how to calculate hp

[cb7greenie]

hey does anybody know how to calculate horsepower without a dyno test?

[Jonas]

weigh your car with you in it. run the quarter mile at a track. use the weight and quarter mile time to calculate horsepower. it wont be exact but its better than nothing.

or get a gtech. again, not totally accurate but something.

[NAiL05]

(tq x something)/5252. Something like that I forgot.

[Jonas]

yes because he has his torque figures handy, right?

[NAiL05]

Yea and he cant spend 40-50 for just a run.

[visualpoet]

weigh your car with you in it. run the quarter mile at a track. use the weight and quarter mile time to calculate horsepower. it wont be exact but its better than nothing.

or get a gtech. again, not totally accurate but something.

The gtech is accurate within 1 hp, I believe. Once it's dialled in with your weight. The accelerometers are pretty sophisticated.
You can find both a calculator for figuring HP and the mathematics involved here
http://www.ajdesigner.com/fl_horsepo...apsed_time.php

They also have some other selections that will figure torque from your peak hp and rpms and other fun things you can do with math.

Do it several times and use the highest times/trap speeds. This will be your actual almost completely accurate whp. (Because you can mess up and not reach your cars full potential, but you can't go past the peak hp. Understand?). Dyno numbers are like lab numbers, they don't factor in rolling friction and wind resistance, so they're a little higher than the whp numbers you get from doing the mathematics.

I make several runs and average it out. That way I'm a little lower than actual, and stand less chance of looking like a liar if I can't hit my best numbers consistently.

[vietkid_2006]

i have the formula somewhere in my aviation text book..but too lazy to get it..lol.

[Jonas]

The gtech is accurate within 1 hp, I believe. Once it's dialled in with your weight. The accelerometers are pretty sophisticated.
You can find both a calculator for figuring HP and the mathematics involved here
http://www.ajdesigner.com/fl_horsepo...apsed_time.php

They also have some other selections that will figure torque from your peak hp and rpms and other fun things you can do with math.

Do it several times and use the highest times/trap speeds. This will be your actual almost completely accurate whp. (Because you can mess up and not reach your cars full potential, but you can't go past the peak hp. Understand?). Dyno numbers are like lab numbers, they don't factor in rolling friction and wind resistance, so they're a little higher than the whp numbers you get from doing the mathematics.

I make several runs and average it out. That way I'm a little lower than actual, and stand less chance of looking like a liar if I can't hit my best numbers consistently.

for it to be accurate you need a completely flat level surface. most roads are not flat and level. many tracks arent even! bumps or slight changes in the angle of the road can have a noticeable effect on the reading. that, or my bone stock f22a was making 140whp back when i got my gtech. you also need to know the exact weight of your car.

[owequitit]

for it to be accurate you need a completely flat level surface. most roads are not flat and level. many tracks arent even! bumps or slight changes in the angle of the road can have a noticeable effect on the reading. that, or my bone stock f22a was making 140whp back when i got my gtech. you also need to know the exact weight of your car.

Having once had a GTECH, it can produce some very accurate but repeatable results.

1) If you are accelerating on an uneven surface, it is as simple as running it both directions and averaging it. That will cancel out the effect of the grade.

That also works for wind.

2) The GTECH has a VERY specific procedure to use for calculating HP. You can get great variation if you don't follow it to a T. When I first got mine, I was consistently getting way high figures, and they were all over the place. but once I learned how to use it, and learned how to be consistent, I was getting realistic and repeatable results to within 1-2HP every time. I think I was getting 98 net HP on a bolt on F22A4. That is reasonable for the speeds I was running. I will discuss net HP momentarily. That is within a 2% error, which is more than reasonable.

3) The GTECH can only measure net HP. Since it is measuring acceleration, it can't measure WHP or Crank HP. Instead what you end up with is NET HP, which is basically HP that is left over to accelerate the car. This means that aerodynamic drag etc is also a part of NET HP. Also, the faster your car goes, and the less extra power it has to accelerate, the lower the net HP number becomes. So even though your car might still be making 200 crank horsepower, if you are at the vehicles top speed, there is 0 net HP, because the car has nothing left to accelerate with. That is somewhat of a disadvantage in terms of comparing to other measured forms of HP.

