Hick727

This is where im gonna post all my basic questions so i dont clog the forum.

1. Whats a 8 3/4 rear? Are there different ones?

2. Whats a Overdrive unit?

3. Whats a 2:76 or 4.10 gear and whats the difference? How do you choose which is best for your car?

4. Whats a Posi Track?



Thats all for now. More will come, i can guarantee. lol

blue 68 gts

1) An 8 3/4 is the measurment of the ring gear on the 3rd member. It's a banjo type rear end meaning it looks like a banjo when its empty.

There are truck, A-body, B-body, C-body, and E-body 8 3/4s



2) An over drive unit is where the engine speed is turning faster then the output shaft on the transmission.

The ratio would be something like .99 : 1, 1:1 is direct drive, 1.01 : 1 is an under drive.



3) 4.10s are better for getting the car out faster, such for drag racing. 2.76s are used more for gas mileage, or smaller engine cars.

The best gear for all around street AND racing would be around 3.55s.



4) Posi traction is a GM term for the 3rd member to put power to the wheel with the least amount of traction. When one wheel spins forward, the other will as well with a posi or sure grip.

"Sure grip" the term for mopar, but there is plenty of different terms for different manufactures.

1966sportfury

good answers, just keep asking that's how we all learn!

posi is gm, sure grip is mopar. limited slip is fomoco (ford), they all do the same job.

the reason i brought this up is allot of ppl think limited slip is a open rear end "one tire fryer" when its not.

scrapmetal

Good answers

Silverick

All good answers with one exception:

An overdrive is when the engine is turning "slower" than the output shaft which coincides with your .99 : 1 example.

78D200

Nick is correct. There are 3 different cases, the case is where the ring and pinion are set-up. 1-3/8" small stem pinion (aka. '741'), 1-3/4" large stem pinion (aka. '742'), and 1-7/8" tapered stem pinion (aka. '489')


Nick is correct on this. I feel though in order to select the proper gear ratio, there are some things that you need to know. 1)In what RPM range does your motor make the most power? 2)Do you have an automatic or manual transmission? If automatic, what is the stall rating of your torque convertor (is it stock or aftermarket)? If manual, which transmission do you have? 3)What is the vehicle that this is all in and what is the primary use of it? 4)What size tires are you planning on running with?

Answers to these questions, and others I'm sure, will help to better determine what is the proper gear ratio for you.

blue 68 gts

You're right. Type different then what I was thinking. Sorry.

OD = more speed, less torque.

Silverick

[quote=Hick727;61356]

3. Whats a 2:76 or 4.10 gear and whats the difference? How do you choose which is best for your car?


A 2.76 to 1 (2.76:1) gear ratio means that it takes 2.76 turns of the driveshaft to turn the axle or wheel 1 complete revolution.

A 4.10 to 1 (4.10:1) gear ratio means that it takes 4.10 turns of the driveshaft to turn the axle or wheel 1 complete revolution.

It can get confusing if you're just learning, because a 2.76 is considered a "higher" ratio and the 4.10 is considered a "lower" gear ratio.

Silverick

No apologies necessary!

I knew that you had it right but, just wrote the wrong word.

Hick727

Whats a pinion? And what do those sizes mean?

And what does size of tires have to do with gears?

And I still dont get what overdrive is

Is there like a book called Muscle Cars For Dummies? Cause i need one

tang72

Good explination but for the record ;Limited Slip differentials (LSDs) is not just a Ford term its a description of the type of action (locking or limited slipping) done. Fords go with Trac lok (Track Lock.) Like it or not i have a blue oval pony... or two. The charger is still my favorite though dont worry.

440roadrunner

"Overdrive" probably comes from "old school" cars and trucks. Up until say, 1972-3? or so, when imports became more popular, VIRTUALLY ALL American cars had a 1:1 transmission ratioo in "high" or cruising gear. That is, the input shaft was coupled directly to the output shaft, so the engine turned the transmission input shaft, the input shaft directly turned the output, and the driveshaft would be rotating at whatever engine RPM was. The gearing in high gear, then, was reflected by the gear ratio in the rear axle, and also affected by tire height.

Now, many present day transmissions don't really have a "straight through" coupling in any gear, but rather are connected with internal gearing. But when original "overdrives" came out, I don't know, "the 50's?" they were an auxiliary gear BEHIND the main part of the transmission, but built into the tail unit of the transmission.

So the original overdrive units, shifted typically by an electric solenoid, either selected the transmission "high gear" and passed the drive "straight through" in 1:1 ratio, or when engaged, added a "higher" (higher speed) gear to the transmission, which, with the same engine RPM, would drive the car faster.

Of course this reduces engine torque, making passing and hills such that you would normall "kick out" the overdrive and drop back to 1:1 "high" gear.

