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Discussion Starter #1
Can my stock 70 standard rear diff be rebuilt as a traction lock? I want it to be posi when I'm all done but was wondering if it's better to rebuild my rear-end or look for a proper posi rear instead???
 

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Hi Dale... Absolutely! Brand-new posi's are 300-400 bucks from summitracing depending on the brand, or you can just wander into your transmission/rearend shop and go:

I want a posi.

They'll just swap out you spider's with a posi unit.

Getting a whole new axle from a real shop like moser or currey would set you back a couple thousand bucks for the whole deal, $1000 or so for just the center section.
 

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Dale, unless they have come out with something in the last few years that I dont know about you will have to change the whole carrier. I dont mean the whole pig but just the carrier that the ring gear bolts to and houses the side and pinion gears. Do you know if you have a 28 or 31 spline pig? If you do you can go junk yarding as the late 60s to early 80s Ford f150 4x4s are a good place to find traction loc pigs. And alot of them had 3.00 to 3.55 gears. The only thing is you will have to swap ou t your yoke from your pass car. If you can find the one you need they are not that expensive to freshen up. Alot cheaper than buying new.mm
 

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Discussion Starter #4
mm - not sure, but if I'm reading the posts correctly, my standard 1970 351C - C4 should probably have the 28 spline...correct? So would the pig from the F150's still work?

Also guys, I don't mean to be an idiot here but the rear axle ratios have always been somewhat confusing to me. If I want a ratio that will give me good gas mileage and top end speed then I need the lower ratio rear correct? And, I don't really want to be buying new rubber ever three blocks, which I probably would with the higher ratio....right?

But, here's the clincher...after changing this beast over to 4V with those nice open chamber heads I just got and sticking a fresh 4 gear behind her, I would still like to be able to make her sqeal a bit if I really really want too. Any suggestions for a good ratio and anyone that maybe can explain the theory behind those gears to me so I'm not such a neophite anymore.

Thanks in advance guys for all the help.
 

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Cruisin' cats generally stick to 3.00:1 or 3.25:1... Maximum of 3.50:1 without overdrive.

If you go to the strip, you might want a 3:73, but you're revvin pretty hard at the end of the quarter.

No matter what gear ratio you pick, you won't have ANY problem breaking em loose on demand.

I bet you'd be REALLY happy with no more than a 3.25:1 rear-end with posi.

Anyone who tells you anything over 3.50 is just burning holes in your pocket book, and your ear drums.
 

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Dale your probably right in that your Cougar is a 28 spline rear end, but Ford sometimes used what they had on hand and you never know what a previous owner did to it. Pull an axle first to be sure. Yes any of the F150 pigs will work as long as it has the right axle spline that you need. That said, the same above hold true that you never know for sure till the axle is pulled. If memory serves correct most F150 4x4s that have the traction loc you desire will be 31 splines, but some of the older 6cly F100s may have the 28 spline axle. Most 2 wd F100s and F150s with 6 cyls and 302 will have 28 splines but most will not have the traction loc. Just to give an example wow strange it can be I once changed a pig for my buddy who had a 82 F100 300-6 with an od stick trans. He wanted to change out the 2.49 pig (yes thats right 2.49) to a 3.00. We had a 3.00 28 spline open pig on hand and decided to swap it into his truck and we pulled a 31 spline 2.49 pig out of that little no spring 6 cly truck. There is a way to tell what is in it from the numbers on the tag on the pig but you need a sharp ford parts man to help decode them. I do agree with Logan that a 3.50 is about as low as you want to go for a gear. I have a 3.25 in mine and it gives good acceleration with out being too short winded for extended 70-80mph runs. I used to have a really sharp manual trans and rear end shop in my area that used to help me out with my 9" pigs when I was racing but alas he quit and opened a computer shop!!!! Dale rember this, bigger # ratio= less top end, smaller # ratio= more top end at the same engine rpm.mm
 

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posi rears

I frequently go to the friendly junkyard across the street and cruise it for 9" posi rears. Like MM says, the best place to look these days is in light trucks.

Passenger cars quit using the 9" in the late 70s, and those that did usually had 'economy gearing' for better gas mileage. My wife's old 77 Marquis had 2.32 gears! Performance was not a high priority! Trucks continued on for several years - until the mid- to late 80s, I think. My latest posi came out of a F-100 that was originally owned by one of the local utility companies or fleets (look for the distinctive colors). It was a six-cylinder stick-shift and it had a 31-spline 3.50 posi! The going rate for this yard is $75 for a complete rear end. Most yards charge more.

