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Old 10-25-2007, 03:07 PM   #1
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Default Stock Greddy - Dyno Questions

To give a quick background. '90 miata with greddy kit. Pretty much stock kit, upgraded the plugs and wires and i have a fm heatshield on it. After breaking it in thoroughly (1300 miles at about 3.5-4 psi) i upped the boost to about 5.5 psi - 6 psi. My oil gauge isn't going crazy even at wot, so i'm assuming i'm not knocking. It pulls pretty good.

Today i put it on a chassis dyno at school, and we were trouble shooting the dyno for a while and finally made a few runs. The way we have it set up it is measuring wheel speed and not engine speed so i don't have a plot right now. (will be making more runs tomorrow and will have a plot, after i convert to engine speed with the gear ratio).

My question is, I showed a max torque of 158.8 ft-lbs of torque, and a max hp of 94. (i think the conversion that is programmed in to calculate hp is way off). Does that torque reading seem pretty accurate? Should it be higher/lower? I am kinda weary to how accurate the school's dyno really is.

Also I know to calculate HP it's hp = 2*pi*N*T So when measuring wheel horsepower is the N = to WHEEL SPEED? or is N = to ENGINE SPEED?

Any help is appreciated!

-Sean
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Old 10-25-2007, 03:31 PM   #2
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Originally Posted by papasmurf View Post
Also I know to calculate HP it's hp = 2*pi*N*T So when measuring wheel horsepower is the N = to WHEEL SPEED? or is N = to ENGINE SPEED?
The only way I know how to calculate horsepower is Horspower=(Torque*RPM)/5252. With your equasion I see Pi in there so I am wondering if tire height should be in there somewhere. I think your equasion to determine Horsepower from wheel speed must be incomplete. I could be wrong though.
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Old 10-25-2007, 03:39 PM   #3
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well, looks like he's trying to calculate RPM from the speed of the drum, which would be 2*pi*r (r being the radius of the drum)...

either way, it ain't right. torque at the drum, times engine speed (rpm) divided by 5250 will give you your torque number.
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Old 10-26-2007, 10:42 AM   #4
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i know what you mean i did a dyno run the other day and it read out at 111hp (dynojet) but i dont think the guy put in my gear ratio and tire size, could that be a reason for low hp? just wanted to jump in and get some answers
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Old 10-26-2007, 10:49 AM   #5
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dynojets calculate the gear ratio from the software. as long as your in 4th gear or below the results will always be the same.
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Old 10-26-2007, 11:52 AM   #6
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Cool, thanks for the replies. I need to check into the software, because i know it was just overhauled last year. They are using labview (from national instruments) to gather the data from a pressure transducer they put on the the back wheel. (We ran it in 5th 4th and 3rd gear....with 3rd gear having the best results) We are putting it back on the dyno today at 10 mst.

I was gonna pay to go put it on a nice dyno at a tuning shop, but although my schools dyno is a lil ghetto, its also $0.00/hr.

It just seemed odd the combination of a high torque value with such a low hp value.

Am I safe to assume i am having no knock by listening and oil gauge alone? I know i should put in a knock sensor/wideband 02, but i won't be able to do that for a few weeks.
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Old 10-26-2007, 12:03 PM   #7
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well it sounds like the TQ they calculated was correct, so what was your torque at 6800RPM? that should be your peak.
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Old 10-26-2007, 01:44 PM   #8
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if say your peak torque hapened at 5K rpm and you kept a minimum of 130 lbft until 6800 then you would have 170 or so whp. But if you only made 110 lbft then you would have 142 whp
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Old 10-26-2007, 05:17 PM   #9
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If you have a full base kit you shouldn't be knocking. I'm pretty sure it's supposed to run at 5.5psi. That's where mine is running anyway.
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