Cspotrun

Ill start off by apologizing for the length of this post but I really would like to see some input on calculating a torque figure from a Horsepower figure. I had my truck dynoed today. Unfortunately they could not get the optical pickup to work and my pull only included a horsepower number. I did some basic math and came up with a torque figure but wanted to get opinions on 2 things. 1. Are my figures correct and valid 2. Are these calculations pretty much what the Dyno software does anyway. First a run down on the truck. Its a 2001 QC LB 4x4 Auto 3.54 rear gear running on 295/75/R16 BFG ATs. I've got an Edge Comp and RV275s as the major power adders. Misc items include BHAF, stock down pipe to 4 inch straight piped exhaust. The wastegate is turnbuckled and I see about 31 psi on spikes and 30 psi sustained on a long highway pull. EGTs are 1250 at 85 mph in 3rd gear with OD locked out. In 4th I can peg the IssPro Pyro by about 95 mph. This is all with the Comp on 5X5. Now on to the Dyno runs. Run was made with the Comp on 5X5. First off all values are SAE corrected and if it matters the dyno operates at 5800 feet above sea level. The dyno plot is speed vs. HP and my max HP was 316 hp at approximately 60 mph. I calulated a torque peak of 771 lb-ft fo torque. Below is my math on how I derived the 771 lb- ft of torque.

295/75/R16 BFG ALL TERRAIN
33.2 inch tire diameter = 104.248 inch circumference

60mph = 316800 ft/hour
316800 ft/hour = 3801600 in/hour
3801600 in/hour = 36467 revs/hour
36467 revs/hour = 608 RPM

316 HP generated at the wheels

316 HP = (Torque X 608 RPM) / 5252
1659632 = Torque X 608
Torque = 2730 lb-ft at the wheels

3.54 Rear gear ratio and a 3rd gear ratio of 1.0 yeilds a 3.54 final ratio

2730 lb-ft/3.54 = 771 lb-ft

The following table is ploted points with the torque value derived from the formula:

Torque = (HP X 5252)/RPM

RPM HP TORQUE
1700 85 262.6
1800 117 341.38
1900 166 458.85
2000 220 577.77
2100 285 712.77
2150 316 771.92
2200 315 719.29
2300 311 680.57
2400 310 651.25
2500 300 606
2600 285 554.37
2700 211 395.77

The RPM points on the above table were pulled from a Speed tire calulator using the tire diameter of 33.2 and the rear gear ratio of 3.54 and a final gear ratio of 3.54.

Gimme your opinions guys

CCB

oestreich84

I know it is wrong but my brain isn't working. I know it is wrong because your TQ and HP peaks are at the same rpm. This will never happen. The only way it could is if your engine produced 1 lb/ft of TQ at 5252rpm. I think we can rule that out It is way to late tonight to be thinking about something like this. I'll get back to you tomorrow.

RowJ

Just eyeballing your figures but I think you math is off starting at 2200 rpm.

Also know your thinking is incorrect. HP & Torque do not follow similar curves across the range of rpm's.

RJ

ajpulley

Dumb question here. How can anyone calculate one from the other at any time? Not trying to be a samarta$$, but I've read this elsewhere and I don't understand it. In order to do so, you are basing one value as a proportianate to the other, and more or less as the same algerbraic expression, i.e. the two will rise and fall equally and would have peaks and dropoffs the same, if you use one to calculate the other, right?

If someone designs and builds an engine, peak HP and Torque are variables not directly related to each other. I might raise HP or Torque disproportianately to the other by tuning the exhaust, or cam, or intake, or fuel, etc. in order to gain more of one over the other. Right? So, even if I took the values at the rear wheels, I would still have a peak torque different from HP in relation to the engine's values. Right? And, my peak differences would be different from another engine, like let's say an International, Duramax, Caterpiller or another model of a Cummins.

Does anyone understand what I am thinking? Whyor what don't I understand?

AJ

JD730

I've dyno'ed my truck twice so far and the guy that does is real nice and he'll let me try different things, like start speed, how I've going to shift through my auto.
What kind of dyno is it? The one I use is a Mustang. How are you running your auto on the dyno? Put it in drive and hammer down and let it shift through?

Check my gallery for the overlay picturs of 2 different dyno graphs, one with the stock spring and one with a 3200 spring, the stock spring plot hp and tq peaked at the same rpm.
The stock spring run was done by putting the trans in drive with the converter lockup switch locked and run. Tq peaked very nicely at 1750, but so did the horsepower. I succepted transmission shifts to be a factor here. The 3200 spring runs where started after the trans was in 3rd gear with the converter locked at around 42mph on the dyno. Tq is low around 1700 because the turbo was still spooling.

I don't know if this will help, but its what I have run into on the dyno. I would like to try a dynojet one day though.

HOHN

Correct. HP is a function of tq and RPM. So HP will rise with RPM, even if tq is constant.

You will sometimes see curves where HP is fairly constant. What's happening in this case is that TQ is falling at the same rate that RPM is gaining, so they cancel each other out and HP remains fairly constant.

