How To Swap Buick V6 3.8 to SBC 350

1986b2000

Not-quite-so-new-guy
Jul 8, 2015
10
1
So with the 350 swap does anyone suggest a specific transmission that bolts right up to existing v6 mounts and used stock driveshaft?
 

Garrett1982

G-Body Guru
May 18, 2014
582
28
Uniontown, Pa
So with the 350 swap does anyone suggest a specific transmission that bolts right up to existing v6 mounts and used stock driveshaft?
Th350 mounts to crossmember in the same spot as the th200 but looks to be slightly longer overall depending on tailshaft length. The 200 4r is the same length as the th200 but has a different mounting location. You would need a different crossmember. Jeg's sells double hump crossmembers for around $125.
image.jpeg
 

Clone TIE Pilot

Comic Book Super Hero
Aug 14, 2011
3,309
113
Galaxy far far away
I did this sawp 9 years ago, the thing is to have a complete donor car. I was lucky to get a good deal on a complete powertrain out of a MCSS for my Regal swap, including complete computer harness, ECM, th2004R, belt drives, emission parts, and trans crossmember, it was everything. Even after doing a engine swap, the 2.41 gearing in the rear axle is still a major handicap as just about every base car with the 3.8 came with that rear. Again I got lucky and got a good deal on a complete posi rearend with 3.73 ratio out of another MCSS a year later.

It was easy to swap out the v6 computer harness for the v8 computer harness which kept the car legal and maintained lockup, the CCC also gas washes the cylinders less than a normal carb because it has finer fuel control.

The handling with a v8 over v6 springs is going to be poorer than stock and expect to do a v8 front spring upgrade soon. Also be a good time to install new upper controls arms designed for tall ball joints to fix the G body front geometry issues so you don't have to use as harsh of a spring rate up front.

The th200c and th2004r use the same length driveshaft but different trans crossmembers. For some reason Turbo Buicks and MCSSs use different style crossmembers and trans mounts despite using th2004rs. MCSS use a on center mount crossmember with a centered trans mount and the ends bolt directly to the frame. Turbo Buicks use a off center trans mount and the ends of the crossmember use rubber mounting bushings on the frame.
 
Last edited:

Austin F

n00b
Jul 1, 2016
1
1
Is there a simple way to rewire the stock olds 3.8 “choke circuit” for a Holley choke, or would it be easier to run my own circuit?
 

84 caballero

Not-quite-so-new-guy
Jul 1, 2020
6
1
Hello I have a 84 gmc caballero and I took the 3.8 v6 out and I’m going with a sbc 400! I can’t find the bulkhead engine harness no we’re to replace with new one!! My old one has been cut!! Can someone help me out please cause I’m stuck!! I’ was told that part changes do to me going to a v8!!! Help please Thxs
 

Attachments

  • 2E45957E-9599-4F4F-941A-4C94F5052CEB.jpeg
    2E45957E-9599-4F4F-941A-4C94F5052CEB.jpeg
    1.4 MB · Views: 240
  • DC7C7295-BE8B-4E71-831E-C5B331A8BBEB.jpeg
    DC7C7295-BE8B-4E71-831E-C5B331A8BBEB.jpeg
    1.1 MB · Views: 248

Chrisk81

n00b
Jan 25, 2022
3
3
This is a "How To" Post to swap the following engine in a G-Body:

From: Buick V6 - 3.8L (231) and 4.1L (252)
To: Small Block Chevy V8 (305, 350, 400, and others)


Disclaimer:
Parts and "How To" are listed in each section. I consider this a continous working and living post. This is by no means a final revision and other topics related to engine swaps will be added. While topics as rear end ratios and transmission are somewhat related, specific details of those topics are beyond the scope of this post and deserve their own unique topic.


Feel free to reply or send a private message in regards to the specific engine swap mentioned. There is more that needs added and if you have specific knowledge, feel free to share.

Suspension:
The Front Coil Springs need replaced with ones used in any V8 G-Body. This assumes you won't be taking out any weight (such as A/C- Heat removal, installation of fiberglass/aluminum body panels, etc) Ones from a Monte Carlo SS are a little stiffer than other V8 G-Bodies.

While you're at it: If you still have the original suspension bushings, ball joints, tie rods, etc, now is a great time to replace all this stuff. Since you already need to change the front springs AND with the engine out, it is much easier to do this work. Also makes it easier to change out the motor mounts.

Mounts:
You will need the proper G-Body Specific Chevy V6/V8 Mounts. When properly installed for a V8, one of the motor mount holes will hang off the frame. Only 3 of the 4 holes on the motor mount are used.

