When entering data into a chassis dyno computor does any one know if you need the rear end ratio information entered? I would think this would have a effect on the torque calulation. :confused:


[Modified by Colevette, 6:44 AM 6/30/2002]

 

Your gear ratio will not affect your torque reading. See this thread for further explanation: http://forums.corvetteforum.com/zerothread?id=254312

 

the final drive gear will affect the chassis dyno results. If you test the car with 3.36 gears, then chage to 4.11 gears and retest, torque will have gone up, horsepower will have gone down.

Think about it. If you do the above decribed gear swap, your 1/4 mile times would change (all other things equal). Your ET would go down (launch) and the top-end would tend to also be reduced (but there are other variables here).

I'd like to know more about this as well.

 

Gearing is not a significant factor in dyno testing with a typical inertia dynamometer. They measure horsepower directly by computing it from the instantaneous rate of angular acceleration of the durm and then relate power to engine RPM on the output graph. Torque is computed for the power and RPM data.

Shorter gearing will allow the engine to rev up quicker, so a bit more power will be absorbed spinning up the rotating mass of the engine and drivetrain in a shorter period of time, but the resulting recorded dyno numbers will not be substantially different than with taller gears because the taller the gearing the higher the speed, which dissipates more power through the tires and diff., so the two effects tend to offset one another.

The reason why shorter gears run quicker in the quarter mile is that they allow the engine to rev quicker so it spends more time in the upper power band, and it's the average power delivered through the gears that determines your speed, and ET.

Duke

 

Duke's got it. The part not mentioned is that there is a pickup for sensing RPM directly. So, the dyno shows you RPM per MPH on the output (at least the dynojet printouts I got showed it as a separate item). It then draws you a graph showing power and torque versus RPM. Could just as easily be versus MPH, but the important one is RPM.

That's how it compensates for gear ratio, tire height, etc. You can also do pulls in whatever transmission gear you want. Both dyno operators I've used recommended using whatever gear is 1:1 in the transmission to minimize drivetrain losses. Serious racers comparing/optimizing transmissions probably pull in all grears.

 

Yea, I always understood the actual power levels to be unaffected by gears; however, Dynojet states that the readings "will be affected by by gearing." "A smaller, numerically higher gear will show more torque and less hp due to power being absorbed........"

All other things being the same, you will propel a 3200 pound automobile down the strip in less time with 4.11's than with 3.08's. How can the actual rear wheel power and torque curves be identicle? Doesn't tht seem to be a contradiction? The same work is being done, but in less time.

What am I missing?




[Modified by Maximillian, 3:11 PM 6/30/2002]

 

Torque or power plotted as a function of engine revs is a function of the engine, alone.

Torque or power plotted as a function of speed is a function of both the engine and gearing. That's why cars have mulitiple gear reductions - to allow high power at lower speeds to achieve brisk acceleration.

A Dynojet equates the instantaneous power measured at the drum with the RPM pickup from the engine, so it plots torque and power at a function of engine speed, not wheel speed. Different final drive gearing will provide slightly different power curves because of inertia issues, but the difference isn't going to be great for the range of typical automotive gearing. However, if you tested in first gear, noticeably less power will be shown due to the power absorbed in spinning up the engine and drivetrain inertia load much quicker. This is a limitation of inertia dynos. A steady state dyno such as a water brake is unaffected by inertia because power readings must be taken at steady speeds.

Consider a railroad locamotive - electric, diesel electric, or steam. They only have one effective gear ratio. All types produce peak torque at zero RPM. From there the torque curve fall gradually, but power continues to increase until the torque falls at a faster percentage than revs rise as a percentage. That is the point of peak power and it can only be achieved at one speed.

Steam locomotives were effectively geared by the size of their drive wheels - say 63" for a freight locomotive that could run up to 60 MPH and 84" for a passenger locomotive that could run 80. Electric traction motors are geared to the axles - about 4:1 for a freight locomotive and 3:1 for passenger.

Just as a 250 RWHP Corvette with a 3.08 axle will have more top speed than the same one with a 4.11, the 4.11 equipped car will pull a heavier trailer up a mountain grade in top gear, because more power is available at all normal highway speeds due to the higher revs per MPH from the shorter gearing.

Imagine your Corvette was like a locomotive and didn't have a multispeed gearbox. They would accelerate much slower, but would have the same top speed. The problem with IC engines is that that their torque curves peak at some mid-range speed and falls as speed goes down and effectively produce zero torque at zero revs. That's why they need a clutch and gearbox, and that's why electric cars (and vintage steam cars) have neither.

Duke



[Modified by SWCDuke, 4:10 PM 6/30/2002]

 

Excellent explanation Duke! In addition, lower gearing will often make for higher trap speeds in the 1/4 mile, even though the car's potential top speed might have been reduced. By increasing initial acceleration, for the reasons that Duke gave, the car would have reached a higher speed early on which it would continue to accellerate from. This is assuming that the lower gearing hasn't caused a substantial reduction in traction that would negate the increase in power to the wheels. Kinda' like getting a "head start". If the car is going 5 mph faster at mid track because of the lower gearing, it will most likely finish the run 5 mph faster also.

 

I once ran my Harley on a dyno-jet chassis dyno. A year later I ran it again. At the end of the run the second time the mechanic looked at the sheet, and pulled the sheet from the last year and said "you changed sprocket size from 53 to 47 tooth from last year". I was amazed. I looked at the sheets and there it was! The computer can some how tell the gear ratio and give results accordingly.

 

Wow that is cool reading Thanks guys. Max I think you are putting "time or et " in a dyno pull. I don't think the dyno cares how long it takes a particular car to rev out? IE: a 4.11 car will rev out faster than a 3.08 but if it was the same motor it would still make the same HP/TQ numbers once there. Time is not an issue on the dyno. IMHO

 

Sorry it took me some time to say thanks to you guys for responses. I was thinking exactly like Max, but Duke you set me straight know and I understand it. Happy 4th of July to all of you.
Thanks Colvette :flag