How to figure out your HP (approximately)
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Joined: Apr 2010
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From: Tampa, FL
How to figure out your HP (approximately)
So I found this formula online, and actually spent about an hour verifying it against the actual curb weights and performance numbers from major car review magazines, and it's actually remarkably accurate. The # is for HP at the WHEELS NOT AT THE CRANK so account for 15-20% loss, and you will have a general idea of how much power you are making!
HP = [(MPH/234)^3 * Weight
Broken down for people like myself who suck at math....
That means take your MPH at the end of your 1/4 mile (the trap speed) and divide it by 234
You will get a # something like .348xxxxxxxx blah blah whatever.
Take that number and multiply it by itself 2 times. (.348 x .348 x .348) and you will get a number similar to .042xxxxx blah blah blah.
Now, take that .042 (or similar) number, and multiply it by the weight of your truck. Not the curb weight, the actual weight. (Curb weight + driver, gas, extras)
You will get a pretty damn close estimate of how much power you have.
I ran the formula for my truck...more on that later....but I also ran it for a '99 Dakota R/T based on a MotorTrend report (1/4 mile time 15.4 @89 mpg) and an ACTUAL weight of 4100 lbs (curb weight is 3850, I accounted for gas, fluids, 150 lb driver) and I got....
220 hp @ the wheels...the truck was rated for 250 hp from the factory. 250hp - 15% loss = 212.5 hp. Damn. Within 7.5 hp. It wouldn't surprise me if that motor made more like 260 hp from the factory, which would be....220 wheel HP!!! Pretty damn close, I would say.
So, for my truck...it was a little harder. I had to use a stupid app on my phone (it used GPS to calculate 1/4 mile times/speed....but seems remarkably accurate and consistent) and my average speed at the end of the 1/4 mile was 81.5 mph. Curb weight for my truck w/ manual trans is 4208. Mine is automatic, add 100 lbs I assume. Call it 4300 for the sake of being simple.
So-
4300 truck
250 lb driver (I'm fat)
130 lbs of gas (I literally just filled up my tank before the test and drove 30 miles) at the time of the test, 22 gallons @ 6 lbs/ gal
20 lb (?) bed liner
20 lbs of sub/amp/etc (it's just a single 10" sub and small box/ amp)
60 lb (?) trailer hitch I added
That's 4780. I'm sure I'm missing something (washer fluid/ trans fluid/ diff/ etc.) so let's call it 4800 lbs to make it easy. I apply the same exact formula, and I get......
202 wheel HP. My truck is rated @ 210 hp factory at the crank, I have added an intake, muffler, plugs, and removed the clutch fan.
210 (factory) hp - 15% loss = 178.5 hp....so if my math serves, my mods have gained me about 25 hp. I think that's probably about right. That means I have 238 hp at the crank.
Is this exact? Of course not. Is it pretty damn close without using a dyno? Yes.
Don't know you 1/4 mile speed? If you have a GPS-enabled Android phone you can download an app called Car Performance for free and it will tell you. It's not 100% accurate....it told me that my 0-60 time was 7.4 seconds and that I did the 1/4 mile in 14.8 seconds once (no way in hell my truck is even close to that fast) but if you make 5-6 runs you will get a really good idea of your trap speeds.
It takes a minute to figure out, but run the formula and post your results here. If you are unsure of how to figure it out, I can help. But you have to know your 1/4 mile speed.
HP = [(MPH/234)^3 * Weight
Broken down for people like myself who suck at math....
That means take your MPH at the end of your 1/4 mile (the trap speed) and divide it by 234
You will get a # something like .348xxxxxxxx blah blah whatever.
Take that number and multiply it by itself 2 times. (.348 x .348 x .348) and you will get a number similar to .042xxxxx blah blah blah.
Now, take that .042 (or similar) number, and multiply it by the weight of your truck. Not the curb weight, the actual weight. (Curb weight + driver, gas, extras)
You will get a pretty damn close estimate of how much power you have.
I ran the formula for my truck...more on that later....but I also ran it for a '99 Dakota R/T based on a MotorTrend report (1/4 mile time 15.4 @89 mpg) and an ACTUAL weight of 4100 lbs (curb weight is 3850, I accounted for gas, fluids, 150 lb driver) and I got....
220 hp @ the wheels...the truck was rated for 250 hp from the factory. 250hp - 15% loss = 212.5 hp. Damn. Within 7.5 hp. It wouldn't surprise me if that motor made more like 260 hp from the factory, which would be....220 wheel HP!!! Pretty damn close, I would say.
So, for my truck...it was a little harder. I had to use a stupid app on my phone (it used GPS to calculate 1/4 mile times/speed....but seems remarkably accurate and consistent) and my average speed at the end of the 1/4 mile was 81.5 mph. Curb weight for my truck w/ manual trans is 4208. Mine is automatic, add 100 lbs I assume. Call it 4300 for the sake of being simple.
So-
4300 truck
250 lb driver (I'm fat)
130 lbs of gas (I literally just filled up my tank before the test and drove 30 miles) at the time of the test, 22 gallons @ 6 lbs/ gal
20 lb (?) bed liner
20 lbs of sub/amp/etc (it's just a single 10" sub and small box/ amp)
60 lb (?) trailer hitch I added
That's 4780. I'm sure I'm missing something (washer fluid/ trans fluid/ diff/ etc.) so let's call it 4800 lbs to make it easy. I apply the same exact formula, and I get......
202 wheel HP. My truck is rated @ 210 hp factory at the crank, I have added an intake, muffler, plugs, and removed the clutch fan.
210 (factory) hp - 15% loss = 178.5 hp....so if my math serves, my mods have gained me about 25 hp. I think that's probably about right. That means I have 238 hp at the crank.
Is this exact? Of course not. Is it pretty damn close without using a dyno? Yes.
Don't know you 1/4 mile speed? If you have a GPS-enabled Android phone you can download an app called Car Performance for free and it will tell you. It's not 100% accurate....it told me that my 0-60 time was 7.4 seconds and that I did the 1/4 mile in 14.8 seconds once (no way in hell my truck is even close to that fast) but if you make 5-6 runs you will get a really good idea of your trap speeds.
It takes a minute to figure out, but run the formula and post your results here. If you are unsure of how to figure it out, I can help. But you have to know your 1/4 mile speed.