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1966 Ford Mustang coupe I6 C4
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Discussion Starter · #1 ·
I don't know a whole lot about rear diffs and axles, so I appreciate how much I've learned on VMF - huge gratitude to each of you who have posted on theses subjects over the years - it's been a great education!

Based on my research, it looks like upgrading my I-6 w/C4 to 5-lugs should involve swapping out my original stock 7.25" rear (2.83 gears) for an 8" w/3.25 gears and disc brakes because I prioritize spirited driving on winding country roads and a little more zip off the line over fuel efficiency on the expressway. I also hope to someday drop in a 289 w/4- or 5-speed (<400hp), and I'd prefer to only have to do this swap once and be able to easily/cheaply maintain my driveline.

Although it seems most simple to just cut off my sleeves, buy a crate 9" full assembly w/Wilwood discs from Currie, bolt it up, and start working on jokes about third-members...that also seems neither cheap nor necessary given my objectives for the vehicle (and my small biceps). I'm budget-conscious, but I'd rather pay a bit more for the "right" solution for my needs that will last me decades than cobble together the cheapest junkyard parts and spend dozens of hours making them work "well-enough" until they don't.

Oh, and safety. My general rule is "Safety Fifth" but because others' lives are at risk when I'm on the road, I guess I should consider safety a high priority when deciding on drum vs. disc brakes...with maintenance being a close 2nd.

Based on today's available tech and economics, I'd love any confirming or alternative opinions as well as any advice on where to get said recommended rear end/parts. Thanks in advance for any/all guidance and further education!

Cheers
 

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Post your location. I have done the 4-5 lug conversions...and I have several cars w/ 6's. I have extra 8 inch rear, but I am in S. Texas. far from just about everywhere. You will have alot to change and many parts to source. When you change rear, you will also need to upgrade driveshaft, and possibly fiddle with yokes for trans you are installing. It just depends on what you plan to install. I suggest 5 lug conversion, but that is me. I make everything ready for the V8, but generally just improve the 6. so here is my list,
5 lug conversion front and rear.
4.5 leaf SD leafs reverse or mid-eye
Moog 8534 coils, possibly cut 1/4 coil or 1/2 coil, but 1/4 at a time.
roller perches.
1' or 7/8 sway
disc brakes up front
export brace
new steering components to include rebuilding steering box
New uppers and lowers a arms
duraspark ignition/ and dizzy
cam of choice
dual out split header
dual exhaust out the rear.
trans of choice, I have 1 with 5 speed, and 2 w/ hopped up C4s.
this is alot, but will make that six run with the big boys.

AS for rear gears, the T-5 car had a 3.00 rear, and it was a little too low, we just installed a 3:50 gear, but have had no seat time to try it out. The C4 cars have a 3:00 and a 2.79 rear gears. One will get the extra 3:25 pumpkin I have yet to install. I suggest 3:25 at the minimum. On 6 cylinder cars that were autos, they got a 2.79 from the factory. 6 cylinder manual trans cars got a 3:20 from the factory. If going w/ overdrive trans then you need 3:50 and up for great results.
 
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1967 Convertible Shelby clone
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Ditto to everything Coupster said. A 9 inch with rear discs is overkill in most of our cars-mainly just bragging rights. As far as handling abuse- you can break anything including 9 inch and toploaders.
 

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1966 Ford Mustang coupe I6 C4
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Discussion Starter · #4 ·
@coupster - this is excellent insight & guidance - thank you. I'm in Carthage, Missouri, so South Texas is just a fun roadtrip away!

My current thinking is to go with Opentracker's Full Suspension Level 2 Kit with some "big pin" V8 spindles from CSRP and Wilwood discs up front. And the more I'm learning on fordsix.com, the more I'm leaning toward building up my 6 rather than swapping in an 8. Vintage Inlines seems to have some excellent products, so your DSII and exhaust recommendations are great confirmation that I may be crazy but not studip.

Re: transmission - I've kinda relegated it to "future phase" b/c while I enjoy driving a stick and would love to have a 4- or 5-speed, it doesn't seem as necessary as upgrading my suspension. Also, I have no idea of the ballpark budget needed to swap out my current C4 and what trans options I should consider (within reason), so it's tough to gauge if it's worth it (and, if so, how to prioritize within my build plans). But you're making it clear that I may need to figure out my transmission aspirations in order to guide my diff decision for the rear. In your experience, is a T-5 the way to go? Or should I just put 3.25 gears behind the C4 and consider myself covered regardless? Any advice and cost #s would definitely be appreciated.

Thanks so much!
 

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Coupster is right on target again. I went the route from 6 to 8 on a budget in the late 80s thru the early 90s. Suspension & brakes first. Obviously interrelated. Remember the sixes brakes & suspension are/were the economy version. Reliable, tried & true tech, but relatively inadequate for performance. Wears at a greater rate in equal use.

