A challenge from Tim to Brent brought this to mind, but it will work for any engine in any car.
1. Install a bicycle speedometer and mount the display so you can record it on a video camera mounted conveniently.
2. Weigh the car, and keep track of additions and subtractions. . Gas is 6 lb. per gal, for example.
3. Select a level road with low traffic.
4. Get up to max speed that interests you, put in neutral, and record the rate of driftdown to about 20 mph.
5. Transfer the mph/seconds change to a spreadsheet. If you can't calculate the horsepower with those numbers, some semi-smart guy here on the Forum can.
You should end up with a constant number - induced drag, which is drag from drivetrain, tire friction, etc.
Over that you will have a variable number that should plot straight on a chart: that's parasitic, or aerodynamic drag.
If there's wind, averaging two way runs will cancel most of it.
Now you have a baseline of horsepower needed to propel your car.
6. Yep, now go repeat the runs, but accelerating.
Your horsepower at any speed is the rate of acceleration, plus the rate of drag.
Put that in your smipe and poke it..
Ralph, I actually have varied access to 3 type of dynos and each have their strengths and weaknesses for me personally. An engine builder I used quite a bit has a Stuksa that I still have the plates to adapt a Model A/B engine to. Since we aren't racing as much now as we were I don't really have any new projects to go test.
What I do own is a Land & Sea dyno built for outboard motors up to 300 hp. For me when I initially bought this was to have the most promise by just using it in-house and have my own baseline for data. The downside to this set-up is two-fold, ...I would need to build the adapters, and the horsepower on this is factored on a hydraulic pressure chart. This set-up has some design flaws because of the varying oil temperature which must all be factored in a chart. If I would have known now what we are using, I would have never purchased this unit.
The 3rd which is becoming much more usuable as far as we are concerned is a chassis dyno. My fee to use it is $125 an hour, with $100 for each subsequent hour. We usually have everything dialed in by the time we get there so an hour is usually a long time for us. My guess is that these type of dynos are more popular than you think and are in many towns. Probably one close to anyone who wants to do some testing. Matter of fact, from time-to-time you see them on eBay for around a grand.
My suggestion is to go find one of those type dynos because that is real-world readings there. Factor in about 7%-10% for slippage and you have hard numbers you can show a customer. Tim suggested I build an engine like his and run it on the dyno. I have no desire as I already have one more of these set-ups than I'll ever use. For the guys that have them, I say roll on and happy motoring.