Hey guys,

I was thinking about how dynojets work, and just had a couple of questions.

I'm pretty sure that they work off of f=ma correct? mass of the drums times its acceleration gives you the torque. Then you can figure out hp with rpm.

But wont the gears in the car mess up readings of the dyno? since 4.10 gears are going to actually be giving a higher reading than say 3.42?

Cause I see that moser and companies rate axles by flywheel tq x tranny and rearend gear ratios, so I assume maybe im thinking right here?

 

Good question.

Inertia dynos measure the power it takes to accelerate the known dyno roll mass in a measured time. Probably they sample the roll rpm 10-100 times/second + or - and figure it over those short intervals.

Because hp is being "measured" (actually calculated from roll acceleration), torque can then be calculated if engine or wheel rpm is known.

There are a lot of things that can vary readings but overall drive ratio (axle times trans) MAINLY changes the engine acceleration rate. As you know, less power gets to the flywheel during fast engine acceleration (say over 600 rpm/second) than at slower (300 rpm/second) due to having to accelerate the rotating/reciprocating engine innerds. Because it's power being measured, only torque at the wheels changes with drive ratio, not the power. OK, driveline losses are generally a little larger in the lower gears, and accelerating the engine quicker reduces the fw or rw power, so folks generally run in a 1:1 trans ratio. My personal preference would be trying to get about 300 engine rpm/second, but you can't maintain that on an inertia only dyno.

Try a search. This has been discussed a lot.

 

See how does the dynojet figure out the amount of torque, from what I gather, cars are putting down much more torque due to gears.

say 600ft lbs of flywheel torque * 1.00 4th gear * 4.11 gears in back = 2466 lb ft of torque

with 3.42 gears thats only 2052

does the dynojet measure this number and then divide by gear ratios to give you a torque number? Thats where I'm confused, cause wouldnt the dyno results be dependant on a user input of gear ratios?


*shrug*

 

The dynojet will establish the overall gear ratio in terms of RPM/MPH. You can usually see it listed on the data sheets.

As oldSS said, it measures power. But simple substitutions can get you from F=MA to E=mc^2. There are a lot of ways they could calc it really, but yes, basically it is derived from f=ma with a lot of messy unit conversions.

Someone did a dyno in each gear and showed the additional friction and inertial losses in the gears lower and higher than 4th. Can't find it though, sorry.

-brent

 

see, on my dyno sheets the rpm/mph is 46.37 thats an m6 with 3.42 gears. Im just curious as to how the dynojet can calculate this. I mean if I go back to the dyno now that ive swapped gears is this number going to change on its own?

Offtopic here: How are you going from f=ma to e=mc^2? 2 totally different equations, one for objects with relatively slow velocities and one for particles near light speed. Newtonian physics and Einsteins are totally different. Just curious though.

 

Because of the inductive pickup on one of your plug wires. It knows how fast you're spinning it and it knows your RPM at any given speed. All it does is calculate torque from HP via TQ = HP*5252/RPM.

Without that hooked up, it cannot calculate torque but it can still measure HP (and will graph it vs MPH instead of RPM).

 

ah i forgot about that pickup

 

Yes, it will change and calculate the new overall gear ratio without any user input. The dyno operater never asks what rear or just as importantly what tire size you have. 4th gear just provide the least frictional losses and a reasonable acceleration rate (read moderate inertial losses of the drivetrain).

Well, I don't want to get to into it to much, but the SI units are actually consistent. One guy demonstrates it like this: http://www.tricountyi.net/~randerse/emc2.htm . You still have to accept the maximum attainable velocity as the speed of light found by Roemer. You then get that annhiliation energy is proportional to mass. There are whole books written about this so i don't want to get into a big discussion about it.

My real point was that starting with F=MA
Power = work/time
work=force*distance=MA*distance
and you come up with
Power = MA*distance/time

Or you could say torque = inertia* angular acceleration
Measure the angular acceleration of the drum, and calculate its inertia based on mass distribution and radius. With torque, an overall gear ratio, the engine speed, and a bunch of unit conversions you can get power that way.

I think that oldSS is right though, they measure power, plot versus rpm, then calculate torque. I don't work for dynojet though, and don't know exactly what goes on in the dynojet calculation software though.
Edit* That is all consistent with what JonA said also. You also have the option of plotting Power versus MPH and a bunch of other stuff.

