Thread: Explain gearing, posi traction etc...

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Jeri, the numbers used to identify the differential gear ratio are an indication of the number of times the crankshaft of the motor turns as opposed to how many times the drive axles in the differential turn. Using Ken's examples, with a 4.11:1 ring and pinion, the crankshaft of the motor is turning four point one one times for each revolution of the drive axles, so that at 3,000 crankshaft rpm's for instance, the drive axles are turning 729.92 times (3,000 divided by 4.11). With this gear, at this crank rpm's and with a tire that measures 88 inches circumference, the vehicle would be traveling at a speed of 60.82 mph. (3,000 divided by 4.11 times 88 divided by 1056). (1056 is the constant in the formula). I used an 88 inch tire because it is 28 inches tall and that is a fairly common size. (28 times pi (3.14159)= 87.96 inches rounded off to 88).

Now, let's look at the work applied to the drive axles with this combination. If a 4-stroke motor fires each cylinder every other crankshaft revolution and you have 8 cylinders, then for each revolution of the drive axles, you'd have the engine crankshaft turning 4.11 times and delivering 16.44 power pulses to the drive axles. (8 times 4.11 divided by 2). We have to divide by 2 because, remember, the crankshaft only makes 1/2 power pulse for each 1 revolution of the crank on any given cylinder, so with one crankshaft revolution of an 8 cylinder motor, you have only 4 cylinders firing. The other 4 will fire on the second revolution of the crank.

Now, with the aforementioned 3.00 gear, you can see that we're going to slow the motor down for the same 60 mph speed of the car. 8 times 3 divided by 2 = 12 power pulses delivered to the drive axles for each revolution of the crankshaft.

So, it's easy to see that if you're using less power pulses to go the same speed, you'll use less fuel. The flip side of it is that you're not delivering as many power pulses to the drive axles, so you won't have as much power to move the car.

This is why you'll see drag strip cars using a very high numerical gear. They have to deliver the maximum number of power pulses to the drive axles in a given distance (1,320 feet). There's more to it, of course. The other thing to consider is that the motor makes more horsepower the higher it is revved (up to a point), so they use a gear that will allow the motor to reach the maximum engine revs right at the finish line.

In your case, with a front-motor, rear-drive car, the optimum choice for a gear will be somewhere around 3.70:1. That will give you reasonable mileage and reasonable acceleration. Everything we do to our cars if they must serve double duty (street and high-performance) is a trade-off.

Everything I've talked about here assumes a 1:1 ratio in the transmission. You want a manual transmission, so you could have a 5 or 6 speed transmission with the 5th and or 6th gear being overdrive, so that you would slow the motor down for mileage in the top gears and still have a low gear ratio in 1st, 2nd, 3rd and 4th to provide acceleration.

As far as the posi, most cars weren't produced with posi because of the additional cost of producing the parts. Grandma doesn't need posi when she's going to the grocery. There are several aftermarket units available which will bolt in and provide you with 2-wheel traction, Auburn www.auburngear.com and Detroit Locker just to name two. Here's a link to a supplier and some very good explantions of available units.
http://jawsgear.net/store/catalog/lockers.php?cPath=31
You can also equalize traction pretty well by replacing the conventional right rear shock only with an air shock, pumping the pressure up until you get the same amount of weight on both rear tires under acceleration.

You see, Jeri, on a front-motor, rear-drive car, the right rear gets light under accereration due to the way the chassis twists. That's why you'll see the right rear tire worn out on a kid's car with an open (non-posi) differential. When you apply more pressure to the tire on the right rear with an air shock, you equalize the pressure on both rear tires and both of them hook equally.

I'll add more to this as time allows, have to run right now