Primer on power and torque specs

From V6SHO
Jump to: navigation, search

Ron Porter's Power/Torque spec primer

After all of the threads on mods, 1/4 mile times, hp improvements, etc, I feel it's time to run through some basics, as there seems to be some misunderstandings out there. I have run through this scenario verbally many, many, times over the last 25 years, so it will get long-winded (as I usually do.:-}) I am not an engineer, so I try to put explanations in laymans terms and keep the math simple.

In the example, I will use Brett's dyno numbers for Run 13 simply because they are handy. The HP equations are

  HP @ flywheel = torque X RPM/5252
  HP @ wheels = HP @ flywheel X 0.83
  HP @ wheels/0.83 = HP @ flywheel

(assuming 17% loss thru the drivetrain). Ed. note: the car in this example is an automatic, and as such has more than 17% loss through the drivetrain due to the torque converter, and it has no flywheel. But this page is purely for pedagogical purposes anyway... Basically, horsepower numbers aren't worth much. Horsepower is a measurement of torque over time and will not necessarily give you the information you need to determine how much how much of a slug in the gut you'll get when you nail the throttle. Actual torque is the key.

Here are Brett's numbers for his peak torque, peak HP, and at 7,000. I have calculated the flywheel HP for comparison purposes. These numbers represent a basically stock SHO motor with mild mods, one that many of us are familiar with.

RPM ft/lb HP@tire HP@flywheel
At torque peak 3000 192.6 110.0 132.5
At old HP peak 5800 168.3 185.9 224.0
At 7000 RPM 7000 127.9 170.5 205.4

We all know how this car feels under acceleration, it's like the rest of ours. Now, I am going to add a hypothetical chip that I call the SUPERDUPERCHIP. All this chip will do is keep the torque curve flat as Kansas from 5,800 to 8,000 RPM. Below 5,800 will be identical to stock. Since it's hypothetical, we make assumptions that the A/C compressor won't be hurled through the hood, etc. Let's put the hypothetical SUPERDUPERCHIP on Brett's car and run the numbers again.

RPM ft/lb HP@tire HP@flywheel
At torque peak 3000 192.6 110.0 132.5
At old HP peak 5800 168.3 185.9 224.0
At 7000 RPM 7000 168.3 224.3 270.3
At 8000 RPM 8000 168.3 256.4 308.9

Wow, I got 71 more horsepower at the wheels and 85 more at the flywheel!! I'm a gonna race me a new Z28 for pink slips!!

Not so fast, buckaroo. If you got in this car and drove it, you would obviously notice no change below 5,800 and not a real change above 5,800 (although this would be a good motor for Blackhawk and twisty roads). What happens is that acceleration simply does not fall off after 5,800 (it basically prolongs (!) the acceleration at the same rate). You would maintain the same acceleration at 5,800 as as 8K. Since virtually all cars lose their rate of acceleration as speed, wind resistance and friction losses mount, this actually isn't all bad. Please note that, even though horsepower has been greatly increased, at no point along the torque curve has the actual torque been increased over the 192.6 ft/lb peak figure, which means this HP gain will do nothing for plastering you into the seat and wrinkling the skin on your face.

The fact is the highest torque reading is still 192.6 ft/lbs. Look at torque ratings for higher cubic inch cars. If you want that socked in the gut feeling, you've got to get the absolute torque numbers up higher across the range. NOS or a supercharger are probably the best ways to kick up torque at a lower rpm.

This brings us to mods like the BoltOn Stage (BOS). My findings echo what Vadim has shown in his earlier dyno numbers. There is an increase in the absolute ft/lbs of torque available in the mid-range, which is noticeable (not face-wrinkling...just noticeable) and produces around 30 HP at 6K. What the mods do is decrease the rate at which the torque drops off after 6K, so Vadim shows a 45-50 HP increase at 7K.

This is what I expected from the BOS, I got it and I'm pleased. A number of you would drive my car and think it was an absolute waste of money, because you are expecting a big increase in the face wrinkling factor. In a race, the higher average torque will let me pull away from stock SHOs, but it won't be extremely obvious from the seat of the pants feel. I will expect slightly higher speeds in the 1/4 mile, but there isn't a big increase in absolute torque to get the ETs down (assuming that you can get traction).

You have to be careful when you look HP numbers to properly interpret what they mean. I've learned a lot over the years about the way performance mods affect a car and how they work or don't work. Putting in cam (for example) that adds 100 HP doesn't do much in a street car if it lowers the peak torque and just moves the power peak up the RPM range.

Steve Reynolds adds:

That's why I like to use the torque per liter rather than HP per liter as an engine measurement. The bad news when it comes to our SHOs is we already have about the best torque per liter available which means there is no cheap and easy way to obtain a significant torque increase. All our increases will be in high RPM improvements.

There are only 4 ways to make a significant increase in low- to mid-RPM performance in our SHOs:

  • NOS
  • Supercharge
  • Turbocharge
  • Add displacement