I need help. ON a thread posted here I learned about this formula stuff where you can calculate torque, HP, etc. given certain things.

I have compiled data on the RX8 and need help understanding some things

first, in this formula

HP = torque X RPM /5252 What RPM do I enter?

I have calculated the rate of acceleration in 5th gear and I am really into this since according to the author, it is a true way to calculate HP.

Is the RPM the max torque RPM of 5500?
Redline RPM of 9000
Horsepower RPM 8500?

Where is the RPM figure? None of the above come out right.

 

This is pulled from the Mazda site:
Auto Manual
Horsepower, SAE net 197 @ 7200 rpm 238 @ 8500 rpm
Torque, SAE net 164 @ 5000 rpm 159 @ 5500 rpm

Don't know if it will help or not, but it does provide the *cough*BS*cough* claimed HP at the rpm they say.

 

The formula you list is used to calculate horsepower given a certain torque output at a certain rpm.

If you used the max torque power and the max torque rpm, you would be calculating how much horsepower the engine is producing at that max torque rpm, not the peak horsepower.

To calculate the horsepower at the redline rpm, you need to know the torque produced at that rpm. The torque figure you use will not be the peak torque output, and the horsepower you calculate at this point won't be the peak power output of the engine.

To cacluate the horsepower at 8500 rpm, you need to know the torque produced at that rpm (which you can figure out, since we know how much hp the engine makes at that rpm). The torque produced at this rpm will not be the peak torque produced.

Does that make sense? I'm not quite sure what you are trying to accomplish, so I can't really dive into much more detail.

 

My goal is to determine mathematically if the new figures are possible. I know that dyno testing determines the specifics of a given car but I want to know if the figures given are even possible. The formulas in the article "most ridiculous item of the day" are said to be based on non-negotiable laws of physics.

So far I have calculated the rate of acceleration on the 6spd in 5th gear to be 0.218 G's.

I am trying this and then want to use the data to compare vehicles - 350Z, S2000, G35, etc.

So often we read one review that shows car A as the winner and car A has a 0-60 of *** but then another article comes out that shows completely different sometimes grossly different figures. I believe that possibly this mathematical method will and could be a way to compare not based on a good day or a certain driver. Maybe I am wrong but certain givens, according to the article, would hold true so...

Here is what I have on the RX8 for anyone that is familiar with this formula stuff:

weight 3053 car + add your body weight
torque 159@5500
HP 238@8500
tire diameter 25.5 inches
center of axle to ground 12.75 inches
highest gear ratio 0.84
ring and pinion ratio 4.44

 

okay I may be stupid. I just realized that whatever figure you put in will result in accurate outcomes for those figures only. So the exact or trueness is dependent on accurate figures.

I did find that if the engine produces 238 HP and 159 ft lbs torque
that there is 147.05 ft lbs. of torque at the max HP RPM.

 

Yup, you got it.

You can probably use that formula and some calculus to get something you can use to compare with other cars over a range of revs, but my brain says it's the weekend and that it doesn't want to think....

 

okay, I think

The manual RX accel rate is (with me in the car) .220

The automatic is .217

Infiniti G35 .263
Cobra .357
S2000 .209

I could go on but I beleive that this method of calculation explains certain things.