harveyf

I referred to the attached document in another thread. Since it is probably of general interest, I posted it as a new thread. As proof that I never throw anything away, I see I printed this out in 2006. My respects go to Mr. John De Armond of the alt.hotrod newsgroup, who originally presented this. It may not be totally accurate but should be useful for making A to B comparisons for those that are modifying their cars.

bonus12

I used to divide the weight of any car by the horsepower and 2.3 (constant) to get the 0-60 time. It's quite a good estimate despite variables light drag coefficient and gear ratios.

So you can switch around that formula to get:
Hp = weight / (0-60 time in seconds x 2.3)


I started with ....

Weight / (2.3 x hp) = 0-60 time in seconds
=
Weight / 2.3 / hp = 0-60 time in seconds
=
Hp = weight / (0-60 time in seconds x 2.3)


For the weight of the car, you can use the curb weight as a starter, or you can look at your last smog receipt, which sometimes says the weight but I'm not sure how precise it is. If the curb weight of the factory stock 1986 944 Turbo is about 3000 lbs, which it is, and you divide that by (a 0-60 time of 6 x 2.3) ... then you get 217 hp...which is actually spot on. It's the same math if you do 3000 / 6 / 2.3 = 217

harveyf

I'm not familiar with that forumula but that's nothing new. Regardless, if you are just trying to see if your latest upgrade has an effect, methods like these can be instructive. I wouldn't personally go for 0 to 60, as how you get off from a standing start and how you accomplish your shifting can be variable. Another thing I haven't mentioned is that to be apples to apples, the ambient temperature much be the same, if you record measurements on different days. There are standard protocols from groups like the Society of Automotive Engineers that are applied to power measurements to get them back to a value at a standard temperature. Anyway, taking your car out and doing some time and speed runs meets all social distancing requirements that I can think of so go for it!

V2Rocket

Cool procedure, thanks for posting.
It checks out in theory since drag racers have been able to estimate ET/MPH from HP/weight or vice versa pretty well over the years too.

Force over distance over time...

FRporscheman

This is how I've boiled it down.
F=m*a
a=(v2-v1)/t ... I recommend using one gear only, so perhaps something like a 30mph-60mph time in 2nd gear
T=F*r ... r is the lever arm between flywheel and contact patch, aka the radius of your tire * 1/the gear ratio * 1/the final drive.

therefore
T=m*(v2-v1)*r/t

note 1: this is your average torque across the rpm range used in the acceleration test. Not peak torque. You can kinda get your peak torque by making the accel test as short as possible and centering it around the rpm where you think the peak is at.
note 2: if you want peak horsepower, you need to measure your torque at redline and multiply that by (redline/5250).
note 3: I recommend 2nd gear because 3rd will get into speeds where drag is a big factor, and 1st will probably accelerate too quickly for accurate timing of the stopwatch, but you can try 1st.
note 4: This post is worth what you paid to read it.

AnthonyGS

You need to account for the torque multiplication if the transmission and final drive ratios too (post above) Other than that, yes this is basic physics.

Dyno operators typically want you to run the car in a transmission gear that is 1:1. I think that’s 4th gear for most 5 speeds.

doolittle

I have been using the Race Timer app to measure modifications that I have made to my car. The 50-75 mph time in 3rd gear (with a note of fuel tank level) is what I am using. I am interested in trying out Harvey's test, too.

harveyf

Nowdays, there are several "automated" apps that will easily give you numbers. They are all useful. It is nice to go back to basic principles, as FRPorscheman has done. It helps keep us grounded.

To go a step further, I have a datalogger that will measure in 1/100th of a second intervals, gps speed and rpm. I also plan to add an ambient air sensor. I will give a plug to Autosportlabs, who sells a reaonsably priced datalogger with built in gps. Using the data logger, you can minimize stopwatch and speedometer errors.

Speaking of basic principles, early in my career I was reviewing calculations prepared by a senior guy. He was deriving section properties of steel beams by hand, rather than referring to standard references. When I asked him why go to the trouble, he said "I like to work from basic principles." That was a little too granular for me but I have to admit that it made a lasting impression on me.

FRporscheman

Even though I have technical degrees and do technical work, I like to keep it simple for as long as I can. It does take longer but it ensures ones understanding of the problem being worked on.

Datalogger sounds like the way to go! I've never used one - sounds perfect for this.

AnthonyGS, I did in fact account for gear & final drive ratios in my post.