I'd suggest you get a code-breaking book that will tell you the door dataplate axle codes. Trucks can get a little weird compared to cars. Trucks will generally have steeper gears than cars, especially when they have small engines or stick shift trannies. That isn't true in all cases though. Some light trucks don't even have 9" rears. Many wimp trucks had the 8.5" integral (non-dropout style) rear. Look under the truck to verify it has a 9", then try to read the tag or dataplate. Generally, with passenger cars, a number for the door code indicates open, and a letter code indicates a posi. I repeat, generally! This 'rule' flies out the window for trucks and vans for some reason!

If you don't have a code book, you'll have to crawl under the truck and look at the center section bolts for the data tag, that is if someone hasn't removed it already. It will not be a stainless tag like GM uses to mark its 'Use limited-slip lube only' so it may be rusty and hard to read depending on where you live. Use a pocket knife to scrape the tag to highlight the letters. Don't pull the tag off! It will break right in the middle of the axle code! The ratio will be in the lower left hand part of the tag, sometimes partially obscured by the nut. 3.00 is 3.00 open, 3L00 means 3.00 posi.

My favorite all-purpose gears are 3.70s and were only available in trucks and vans from the factory. Beta Cat now has those in it. It is only taching 4600 at the end of the quarter mile at 95 mph. Both of my 73 Cougs (351C-4Vs) came with 3.25 posi. My wife's 70 Mach (351C-4V) came with a 3.25 open. Go figure. Her (all stock) car got almost 20mpg on the highway on our trip to Charlotte for the 30th anniv Mustang show. Cars with Holley carbs or other performance mods will generally do worse. My first Cat (72 Standard, 351C-2V) got around 18mpg highway with whatever gears it had. I wasn't into cars back then so I really don't know, but probably 2.75s or 3.00s. The difference in gas mileage between the wife's Mach and my first Cat was probably attributed to my car being out of tune (I couldn't set the timing right with that blankety-blank-blanking A/C in the way :mad: ) and my Cat being a low-compression 2V.
 

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gears

Dale,
I am by no means an engineer but I will try to explain gear ratios in simple terms. Gear ratio's are a measure of the realtionship between engine RPM and axle RPM. Assuming that your transmission's final drive ratio is 1:1 ( every time the engine spins once the transmission spins once ), a 3.00 rear will require that the engine spin 3 times to get the axle to spin once. A rear with a 2.00 ratio will require the engine to spin twice to turn the axle once, and a 4.00 will require the engine to turn four times to get one revolution on the axle. Things such as tire size and overdrive gears can effect the relationship as well, but lets not complicate things. When someone refers to a "high" ratio gear, they are referring to the higher number of RPM's the engine must turn to get the axle to turn. Higher ratio gears allow for more rapid increases in RPM, therefore providing greater torque and horsepower at the rear wheels ( horsepower being a product of torque and RPM ). The downside to this is that the engine will be revving higher at lower highway speeds than a "low" ratio rear. Lower gear ratios allow the engine to spin fewer times to get the axle to spin. This provides better fuel economy and longer engine life than the higher revving gears.
I have had several different gears in my 69 convertible. Right now I have 3.70 gears in my 28 spline nine inch rear. I have a mildly modified 351 W 4-V. The 3.70 gears are great for street/strip cruising. The engine gets into it's powerband quickly and runs out of steam at around 90 mph and around 5500 RPM. The downside to the 3.70 is that my gas mileage is pretty poor and highway cruising can be a burden. In order to maintain 65-70 MPH, I must keep the RPM's between 3500 and 4000. Not good for long runs. Personally if I were to do it again ( and I probably will sooner or later ) I'd go with either a 3.25 or a 3.50, leaning more to the 3.25. With a stout cleveland and a four speed, you will be able to spin the tires easliy with just about anything. The mid range posi's are a good balance of performance and driveability. Good luck with whatever you decide.
Paul G.
http://home.sprynet.com/~pamar/cougar.html

PS-I believe that only the boss 302's and 428's got 31 spline rears in 70, but I could be wrong, and it can't hurt to check
 

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Calculating engine rpm at speed...

Just a little basic algebra here...

Pi times tire diameter gives tire circumference. (3.1416 * D)
5280 feet per mile times 12 gives inches per mile. (5280 * 12) = 63360 inches per mile.

Therefore 63360 / (3.1416 * D) = tire revolutions per mile.

60mph = 1mile per minute.

Therefore 60mph = 63360 / (3.1416 * D) = tire revolutions per minute.

