I was talking this over with a mechanic friend of mine, since fuel mileage is important to me. He told me that if I intended to keep the old 6.2, or since I'm somewhat of an apocalypse nut an older diesel motor, that I should put an forced induction system on it. The two words that left his mouth actually made me cringe a little. He suggested a twin turbo set up, along with a few other gas saving mods that one can put on a motor but those aren't that big a deal.
In my opinion, anybody telling you to install twin turbos for better fuel mileage has a screw loose in his head.
A turbocharger, better put - and exhaust-driven supercharger . . . was originally introduced to reclaim power lost at high altitudes. Now, it's more often used to make a small engine perform like a bigger one.
Problem with the 6. 2i is -it's a light-duy engine. GM first had it designed to match the power of a 305 gas engine. Thin heads, lightweight block and a cast iron crankshaft.
Why anybody would thing installing twin-turbos would make it more fuel effiicient (and hold up under the boost) is beyond me.
NOTE - Banks did make twin-turbo kits for 6.2s for use on boats when engine life and fuel efficiency was not a concern- just ultimate power. I robbed one of those kits and installed on one of my 6.2s, but left one turbo off.
At sea level, you're not going to see much of a fuel mileage gain with a turbo added. It WILL enable you to turn the pump up, and make more power. Wiith that added fuel delivery, the engine WILL be more efficient, then if you'd done the turn-up with no turbo.
What usually happens when you add a turbo is - you go faster, so it's hard to see overall fuel savings.
6.2 needs to run close to 2000 RPM to make the best fuel mileage. No way around that. Do NOT use big wide tires and get your RPMs down if you want fuel miileage. A beefed up 700R4 or 4L60 works nicely. So can a 4L80 except is needs an expensive controller to install it. If you got a non-lockup TH400, you mgiht get 20 MPG once in awhile with 3.08 axles. If you've got 3.73s or 4.10s - forget it.
If you want manual shift - there are many options. GM sold many 6.2s with four-speed manual overdrive transmissions made by New Process. I have two, and they work real nice. If you' ve got the big HD Munice cast-iron 4 speed - with granny no-synchro low- and not overdrive - you can install a Ranger overdrive unit in front of it. Or insall a later 5 speed.
One of my 6.2 rigs has a small motor-home body on it. It's an 86 K5 Blazer.
When all original - 6.2 non turbo and 3.08 axles and 700R4 trans it got 21 MPG at best.
Same with turbo - it still got a best of 21 MPG but I always climbed hills faster.
When I installed the motorhome body, and weight climbed to 8000 lbs. mileage dropped to 16 MPG.
Trans blew. 700R4 could not handle the turbo power and 3.08 axles. So, I am now in the middle of putting in heavier 3.73 axles and a beefed up 4L60 trans with a custom built 6 lug torque-converter. I am hoping for a slight MPG improvement since the engine won't be lugging so much. Too much RPM s and too low RPMs (with lugging) both hurt fuel mileage.