Stick with an 8in with drum brakes. I converted mine to mid-sized station wagon brakes. Was cheap & easy and added braking power. Still running them with the V8. See if you can find someone going from the stock disc brakes to aftermarket and make an offer for the stock pieces based on condition & completeness. Somebody correct me, but the V8 spindles are the same between drum & disc early cars. It's the brackets that make the conversion possible.

Good advice on springs, though I haven't looked up the coil specs. I used 67/68 big block coils. I felt 620lb coils marketed across the board were overkill. Cut them down slowly until I got the ride height desired. I am one of the few who will not endorse Eaton leaf springs from my experience with a set. A GT steering box & front anti-roll bar with a 5/8 rear bar. Mostly stock stuff that people were casting off for a song back then.

Export and the cross brace to stiffen things. Recently added Zrays lower brace and felt an immediate difference.

Top loader 4 speed and 3.55 Traction-loc gears worked great.

Was local modified class champion a couple of years. Key was tight courses favored the six's low-end torque and regular attendance for points. Will add more later, have a call to make tonight.
 

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I don't think you can make a great choice on rear end gears until you have the final decision made on the transmission. You can throw around the idea of a T4 or T5 all you want, but in the end its completely irrelevant without knowing the gear ratios for that particular transmission. The same goes for your engine, and its overall characteristics....without having the final setup decided on, its not worth wasting money on gears. Ok, so part of the handling upgrades require v8 spindles...I understand that, its better steering geometry. If you are going to focus on suspension first then v8 spindles are a must...the rear end is NOT a must though until you have finalized the drivetrain. Sure...you may end up with 5 lug up front and 4 lug in the back for awhile...but that doesn't much matter other than cosmetically(for that matter 4 lug I6 front hubs may well fit on v8 spindle, never tried but I guess it could be possible). The point is you don't have enough information yet to make a smart choice on gear ratios, if you MUST swap axles immediately, just swap to a stock 8" axle and leave the gears alone until you have enough information to make an educated choice....personally though, I wouldn't touch the suspension and steering until you finalize the drivetrain or at least have a plan you aren't going to deviate from that includes the exact transmission you are going to use and all of its gear ratios.
 

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Coupster:
"On 6 cylinder cars that were autos, they got a 2.79 from the factory. 6 cylinder manual trans cars got a 3:20 from the factory. If going w/ overdrive trans then you need 3:50 and up for great results."

My 1966 convertible 200ci with C4 autotrans came with a 3.2 rear end. Perhaps it was an option? Anyways the car accelerates pretty good for what it is. I'm often asked if I have a 289.
 

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Completed the call.

A T-5 would be do well behind a 6. The OD will help on the highway with the six's low rpm power band if you go 3.55 or higher numerically on rear end and would support the conversion to V8.

Six bottom end was stock with about 150k miles. Couldn't blow up it up with those 6 main bearings. Autolite 2100 2 bbl carb (jetted down) on a 67-69 head. I can give you more details on this if you like, but there may be better options now. DS system, Petronics replaced the points. Clifford manifold splitter with stock manifold. Headers and cam would have been nice. If I were to rebuild the motor, balancing and blueprinting would be where I'd spend money. I ran across a low mile 5.0 complete for cheaper than I could build a lawnmower motor when I wasn't even looking so the last thing in the conversion was the motor.

Which brings me to my overall point is to build the body stiffening, suspension and brakes with future conversion to V8 or not in mind. It will make the six fun to drive in the meantime and allow you to eat that elephant in manageable bites. I've lost count of the people who did the engine first and were not happy/broke stuff and created something dangerous to drive. Build a good foundation first.

Brit sports cars like an MGs or MG Midgets were very under powered, but they were a blast to drive in the twists and turns.

P.S. Get a later model 8 in unit with the webbing. Early ones don't have the webbing and casing is weaker. Unless you run super sticky tires or slicks, you'll probably never get the traction/grip with a V-8 to break one that's in good condition.
 

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Coupster:
"On 6 cylinder cars that were autos, they got a 2.79 from the factory. 6 cylinder manual trans cars got a 3:20 from the factory. If going w/ overdrive trans then you need 3:50 and up for great results."

