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Offline popbumper

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Standard shift 6.75" to 8" rear end swap questions
« on: December 10, 2014, 02:15:00 PM »
Some quick questions, My wagon is a standard equipped with the stock 6.75" rear end. I pulled an 8" rear end off of a '78 hatchback, and have it stored.

1) Is it a direct swap in with no mods to any major components?
2) Does the drive shaft have to change?
3) What performance decrease/increase am I looking at with this change in gearing?

I have a stock 2.3L, no mods, so it's not in any way "race ready". Just curious if putting an 8" in is a good idea for any reason. It's the only area of my car that has NOT been touched yet, and I'd love to just be able to swap in a different restored rear end with new rear springs rather than pulling the original and putting the car up on blocks for a while.

Thank you!

Chris
Restoring a 1976 MPG wagon - purchased 6/08

Offline dick1172762

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #1 on: December 10, 2014, 04:08:41 PM »
Go to "General Help" then go down 15 post to mustang II rear end question. Its all there about the 8" rear end.
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Offline bbobcat75

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #2 on: December 11, 2014, 08:28:05 AM »
whats the gear ratios in the rears?  pretty easy swap!
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Offline popbumper

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #3 on: December 14, 2014, 06:57:47 PM »
Thank you Dick - but what I would also like to know is what can  expect from a performance perspective? How will an 8" rear "behave" as opposed to the 6.75"? I know the ratios are different, but I don't quite understand how things will "perform". I'm just running a stock 2.3, nothing built up.

Chris
Restoring a 1976 MPG wagon - purchased 6/08

Offline dick1172762

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #4 on: December 15, 2014, 08:18:54 AM »
The only real advantage is you can get posi's for the 8". This is great for a hot rod Pinto, but for your wagon, its a waste of time. 6 3/4" rear ends are plenty strong for what your going to do with your Pinto. Pull the back cover off and change the lube because it could be the same lube it came with when new.
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Offline Pintocrazed

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #5 on: December 15, 2014, 08:28:03 AM »
ILL HAVE TO SECOND DICK ON THAT.STOCK MOTOR WILL NEVER PUT THE LITTLE REAREND IN A BIG ENOUGH BIND TO BREAK.JUST KEEP IT SEALED AND WELL LUBED

Offline russosborne

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #6 on: December 16, 2014, 12:19:06 AM »
Problem is that if the stock rear does break, even from old age, it can be hard to fix(finding new parts).
You will never break the 8inch with the 2.3 unless you go crazy with turbos (400+HP).
Probably the simplest swap there is on these things.
Since you already bought it you might as well use it. At least in my opinion.
Me, I've bought an 8inch, but now I need a 9inch with the engine I plan on running.
Russ
In Glendale, Arizona

RIP Casey, Mallory, Abby, and Sadie. We miss you.

79 Pinto ESS fully caged fun car. In progress. 8inch 4.10 gears. 351C and a T5 waiting to go in.

Offline Srt

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #7 on: December 16, 2014, 09:07:32 PM »
ILL HAVE TO SECOND DICK ON THAT.STOCK MOTOR WILL NEVER PUT THE LITTLE REAREND IN A BIG ENOUGH BIND TO BREAK.JUST KEEP IT SEALED AND WELL LUBED

I had 6.75 in my turbo car for years and it worked just fine.
the only substitute for cubic inches is BOOST!!!

Offline popbumper

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #8 on: December 17, 2014, 12:59:28 PM »
Cool Russ - but how will the different 8" gear ratio affect my driving as opposed to the 6.75" rear end? Can anyone tell me that?

Chris
Restoring a 1976 MPG wagon - purchased 6/08

Offline dick1172762

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #9 on: December 17, 2014, 01:32:56 PM »
If the ratios are the same there will be no change.  You need to post your gear ratios for both rear ends to answer your question.
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Offline russosborne

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #10 on: December 17, 2014, 02:23:50 PM »
Yep, it totally depends on what the ratios are. Lower numbers (2.7, 3.0, etc) are more of a highway gear. Better gas mileage on the freeway, but acceleration is slower.
Higher numbers like 3.5, 3.9, 4.11, much quicker off the line, but not good on freeways.

Ratios are kind of confusing. Lower numeric numbers equals slower engine speeds compared to a higher ratio at the same mph. Wild butt example, going 65 with a 4.11 ratio you might have an rpm of 5k. With a 2.75 it might be closer to 2K. Hope that makes sense. Those numbers are  not real, but they do give you an idea of what I am talking about.

You have to know what both rears have to compare. However,odds are you could buy the same ratio gear for the 8 inch rear as what the 6.75 has if you like the current ratio. Lots of available ratios for the 8 inch.

