Thread: Explain gearing, posi traction etc...

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"Here's a link to a pontiac gear ratio explanation that I found...is this accurate?"

Yes, an excellent article.

What we may not have addressed is the actual role played by each piece and compound gearing.

Basically, the power is taken off the end of the crankshaft and transmitted to a flywheel which is bolted to the end of the crank. Bolted to the flywheel is a pressure plate. There are several types, including a Long style, a Borg & Beck style and a diaphragm style. The Long and B&B use a number of springs around the perimeter of the unit (they look pretty much like a valve spring) to exert pressure to clamp the clutch plate between the pressure plate and the flywheel. The diaphragm style uses a single large diameter diaphragm spring which exerts the pressure to clamp the clutch plate against the face of the flywheel.
http://auto.howstuffworks.com/clutch3.htm
The input shaft of the transmission has splines on it which engage with matching splines in the center of the clutch plate, so that when you let the clutch pedal out, the clutch plate is clamped between the flywheel and the pressure plate and rotational force from the crank is transmitted to the transmission input shaft. Your gear selector is used to select different gears within the transmission case, depending on whether you are starting from a standstill or accelerating through the gears. At the rear of the transmission is the output shaft. A splined yoke is engaged onto the splined end of the output shaft. Your driveshaft is connected to the splined yoke. On the other end (rear) of the driveshaft is another yoke which connects to the pinion gear. The pinion is splined and the rear yoke is splined to match. The pinion gear runs on the outer edge of the ring gear, which is more or less attached to the axles which are attached to the wheel and tire of the car.

Each gear in the transmission has a different number of teeth so that each gear is a different ratio. In order to determine your final drive ratio, you must multiply the transmission gear times the differential gear. In other words, in a traditional 4-speed transmission, fourth gear will be 1:1 (If you are taching 3,000 rpm's with the motor, the trans output shaft will also be turning 3,000 and so will the pinion in the differential. If we had the aforementioned 4.11 ring and pinion gearset in the differential, then the pinion would be turning 3,000, but the ring gear which is driven by the pinion would only be turning 729.92 rpm's. This would result in the 60.82 mph speed of the car like I said in the other post. But, let's say that you shifted down to 3rd gear in the transmission and that the ratio of 3rd gear is 1.23:1. You'd multiply your differential gear ratio (4.11) times the 3rd gear ratio in the trans and get a final drive ratio of 5.05:1 (4.11 times 1.23).
Now, with the trans in 3rd gear and taching the motor at 3,000 rpm's, the pinion would be turning 594.05 rpm's (3,000 divided by 5.05), resulting a car speed of 49.50 mph. Now, lets say you down-shifted again to 2nd gear, which has a ratio of 1.61:1. Multiply 4.11 times 1.61 and you have a new final drive in 2nd gear of 6.62:1. Now, with the trans in 2nd gear and taching the motor at 3,000 rpm's, the pinion would be turning 453.17 rpm's (3,000 divided by 6.62), resulting in a car speed of 37.76 mph.

So, to make a long story short (oops, too late ), we might consider that the pinion gear is directly connected to the motor's crankshaft and that the ring gear is directly connected to the cars tire. We get the ring and pinion gear ratio by dividing the number of pinion teeth into the number of ring gear teeth. For instance, the 4.11:1 ratio we've been playing with here would be achieved with a pinion having 9 teeth and a ring gear having 37 teeth (37 divided by 9 = 4.11)

Here's a calculator compliments of Smokemup that you can play with and see the different combinations and how they affect power and speed...
http://www.smokemup.com/auto_math/mph_range2.php
Here's a link to the Pontiac Trans Am page that lists different transmissions and their ratios for you to play with...
http://www.thepontiactransampage.com/transgears.html

I know this is a lot to throw at you, but you asked for it

jeri the posi. trac. controls the rear wheels when they start to spin. They cant be locked together all the time or you couldn't get around corners, so they have a certain % of slippage built into them. if you spin one wheel over that % then it will engage the other wheel and then you have two wheels pulling.
as far as the gear goes, about all you can do is, decide which trans. you're gonna run. what size tire you gonna run and how many rpm's you want to turn in hi gear going down the road at what ever speed you drive and install the gear that will do that. im sure there is nothing new in there, but what the hell.

ps: nice shop,

Good grief guys! LOL!!! You're just blowing me away with the info!!!! I'm thinking that i've got alot of work for the year 2006 without ever even touching the body of the 78. As Pat so nicely pointed out...I have no tools or shelving in my garage. So, I think my 06 budget is going to go toward some maintenance on the 81 Turbo, more tools (I have ONE Phillips head and ONE flathead screwdriver...a set of Allen wrenches and one small craftsman set of socket wrenches LOL...I also need some shelving, jacks & jackstands, possibly and engine stand and i'd love to have a set of those car dolly's to put under the tires to move the cars around the shop.

I'm thinking that could blow my budget for the year...but I also think I have enough work for the rest of this year and NEXT year stripping the 81 & 78, putting together a specific plan for the 78 and getting my shop organized...

I'm going to print this thread and start a notebook...I've got to go back and find that thread about putting together a build plan too.

Thanks again,
Jeri

Here's a pic of the 455 engine block stuffed in the trunk of the 78...lol...this should be fun...if you know how small those trunks are