Passenger side engine cradle, viewed from front, with and without the mount installed. Motor mount holes are marked. The top holes on the mount are spaced 2-5/16" centers and the bottom hole are 1-3/4" centers (only one bottom hole is used).

motor_mount_holes.jpg
Motor_Mount_1_resized.JPG



While you're at it: If you bought motor mounts form a junkyard and have the old rubber bushings, drill open the old mounts and replace with new Energy Suspension (or your favorite equivalent) polyurethane inserts. Chances are the 20+ year old rubber is well worn. Instructions are included with the new bushings

Transmission and Cross-Member:
Most V6 cars got a THM200C (3 speed). These make a great door stop or practice for learning to rebuild a transmission (like I did). It most likely won't withstand any power from a V8. Besides, it has the wrong bolt pattern. You will need a specific SBC bolt pattern OR a dual bolt pattern transmission. A THM350 (3 spd) with Chevy Bolt Pattern will work with the THM200C cross-member.

If using the THM200-4R (4-Speed overdrive), you will need the specific cross-member. Any G-Body built before 1984 will need a mounting point for the TH200-4R cross-member added to the frame by welding in a piece of steel. It will not bolt in otherwise.

Along with rear differential, the transmission you chose depends on budget and goals and therefore further discussion goes beyond the scope of an engine swap.

Electrical:
ALL G-bodies have a wire harness that mounts to a bulkhead on the driver's side, right under the wiper motor. This does NOT include the ECM wire harness on 81 and later gas powered G-bodies. The harness has 2 pieces. The left half (viewed from the front of the car) is for the engine. The right half (which can be pried out, carefully) is for lighting and horns. The picture below shows the ENGINE harness with the lighting/horn harness removed. The black plastic wire loom is removed for clarity.
normal_engine_harness.jpg


LEGEND:

PURPLE - Starter Solenoid (has a ring terminal at the end)
PINK - Distributor (most likely a slip on terminal)
RED (2) - Starter at the battery terminal. Have larger ring terminals with fuse link. Has a splice for the HVAC system.
GREEN - Temperature sending unit
BROWN - Alternator sending unit

The following three work together in one circuit and depends on weather you have guages or idiot lights

LT BLUE - Choke heater
LT BROWN - Oil Pressure Light
BROWN/WHITE Oil Pressure Light


Fuel Line:
You will need to swap the fuel line so that it routes to the passenger side. Given the car's age, now would be the time to run new fuel line from the tank to the engine. If keeping the emissions controls, you will need at least 2 lines, one for the fuel and the other for the charcoal canister. If using a fuel pump with a return OR if you decide on fuel injection, a third line will be needed for a return.

Cooling:
You will also need the radiator, fan shroud, fan, hoses for a V8 G-body. The radiator for a V6 will not be sufficient.

Also the proper water pump is needed for the Chevy V8. Be sure to use the proper water pump for the belt drive system you decide to use (serpentine or v-belts). The 2 work in opposite directions.

Revs
REV A - 09/11/2007 - First Release
REV A1 - 09/12/2007 - Text font and color changes
REV B - 9/14/2007 - Combined Transmission and Cross-member. Added notes about pre-84 frame for TH2004R. Added Fuel Line and Cooling sections.

Giving Credit where due:
85 Cutlass Brougham - info on pre-84 frame for transmission cross-member and fuel lines.
THANK YOU!!! Swapping a 350 into a 85 ELCO and this thread has so much i am wondering about.
 
  • Like
Reactions: 1 users

MrSony

Geezer
Nov 15, 2014
6,434
113
Des Moines, Iowa
THANK YOU!!! Swapping a 350 into a 85 ELCO and this thread has so much i am wondering about.
if you have a 229 v6 (distributor in rear) its v6 out v8 in. the 231/252 (distributor in front) just get everything from a v8 car.
 

Chrisk81

n00b
Jan 25, 2022
3
3
if you have a 229 v6 (distributor in rear) its v6 out v8 in. the 231/252 (distributor in front) just get everything from a v8 car.
Ok thanks. What i pulled was indeed a DIST in rear setup.
 

Attachments

  • MWVSE2586[1].JPG
    MWVSE2586[1].JPG
    2.8 MB · Views: 45

MrSony

Geezer
Nov 15, 2014
6,434
113
Des Moines, Iowa
Ok thanks. What i pulled was indeed a DIST in rear setup.
you will pretty much need just a v8 size radiator and fanshroud then. most if not all wiring, starter, front pulleys etc should swap over. the transmission as well.
 

GBodyForum is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are trademarks of Amazon.com, Inc. or its affiliates.

Please support GBodyForum Sponsors

Classic Truck Consoles Dixie Restoration Depot Mike's Montes UMI Performance

Contact Admin@GBodyForum.com for info on becoming a sponsor