My 1966 convertible 200ci with C4 autotrans came with a 3.2 rear end. Perhaps it was an option? Anyways the car accelerates pretty good for what it is. I'm often asked if I have a 289.
There was a more uncommon 7.25" rear end with shorter gearing if I recall correctly. What trans do intend to go with? What is the first gear ratio of that trans? An I6 has a fairly flat torque curve and a fairly low redline. This means if your rear end gear ratio is 3.73 or 4.11 and your first gear ratio for a potential T5 is most commony 3.35. So you have 3.35 x 3.73 = 12.4955. This is not a very good 1st gear gear for a low-revving engine that already has decent low end torque. Sure, its snappy and makes your I6 feel a bit faster off the line...but you will have to shift to 2nd gear before you have gone much more than about 35-40 feet. A rear end gear ratio of 3.27 or 3.55 is much more suited to the engine. Now, if the engine were instead a 289 with a 6000 RPM redline, that 1st gear is significantly more useful(still not overly useful though, a t5 with a 2.89 first gear would be more suitable to a 289). I am not a big fan of having to shift immediately after launch so I can spin my wheels more, which is already wasted time....on the other hand if you had a 4 cylinder that didn't produce a whole lot of low end torque then a 3.35/3.73 combo would be a near perfect match
 

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the combo I listed above is the same that I am running on my SBF's and my only BBF....same set up, just did not cut coils for v8's, and BB has 9 inch and power steering. Some have the granada discs, some have factory discs. I forgot to mention that ALL cars get "the drop"...shelby drop....hehe

as to my opinion for the OP... as stated above, stick w/ C4/small rear now, and focus on the suspension, steering, front end for now. Make it stop and steer. More time will open up more options for future parts. Also just pull the trigger on the T-5. C4 can be built tough, but it takes away more ponies from the 6. Put the t-5 on your list from the get-go. More expensive, but more fun for sure. And I disagree on the rear sway...but that is me...
 
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Coupster:
"On 6 cylinder cars that were autos, they got a 2.79 from the factory. 6 cylinder manual trans cars got a 3:20 from the factory. If going w/ overdrive trans then you need 3:50 and up for great results."

My 1966 convertible 200ci with C4 autotrans came with a 3.2 rear end. Perhaps it was an option? Anyways the car accelerates pretty good for what it is. I'm often asked if I have a 289.
Yes, the 3.20 was optional with the 200/C4. My Sprint 200 has 3.20.
 

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1966 Ford Mustang coupe I6 C4
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Discussion Starter · #14 ·
Thanks so much for the guidance, Gentlemen! I just shot a quote request to Modern Driveline for their 6-cyl C4-to-T5 conversion pkg. I now understand why spec'ing the future tranny is helpful/necessary in order to properly spec the gear ratio in an 8" rear.

That said, I'm definitely doing the Arning Drop after I figure out how to tackle my rusty front frame rails and then follow @coupster's advice and "focus on the suspension, steering, front end for now."

Re: steering - should I be considering an upgrade to power steering while I'm putting in new tie rod ends, idler arm, etc? It looks like a couple decent options are Opentracker's Borgeson kit, and Mike Maier has a bit pricier KRC kit.
 

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Discussion Starter · #15 ·
P.S. Get a later model 8 in unit with the webbing. Early ones don't have the webbing and casing is weaker. Unless you run super sticky tires or slicks, you'll probably never get the traction/grip with a V-8 to break one that's in good condition.
Thanks for all your insights, @sotexyellow! What's the first year of the "later" webbing 8"? There's a "Complete drum to drum 8 inch rear end with gas saving 2.79.1 ratio third member" out of a '67 Mustang for $400 near me...but I'm not sure if that's a good deal, or if the slightly wider '67 rear will easily bolt under my '66...but maybe that's as easy as wheel backspacing (something I'm also trying to learn as much about as I can!)... Any thoughts are welcome & appreciated!!
 

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Thanks for all your insights, @sotexyellow! What's the first year of the "later" webbing 8"? There's a "Complete drum to drum 8 inch rear end with gas saving 2.79.1 ratio third member" out of a '67 Mustang for $400 near me...but I'm not sure if that's a good deal, or if the slightly wider '67 rear will easily bolt under my '66...but maybe that's as easy as wheel backspacing (something I'm also trying to learn as much about as I can!)... Any thoughts are welcome & appreciated!!
I currently have a '67 8" sitting under my '66, be aware it is 2" wider than the '66 rear end, which may cause fitment issues depending on the wheels used. I am using late-model wheels from an '05 v6 with 1" spacers so fitment is fine for me...but if you run something like a 17x8 wheel with a 4.25" backspacing that wider rear end is sure to be a problem.

P.S. I am currently looking to change it out for a stock 66' width rear end instead because it limits me a lot on wheel choice...so I will warn you now just because you may be happy with it with some wheels you find(or combination of wheel and spacer) you will be less than happy if you choose to swap to something else
 

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400 IS TOO MUCH FOR 67 UP 2:79 OPEN..... WAIT..... sorry not yelling....
 

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yup- 400 is too much for an 8 inch with 2.79. Be patient and keep looking, they are not that rare.
 

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I sold my stock 8" with 2.79 for $110 a few years ago, and it sat on the market for 3 weeks before someone bought it.

But yeah, just go with a v8 spec 8" with drums until you figure out your trans.
 

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Is your goal to convert to 5-lug for wheel choices or.......?
 
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