JeepLab Prodigy Install LIVE

[UselessPickles]

the axle gear ratio is a non factor

As Jessee's dyno results show, overall gear ratio does matter on that dyno, the way the dyno run was performed. The results are very different between 3rd and 4th gear. There is no way you can look at those results and conclude that the gear ratio is a non factor.

A difference in axle ratio causes a difference in overall gear ratio essentially just the same way a different transmission gear causes a difference in overal gear ratio. If axle ratio was a non factor, then transmission ratio would also be a non factor. If the overall gear ratio matters, then the entire combination of transmission gear, axle ratio, and tire size all matter.

[JeepLab]

As Jessee's dyno results show, overall gear ratio does matter on that dyno, the way the dyno run was performed. The results are very different between 3rd and 4th gear. There is no way you can look at those results and conclude that the gear ratio is a non factor.

A difference in axle ratio causes a difference in overall gear ratio essentially just the same way a different transmission gear causes a difference in overal gear ratio. If axle ratio was a non factor, then transmission ratio would also be a non factor. If the overall gear ratio matters, then the entire combination of transmission gear, axle ratio, and tire size all matter.

I said Axle Gear is a non factor. Not transmission gear. The dyno connects to the cpu in the dash and uses the RPM of the engine to correct for the speed of the tumbler

Stock jeeps get dyno'd in 3rd gear because the stock tune hits the speed limiter in 4th.

Modded Jeeps get dyno'd in 4th, as it is closest to 1 to 1 ratio. I have never heard anywere of a jeep getting dyno'd in 5th gear.

Ive been at this a while, and have interviewed and worked with every power mod manufacturer. They all say the same thing.

Dynojet both drum speed and engine speed are measured and used to take out any effects of gear torque multiplication.

[UselessPickles]

So there's a catch 22 here.

A steady state dyno chart is the only way to get guaranteed consistent and comparable dyno results (regardless of gears, tires, etc).

But a steady state dyno chart is arguably not "realistic", because we use the torque/power of our engines primarily to ACCELERATE... not to maintain steady state speed. A tiny fraction of the peak power is actually required to maintain steady speed. A steady state dyno chart will show the highest results of all, but you will never experience that amount of torque/power while accelerating on the street.

When accelerating on the street, some of that torque/power gets used up to accelerate the drivetrain and wheels. The faster you accelerate (i.e., lower transmission gear), the more of the torque/power is lost to accelerating the drivetrain and wheels.

With a turbo, the effect is compounded by the turbo spooling more slowly when under less load in lower transmission gears. Or you can look at it from the other direction and be happy that with higher transmission gears, the turbo has the advantage of spooling up more under higher load to produce some more power, making up for some of the loss of torque multiplication from upshifting to a higher gear

(I've actually noticed that compared to my expectations based on how 1st and 2nd gear feel, 3rd pulls VERY strong all the way up to redline, without ever feeling like it's losing steam due to increasing air resistance.)


So what people really care about is a representation of how the vehicle will accelerate on the street. It makes sense that the best dyno test would be one that mimics acceleration on the street.

Load bearing dynos can be configured to perform sweep tests that simulate actual acceleration on the street, by dynamically adjusting the load of the dyno throughout the test to simulate both the load of the weight of the vehicle, and the load of air resistance (which increases with speed, proportional to the square of speed). This only works well if the dyno is accurately configured with the vehicle's weight and aerodynamic drag constant (drag area). I know Mustang dynos work this way. I'm not sure if the load bearing Dynojet dynos can do this. Even if you configure the dyno accurately, you still have problems. Which gear do you use? Do you simulate a fast 1st gear acceleration, which will give the lowest results (due to larger rotational inertia losses from the drivetrain, and slower spooling turbo with respect to engine speed), but most accurately represent the most acceleration you'll ever feel from the vehicle? Do you simulate acceleration the highest gear that can reach the rev limiter, which would give the highest results (least rotational inertia loss, turbo spooled more fully at lower rpms), but not accurately represent the power you will use in everyday driving? Or do you go with some middle gear for a balance of impressive numbers and realistic representation of what you'll experience most often on the road? Different jeeps with different tires and axle ratios would also produce different results on this type of dyno pull, which would accurately reflect their affect on real world driving performance.