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Discussion Starter · #1 ·
Ok i am changing my I6 66 stang to a 5 bolt setup and installing disk brakes in it (eventualy it will get a 302 in it, but not yet) well it had the 8 inch rear end in it?
i dunno what it is. anyway i was told to change it to a 9" which is fine but i have no idea what exaclty that entails? should i just go grab the entire rear of a donar car? what different parts are needed?

yet again... clueless
 

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I posted a simmilar question on this subject 2 days ago titled " 8"vs9" rear ends " som of the replies migh help you .. I think it is on page 2 of the posts. But to sum things up if you don't plan on going crazy with the HP your 8" should be fine.
 

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The 65/66 9" rears only came in the "K" code and Shelby mustangs they are very rare and quite expensive, I have seen them going for a 1000$ with no axles or center section. there are other cars that had the same width but more than likely will not have the same axel tube diameter to work with your stock shock plate`s. The nominal width for a 65/66 9" rear is 52" flange to flange I believe. Your 8" should be able to handle most power levels, just get a posi and new axels with a 5 lug pattern and put on that front disk conversion and you have a 5 lug car. Probably cheaper than the cost and hassle of switching to a 9".
 

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If it is an I6 it is probably that 7 5/8" or whatever size it is rearend. Not an 8". Easy way to tell would be to look at it from the rear of the car and if it has a bolt on cover on the back of it it is that 7 5/8" setup. It pretty much has to be if it is a 4 lug car. If not it is surely an 8" rearend.
 

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for the average user, and even the peformance freak, there is vertually no gains to be had from the 9" rear end swap. in fact, the 8" can be made relatively bullit-proof (like the 9" at least) by purchasing a quality posi set-up. in addition, the majority of the 9" rear ends, with the exception of the very rare (and expensive) will be too wide for your '66. not to mention, the 9" rear will add about 100 unneeded pounds tot he back of your car.
bottom line, keep your 8", dump $300-$500 into a good posi kit complete with a better responding gear ratios.
what transmission are you using and will it be the same once the 302 is installed? pick your gears wisely from that point.
 

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Don't bother with the 9". Unless you have 500+ hp, no problem with a good 8". First, make sure it is an 8". I would guess the 6cyl might not be. If it is, you can simply have the axle flanges redrilled to the 5-lug.
If it isn't an 8", get one. They are cheap and easy to find. (I have 2 for sale). Then with some gears and a Trac-Loc you are on your way...
 

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Discussion Starter · #12 ·
sinc this is first and formost my daily driver, would it be a good iddea to go with 4.11? expecialy when i will have to wait on the tranny install for a while, just dont have the cash to do it all at once. what would be the best ratio to use if i lookign for decent gas milage, but want to still lay down some hurt every once in a while
 

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Is there really 100lbs difference between a 8" and a 9"? The housings have to be similar in weight(within a few pounds). It doesn't seem like there would be 100 lbs difference between the pigs.
 

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the T-5 will work great for you, i will be doing the same swap shortly. like nastystang said, put the money where it should be into the T-5 swap.
by all means WAIT on the 4.11's until after the tranny is in place. your C4 will spint he engine soo high that you'll probably not want to go over 50 mph! even as a daily driver, the 4.11's behind the T-5 will probably put you at about 3000 rpm when driving 60 mph or so. the 4.11's will definitely make you scream off the line assuming you have and engine that is set up compatibly. what kind of T-5 will you be putting in? what will the low gear be? if you are really worried about overall drivability, you can't go wrong with a set of 3.73 gears.
not too long ago i found a site on the net that compared the specs for the two rear ends. i am almost certain in the weight difference (otherwise i would have no reason to remember it).
good luck.
 

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nastystang...did you get your speedometer re-calibrated after the install of the rear-end gears? also, your transmission is the worldclass T-5 (.063 5th gear), is it not?

also, were you the one who installed the 4.11's? my friend and i have another mustang (89 body with a a 347 stroker mounted up to a worldclass T-5 with 3.73 gears) which runs approximately 2300 rpm at 70 mph.
 

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When you change gear ratio`s all you have to do is switch the right speedo gear. I use the white gear with my 4:11`s, my speedo might be a little off, could be closer to 2500 RPM`s @ 70, keep in mind the T5Z has an .063 5th, others have an .068 or something even a little less like an .073 or something, that might be where the RPM difference is coming from, that and tire size, the smaller the OD = more RPM`s, the bigger the OD = fewer RPM`s
 
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