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Greetings. I have a 1966 couple, C code, that I purchased a couple years ago.

Looked long and hard before finding the right Mustang, and I’ve put a decent amount of time working on the vehicle myself as well as a few dollars with a few trusted shops.

Most recently I had a new motor put in. The general specs are:
289 block stroked to a 331
Forged eagle crank and rods
Mahle lightweight pistons
Custom Bullet hydraulic roller cam

I’m still breaking the engine in, and ha e the original C4 transmission in place. There is some leakage with the transmission and while I’m fairly certain a C4 should be a solid choice for the car, I’m curious about a few things:
1. She runs much more smoothly when I drop onto low, shifting the automatic up as I gain speed. No idea why, hoping someone can tell me
2. I’d like to just drive her a bit, and am looking for a “quick fix” for the leakage. It’s up near the front of the pan. While I’m mindful of treating the car right, I am hoping there is a stop leak hack suggestion out there.
3. Finally, in the near future I want to complete the drivability part of the build. I redid the suspension and the car sits low, handles well. I like to drive and want to put a manual transmission in. Any suggestions on a solid manual to put in, as well as estimated cost to do so? I do what I can myself on the car, but the transmission is beyond me.

Thanks!
 

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Don't know what rear gear you're running or how you plan to drive the car, but for the street and boulevard cruising, the C4 is great. You can shift it manually, especially when built well with a 67-up valve body they can live a long time. I have three. For a period correct manual and no overdrive, a Top Loader 4 speed is a direct swap, but many opt for a T-5 or for extra durability in high hp setups, a World Class T-5 from a Cobra etc.
 

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T5 is a good trans (the world class version), depending on your diff gear ratio. Top loader is cool too. It’ll probably come down to preference. They’re are a ton on this forum who are Top Loader-or-die types, with just as many of Overdrive-or-no-drive types too. T5 or top loader go right it, and work with either hydraulic, cable, or z-bar actuation, which will also be a matter of preference.

I have a cable actuated T5, with a 3.70 rear gear. All behind a decent 347. I like my cable set up, but if I had to do it again, I’d probably go hydraulic. Cables can be a pain with some headers.

If you’re handy with tools, I think you could manage the swap, unless you have a bad back/neck/shoulders, then I would have a shop do it.

Good luck.


Sent from the interwebs
 

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If you decide to go with a T5 be sure and get the "z" model as in T5z. The z has a 2.95 1st gear ratio vs. 3.35 in a non-z T5.
When you add a lower rear axle gear ratio such as 3.55 to take advantage of the benefits of the T5 swap a 3.35 1st gear becomes almost useless and you will be shifting into 2nd almost as soon as the car is rolling.
And don't accept the standard .63 overdrive ratio either. There is a huge RPM drop when you shift from 4th to 5th with .63 OD. I had my T5z built with the .80 OD and combined with my 3.73 rear end it is perfect.
 

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One thing to note on your OEM 1965 C4...it has 2 automatic settings. If you want to use all 3 gears, select the green dot. If you select the first detent past neutral, it only shifts between 2nd and 3rd gear. Think of this as a very early "economy mode".

Other thing...when I put a stroker in front of my OEM C4...it lasted about 2 weeks before it started slipping.

C4's can handle a lot of power if built right. However, 54 year old stock c4's don't handle an extra 150 HP well in my experience.

Agree with the above on the T5. All in if you have someone do it for you...I think you'll be looking at 3 or 4 grand? Just a guess.

Phil
 

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First off. Welcome to the forum! Your C4 is an excellent transmission & can be built to handle anything a 331 can throw at it without breaking a sweat for a lot less $ than a 5 speed conversion will cost. If you are considering a rebuild, Red Eagle clutches & bands, Kolene treated steels, a TransGo reprogramming kit & a Sonnax "C" Intermediate Servo Piston. This will wake your transmission up. I dont regret doing mine. My 331 looks very similar to yours also with AFR Heads & with a 2500 stall converter I'm very happy.
 

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Discussion Starter #7
Thanks. I had been driving this car for a year without realizing the green dot placement provided a start in first gear.

I am fairly handy working on the car, but the transmission swap would be beyond my skill (and tool) set.

I think I’m going to give her a go in the C4 and see how things hold up. If there’s a problem, I’ll have a fun decision to make.
 

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As for the leak, one sneaky and simple one is the seal for your filler tube. They tend to dribble down the front of the case almost invisibly, to drip off the front/pan area. It's a very easy fix, relatively speaking.

I really love my old C4. It's a very good choice for a car that has a broad-shouldered powerband. For an automatic, they don't come much simpler, lighter, cheaper, or more rugged. Shift quality is also excellent and predictable, once you've done some very basic upgrades. A shift kit and better friction materials are a must, when you pair this transmission with 300+ horsepower, unless you intend to always drive like a grandma. That being said, once done, this transmission will serve well for just about any purpose.
 

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I will second Grimbrand on the filler tube seal. Its an O-ring that over the years of hot-cold-hot-cold hardens & flattens out. A new O-ring smeared with vaseline should see you right.
Cheers. Gaz.
 
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