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Old 10-01-2006, 09:17 PM
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TH350 to TH200r4 Conversion

Hi, newbie here.

I'm researching swapping the TH350 in my 250ci daily driver '74 Nova with a TH200r4. The car has 46K original miles on it.

I don't plan to drag the car but will be looking into some 6=8 upgrades and swapping in a 8.5" 10 bolt 2.73 posi-traction rearend in place of the open 3.08 rear end.

I've read a few articles on line about it and sounds like pretty much a direct swap but wonder if anyone here has done it and how much difficult/unexpected stuff had popped up.

A couple things I wonder about are:
-Can I use the same torque converter?
-Will I have to get a different crossmember?
-Can I use the same column shifter?

Any help or advice is appreciated.

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Old 10-01-2006, 11:45 PM
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Quote:
Originally Posted by Nova250-6
A couple things I wonder about are:
-Can I use the same torque converter?
-Will I have to get a different crossmember?
-Can I use the same column shifter?

Any help or advice is appreciated.

No, you can't use the same converter. Different input shafts and the 200-4r is a lockup converter.

You probably won't have to get a different crossmember. The 4r will bolt to your existing crossmember, but the mount is about 4" further back than it was on the 350. Many GM cars (but I don't think the Xbody) have multiple holes drilled in the frame to accomodate this exact thing. For instance a Monte Carlo might have had a TH350, but an Olds Cutlass might have a 4r. GM simply equipped both cars with extra holes in the frame to make things easier. You might have to get clever depending on what your frame looks like, but its pretty straightforward.

You can use the same column shifter, but of course the letters and numbers won't line up. You also might find that the throw of the shifter isn't far enough to go all the way down to first. I went the opposite way; from a 200C three speed to a 350 and I noticed that it was much shorter between detents so you may have the opposite problem. The solution is pretty simple; just get the whole set of shifter arms and pivots from the 4r from the column down to the tranny. Its a simple matter of length of the lever

A couple other considerations:

-also get the TV cable and bracket from the carb of the donor car. The 350 uses a vacuum signal and no cable. The 4r uses a cable but no vacuum.
-Double check your driveshaft, but you should be able to use your stock driveshaft. The overall length of the two trannys is within like 1/8" so you should be fine, but double check the splines and diameter.
-You'll also need the proper speedo gear.
-You're also headed for a big mismatch with a .67:1 overdrive and 2.73 gears. Your highway RPMs with the overdrive and 2.73 will be 1500 rpms. If you want to drop a little highway RPMs, I would add the 4r tranny and swap to 3.73s... seriously. The final drive will make those 3.73s act like 2.50s and your highway RPMs will be more like 2000. I dealt with a 2.73 rear and a 4r for a couple months before I yanked the rear in favor of 4.10s. At 75 mph it wouldn't stay in OD, it kept hunting in and out because it was lugging.
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Old 10-02-2006, 10:33 PM
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Your Nova has only one set of crossmember holes, but from what I've heard, the Turbo-400 crossmember is the one to use. It places the mount a few inches to the rear, and about 1/2" to the right. You may have to redrill or slot the holes to get it back to the center. You can get one at an early Camaro restoration supply place, and it bolts to the same place your current one does. Aftermarket suppliers (Bow Tie Overdrives, etc.) make crossmembers especially designed for the swap, also.

I'd try it with the 3.08's before swapping gears. My T-bird ('88, 302) came with AOD and 2.73's, and it would run 75mph all day without dropping out of OD except on a serious hill, but by that time I would have shifted it out manually anyway. It'd probably have been just as happy with 3.08's.

Last edited by jimfulco; 10-02-2006 at 10:41 PM.
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Old 10-03-2006, 09:53 PM
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You also need to address the converter lock-up which was computer controlled.

In previous threads, I was advised that all you have to do is run a switched 12v to the upper left plug on the terminal located on the left side of the 200R4. I just did that on my '72 and ran the 12v through at on-off toggle switch on the console. I'm going to run with 3.08 gears. Can't tell you the results because the car isn't on the road yet.

