Being a retired ME, I'm always glad to get feedback and constructive criticism from a fellow engineer.
BTW, it was a "long time ago, in a galaxy far, far away" when I last crunched eng'g. equations(especially ones with exponents!). So it was definitely an interesting trip traveling thru virtual la-la land to shake the dust off my rusty thought processes, and see if they still functioned. Unfortunately, there appears to be more rust than I anticipated.

 

I didnt want to directly address it, but horsepower and torque are not both measured quantities. A dyno measures either one or the other depending on its type and computes the other. They are numerically derived. You do not use one or the other to accelerate, acceleration is caused by a net force, and horsepower is not a force, it is derived from the force of torque, which u correctly identified as a rotational force multiplied by the moment arm CONSTANT. I can derive the torsional force on the left foglight from the engine and dyno information as well as some physical measurments and constants and plot it at the same time as horsepower or torque (depending on the dyno), but it doesnt have anything to do with the car accelerating.

Horsepower is power, which is work divided time or force multiplied by velocity. Velocity isnt an apt reference of horsepower because it has nothing to do with the generation of it, it just describes a body being moved by the real real output of an engine. RPM is a characteristic of and engine and a necssary coorlation.

If the aero hp is greater than the rolling hp, that simply means the dominate retarding force is aero or frictional based, nothing more, nothing less. Acceleration is provided by force, abiet over a constant moment arm, Newtonian mechanics are pretty clear about this.

I really like the origional concepts and the equations, very interesting points and math, the final application just doesnt make sense.

 

(While violently bashing head against top of desk yelling with a crazy person voice) " Make it stop!!, Make it stop!!!!!" ( Reaches for revolver in comp. desk drawer) " I cant take all of the math and big words i dont understand!!!!!!" ( ***** the gun and points to temple) " Bang!!!!!!!" ( Blood and small pieces of brain fly against the wall. Head falls and hits desk.)
The pain is now over!


LMFAO!!!!!!!!!

On a side note, this thread is good stuff. Just wanted to make you laugh!
Please continue

 

Velocity by itself as a direct reference, no. But when multiplied by the force required to maintain that velocity, yes. It's implied that it takes more force to go faster, but in casual to lightly technical discussions people do not generally link the two in as direct a relation as exists.


I'm going to have to ponder this piece a bit . . . force ÷ distance . . . moment of some sort, probably torsion . . . torque per unit area


Norm

 

"...horsepower and torque are not both measured quantities. A dyno measures either one or the other depending on its type and computes the other."
I never stated that horsepower and torque are both measured simultaneously on a dyno.

"You do not use one or the other to accelerate....."
Then, what accelerates the car???

"...the force of torque, which u correctly identified as a rotational force multiplied by the moment arm CONSTANT."
The definition of torque is a vector(straight line, not rotational) force multiplied by a moment arm.
Rotational force = torque.

"I can derive the torsional force on the left foglight from the engine and dyno information as well as some physical measurments and constants and plot it at the same time as horsepower or torque (depending on the dyno), but it doesnt have anything to do with the car accelerating."
What is your point???

"Horsepower is power, which is work divided time or force multiplied by velocity. Velocity isnt an apt reference of horsepower because it has nothing to do with the generation of it, it just describes a body being moved by the real real output of an engine."
1 hp = 33,000 ft-lb/min
The units for horsepower can be changed, as long as they remain the product of force and distance over time, and any necessary unit conversion constants are used. Below are the units for horsepower best suited for working with aerodynamics.
1 hp = 33,000 ft-lb/min x (1 mile/5280 ft) x (60 min/hr)
1 hp = 375 lb-mph => 1 hp can generate 375 lb of force at 1 mph, or 1lb of force at 375 mph.
Converting this equation gives:
Power(hp) = {lb(force) x speed(mph)}/375
From this one can see that if the total aerodynamic force on a car is 1000 lb at 100 mph, then the car's engine will have to produce approx. 267 hp to overcome this force.
Thus, the aerodynamic hp for that car at 100 mph is 267 hp.

"If the aero hp is greater than the rolling hp, that simply means the dominate retarding force is aero or frictional based, nothing more, nothing less."
Totally agree. That's exactly what I stated.

"Acceleration is provided by force, abiet over a constant moment arm, Newtonian mechanics are pretty clear about this."
Now you're seeing things my way.

"I really like the origional concepts and the equations, very interesting points and math, the final application just doesnt make sense."
Thanks for the kudos. However, I have to respectfully disagree with your final comment.

 

Throwing money at something to get the results you want is always better than some number crunching spreadsheet, right guys?

money always seems to be the "limiting" factor. sigh...

Better yet, throw my numbers into your spreadsheet and tell me what my topend is. I'd love to know. 27'' tall tires, 4.10 gearing, 3650 lbs, and the hp/tq numbers in my sig. My car also has a chin spoiler so that affects the aero somewhat.

 

What rpm's correspond to your HP and torque numbers? (My sheet fits a torque curve to those points.) What is your redline (or fuel cut rpm).

What are the tire sizes? Both front and rear if they are different. (It works from conventional tire size data and computes what it needs.)

3650 lbs sounds a little high - do you have a heavy sound system? If so, that changes the weight distribution a little. BTW, what's a good estimate of driver weight?

I've got data or at least reasonable estimates for the rest.


Norm

 

Thanks Norm. I'm making 337.4hp at 6147 RPM/119MPH and 328.0tq at 4262 RPM/83.4MPH. My tires are 255/35R20 front and 275/35R20 rear. Rev limiter set to 6800 RPM and she pulls all the way to to it. I'm 215lbs. and yes, I have some sound equipment. I'm guessing the car weighs 3650 because that's what they used when the car was dyno'd. Does a mustang dyno weigh the car?? Maybe they guesstimated...

If you can plug that in and provide me an estimated top speed I can tell all the kids that ask me "how fast will it go?" a real answer. lol!

 

I'm seeing 155-ish being likely if those HP & T numbers were converted back to and reported as engine output, maybe 163 if they are rear wheel numbers that don't consider powertrain losses. That can be compared with 145 for a stock GT (3.55's) using the same spreadsheet. I did knock 0.01 off your drag coefficient relative to stock as a WAG (0.38 vs 0.39). Another 0.010 off is worth about 2 more mph.

I don't know how those guys came up with that weight either, but maybe somebody more familiar with that setup can chime in.


Norm

 

Those are rear wheel hp/tq numbers on a mustang dyno, which simulates "road-like" conditions so they're fairly real world numbers. I figured I was around 160-ish or so. Not sure about the weight but I thought these cars were ~3600lbs from the factory. This is a fully loaded car with a likely 30-40lb. sub box and heavy 5-channel 1000watt amp in the trunk. I've shaved some weight via the exhaust system, driveshaft but added some with the sound system components, MGW shifter (that thing weighed MUCH more than the stock unit, but also works MUCH better) and 20'' wheels. I have my hp/tq numbers where I want them for a N/A car, now I need to put it on a diet! (along with myself). I plan to swap some lightweight 18'' wheels/tires on the car after the first of the year. 20'' wheels are too heavy and 20'' tires are too expensive! I'm trying to make my current ones last until then...

Thanks for doing the math for me, Norm.