Transmission Replacement

[ssn696]

Actually, the 200-4R has the same 1st gear ratio as a TH350. So, right away, you don’t need to change the rear ratio. But, it would make the acceleration snapper with the little 3.8, with little penalty on the highway because of the overdrive. Budget $1500/$2000 for a mild rebuild. More if you plan on 300+ HP. And you’ll need to arrange for locking the torque converter without a computer. There’s a link on my ‘About’ page for that.

 

[ssn696]

If you will keep the 350 mild, you might ignore the naysayers and consider a 700-R4 (4L60). GM put these in trucks and in the fatsos like the Caprice as a compromise - steep 3.06 first gear gets the lard moving with highway-friendly overdrive. This trans is Chevy-only block pattern, but there are adapter plates. Unless you’re going to hammer on it, this trans might let you skip the gear change. Still needs a lockup kit.

 

[Clone TIE Pilot]

To swap in a 200 4R or a 700 4R, you will need the correct trans crossmembers, either new aftermarket or used factory. One big issue is that pre 1984 G bodies do not have the correct crossmember mounts on the frame for later OD transmissions. You will need to weld on the correct crossmember mounts onto the frame.

Pre 84 short frame mount.

Post 84 extended frame mount.

Basically, the post 84s are a little more boxed from the factory.

 

[64nailhead]

No one here has pointed you in the wrong direction, really. Either a 700-R4 or 200-4R will work just fine. I'd recommend to you to come up with a total plan before you start buying parts that might not suit your end goal of cruise and occasionally run down the strip.

1st - determine the size wheel and tire you will run - you cannot choose a correct rear gear ratio if you change your mind from running a 25" tire to a 28" tire. 2nd - you mentioned a 350 install- mild 300-350HP? Need to know this for transmission.

 

[MrSony]

With a 200r4 a 3.42 rear gear (standard grand national setup) and a standard 235/60/15 tire size, you'd be at about 2100rpm give or take on the highway at 70mph. Upgrading the rear gears and adding transmission gears go hand in hand. you could even go to a 4.10 and make that 3.8 accelerate like a 305 lol.

 

[olds307 and 403]

The 3.42 gear is good improvement and all around gear. With lock up, you will run the same rpm as your current rear gears, probably 2.41. The 2004R, other than crossmember position is a direct swap for a short tail TH250/350. You probably have a TH250, which isn't worth building. Look for a stud by the cooler lines on the trans case. The 700R4 also will need the extensions and the drive shaft shortened. It has a very deep first gear. The 2004R has better gearing, including first than your current or the 700R4 trans. The 700R4 trans is much easier to find and build.

 

[ssn696]

To swap in a 200 4R or a 700 4R, you will need the correct trans crossmembers, either new aftermarket or used factory. One big issue is that pre 1984 G bodies do not have the correct crossmember mounts on the frame for later OD transmissions. You will need to weld on the correct crossmember mounts onto the frame.

Pre 84 short frame mount.

Post 84 extended frame mount.

Basically, the post 84s are a little more boxed from the factory.

Frame pad extensions are available.

 

[DRIVEN]

3.42 or 3.73 is a good all around match with a 200-4r. Do some math or use a gear calculator to see what fits what you want to do with your car. For example; my '79 Cutlass was a 305/TH200/2.29 combo. By going 200-4r and 3.42 I was able to have the exact same freeway cruise rpm but much better acceleration. It felt like a 75hp bolt-on but still gets 20mpg on the open road. All these years later I still have the 305 because it went from miserably gutless to tolerable. All due to gearing and leverage.

 

[Gonzo1970]

305, 200-4R, 3.73's.. Thats where I'm goin...

-Gonz

 

[Texas82GP]

I have a 4L60 behind my 3.8.

Does a Series II 3.8 have a BOP bellhousing bolt pattern?