On my '86, I used a TCI kit. I also put the toggle switch in the power line. It helps if the trans starts "hunting" as mentioned above. I'm running a 3.42 gear and a warmed over 406. Mileage still sucks.
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Old 10-03-2006, 10:18 PM
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I recently converted a '64 El Camino from Powerglide to a 200-4r.
I bought the trans,torque converter,crossmember & TV cable setup from Bowtie Overdrives.
The El Camino has 3.08 gears and 26.4" tall rear tires and in overdrive is only running a little over 1600 RPM at 70 MPH.
I feel that this is too low of an engine speed. I think I'm going to change to 3.73 rear end gears soon. This change should bring the RPM @ 70 up to about 2200. I considered 3.55 gears, but decided on 3.73's so I could go up a little on the rear tire diameter without getting back gown too low in RPM @ 70.
BTW. I ended up not using Bowtie Overdrives crossmember, for two reasons.
1. It seemed a little wimpy.
2. It was a little crooked.
I welded a piece of 1/4" angle iron to the frame at the appropriate spot on each side to be able to use the original cross member. Of course, that may not be an option with your Nova, being a Unibody car. The El Camino has a full frame.
JA
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Old 10-04-2006, 03:48 AM
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Thanks guys! A lot to consider that's for sure.

Based on your RPM range inputs, I found this calculator online and compared some of the options between transmissions/rearends.

csgnetwork.com Engine RPM Calculator

Based on the calculations, I'm inclined to stick with the 3.08 rear gear with the OD transmission for now. The 250 inline 6 is a high torque low rpm engine and I might get away using the 3.08 rear gears.

Worst case I'd have to drop it down into a lower 1:1 final gear which is where I'm presently at driving in town and at highway speeds.

Here are a couple of different rpm scenerios for highway and around town I ran for the .63 OD and 1:1 final ratios with the two rearend gearsets.

Again, thanks for the info!


Last edited by Nova250-6; 10-04-2006 at 03:58 AM.
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Old 12-13-2011, 07:06 AM
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Yes, the 2004R is electrically controled lockup (via the PCM) and when doing a changeover you must make sure that lockup DOES work. The OD section of the trans will overheat without it.

as for the numbers and letters not lining up... If memory serves.. TechPak made a pop-in indicator for swaps. Probably still available.


Quote:
Originally Posted by curtis73
No, you can't use the same converter. Different input shafts and the 200-4r is a lockup converter.

You can use the same column shifter, but of course the letters and numbers won't line up. You also might find that the throw of the shifter isn't far enough to go all the way down to first. I went the opposite way; from a 200C three speed to a 350 and I noticed that it was much shorter between detents so you may have the opposite problem. The solution is pretty simple; just get the whole set of shifter arms and pivots from the 4r from the column down to the tranny. Its a simple matter of length of the lever

A couple other considerations:

-also get the TV cable and bracket from the carb of the donor car. The 350 uses a vacuum signal and no cable. The 4r uses a cable but no vacuum.
-Double check your driveshaft, but you should be able to use your stock driveshaft. The overall length of the two trannys is within like 1/8" so you should be fine, but double check the splines and diameter.
-You'll also need the proper speedo gear.
-You're also headed for a big mismatch with a .67:1 overdrive and 2.73 gears. Your highway RPMs with the overdrive and 2.73 will be 1500 rpms. If you want to drop a little highway RPMs, I would add the 4r tranny and swap to 3.73s... seriously. The final drive will make those 3.73s act like 2.50s and your highway RPMs will be more like 2000. I dealt with a 2.73 rear and a 4r for a couple months before I yanked the rear in favor of 4.10s. At 75 mph it wouldn't stay in OD, it kept hunting in and out because it was lugging.
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Old 12-13-2011, 07:26 AM
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