Tech Article - HP Calculations (PI News, Vol 9, 1982)

Nice TX Rocky will apply..

did you do yours?

Matthias

way back when, I used a cassette tape deck to record the acclerating runs. I would yell "mark" at each 1K rpms, then go back home and listen with stop watch to get data.


for years, I enjoy playing the tape just to hear the engine spooling up

I am at a real Dyno today. Hope to have real, measured numbers in about 45 minutes. Then I can compare them against calculations.....

I can probably use an iPhone to record the "mark" calls instead of JFB's cassette recorder trick!

I am going to attribute to me not being able to concentrate, but not only do I not grasp the given calculations, but if I just take them at face value, put into excel, I am not getting meaningful output

Hmmmmm. We are either smarter now, or then. I haven't really studied this in detail, so even though I posted it, I can't say much yet.

I did run my car at the dyno, and got this:

Max HP: 272
Max Torque: 298 ft-lb

I am happy with this on my 100K mile engine...

Pix to follow.

Rocky

Your car still feels pretty strong. Feels more than 272hp.

272HP ain't bad at all

was that at the rear wheel or expected at the engine?

If you do an excel and get good results, would you mind sending it to me.

If you want what I have been messing with today,

My initial review of the Ron Graves's PI article is not being able to grasp the concept of calculating torque. when I made an excel work sheet, my resulting HP numbers were 10X expected. not knowing how he derived his equation, but I expect the 3.2 contant should have been 32.2 (the conversion of pounds weight into mass slugs)

my approach to calculating HP based upon acceleration times is the calculate the instanaous Kenetic energy at each 1K rpm, then the power from delta KE and time.

surprisingly the KE method gave me vary close results to Ron's equation (devided by 10).

So from what I have been able to quickly glean, the Mustang Dyno ("Load Dyno") measures Rear Wheel Horsepower. The expected engine dyno is the RWHP divided by .85 (basically 15% greater as an approximation).

Anyway - my results are attached. I have to say, I am happy with the torque characteristics - pretty flat across the board.

I also will say, some of these Turbo'ed cars can really put out the HP. A couple of the cars on the list were little Japenese cars with Turbos on them.

The second one down (over 1000 ft-lbs of TQ) was a big diesel pick-up.

It was quite funny on our dyno days when some of the Japanese cars with the smallest displacement of the day had some of the largest numbers. We always did our dyno days with HP, Torque, and HP/Cubic Inch. It really made people appreciate all the different motors. It was a lot of work but I miss the dyno days. We had a guy with at GT500 who would consistently get highest HP of the day. every year it would go up by roughly 100 HP. On the last dyno day we did he managed 750 RWHP. He was disappointed when we closed down because he said he "went big" this year.

I don’t know whether to THANK or curse Rocky!

My perception of Ron Graves’s incoherent approach to calculating the Power measured to accelerate an object first required me to review the definitions of Energy, Work and Power. It took awhile but at least I did recall the physics basics for solving such a problem without having to deal with differentials and integrals (I hated those things).

So in my mind, I derived the equations to allow the simple calculation of Power similar to Ron. This is where my real dilemma began! Even Ron mentioned his quandary since the calculation of power to get a 3300 pound object to 60 mph in 4 seconds was about the publish horsepower of an Escort. So for last couple days I had to find out why. I eventual found documentation and discussion with other engineer that explained it. Ron hinted upon it, but I don’t think he grasped the magnitude.

Unfortunately to determine an equation for rotational inertia power, some advance math is needed along with a lot of measurements and weights of the DRIVE TRAIN. As Ron implied the comparison of the horsepower measured on a dynamometer is basically at constant speed (or changing slowly) where in a real world drag strip measurement of performance, the power need to spool up all the drive train is significant. The “rule of thumb” used is nearly 40% horsepower is absorbed into spinning up the drive train. Once at a constant speed, that power can then be applied to the road.

So in Ron’s case, taken his measured accelerating 230 HP, assuming about 30% went into spooling up, gives an engine HP rating of over 300 HP thus agreeing with Ford’s published value.

So I will thank Rocky and Ron for making me learn something this week. For me, to learn something I have put my hands on it or in this case, get the math to add up.

quote:

I don’t know whether to THANK or curse Rocky!

I didn't mean to cause such a stir!

I do recall a previous discussion of an aluminum flywheel. I was in disbelief that shaving a ~10 lbs. off the mass of the rotating assembly could make a significant difference in a car's rev'ing ability, but this seems to fall in line with your conclusion.

It is very interesting, and it is good to have a lucid explanation.

Thanks - I am glad you got into the math.

Rocky