V8 (350CI) to V6 (4.3L) Swap - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-05-2012, 06:02 PM
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V8 (350CI) to V6 (4.3L) Swap

Hi All ~
I have a '97 4.3L V6 (400K miles but smooth as a sewing machine) that I'd like to drop into a great '91 C1500 body that now houses a 350CI V8. The body on my '97 C1500 donor is crap and the V6 gets 24MPG (yes... really, all day, everyday)... thus the "upgrade". I've heard the 350CI is about the same HP (~ 200+) but mileage is more like 14MPG for same body.

The V6 has the 4L60E (electronic version of the, superb, TH700R) trans.
Feels like this should be drop-in swap. Has anyone done similar conversion and what "challenges" should I be aware of?

Will also upgrade the V6 to full headers and stainless (Borla?) exhaust. No emissions inspections in MA for vehicles older than 13 yrs... so I'm good there (maybe dump the cat?).

Thanks
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Old 04-06-2012, 05:24 AM
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Sorry I can't be of better news but if your 4.3 has that many miles on it you may as well rebuild it while it's out. Even so it may not use oil and it may not smoke it's a very safe bet it's wore out. When I got my truck the 4.3 had 260,000 miles on it. The former owner said it had been overhualed. When I tore it down I found all GM parts which told me it had never been rebuilt.
This engine didn't use oil and it didn't smoke. When I took the block to the machine shop and the machinist mic'd the bores I'm yet to figure out how it even ran. Stock bore size is 4 inches, the number 1 and 6 cylinders came out at 4.011, Cylinders 2 and 3 came out at 4.009 and cylinders 4 and 5 came out at 4.007. However, the bearings still looked okay, but they also had obvious wear. Next is you have to tell me whats the secret of getting 24 mpg. My father has a 1998 GMC Sierra, on it's best day it might get 15 to 16 mpg. It has 200,000 miles on it, the exhaust has been opened up with a high flow cat and turbo muffler, the restrictor removed from the throttle body, a new timing chain installed, spiral core spark plug wires, K and N air filter. All synthetic lubes, 3.42 rear gears and a 4L60E trans. My truck, which is a 1992 short wheel base sport side standard cab has the 4.3 with TBI. I bumped up the compression ratio, gave it a tight quench, new injectors, duel exhaust, a isky roller cam with RV specs for mileage and torque. Edelbrock intake, MSD ignition components, roller rockers, new ferrea valves, balanced rotating assembly, all synthetic lubes, 3.42 gears, 5 speed manual trans and on it's best day it gets 18 to 19 mpg. However, the swap isn't very hard, but to use the injection on the 97 engine you'll need the fuel pump, computer and the wiring harness from the 97 to the 91 and that is a lot of work. Sorry to say it's not a straight forward swap. If the truck already has a 350 I would stick with it and add better parts to get the mileage and power up. The 350 is capable of knocking off some pretty impressive mileage numbers when the right combo of parts are used.
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Old 04-06-2012, 06:49 AM
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Quote:
Originally Posted by Vortec
Hi All ~
I have a '97 4.3L V6 (400K miles but smooth as a sewing machine) that I'd like to drop into a great '91 C1500 body that now houses a 350CI V8. The body on my '97 C1500 donor is crap and the V6 gets 24MPG (yes... really, all day, everyday)... thus the "upgrade". I've heard the 350CI is about the same HP (~ 200+) but mileage is more like 14MPG for same body.

The V6 has the 4L60E (electronic version of the, superb, TH700R) trans.
Feels like this should be drop-in swap. Has anyone done similar conversion and what "challenges" should I be aware of?

Will also upgrade the V6 to full headers and stainless (Borla?) exhaust. No emissions inspections in MA for vehicles older than 13 yrs... so I'm good there (maybe dump the cat?).

Thanks
Unless you drive on a downhill slope at 50mph you are NOT getting 24MPG. I have owned a couple 4.3L engines and they are no better than the 350 for mileage.
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Old 04-06-2012, 08:14 AM
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ive had 3 different 4.3's and all got 14mpg. but when i swapped in a 350 i got 20mpg and that was with a carb. and they were s10's with 4.3's getting 14 mpg, i couldnt imagine how bad it would be in a c1500
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Old 04-06-2012, 02:21 PM
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Not sure what went wrong for you guys, but we had a safari van, 25 mpg all day, 4.3 v6, 4l60e.
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Old 04-08-2012, 12:50 PM
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To all you guys getting 14-16MPH from a 4.3L V6, check your plugs... you're only running on 4 cylinders.

I monitor my C1500 regularly (not religiously) and continue to get 24MPG daily roundtrip to Boston from south of Plymouth, MA (50 miles each way). And this from an engine with 380,000+ miles on it @ highway speed (and if you've ever driven in or near Boston, you know what that means).

Seriously... fix your vehicles!

I appreciate the indications of what can be had from a 350CI so maybe I'll keep what's in the newly purchased truck and see what it can do. Especially since it sounds like a good deal of work to swap it for the V6.
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