Okay, say you could build two identical turbo Miatas except for the rear end. One has a 3.9 r&p and the other has a 3.6. If you were on the dyno or the drag strip, or a fifth gear pull side by side, wouldn't the 3.9 car have an advantage over the 3.6 car? On the dyno do they adjust somehow so they would have the same out come? I'm having trouble understanding this.

 

Gearing doesn't matter so much on the dyno, as long as you're in the 1:1 gear of the transmission to eliminate as much drivetrain loss as possible.

It'll affect the results a little, but I wouldn't be worried about the two.

 

one will have quicker acceleration but die fast
the other will have slower acceleration and die later
so in all, it depends what you will be using the car for...
one is awesome for cruising, the other is a gear shifter's wet dream.

 

5th gear pull side by side, the 3.9 will out accelerate the 3.6 until it starts bouncing off of the rev limiter, at which point acceleration will stop. The 3.6 will continue to accelerate up to the speed of the 3.9 and then beyond the speed of the 3.9, eventually passing the 3.9 car.

This assumes that both cars are capable of reaching the rev limiter in 5th gear.

On the dyno, the machine does not generaly output wheel torque, so there will be no significant differences. (Wheel horsepower remains identical, regardless of whether it's a 1:1, 3.6:1, 3.9:1, or 999,999,999:1 rear end). The machine measures wheel horsepower and uses wheel horsepower and engine speed to determine engine torque.

At the drag strip the 3.9:1 car on paper is the clear winner. Things get complicated when you add a turbocharger to each car though. If we assume that each car has perfect grip (a poor assumption at best) you run into small issues at each shift, especially in the first few gears where the tiniest changes in speed are multiplied greatly at the end of the track. Here, the 3.9 car is shifting more often, and as such, is lifting off of the throttle more often, and having to rebuild lost boost pressure more often.

 

Unless you have an absolute fire breathing monster, a 3.6 gear in a Miata is not a good gear for the drag strip. I would say a 3.9 is too tall for most of the cars here to produce the best results on the drag strip as well. There are many factors to consider when you select the right gear.

The simplified version; you want to be just past peak horsepower when you go through the traps in 1:1 gear. What that gear is depends on rpm range, tires size, and power to weight ratio predominately.

 

I feel like his question was more "how does the dyno compensate for different gear ratios?" rather than "which car would be faster?"

If that is the case....

The dyno operator does not have to enter a gear ratio. The dyno itself compensates for different gear ratios based on the change in speed that the roller is turning vs the climb in RPMS. While the 3.90 car climbs rpms faster, the dyno also sees a smaller total change in speed. When the dyno calculates power the end result between the 2 cars will be the same.

Example:
Tall gear car goes 20mph to 120mph in 10 seconds
Short gear car goes 16mph to 96mph in 8 seconds

While they both went 2000rpms to 7000rpms, the taller geared car took longer to get there, but it also gained more speed so the dyno can calculate that the cars made the same power.

 

However, many will dyno a car in a really high gear if they want to show the best possible spool (you can get full boost pretty low in the rpm band if you floor it in 4th) and conversely use a nice low gear for max HP runs because the lower load lets you get just a tad more aggressive with timing.

Sad but true.

 

Power is conserved with gearing. A 3.9 and 3.6 put the same power to the ground, but the 3.6 has lower thrust force. Dyno is either going to figure out the gearing based on drum speed vs rpm or with the gear ratio and tire size punched in an no rpm signal.

 

I assumed (especially in Miatas) that a higher gear was used to prevent wheelspin, no?

 

On the dyno? no. You use your 1:1 gear to minimize gear loss.

 

It totally depends on the dyno. DynoJets work that way, but a hub dyno like the DynaPack directly measures the torque output of the rear axles using hydraulic pumps and load cells. For those, the dyno operator needs to input the rear end ratio (and gearbox ratio if not in a 1:1 gear) in order to get the engine torque/power numbers out.

--Ian

 

Thats backwards.

If you dont believe me. Get a car with a real CVT one that acts like a snowmobile CVT and run int on all the dyno's the graph that make sense (hp or torque) is the graph that the dyno directly measures.

 

Yeah , that was kinda where I was going with this. All the explanations helped. Thanks.

 

Well... that's mostly my reason.

In other/most cases, i'd imagine it's because they're faster on the track and less of a pain in the dick to drive once they're making power with the lower numerically gear. Riding 2/3 is better than ******* around with shifting a million times a lap.