I have been thinking about this for a while now. They always dyno the car in 1 to 1 ratio ( 4th gear or high gear in auto) If you have say a 3.08 rear end and then change the rear to 4.10 without doing any other modification will you get higher torque and horsepower numbers ?

I am thinking that you will get higher numbers and it will not be just a small difference but a rather large one. The tranny and rear are just torque multipliers so if you are at 1 to 1 on the tranny and 3.08 at the rear then the engines torque is multiplied by 3 or so and if you change it to 4.10 then it is multipied by 4 or so.

Now I know the computer monitors the RPM of the motor so it can compensate for that difference but the torque the dyno actually "feels" is going to substantially higher with the 4.10 rear end so it will spit out higher RWT and RWHP numbers wouldn't it ?

The way to get the same torque multiplier out of the 3.08 as the 4.10 would be to do the dyno pull in third gear but they don't do that.

 

Wayne

I think the computer cross references your tire speed and rpm's to 'balance' it out. Diff size (diam) tires would also change the readings.

 

There are a lot of things that can vary a chassis dyno results.

How tight the car is tied down.
Air pressure in the tires.
Gear the car is operated in.
Rate of accelaration.
Tempture and humidity.
Weather your torque converter locks up or not if it's an automatic.

Our local shop watches your tire speed ratings and will not allow you to exceed them. I have seen cars dynoed there in a lower gear to get a full RPM run.

 

Chris I think you are right, actually I think I figured it out and it wil not make any difference what rear gear you have and I think the only reason for 1 to 1 ration is so you don't spin the tires on the dyno, however you still would with enough horsepower and 4.56 rear end.

The dyno can only use time to measure how fast the car spins the drum. So how fast the drum accelerates and what it's starting RPM and ending RPM in relation to what the engines starting RPM and ending RPM are determine the HP and torque numbers.

So if, for ease of explantion you had a 2.0 rear end and the drum was the same diameter as the rear wheels at 2000 RPM ( engine speed ) the drum would spin at 1000 RPM. So now if you take the motor to 6000 RPM the drum would spin to 3000 RPM. Now you have to look at how long that took, for ease of explantion we will say 1 minute.

Now you change to a 4.0 rear end and the starting and finishing RPMs of the motor are the same 2000 - 6000 RPM
but the drum now spins starting a 500 RPM and finishing at 1500 RPM. So I guess the computer has a formula that says that should only take 1/2 a minute so everything works out equal in the HP/torque departement between the two rear ends.

The dyno will do the same thing only break it down into much smaller time/RPM increments so you can get the proper graph.

Slow day at work

 

i think i have seen dyno graphs that give you an entire curve from, say 1800 up to redline of 6000+ and the graph includes the little hitches in it which are obviously the shift points.

 

The dyno operator figures out your overall gear ratio through comparing the tire and engine speeds. That's the only ratio that matters for computing your car's power.

Dyno tests are usually run in 1:1 (if such a gear exists) because the transmission is much more efficient in that gear. Everyone likes to see big numbers .

 

Yeah, they compensate for the gear ratio. As a matter of fact, going from 3.36 to 4.1 gears will show a loss of almost 20 hp.