Transmission Ratios and Posi

[Stallion]

I was just reading about ratios in the transmissions. I assume that these are the gear ratios in different speeds. That's what I got out of context. But, can you elaborate on this subject? For instance, why do you want a close ratio in your transmission?

Also, can somebody explain the term "posi" to me? What exactly is it? I couldn't find it in the automotive dictionary Ken supplied me with.

Thanks!

TR

 

[Verle]

Last one first: Posi is short for Positrac. A positraction differential assures power is applied to both rear wheels. A car with an open or non-posi differential will spin just one wheel if that tire is on water, ice or other low traction surface.

The transmission ratios referred to are the mechanical ratio of each gear, 1st, 2nd, 3rd, etc.
A Muncie close ratio 4 speed has ratios of:
1st 2.20
2nd 1.68
3rd 1.31
4th 1.00

A close ratio transmission is desirable for maximum acceleration. Any engine makes the most power in a certain RPM range. Let's say my engine makes maximum torque at 3500 and maximum horsepower at 5800. If I shift when the tachometer reaches 5800 or 6000, I will get best acceleration if the next gear keeps RPM at or above 3500.

Verle

 

[Stallion]

Okay, I see. So the "difference" of gear ratios should be the least possible because then it won't bog down the engine (on the extreme far ratios) and it can maintain a certain RPM.

Thanks!

TR

 

[Stallion]

I have another question about this. If it is an open differential, then the wheels are independent as you said. But, if there is posi then they aren't, right? But, I thought that was the whole purpose of a differential. To have independent wheels, right? How does that?

And, is postraction wanted in your car? Would you want dependence with little or no traction?

Thanks!

TR

 

[JohnZ]

You really need a diagram of how a differential works, which can be found at www.howthingswork.com, to understand how the power coming into the differential through the pinion gear (which is attached to the driveshaft) gets transferred from the ring gear through the differential case and then to the axle shafts through the differential side gears.

With a conventional "open" (non-Positraction) differential, whichever wheel is on the lowest-friction road surface (water, snow, ice, etc.) will spin first, and will get most of the torque. With a Positraction (limited-slip) differential, there are spring-loaded internal clutch packs on both sides of the differential; when one wheel tries to spin faster than the other, the clutches come into play and transfer some power to the wheel that's NOT spinning.

 

[Stallion]

Oh, I see what you mean, JohnZ. I'll go over to that site and look over differentials and how they work. Thanks!

TR