conditions are controlled *enough* for my comfort). However, that goal is itself only transient; I am actually only interested in how a mod will affect my 1/4 time/trap, or how it will affect drivability.

Horsepower figures, IMO are largely irrelevant. I'm concerned about trap speed. I understand not every one tracks his car, but I do, with enthusiasm. Ultimately, I will measure how my car is performing by the trap speed. Or, in the case of my daily commute, if the function of driving my car is more enjoyable.

But I must admit, I really am a geek, and a numbers geek at that. If I had the UpRev, and the similar modelling software, I would probably do what you are doing now. In fact, whenever I get a dyno done, I spend hours just playing around with winpep look at the data in every way imaginable.

 

Trey,

Did you ever get your A/F sorted out?

 

Unfortunately, no, but that deserves it's own dedicated thread. Check back in a couple hours(on the phone right now) on my mrev/stillen thread. I'll post my update there.

 

Well if you're just interested in a double-integrated version of power (your trap speed), then you'd be in heaven with the Cipher. Datalog speed versus time over some long range (taking the launch out of the equation) and integrate it. That's far easier than differentiating velocity versus time to extrapolate power. I have to do some tricky digital filtering to get nice power curves out in the end.

 

Remember a Dyno is a tuning tool, you can set a dyno up to give you any numbers you want..especially a mustang Dyno. There are all kinds of calibration settings to these things. I dont pay attention to dyno number that much cause so much can factor into them, like a I said its a GREAT GREAT tuning tool.

The best way to get true #'s on what your car is putting down is take it down the local dragway once or twice usually cost 15 bucks... dont launch the car hard to make sure u dont break anything (very sticky starting line, IRS tends to really hop, remember u r not going for time, but MPH so u dont want to spin at all) These cars stock once u get rolling u can put it to the floor the rest of the way. I suggest power shifting to get the most MPH.

Most dragways have a scale as well, if not take your car through a truck stop and have it weighed with you in it, use this formula...

hp = weight * (speed / 234)3 For you non math majors

lets say my car weighs 3400 pounds with me in it.. and i run 98 MPH in the 1/4 well take 98/234 that equals .419 rounded off... take that to the 3rd power (times itself 3 times .419x.419.x419= 0.074) rememer dont x's it by 3 or u will come out wrong. now take your weight 3400x.0.074 and the number you get is the power your putting to the ground on that run. which i get 250 RWHP. Try to note the weather conditions, note if you get a head wind or a tail wind, get the DA (Density Altitude) ask any major drag racer at the track and he can tell you. and you can correct to SAE HP.. (seal level)

Hope this might help some of you guys out.

 

That calculated number is a very rough estimate at best, and is just empirically fudged. I'd take a dyno number as absolute long before that number back-calculated from a single datapoint.

 

why do people speak of the mustang as if it's a crap car? those cars respond so much better to mods than just about anything else out there. and that includes the "glorified H" emblem cars. those are pretty good numbers.

 

dis regaurd. im an ***. saw mustang and dyno. i appologize. grew up in the back seat of a mustang. got a little choked up about it. i will slap myself later.

 

Not exactly what I was hoping for...

I started a new thread in the Engine, Drivetrain, etc. forum at Dyno Stock 05 6MT Before/After MREV+