What it all really comes down to is how do you want the car to be? A stoplight to stoplight cruiser is fine with something like 4.11s, but without overdrive not great for hoping on the frewway for a long trip. And vice versa, something setup with 2.75 gears is not much fun off the line, but great for long distance freeway driving. Something in the middle will give you some of both worlds.
Russ
In Glendale, Arizona

RIP Casey, Mallory, Abby, and Sadie. We miss you.

79 Pinto ESS fully caged fun car. In progress. 8inch 4.10 gears. 351C and a T5 waiting to go in.

Offline 71HANTO

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #11 on: December 17, 2014, 03:12:02 PM »
Just to throw this into the mix...the 8 inch is 30+ pounds heavier. That is un-sprung weight. The axles and gears are also heavier which saps a slight amount of horsepower/torque that gets to the ground given the same ratios. It is not much but a 6.75 will get SLIGHTLY better gas mileage and is SLIGHTLY faster in acceleration. That being said, most of the big shops I have dealt with won't touch rebuilding a 6.75.


71HANTO
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Offline Pintocrazed

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #12 on: December 17, 2014, 03:17:07 PM »
I HAVE 3.40'S IN MY 73 RUNABOUT BUT CAN GET A 8" WITH THE SAME GEARS FOR FREE.MY 2.0 WILL EVENTUALLY BE BORED .30 OVER WITH BIGGER CAM AND HEADER.THINK THE EXTRA WEIGH FROM THE 8" WILL HURT?

Offline russosborne

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #13 on: December 17, 2014, 11:03:47 PM »
Get the 8 inch and hang on to it until the 6.75 inch breaks if nothing else.
Probably overkill, but it is nice to have a spare ready to go if you need one.
Russ
In Glendale, Arizona

RIP Casey, Mallory, Abby, and Sadie. We miss you.

79 Pinto ESS fully caged fun car. In progress. 8inch 4.10 gears. 351C and a T5 waiting to go in.

Offline pintoguy76

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #14 on: January 07, 2015, 09:02:29 AM »
I put an 8" from my old 76 pinto sedan into my 74 wagon. I don't remember having any problems with the swap, it pretty much all bolted right in. I probably used the driveshaft from the 76, but I dont remember for sure.
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Offline Wittsend

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #15 on: January 07, 2015, 11:08:42 AM »
Was it ever concluded what these specific ratios are in the 6-3/4" and the 8"?

I'd say of all the factory ratios that came with the 8" (3.00, 3.40, 3.55 - less a very rare 3.25) 3.40's are probably the best to get. I'd use them especially if you are running a 4 speed (no overdrive). I had 3.00's with my T-5/turbo 2.3 and the gearing just seemed all wrong..., in every gear.  This regardless of the 175-70-13" or the 215-60-14" tires I ran.  3.40's made the car much more "drivable" in everyday use. Currently 2,600 RPM @ 65 MPH in 5th gear with 175-70-13" tires.

 For comparison the Turbo Coupes were dragging around about 1,000 extra pounds and only ran 3.55's (manual) with 225-60-16" tires. The turbo motor can probably deal with 3.25's. This makes 1st gear less "granny" and would drop the freeway RPM.  I realize the post deals with a stock motor and a 4 speed, but where ratios had a negative effect on the turbo/T-5 they would be more so on the stock car.

Offline Srt

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #16 on: January 08, 2015, 01:12:06 AM »
Was it ever concluded what these specific ratios are in the 6-3/4" and the 8"?

I'd say of all the factory ratios that came with the 8" (3.00, 3.40, 3.55 - less a very rare 3.25) 3.40's are probably the best to get. I'd use them especially if you are running a 4 speed (no overdrive). I had 3.00's with my T-5/turbo 2.3 and the gearing just seemed all wrong..., in every gear.  This regardless of the 175-70-13" or the 215-60-14" tires I ran.  3.40's made the car much more "drivable" in everyday use. Currently 2,600 RPM @ 65 MPH in 5th gear with 175-70-13" tires.

 For comparison the Turbo Coupes were dragging around about 1,000 extra pounds and only ran 3.55's (manual) with 225-60-16" tires. The turbo motor can probably deal with 3.25's. This makes 1st gear less "granny" and would drop the freeway RPM.  I realize the post deals with a stock motor and a 4 speed, but where ratios had a negative effect on the turbo/T-5 they would be more so on the stock car.
IIRC the 6.75" had ratios of: 3:18-3:40-3:55-3:73
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Offline Wittsend

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Re: Standard shift 6.75" to 8" rear end swap questions
« Reply #17 on: January 08, 2015, 11:15:26 AM »
"IIRC the 6.75" had ratios of: 3:18-3:40-3:55-3:73"

Yes, Thank you. And if I recall correctly there was at least one 2. (something) ratio as well???  However, I was actually asking regarding the ratios of the original posters car and the rear end he was getting (sorry if my question was confusing).   BTW, if they were running 13" tires and mileage was more important than scooting around I'd go with the 3.18, otherwise I'd still go with the 3.40 especially if they up-size the tires.