Question about C3 upgrade
Thread Starter
4th Gear
Joined: Sep 2024
Posts: 4
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Question about C3 upgrade
Hello so I have a 76 C3 and my transmission is slipping. Should I rebuild the L48 transmission or upgrade it to a TH400 turbo instead? Or what would be the best upgrade option if not the TH400? I have a 350 engine in it at the moment. Thank you in advance for your advice and feedback.
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
th350 is plenty strong for a stock L48 engine.
it can be improved by "kits" of parts and a good trans guy.
anyway, an overdrive trans is the best option.
2004R is pretty much bolt in.
you get a peppier 1st gear and overdrive for freeway
it can be improved by "kits" of parts and a good trans guy.
anyway, an overdrive trans is the best option.
2004R is pretty much bolt in.
you get a peppier 1st gear and overdrive for freeway
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
The th350 is a great transmission and uses less power to move the car that the th400 does.
My th350 has a shift kit in it and a 3000 rpm stall behind a 440 hp engine and has never let me down.
My th350 has a shift kit in it and a 3000 rpm stall behind a 440 hp engine and has never let me down.
Le Mans Master
Joined: Jan 2001
Posts: 6,684
Likes: 1,364
David76vet
If I was going to spend money on a tranny I'd go to a 200-4R and get some overdrive and a better first gear (as mentioned above).
It will be a lot more funner and since you're going to spend the money anyway might as well get some improvement out of it.
If I was going to spend money on a tranny I'd go to a 200-4R and get some overdrive and a better first gear (as mentioned above).
It will be a lot more funner and since you're going to spend the money anyway might as well get some improvement out of it.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
A 4th gear would be handy if I drove it long distances. But I don't and I like the available RPMs when I stomp on it to accelerate. So with the 3.55 rear diff the TH350 does well. With the 3.08 not as good, but still ok.
The high stall torque converter is a game changer though for launching from low speeds or a stop.
Pro
Joined: Aug 2000
Posts: 610
Likes: 167
From: Vancouver B.C.
My 72 is a 4 speed manual car, but from what I know the TH400 is a longer transmission than the TH350. You may have to relocate the crossmember and get a shorter driveshaft. Someone else will know this answer, was the TH400 crossmember in a different location for the TH400 vs Muncie/TH350, or was a different crossmember used to accommodate the different length? As others have mentioned, if staying with an automatic, I’d upgrade to a 4 speed auto. - 200r4 direct swap.
Melting Slicks
Joined: Aug 2016
Posts: 2,337
Likes: 1,166
From: CLEVELAND STUCK IN OHIO
2024 C3 of the Year Finalist- Modified
If you're staying with automatic transmission the 200R4 is the easiest swap and can handle anything your L48 can give it.
Its got a 1:82 first gear, and a .67 overdrive forth gear. Match up your preferred RPM stall converter and away you go...
Its got a 1:82 first gear, and a .67 overdrive forth gear. Match up your preferred RPM stall converter and away you go...
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Le Mans Master
Joined: Jan 2001
Posts: 6,684
Likes: 1,364
David76vet
C3s used the same crossmember location for M-21s, THM-350s, & THM-400s. The overall lengths of the trannies are about the same. The M-21 and the Turbo-350 just used a forward facing bracket to hold up the *** end because of where the rear tranny support is for the Manuals & the Turbo-350. The 400 sat directly on the crossmember and the 200-4R also sits directly on the crossmember.
Reportedly the 76 came with either a Turbo-350 or a Turbo 400. You can use that same 350 U-joint with the 200-4R....... or if it was a Turbo-400 you can use a conversion U-joint since the 200-4r will have Turbo-350 sized output yoke. No biggie either way.
Your L48 should have come with the Turbo-350 so you should be able to use your output yoke and u-joint with the 200-4R since the driveshaft is in the sameplace.
If there is one thing I would recommend, since you already have a removable crossmember, is to replace that with one of those crossmembers where the exhaust passes under..... you're exhaust will be straighter and a lot easier to R&R if necessary.
C3s used the same crossmember location for M-21s, THM-350s, & THM-400s. The overall lengths of the trannies are about the same. The M-21 and the Turbo-350 just used a forward facing bracket to hold up the *** end because of where the rear tranny support is for the Manuals & the Turbo-350. The 400 sat directly on the crossmember and the 200-4R also sits directly on the crossmember.
Reportedly the 76 came with either a Turbo-350 or a Turbo 400. You can use that same 350 U-joint with the 200-4R....... or if it was a Turbo-400 you can use a conversion U-joint since the 200-4r will have Turbo-350 sized output yoke. No biggie either way.
Your L48 should have come with the Turbo-350 so you should be able to use your output yoke and u-joint with the 200-4R since the driveshaft is in the sameplace.
If there is one thing I would recommend, since you already have a removable crossmember, is to replace that with one of those crossmembers where the exhaust passes under..... you're exhaust will be straighter and a lot easier to R&R if necessary.
My 72 is a 4 speed manual car, but from what I know the TH400 is a longer transmission than the TH350. You may have to relocate the crossmember and get a shorter driveshaft. Someone else will know this answer, was the TH400 crossmember in a different location for the TH400 vs Muncie/TH350, or was a different crossmember used to accommodate the different length? As others have mentioned, if staying with an automatic, I’d upgrade to a 4 speed auto. - 200r4 direct swap.
Last edited by carriljc; Oct 25, 2024 at 06:58 PM.