You're right though, it all comes back to F=MA.



-brent

 

My real point was that starting with F=MA
Power = work/time
work=force*distance=MA*distance
and you come up with
Power = MA*distance/time

Or you could say torque = inertia* angular acceleration
Measure the angular acceleration of the drum, and calculate its inertia based on mass distribution and radius. With torque, an overall gear ratio, the engine speed, and a bunch of unit conversions you can get power that way.
======================================

you would need to know the Chassis Dyno's rotating roller
weight in Lbs and the roller diameter (or Radius) ??

find the Polar Moment of Inertia of the rotating Roller

PMI = Roller_Radius * Roller_Radius * Lbs * Shape_Factor * .002590083

where Shape_Factor could be .5 for Flywheel or all solid shafts
where PMI is in Inch-Lbs. per Second squared
.002590083 = Constant
Radius is in Inches

then to find HorsePower

HP = PMI * Instantaneous_RPM * RPM/SEC_Acceleration * .000001662

where PMI is in Inch-Lbs. per Second squared
where RPM/SEC_Acceleration of the Roller
where Instantaneous_RPM is of the Roller
.000001662 = Constant



you could also solve for Torque

Torque = PMI * RPM/SEC_Acceleration * .008726646

where PMI is in Inch-Lbs. per Second squared
where RPM/SEC_Acceleration of the Roller
where Instantaneous_RPM is of the Roller
.008726646 = Constant


torque = inertia * angular acceleration


Engine Dynos record Torque times RPM to calculate HP

Chassis Dynos record HP and RPM and calculate Torque

but in above equations, it should be possible for Chassis Dyno to calculate Torque directly as well as HP

 

Wow! My head hurts just reading that!

 

""But wont the gears in the car mess up readings of the dyno? since 4.10 gears are going to actually be giving a higher reading than say 3.42?""

I was the one that showed the hp in all gears and hp goes up slightly with taller gears (...but of course, approaches a limit of what the true hp is) and goes down in hp slightly with lower gears due mainly to the inertial resistance of the engine itself which increases the lower you gear your car down as compared to the same wheel speed if you were in a higher gear. Anotherwords, for the same forward wheel speed gain in rpm, which is an increase in potential or kinetic energy, you have to pump in more and more engine rpm too in the lower gears so you are diverting more power into the internal rotating energy of the car than into it's external vehicle or wheel speed based energy which actually accelerates the car.

 

PMI = Flywheel_Radius * Flywheel_Radius * Lbs * Shape_Factor * .002590083

where Shape_Factor could be .5 for Flywheel
where PMI is in Inch-Lbs. per Second squared
.002590083 = Constant
Radius is in Inches

then to find HorsePower "Lost" to rotational acceleration

HP = PMI * Instantaneous_RPM * RPM/SEC_Acceleration * .000001662

where PMI is in Inch-Lbs. per Second squared
where RPM/SEC_Acceleration of the Flywheel
where Instantaneous_RPM is of the Flywheel
.000001662 = Constant

Example=> 10.0" dia Flywheel , 8000 Engine RPM, and 600 Rpm/Sec test rate on engine Dyno

Flywheel Radius = 5.0 inches and 30.0 Lbs. Weight

PMI = 5 * 5 * 30 * .5 * .002590083
PMI= .9713

HP_Lost = PMI * 8000 * 600 * .000001662
HP_Lost = 7.748 hp @ 600 Rpm/Sec accel rate on engine dyno

RaceCar down DragStrip in 1st gear ( 2.56 Ratio)
with Differential Ratio ( 4.86) = Total Ratio = 12.44:1

in 1st gear peak Engine Rpm/Sec Rate might be close to 5000 Rpm/Sec like in ProStock

so;
HP_Lost = .9713 * 8000 * 5000 * .000001662
HP_Lost = 64.57 HP

64.57 minus 7.748 = 56.82 more HP loss because of 1st gear engine acceleration -vs- dyno acceleration

as engine goes into high gear down the DragStrip
engine Rpm/Sec accel rate might go closer back to 600 rpm/sec like dyno and loose only 7.748 HP for most of the high gear

this is just a Flywheel example
on a DragStrip run, you would have to account for all rotating objects that are experiencing acceleration/deceleration