Are you following me so far? Good.

A P215/70/14 tire is about 26.5" tall.
Plug that value in for D and run the formula.

63360 / (3.1416 * 26.5) = 63360 / 83.25 = 761 tire revolutions per mile.
That means that at 60mph, this tire is rotating 761 times per minute.

now we throw gear ratio into the equation...

To determine driveshaft speed, you must multiply by the rear end ratio.
So, at 60mph, with a 26.5" tire, your driveshaft RPM values would be:
761 * 2.75 = 2093
761 * 3.00 = 2283
761 * 3.25 = 2473
761 * 3.50 = 2663
761 * 3.70 = 2815
761 * 3.91 = 2975
761 * 4.11 = 3128

With most three speed automatics or conventional stick shift transmissions the ratio of engine rpm to driveshaft rpm is 1:1. That means you can stop now. The driveshaft speed equals your engine speed.

If you're running an overdrive, that means the engine is turning slower than driveshaft rpm, so that correction factor can be thrown in now as well.

I think that an AOD automatic has a ratio of .78, so multiply that driveshaft rpm by the AOD ratio to get the engine speeds. (Hey, what do I know about these new-fangled things... I'm a Senior Motorhead! ;) )

So, with a 26.5" tire and 3.70 rear gears and an AOD transmission:

761 * 3.70 * .78 = 2196 rpm at 60 mph.

I hope this math refresher helps all you Cats!
 

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algebra

And I thought I was making it sound complicated. Actually, the math lesson is helpful for anyone looking to modify their drivetrain. When I was looking to swap out my 3.00 I created a spreadsheet that allows you to input desired gear ratio and tire size and gives you your MPH through the entire RPM range. However, it is based on a 1:1 engine/trans ratio and assumes you are in final drive gear. It would be easy enough to adjust for an AOD. I was surprised how little tire diameter effected the outcome. I was condidering getting larger diameter tires to try to improve my highway RPM's but I would have had to tub the car to make a difference. If anyone is interested in the spreadsheet let me know and I'll send it off list.
Paul G.
http://home.sprynet.com/~pamar/cougar.html
 

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Discussion Starter #13
Rear Diff Ratio's

Hey Guy's...been away on holidays and just got back to the puter to check out the postings.

Nice Job Guy's...now that's useable information, I just knew you all had the wisdom that I so desperately needed. :D I think I finally understand the whole concept of rear axle ratio's now with this new found info.

Thanks a bunch and I'll let you all know what I finally come up with....

BTW, Paul....I would appreciate a copy of your spreadsheet as well if you don't mind.

Thanks again everyone...

Dale
 

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oh brother

Cruisin' cats generally stick to 3.00:1 or 3.25:1... Maximum of 3.50:1 without overdrive.

If you go to the strip, you might want a 3:73, but you're revvin pretty hard at the end of the quarter.
:rolleyes: :rolleyes: :confused:

3.73's at the strip...... :confused: On a stock Camaro or 5.0.

Man when I had 3.00's in my car I could do 100mph in 2nd gear.
:D Who the hell wants that?? :confused: I hated it!

4.88's and 28" tall slicks work real nice at the track. 5.14's are pushing it unless your into the high rpm's or bigger slicks.

Logan drives like a grandpa. :cool:

I wouldn't go for less then 4.10's if you have cash to choose just for street/strip. That was the stock drag pack ratio for a bunch of Fords in the good old days.
 

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Buddy... 99% of these cars are cruisers, not strip machines, hence the recommendations...

I really don't enjoy turning 4000rpm's cruising at 60mph... Yeeesh.

At least I drive a ford... Chebby boy. :rolleyes:
 

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Taller gears

Chebby boy? That made me grin. :D

I used to have a 74 (shhhh) Camaro that I ended up back-halfing to turn it in to a race-only car. I was running a 408 inch BBC (+.060 over 396), TH400 and 9" rear. I used 5.14s 'cause I wanted to get that 3700 lb bulk moving off the line. I was also running a 4000rpm stall speed converter and 13-31 slicks (thirteen inches wide, and 31 inch tall). I was shifting at 6200 and went through the eyes at almost 7000 rpm. This all translated to a [email protected] Drop that mph in half and you'll see that I would have been turning 3500rpm @ 60mph -- with 31" tall tires.
 

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hahhahah... I scared 'em right into goin' out and buyin' a brand spankin' new Lariat Supercrew... :D

Welcome back our long lost compatriat... Now just go sell that Jeep thingy and the Blazer and you'll be good to go.
 
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