Question on Dyno Results
Race Director
Joined: Sep 1999
Posts: 11,361
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From: Plano TX
http://www.dragsource.com/index.php?...no=infoinertia
Do you have an inertia dyno (like a dynojet)? If so, make a pull without hooking up the ignition probe. You will get a graph of horsepower vs road speed. No torque numbers will be available.
What you're saying above is absolutely correct for an absorption type dyno. An inertia dyno is a completely different principle (based on kinetic energy rather than applied force).
Do you have an inertia dyno (like a dynojet)? If so, make a pull without hooking up the ignition probe. You will get a graph of horsepower vs road speed. No torque numbers will be available.
What you're saying above is absolutely correct for an absorption type dyno. An inertia dyno is a completely different principle (based on kinetic energy rather than applied force).
Le Mans Master
Joined: Apr 2007
Posts: 7,353
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From: Graceland in a Not Correctly Restored Stingray
Clarification
Thrust to power conversion, HP = lbf x fps / 550.
Torque to power conversion, HP = lbf x rpm / 5252.
The fact that thrust is a linear force and that torque is a rotational force aside, it's worth noting that both formulae include a variable for foot-pounds.
Torque to power conversion, HP = lbf x rpm / 5252.
The fact that thrust is a linear force and that torque is a rotational force aside, it's worth noting that both formulae include a variable for foot-pounds.
Last edited by TheSkunkWorks; May 12, 2011 at 04:31 PM.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
Found this explanation in Wikipedia. Even websites that advertise Dynojet chassis dynos still admit they are measuring acceleration of the inertia drum and using computers and software to calculate horsepower. I retract my earlier confession and am sticking with Manuel. Sorry Zwede.
Then if you hook up the ignition probe the computer also knows the rpm, so it can calculate torque from horsepower.
Thread Starter
Bud2
Joined: Jul 2005
Posts: 1,237
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From: Warrnambool Victoria
My head hurts!
Are you smart guys trying to do my head in? Dumb it down a bit would ya? Fact is I don't care whether the egg came first or the chicken, I'm just wondering what happens when you change gear ratios. Motorhead said it doesn't change - or at least not much. This is what I don't understand. The force applied to the rollers in 3rd would be different than in 4th. So how does the computer differentiate between the two to arrive at horsepower of the engine. My brain is thinking that the dyno would show more HP in 3rd gear because it would spin up quicker than in 4th. Is that true or not???
Bud.
Bud.
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
NP here but think about what HP is. HP can not be measured without first knowing torque. I have been through this kind of discussion many times, but this is something I deal with at work everyday and we have 22 dyno's, yes that is really 22, so I really am not talking from hear say or advertisements here. Have a great day!!
You are exactly right. Anyone that disagrees need to look back to the history of how they first started calculating HP and where the 5252 number came from. It was and still is an arbitrary number. When they say an inertial dyno calculates HP they fail to realize the computers program does use calculations based upon the givens in different mathematical equations. Since HP is only a calculation based on torque and RPM, the factors present in a program to derive HP without torque must resolve those factors somewhere along the way using the torque to HP conversion, otherwise HP would never be a constant in different type of dynos. The program writers must have the equations to write the program to get the desired outcome. It is way more complicated than it seems on the surface. The dyno numbers must resolve into HP from either torque or a suitable substituted number. And yes Thrust and torque or basically the same. One uses rotational axis pressures and the other uses directional force pressures. Either one can be measured. The HP gets calculated.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
Are you smart guys trying to do my head in? Dumb it down a bit would ya? Fact is I don't care whether the egg came first or the chicken, I'm just wondering what happens when you change gear ratios. Motorhead said it doesn't change - or at least not much. This is what I don't understand. The force applied to the rollers in 3rd would be different than in 4th. So how does the computer differentiate between the two to arrive at horsepower of the engine. My brain is thinking that the dyno would show more HP in 3rd gear because it would spin up quicker than in 4th. Is that true or not???
Bud.
Bud.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
You are exactly right. Anyone that disagrees need to look back to the history of how they first started calculating HP and where the 5252 number came from. It was and still is an arbitrary number. When they say an inertial dyno calculates HP they fail to realize the computers program does use calculations based upon the givens in different mathematical equations.
And this is why I'm not a teacher. People don't listen, they just decide that whatever they happen to believe has to be the truth.
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Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I'm with ya, sometimes you just gotta let it go. I know how it works, it is written in black and white probably 1000 times on the internet and I can't find one that says a DynoJet chassis dyno measures torque and calculates HP from that.
Back it up is all I can say or.......I;m guessing most of this just gets in there an bounces around
Back it up is all I can say or.......I;m guessing most of this just gets in there an bounces around
Le Mans Master
Joined: Apr 2007
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From: Graceland in a Not Correctly Restored Stingray
Actually, the 5252 constant comes from 33,000 lbf/min / 2 x pi in Watt's refined calculation, tho it's highly arguable the particular horse used to establish the baseline in the first place was a bit of an arbitrary selection.
Is that HP in Shetlands or Clydesdales??? ...and was he down in the mouth or feeling his oats that day???
Is that HP in Shetlands or Clydesdales??? ...and was he down in the mouth or feeling his oats that day???
Last edited by TheSkunkWorks; May 12, 2011 at 10:49 PM.
Burning Brakes
Joined: Dec 2003
Posts: 1,004
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From: Adelaide South Australia
'cos they provide more torque!
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
http://www.bristoldyno.com/info/whatis.htm
Burning Brakes
Joined: Dec 2003
Posts: 1,004
Likes: 7
From: Adelaide South Australia
Just to throw a curve ball here.....
The LeMans 24 hour race in France is the epitome of endurance racing in the world.
The last few years have seen the outright wins go to the turbo diesel prototype cars. One of the reasons is that they provide much more torque at lower rpm to allow accelleration out of the tighter corners, giving them better opportunities to overtake... (bear in mind that rear diff gearing is optimised for high speeds down the Mulsanne straight)
Of course the biggest advantage to the diesels, is that as they do not rev as high, their reliability is enhanced, as is the fuel economy, resulting in less pit stops.
All this talk about torque vs HP can get untidy.
I've come to realise that the best scenario is lots of torque at higher rpm with matched gearing to provide the best performance.
Good strong torque at higher rpm equates to high horsepower!
and a bloody big grin factor!
Last edited by OzzyTom; May 13, 2011 at 02:06 AM.
Drifting
Joined: Oct 2008
Posts: 1,936
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From: Rochester Hills Michigan
The units of horsepower can be converted to thrust times speed.
When you change the gears, you get more thrust, but less speed.
Thrust times speed will be the same (very close anyway) for both.
(The dyno determines thrust by how fast the drum accelerates.)
Thread Starter
Bud2
Joined: Jul 2005
Posts: 1,237
Likes: 4
From: Warrnambool Victoria
Thanks for all the help....
I THINK Swede gave me a handle to grip onto. I haven't been thinking about the 'speed' of the rollers. But I'm gonna have to go away and think about this for a while.
Bud.
Bud.
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Once you get that one the next one is if a 400HP motor makes 350RWHP on a chassis dyno and then you drop an 800HP motor into the same car would that now make 750RWHP on a chassis dyno ? ?? 
Race Director
Joined: Sep 1999
Posts: 11,361
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From: Plano TX
