C4 toC6?? - Vintage Mustang Forums
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post #1 of 8 (permalink) Old 04-23-2019, 08:13 PM Thread Starter
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C4 toC6??

I’ve got my 66 mustang with no engine or tranny inside. I bought a 302 but now I’m looking for a transmission and after looking on forums they said a c4 would be good since that’s what it came with it but I can’t find one anywhere and I’d rather not spend $1500 for a rebuilt one if there’s other options. So can I get a small block c6? Or maybe an aod? If so what do I need? My car is just for driving around, not racing.
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post #2 of 8 (permalink) Old 04-23-2019, 09:05 PM
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I don't see your location but I can't believe that it's difficult to find a C4. Since you don't have a transmission to rebuild you may as well go all out and get the 4R70W.
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post #3 of 8 (permalink) Old 04-23-2019, 09:26 PM
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C6 is a much heavier transmission, and while functionally identical to the C4, it steals a lot more power. It's a lousy choice for a smallblock. The C4 is ubiquitous. It came in cars, trucks, vans - anything with a 6 banger all the way to the 351, from mid 60s all the way into the 80s.

Any salvage yard that has older cars will have a few. It would be an excellent choice, even if it needs a rebuild.

If you don't care as much about originality, and want to upgrade the rear gears too, you could go to an AOD - or better yet, a 4R70W. There will be some other considerations with these 4 speed overdrive transmissions. They both require a different transmission mount from the C4, and they may require a different length of driveshaft. The 4R70W is better in most every way, with a lower 1st and 2nd gear, and better durability. It does require an electronic controller which will set you back about $800 or so. It's an updated version of the AOD, and features a lot of upgrades that affect reliability and shift quality.

The C4 is a fantastic transmission. It's tough, simple, lightweight, with good shift quality and it's very easy to rebuild. But in the end, it's just a 3 speed.

If you're interested in maintaining originality, or just want to keep things simple and easy, take the time to find a C4. They're around if you look.

If you're more interested in hot performance while maintaining economy, and don't mind spending a lot more, step up to the 4R70W, and enjoy better mileage along with hotter performance. Running 3:50-4:10 rear gears with an overdrive transmission is pretty sweet. Running an overdrive transmission with 2.79 gears really lugs things when you're in 4th.

Best wishes!

Last edited by Grimbrand; 04-23-2019 at 09:35 PM.
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post #4 of 8 (permalink) Old 04-23-2019, 09:35 PM
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Quote:
Originally Posted by Grimbrand View Post
C6 is a much heavier transmission, and while functionally identical to the C4, it steals a lot more power. It's a lousy choice for a smallblock. The C4 is ubiquitous. It came in cars, trucks, vans - anything with a 6 banger all the way to the 351, from mid 60s all the way into the 80s.

Any salvage yard that has older cars will have a few. It would be an excellent choice, even if it needs a rebuild.

If you don't care as much about originality, and want to upgrade the rear gears too, you could go to an AOD - or better yet, a 4R70W. There will be some other considerations with these 4 speed overdrive transmissions. They both require a different transmission mount from the C4, and they may require a different length of driveshaft. The 4R70W is better in most every way, with a lower 1st and 2nd gear, and better durability. It does require an electronic controller which will set you back about $800 or so. It's an updated version of the AOD, and features a lot of upgrades that affect reliability and shift quality.

The C4 is a fantastic transmission. It's tough, simple, lightweight, with good shift quality and it's very easy to rebuild. But in the end, it's just a 3 speed.

If you're interested in maintaining originality, or just want to keep things simple and easy, take the time to find a C4. They're around if you look.

If you're more interested in hot performance while maintaining economy, and don't mind spending a lot more, stepping up to the 4R70W, and enjoy better mileage along with hotter performance. Running 3:50-4:10 rear gears with an overdrive transmission is pretty sweet. Running an overdrive transmission with 2.79 gears really lugs things when you're in 4th.

Best wishes!

^^ This


Other than expense (not taken lightly), there are aren't any downsides to the 4R70W. It makes the car more pleasant to spend time in, no matter the driving situation. Much better acceleration and much better fuel economy, while gaining the ability to cruise at the modern speed of traffic without over taxing the engine. Worthy of consideration.
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post #5 of 8 (permalink) Old 04-23-2019, 09:57 PM
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No reason to ever do this.

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post #6 of 8 (permalink) Old 05-16-2019, 06:41 AM
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Welcome to the forum, Post your location. Dependent on your time frame, I have a C4 that is currently being rebuilt that 'may' not be used. I am sure I can part with it for less than the price you have been quoted, if it's decided to not be used.

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post #7 of 8 (permalink) Old 05-16-2019, 09:06 AM
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I don't understand your response

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Originally Posted by jdub View Post
No reason to ever do this.
No reason to do what? If you're referring to replacing the C4 with a C6, I agree, for the reasons already cited.

If you're referring to replacing the C4 with an overdrive transmission, then there is a really good reason to do this: if you've changed the rear end gearing from 2.79 to something higher, say 3.50. Otherwise at normal highway speeds the engine rpms will be much higher.
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post #8 of 8 (permalink) Old 05-16-2019, 01:15 PM
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I held on to a c-6 I had that was bolted to a 351CJ, thinking I could get big bucks for it. The longer I held it the less it was worth, finally sold it to a trans rebuilt guy so he would have one in stock.
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