diesels on the dyno.. questions ??

[LvTitan]

i've always been under the impressions that dynos MEASURE torque, and that they CALCULATE horsepower from this measurement..
so TORQUE is actually MEASURED, and HP is determined w/mathematical equation ... have i been misinformed ??

there are two questions i'd like to get informed answers to..

1. i went to make some dyno pulls today to see the difference with my new tranny/converter setup since my last tranny was slipping badly on the dyno last time...
when i got it all strapped down, the guy asked me where he could hook up his "tach" lead to..(spark plug wire for instance haha).. i told them they needed to hook up an optical to the crank dampr..well their optical was broken and i had no idea where to get a rpm signal w/out tearing a bunch of **** open to get to an injector signal or crank position signal..
he told me """ no problem, we can still dyno it, but we just won't be able to get a torque reading, just HP"""...
how is it that they can dyno it, get a HP reading w/out a tach signal, but they can't get a torque reading w/out tach signal ?? i know that the formula to calculate HP from torque is based on an RPM constant... so question 1 is : why can i get a HP reading w/out knowing the torque, and why is rpm necessary for torque but not HP (isn't torque NEEDED for the formula to work) ??????

question 2 :: since HP is derived from a mathematical formula based on torque (assuming i'm correct in my above thinking) and the dyno does not know whether the vehicle being dyno'd is a gas or diesel (or does it) how come it can correctly give hp/torque relationship ??
let me explain my question.. let's say truck "D" is a turbo diesel that makes 500HP and truck "G" is a turbo gasser that makes 500HP also.

"G" pulls on dyno and makes his 500hp pull.. torque reading is ~500
"D" pulls on and makes his 500hp pull.. torque reading comes back at 1200

or even backward might makes more sense:
"G" makes a pull of 500 FTLBS... hp calculates out to 500
"D" makes a pull of 1200FTLBS... hp calculates ALSO to 500...

how is it possible that it KNOWS to have different HP figures when both trucks made DRASTICALLY DIFFERENT torque..
does the dyno have to be told that the vehicle is diesel vs. gas and does it use a different formulat to calculate the HP accordingly ??

does anyone know the answer to my questions, or am i just trippin ??

[TXHNTR]

You must be using Frankies Dyno...:hehe::hehe::hehe:

[Sparky]

Just trippin!

[ralphinnj]

It's a little tricky, but goes something like this. Dynos, in general, measure torque delivered to the roller. They do that by measuring the acceleration of the roller and knowing its resistance (load). So, for the actual roller (the drum your wheels go on), the machine knows what torque and HP it "felt."

HP is a measure of energy delivered per time. Torque is a force, and has no time component. The foot-pounds the rollers measure are related to the foot pounds the engine delivers to the wheels by the ratios of the gears/tire diameters (purely geometric relationship). So, the dyno does measure torque. Then it converts that to a power (HP), which is not a function of the geometry. Now to get to the actual torque the engine delivered to the wheels, you need the engine RPM to convert back. A little tricky, but that is what is happening.

So some people say HP is measured, others say torque. Technically, you can't measure HP. You actually measure torque most of the time. There is a dyno that measures HP directly. Conceptually, imagine a rope down in an underground hole under the dyno with a weight on it. If you measure the time it takes to lift it a certain distance (which would happen as you spin the rollers if the rope is tied to the rollers), you can calculate HP directly. I think some dynos work on this principle as well.

Bottom line is that you need engine RPM to get the vehicle's true torque.

Ralph

ps, in your example, the D truck would deliver that HP at a much lower RPM.....that is the difference.

[LvTitan]

ralph i follow all of that perfectly, but answer me this because we have both seen it..
and since theoretically and mathematically the tq/hp relationship is relative...

how come the tq peak is usually at a different RPM from the HP peak, are these graphs using a different formula ??
you see two totally different looking graphs for tq/hp on some cars, but how come they can be measured separately at different rpms for their peak ??

[LvTitan]

and one of the reasons i ask is because now with the NOS hooked up, i don't really think i will see hardly any gain in my PEAK hp figure, since i'm out of fuel and nos won't help there, but i'm going to hit the unit at low rpm to spool the turbo and burn all that fuel at the low end thus increasing the pull at lower rpms..

i'm assuming i won't really see much gain in PEAK HP, but i'm expecting to see a healthy gain in PEAK TQ because my power at lower rpm should be quite higher correct ??

[PSDPlayer]

You could have easily clipped onto one of the white wires going to any of the injectors.

[LvTitan]

doug that's exactly what crossed my mind when i was there, i called several people to try to find out exactly which of the wires to "loop" to get a tach signal from either an injector lead or CPS even..

where's the best place to "grab" the white wire signal ? coming out of the valve cover, or near the FICM ??