Horsepower and Torque
10-03-2019, 12:25 AM
#1
Rennlist Member
Thread Starter
Horsepower and Torque
Apparently the thread discussing HP and Torque was closed. Since there appeared to be a fair amount of interest, I'll try to simplify the subject. Perhaps it will provide some clarity.
TORQUE is simply a measure of FORCE - and only FORCE.
HORSEPOWER is TORQUE with a TIME COMPONENT added. In our world, horsepower is the measurement that we're always interested in.
A practical example: Let's say I have a 6 foot breaker bar attached to the rear wheel center nut on my race car. Let's also say I weigh 200 lbs. If I put my full weight on that breaker bar, my car will move quite easily. Why? Because I've put a whopping 1200 ft. lbs. (200# x 6 feet) of torque to the rear wheel! HOWEVER.... even with 1200 ft. lbs. of torque, it will take a looong time to get the car down the straight on any race track, or the on-ramp to the highway. Assuming I could rotate the breaker bar twice in a minute, the calculated HP in this example would be (1200x2)/5250=.5HP That's one half HP!
We always need to add that time component to make it relevant to our interests. Remember, measuring torque over time is HORSEPOWER.
If you have lots of torque at low RPM, you'll have lots of HP there. If your torque doesn't fall off dramatically up high, you'll have lots of horsepower at high RPM.
A street car may be lots of fun with low RPM torque since it will have good HP down low and be quite snappy. A race car may only need HP at high RPM since RPM drops between gears may keep the engine within the 'good' power range. Example: I was driving a 924 D/P at Road America, and downshifted to 2nd (IIRC) for turn five. Going up the hill I'd upshift to 3rd at the 8000 shift point and the car would fall on it's face. The gear drop RPM was outside the rather peaky power band! I found that simply raising my shift point by 200RPM, I'd scoot right up the hill. I now stayed within the power band!
It's only horsepower that matters, whether you're at low RPM or high. Is peak HP relevant? Maybe... What's more relevant is the average HP within the power band you want to run.
TORQUE is simply a measure of FORCE - and only FORCE.
HORSEPOWER is TORQUE with a TIME COMPONENT added. In our world, horsepower is the measurement that we're always interested in.
A practical example: Let's say I have a 6 foot breaker bar attached to the rear wheel center nut on my race car. Let's also say I weigh 200 lbs. If I put my full weight on that breaker bar, my car will move quite easily. Why? Because I've put a whopping 1200 ft. lbs. (200# x 6 feet) of torque to the rear wheel! HOWEVER.... even with 1200 ft. lbs. of torque, it will take a looong time to get the car down the straight on any race track, or the on-ramp to the highway. Assuming I could rotate the breaker bar twice in a minute, the calculated HP in this example would be (1200x2)/5250=.5HP That's one half HP!
We always need to add that time component to make it relevant to our interests. Remember, measuring torque over time is HORSEPOWER.
If you have lots of torque at low RPM, you'll have lots of HP there. If your torque doesn't fall off dramatically up high, you'll have lots of horsepower at high RPM.
A street car may be lots of fun with low RPM torque since it will have good HP down low and be quite snappy. A race car may only need HP at high RPM since RPM drops between gears may keep the engine within the 'good' power range. Example: I was driving a 924 D/P at Road America, and downshifted to 2nd (IIRC) for turn five. Going up the hill I'd upshift to 3rd at the 8000 shift point and the car would fall on it's face. The gear drop RPM was outside the rather peaky power band! I found that simply raising my shift point by 200RPM, I'd scoot right up the hill. I now stayed within the power band!
It's only horsepower that matters, whether you're at low RPM or high. Is peak HP relevant? Maybe... What's more relevant is the average HP within the power band you want to run.
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hacker-pschorr (02-11-2020)
10-03-2019, 08:11 AM
#2
Rennlist Member
Explained in a good way 👍
10-03-2019, 08:40 AM
#3
Rennlist Member
Good explanation.
I remember Smokey Yunick saying, for the fastest cars, “It is better to make torque at a higher RPM than a lower one so that we can take advantage of gearing.”
I remember Smokey Yunick saying, for the fastest cars, “It is better to make torque at a higher RPM than a lower one so that we can take advantage of gearing.”
Last edited by Mike Murphy; 10-05-2019 at 10:44 AM.
10-04-2019, 06:32 PM
#4
Rennlist Member
I have looked at shift points from an equivalent wheel speed vs torque perspective. I base it off the latest dyno chart I have. There are instances with these cars where you get stuck in no man's land from a gear choice point of view, either lugging or bouncing it off the rev limiter. From the charts I have built it is best from a wheel torque perspective to bounce it off the rev limiter.
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Mike Murphy (10-04-2019)
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Mike Murphy (10-05-2019)
10-07-2019, 08:38 AM
#6
Drifting
My car is a stock '89 with a Fast 44 DME chip, 3" exhaust with cat delete, and SFR stage 2 header; I run in PCA stock class, so boost levels, gearing, etc. are stock.
In practice, I'll run the car up to ~5,800 rpm before shifting. No reason to stress the engine for just practice.
For qualifying and races, I'll shift just under the (factory) rev limiter. My lap times drop ~0.75 sec on a 1:26 lap, which is a significant amount.