Hello Stangnet!

I've noticed the C4 in my '67 vert is starting to take a while to engage when cold. I realize that this may mean a rebuild to replace all the worn junk inside. Not sure of the history of the tranny, but suspect it has never been rebuilt.

Right now, I have the numbers-matching 289 bored to a 302, but I believe the only upgrade is the cam. Not too familiar with driveline. Sounds beefy, but not much power. Low to mid 200's HP maybe? Very soon, I plan to replace the exhaust manifolds with headers, and put a new kit on the existing engine until i get up to around 350 HP.

My question is, should I spend the money rebuilding the C4 if that's what it needs, or should I just replace it with a C6? I know a threshold exists where I should move up to the heavier duty tranny.

Please advise. Thank you!
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66 Signalflare

Active User
May 24, 2016
Not sure but you can get the rebuild kits on-line from $99 to $300 neighborhood. I bought my Mustang in 1989 and it had the small six with the C4. 24,000 miles on the car. Now I've got 35,000 on the car with a 270HP V8 in front of the C4 vice the small six cylinder.
You guys are a bit over my head, haha. I don't have the knowledge or time to rebuild my c4, so it looks like I'll be taking it to the shop to have them do that, with the understanding that I want it to be able to handle about 400hp in the future.

Any advice on what I should let them charge me? Are there differences between a typical rebuild and a performance rebuild?

Thanks in advance!