This definition became further blurred by such transmissions as the Chrysler 4 speed "overdrive" transmission, which in reality did the following:

Chrysler took a "regular" A-833 transmission, used in tons of Mopars over the years, and "re geared" third gear to be an "overdrive" gear. THEN they simply swapped the 3-4 shift lever upside down, so that:

When you shifted, you shifted the "4 speed" shifter normally in reverse, and normally, in first, or second BUT

because the 3-4 shift lever was upside down, when you shifted from 2-3 YOU WERE REALLY putting the transmission into the OLD "4th" or high, or 1:1, or "straight through" gear.

Last, when you put the lever in "4th" you were REALLY putting the transmission into the old "3rd" gear position, which was now re-geared into a "faster" overdrive gear.

Gear ratios are easy. They are simply mathematical ratios. If you take a gear or pulley that is 4 inches in diameter, and drive another gear or pulley that is 8 inches in diameter, one is TWICE as large as the other. If the pulley on the motor is 4 inches, and the pulley on the "driven" (load) is 8 inches, that is a 4/8, or 1:2 ratio. Since the motor is turning the load slower, it's often expressed as 2:1, because the motor must turn TWICE for every ONE of the load.

Same thing in rear gears. If you have a conventional "old school" 3 or 4 speed that is 1:1 (straight through) in "high" gear, you say the "final drive" is whatever the rear gear is, let's say 3.70 This simply means that the engine and driveshaft must turn 3.7 times for every 1 time that the rear axles turn.

Hope this helps, hope you didn't get lost

Hick727

..........so it makes you go faster...?

Haha no I kinda get what your saying. Got lost a couple times though. Ill learn someday

tang72

overdrive= basically i gear that will keep the rpms low. Out of power band and bad for acceleration but saves on gas/ makes it much easier to cruise at a higher speed without blowing something up- Generally speaking. Its complicated... From a performance standpoint its irrelevant and i get by just fine without overdrive in either of my classics but i dont want to cruise above 60 or so because i hate to let it wind so high for so long.

Hick727

So if you like going fast, get a tranny with overdrive?? Do they come in automatics?

tang72

Well if your racing you would not go into an overdrive gear because you want to keep the rpms up. its really only useful when cruising but old cars where not really built for cruising down the road at 90 for hours on end they when built for 55 mph highways. They might be a little looser than your used to at sustained high speeds. Overdrive wont really effect acceleration anyways. It wont feel any faster with overdrive but it will let you stay at a higher speed at a lower rpm. -and yea its just a numerically low gear ratio so it would be available in auto or manual. A lot of car guys dont really think overdrive is a big deal either way, especially those with a budget.

1966sportfury

the higher numerical (4.56) gear the quicker you can get going with limited top speed, the lower the number (2.73) the slower you get off the line, but "can" reach higher top speeds.
over drive is made to improve fuel mileage. also lead to longer engine life. yes you can reach higher top speeds with overdrive if the engine has enough power to keep to going. in racing 1/4 mile you will not have enough time or low RPM for the tranny to get into OD.

78D200

The pinion is the input gear for the rear axle (front axle as well in 4x4 and AWD vehicles). There is a yoke that engages it by the splines and is held on with a nut. The driveshaft is connected to the yoke/pinion.

The sizes that I mentioned are the diameters of the pinion gear stem. The larger the pinion stem, the stronger it is.

By changing the size (height) of the tires that are on your vehicle, you can change the speeds at which you are driving at. Lets say your car normally comes with 26" tall tires. You pull them off and install some tires that are 28" tall; because you have a larger diameter, the car will travel further during one revolution than it would have with the 26" tire. This will cause you to actually go faster by a certain percentage (all based on the difference between the original tire diameter and the new, larger tire diameter). If you install a 24" tall tire on your car, you will travel less distance do to the smaller diameter of the 24" tire compaired to the 26" tire. This will in turn cause you to travel slower by a certain percentage (once again all based on the difference between the original tire diameter and the new. smaller tire diameter).

The change in speed due to change in tire size is really just some simple physics.

78D200

There are a lot of great books out there (none in the "for dummies" line yet). Go to your local book store (like Barnes & Noble) and walk around the automotive section, of books, not magazines.

tang72

Its overwhelming at first but after a while things just start to click. Once your more accustomed to the basics of how stuff works it becomes a lot easier to decide whats good or bad for your build. Dont worry too much about it everyone is still learning.

jamesharley

Nice answers....I m agree with us.....Good Job Keep it up!!

440roadrunner

This is incorrect. The overdrive when engaged, turns the driveshaft FASTER than the engine, or to put it another way, allows the engine to turn SLOWER than the output.

blue 68 gts

If you read earlier up, you'll see we fixed that mistake.