Since the overwhelming majority of CTDs spin less than 5252 rpm, TQ should ALWAYS ALWAYS be higher than HP. How much higher depends on RPM.

That's why you usually see 12Vs with more tq for a given HP-- they are optimized (in factory form) for lower RPM.

jh

HOHN

The formula is hp=(tq X rpm)/5252

In English, this means that HP is a constant function of TQ and RPM-- EQUALLY. So, to double HP, you can EITHER double the tq at a constant rpm, or you can double the RPM at a constant tq.

Using the formula, you can see that HP will be exactly HALF the engine tq at 3636 engine rpm. Few CTDs are making good tq at that rpm, but the formula doesn't care if it's gas or diesel.

The flip side to that coin is that you can DOUBLE hp, just if you can make the same tq at twice the RPM.

For example, my truck makes peak tq at 2K rpm. It makes 9004lb-ft at @k rpm, which the formula tells us is 344HP.

If I could make the same tq at 4K rpm, though I would have DOUBLE THAT: in other words, 688HP!

If I could make the same 904lb-ft at 6000rpm, I would have three times that, or 1032HP!!!


From the formula, you can see that you can calculate ANY of the three values (tq, hp, or rpm) if you know the other two! But you HAVE to have two of the three pieces of information.

The reason many dyno curves lack TQ is because they do not have an RPM pickup for the engine (which is more involved on a diesel with no electronic ignition system). On these dynos, they are measuring HP, and get the "speed" for the dyno curve directly off the dyno rollers.

With an RPM pickup, the dyno could convert the HP to a tq curve, because then it would "know" rpm as well.


In terms of the algebra, you can see that HP is a DIRECT PROPORTION to tq and RPM. The fact that it is divided by a constant (the 5252) doesn't change this.

it doesn't get simpler than that: HP is the product of tq and RPM.

natlchamp2k4

so does 2+2=4?@ 2k rpm, and 8? @ 6k rpm?
just kidding

This is very interesting

Cspotrun

So Hohn, are you saying my figures are correct or because I do not have the actual RPM that they are off by a percentage too great to conside viable? With the Speed given RPM is mearly figuring out the gearing and the tire size so I would guess that if any falibilty lies in the amount of deflection the tires would see under the weight of my truck and thus wouldn't have the exact diameter nor circumference calculated in my "perfect" tire size. That was the only flaw I could see.

As an aside I searched for some comparable Dyno graphs of trucks with similar mods and most are at a lower peak torque value. I do feel the truck makes an honest 600 lb-ft but with out hard numbers I find it hard to feel cofident about it.

JD730- the dyno was a Dynojet, unfortunatley in my town noone has a Mustang dyno, nor much experience with Diesel anything. However, I did dyno in 3rd gear with the convertor locked up as stated in my original post.

Keep the comments coming and why not throw some guesses in to the mix on what my truck may or maynot be making ill see if i cant post the dyno run as well.

Dyno Scan is pending appoval but should be available soon here is the link to it Dyno Scan

RonA

Your numbers are correct. I prefer to convert tire numbers to ft. divide Mile(5280) by tire circ. = tire revs per mile x gearing = eng revs per mile. Once you know rpm at 60mph you can find others easily. 70 divided by 60 = 1.1666 x rpm at 60 = rpm at 70 and so on. My truck has peak torque and hp at around 2150 rpm also.

ajpulley

Hohn, thanks for explaining that. I now understand, well, sort of. So, if I take some numbers from any given manufacturer and create a HP and torque curve from them, it would be the same as what a dyno measures, more or less (calculated vs. measured +/-)? Well, if one could measure the engine and not the rear wheels. Speaking of which, is that how manufacturers come up with a HP and torque curve, or do they actually measure what an engine can produce at the crank shaft when they build one?

Ok, enough frick'n questions now.

Ph4tty

Hohn is right. The dyno measures torque and calculates hp using the above formula. This formula also explains why 600 cc sportsbikes make 100+ hp while big v-twins usually make 60-70. Some 600's turn to 16k rpms while the cruisers redline 5-6k rpms. So with substantially less torque at any rpms- these little sportsbikes get rated at a higher hp than a v-twin that has more torque due to the higher redline.

Another point is that on engines that rev high enough- the hp and torque lines will intersect at 5252 rpm. Just fyi.

HOHN

You can convert your "road speed" graph to an "engine rpm" graph if you run the numbers for tire revs/mile, axle gearing, tranny gearing, etc.

Most dynos will give you a spreadsheet with the datapoints (the actual numbers that went into generating the graph). You can take these and put them into a spreadsheet and convert to actual HP pretty easily, but it can be tedious.

As for tire squat on the dyno, you should have your tires up to higher pressure. Low pressure may marginally help traction, but most of us aren't that high to where we're traction-challenged. Low pressure will cost you some HP on the dyno, because the rolling resistance goes up so much.

If you have stock E rated tires, run the rears up to 65-80psi before you dyno it.