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 ??

 

Danny:

I think I can answer your question. First the math:

HP=(Torque X RPM)/5252

This is a well known relationship and you alluded to it in your post. Now the second part of you question is how the dyno generates the data to get torque and HP numbers.

1) In your case I believe this is rooted in the type of dyno being used (and there are several types) but essentially I believe the type of dyno you were using was essentially a generator which means it actually looks at how much power is being produced and derives the HP value directly and needs engine rpm to calculate the torque value.

2) The explanation to the different values comes from the engine RPM, the gasser was probably running at 6000+ rpm to make the same number. while the diesel was probably pulling at 3400 rpm. Hence the difference in torque while the same HP numbers were generated.

The dyno doesn't "know" anything, it's just a generator that allows you to attach different engines to it. The key is in how fast the engine is turning. That is why you were told that HP was available and torque was not without an accurate engine RPM.

I hope this makes sense.

 

Yep, good explanation, except if the dyno is measuring at the wheels I'm pretty sure they use tire speed. My experience with HP and torque measurements is mostly with electric motors though.

HP = Torque x Speed

 

no he's right with the 5252 equation:
TQ X RPM/5252 = HP
HP X 5252/TQ = RPM
HP X 5252/RPM = TQ

anytime the peak power is under 5252rpm, the tq will always be higher than the hp
anytime the peak power is over 5252 rpm the tq will always be lower than the hp

and that's why a cummins making peak power at 1900rpm makes 2.7times as much torque as it does power..
and why my PSD at 3100 rpm makes 1.6 times as much torque as hp..

and why my older PCM peaking at 3100rpm will make more HP, but less TQ than the newer (05+) trucks that peak around 2000-2200

 

i've learned a lot about this in the last few hours..

but i'm still wondering why every answer i get says you MUST HAVE TORQUE TO CALCULATE HP,

yet i clearly got a HP reading today, but w/out rpm did not have tq reading..
someone is wrong

 

This should clear it up for you http://www.bristoldyno.com/info/whatis.htm

There is an explanation of how a inertia dyno meter works in figuring HP with out a torque reading.

 

I'm in 8th grade and currently learning Physics and also taking an Algerbra class. Its interesting to see how Physics and math play a big role in the automotive world. SANDDEMON08, great website, it gave me a much better understanding of how everything works.

 

I used to work for a Caterpillar dealership and they had two dynos for the service department. One was a chassis dyno (Class 6-8 trucks) and one was a engine dyno. Both used water for the "drag" and measured the pressure of the water. The chassis dyno had a busted drain that would force water to the parking lot when it was underload. Looked like Yellowstone Nat. Park when a truck was running on it....lots of steaming hot water on the ground. Only place I've seen what happens when a 600 Hp 3406E breaks a U-Joint

 

Gald to see we have younger generation users invloved here.

 

The most interesting thing is, I didn't even think about Physics and Algebra in 8th grade! Am I THAT old?? Dang things in schools have changed!!

 

Must be pretty basic physics

My son is a junior in high school and taking Physics 2 College/AP course with Calculus College/Ap and those are things ive talked about with him and its come up in his classes

Bruno

 

The reason is (HP= TQ x RPM / 5252) always holds assuming TQ is measured in ft-lbs. The dyno is always measuring torque (on a non-(mass acceleration type dyno)) and it measures its own roller speed. Since it has its own TQ and RPM it uses that to calculate power. Since power always equals power (there is no gearing or leverage effect) the power at the roller = power at the wheel = power at the crank (less driveline losses). If the computer knowns the crank RPM it can simply take roller TQ and run it through your formula above.

A TQ vs RPM curve can still be done if you know the HP generated at the roller speed but you will need to do the calculations.

 

Least you can make the Dyno LMAO! I'm so busy I dont have the time.. Plus I'm really scared to take my truck to the Dodge Dealer and let them yahOOO's strap it down..

Yes.. It's the only Dyno in my area.. And it JUST had to be at a Dodge House.. Can't find anyone within a 60 mile radius with a Dyno capable of doing a Diesel

 

No need to play games.............

8th graders don't take Physics AND they don't own Excurisons that have 135k miles on them that they bought new ..................

https://www.ford-trucks.com/forums/700487-how-disappointed-would-i-be-with-a-5-4l.html#post5641805


Previous post this year .......
I have a <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com[img] /><st1:NumConv6p0 sch=[/img]2000</st1:NumConv6p0> Excursion Limited <st1:NumConv6p0 sch="1" val="4">4</st1:NumConv6p0>x<st1:NumConv6p0 sch="1" val="2">2</st1:NumConv6p0> with the <st1:NumConv6p6 sch="4" val="5.4">5.4</st1:NumConv6p6> and it currently has <st1:NumConv6p6 sch="4" val="135,000"><st1:NumConv6p0 sch="1" val="135,000">135,000</st1:NumConv6p0></st1:NumConv6p6> miles, its been great since I bought it brand new. I don't think you'll regret it (hence evrybodies different) but if you expect for it to be quick and you want to hallass in it you'll probably be better off with a V<st1:NumConv6p0 sch="1" val="10">10</st1:NumConv6p0> (although I do on occasion race somebody on the street like another Superduty or once something like a <st1:NumConv6p0 sch="1" val="2001">2001</st1:NumConv6p0> Mustang with a V<st1:NumConv6p0 sch="1" val="6">6</st1:NumConv6p0> and I beat both) but for general use you'll be fine.<O</O

 

I am not specifcally taking a Physics class, its just a regular Pre/AP Science class. We just happened to be on the section of Physics. The Algerbra class is specifically an Algerbra Class with regular Pre/AP 8th grade math in the mix.
Great thread, it was very useful and purposeful. Good to know this stuff.