4) Jonas is correct. You have to know your weight as accurately as possible. A good way to get that is to either go to a set of truck scales and have the vehicle weighed, or go to a landfill and have them weigh the car. Usually, they will do that for a couple bucks.

There are some ways to work around the disadvantages of the GTECH, and even get quite accurate results from them.

1) Run in a consistent location. Run both ways. This way you average out the effects of wind, road grade, and you also minimize any other variables.

2) Follow the GTECH dyno instructions explicitly. A small deviation from the calibration or usage procedures can have a significant effect on measured results.

3) If you want to get really technical, you can take temperature and humidity measurements with each run. You can also measure your pressure altitude, and use these inputs to compensate for density altitude. There isn't a hugely easy way to compensate for humidity, but the temp and pressure altitude can be used to correct everything back to sea level. You can then approach the results with an understanding of how humidity affects your results, and make an educated comparison of the two.

4) If you want to get REALLY, REALLY complex, you can compute the Cd and flat plate area of the car, and plug that into another formula to figure out approximately how much WHP you are losing to drag at any specific speed. That can then be used to adjust your NET HP to WHP.

You could get into doing the same thing to take into account gearing etc. You could REALLY make it complicated.

These would probably put you reasonably close to what a dyno is going to give you.

Of course, you can also just use it as a baseline comparison of other reasonably identical runs, so that you have some baseline information that gives you an idea where you are. The important thing with dynos is to be able to get consistently accurate results, and then take that information as a base to determine whether any given change makes a difference and how much.

The GTECH is a great tool, IF its capabilities are applied and interpreted correctly. The technology has been used in airplanes for years, but you have to be very consistent and scientific when applying it.

[Jonas]

that being said. it is quicker and easier to just pay for a dyno pull. lol

but yes, the gtech is a great tool if you want to see how different mods affect your car. get the figures consistent and repeatable, do the mods, and calculate the change. the gtech won't really be useful for hp figures and bragging rights, simply because it often doesn't give 'dyno figures'. like owequitit said it doesnt necessarily measure whp or crank hp. 98hp with the gtech might show up as 115 on a dyno (not that either is worth bragging about... but 98 sounds pathetic).

[visualpoet]

that being said. it is quicker and easier to just pay for a dyno pull. lol

but yes, the gtech is a great tool if you want to see how different mods affect your car. get the figures consistent and repeatable, do the mods, and calculate the change. the gtech won't really be useful for hp figures and bragging rights, simply because it often doesn't give 'dyno figures'. like owequitit said it doesnt necessarily measure whp or crank hp. 98hp with the gtech might show up as 115 on a dyno (not that either is worth bragging about... but 98 sounds pathetic).

I don't know about the gtech (I assume it will be the same though) but I've got the Beltronics Vector FX1 and it lets you adjust your hp to match what it should be. When I got mine and set it up, the numbers it came up with were on the low side for the time and trap speed I was running, so after doing the math, I corrected the numbers.

The 1/4 mile time is accurate within .1s out of the box, and within .05s after you tweak the settings to match your car (its adjustable to compensate for pitch, rollout ands some other things). The accelerometers also tend to score a little higher than the track. This is due to the fact that the track averages your speed over the last few feet( This is what I've been told), where the meters take it at the exact end of the 1/4 mile. I don't find it to be a big deal. It's less than a tenth of a second, and I'm not that consistent anyway.

If you want something to use that's an excellent tool, an accelerometer is worth $50 to $100. You can get one cheaper off of CL or ebay. I've found mine to be very useful. Like any other precision tool, you need to calibrate it, adjust it and use it correctly to get accurate readings.

[Jonas]

the gtech i used was also the original model. one button, and settings were changed by tipping it. very frustrating to use.

[visualpoet]

the gtech i used was also the original model. one button, and settings were changed by tipping it. very frustrating to use.

I've heard the first gen gtech was a pita, and also was pretty inaccurate. I know people that have used that one and newer ones and ran them together. The new ones are much much better. My biggest complaint about my fx1 is that I pulled the wires out of the cig plug somehow and can't find my soldering iron! I've thought about mounting it permanently in the car, but my buddies love to throw it in their cars to play with once in a while.

[cb7greenie]

thnks guys appreciate it

[94b20gsr]

(tq x something)/5252. Something like that I forgot.

Just for reference its tourqe x rpm/5252