What is everyone running in your K20s?

[RaptorSentry]

Currently running a 350 with the TH350 transmission, wondering what everyone is running.

I would like to improve fuel economy and ease of use by either modifying the original engine/transmission or replacing the engine/transmission. Not looking to tow, but still be able to haul a load of gravel in the bed with ease. Here are a couple of options I am looking at and would like some feedback:

1. Keep the 350 and install fuel injection kit, such as the Edelbrock Pro-Flo 4 EFI Kit. Then add a gear vendor Under/Overdrive.

2. Do the same as #1 with the engine and replace the transmission with a 200-4R, 700R4/4L60, 4L60/65-E, 4L80/85-E, 6L80/90-E, or the 8L90.

3. Go with a 6.2L Banks Turbo Diesel with any of the above transmissions.

4. Go with the 6.5L Turbo Diesel with any of the above transmissions.

5. Finally, this is Chevy Performance recommendations, is to go with the LS3 with 4L70E.

I am thinking the 6.2L Banks Turbo Diesel with the 700R4, would give me the best fuel economy (Annual Fuel cost) and still have enough power to haul a bed full of gravel.

Any thoughts?

[Fbks_Cruiser]

Is your current engine/trans worn or tired? Looking for more power than what you have now? Or are you just looking for a mpg improvement? If the answer to the first two questions is no, then it is time to get out the calculator. It will cost you thousands of dollars to do the changeover you are thinking about. How many miles do you drive in a year? How many years will it take just to break even?

I have the 350 with edelbrock 1406 and HEI , TH350, NP205 4:11 gears, 235/85R16 tires and average about 12mpg city/hi-way combined. In the winter with 4wd engaged a lot and a bit of warmup in the mornings it gets about 10 mpg. At work we have a 2017 4wd 2500. It only gets 14 mpg, the diesel is only rated to get 16 combined. It would take a really long time to recoup the investment for a gain of only 3-4 mpg at best unless you drive A LOT.

[RaptorSentry]

The engine just came out of rebuild, however the transmission has not been touched. Long story for a different time why I rebuilt it vs. making this decision to entertain an engine and transmission swap earlier.

Biggest reason I looking at this is currently the machinist who rebuilt the engine wants me to run non-ethanol in the engine, which is costing me $5/gal and getting 10MPG. I'd rather be running 87 octane or diesel as they are more readily available where the truck is. Along with that I want to move away from the carburetor to allow for easier starts, drive ability, and don't have to worry about tuning the carburetor when I through different altitudes bands.

I'm looking at doing a frame off resto-mod in the "near" future to deal with body issues and freshen up the truck, so when I hand it down to my boys I know they don't have to worry about it. Hence the reason I am asking now, so when it comes time to do the resto-mod, if I think it would be a smart thing to do I can change the engine and transmission at that point.

[special-K]

These trucks were designed to haul. You're good to go. I run the same thing you have in mine. I've had it since '01 and it never complained about towing or hauling anything. It gets the same mileage as my '95 K2500 Suburban with 350/TBI, 4L80E, same ratio, same tire size, and better aerodynamics. I run ethanol in both. I rebuilt the engine in the '72 in '91 when it was in another K.20 for 10 years.

[RaptorSentry]

Special-K so you are saying the overdrive in the Suburban did not have an increase in fuel economy compared to your '72? Did it at least decrease the RPM while at highway speeds and there by decrease wear & tear on the engine & transmission?

[special-K]

Yes. They both have 4.10s. I drive the '72 at 60 mph on the big hiways, or try to keep it there. The Suburban I try to keep at 65 mph. I don't know too many people who don't make up for the reduced RPMs of overdrive by driving faster. I drive mixed road types. My runs up or down the interstate are usually about 20 miles +/-. I'm mostly on 2-lanes, some country roads (40-50 depending on road/location) and some state hwys where I drive 60 max, usually 50 or 55 speed limit. If I drive the Suburban at 70 or better mileage drops 2-3 mpg. Light foot on the go pedal is your best bet for less fuel consumption. I try to not let the overall high tension of my surroundings effect me. They want to go faster? Have at it. "This right here is my speed. Don't like it? You shoulda left earlier, you'd be ahead of me". Ride my tail, I slow down.

[Ol Blue K20]

I'm running a 396 bored .030 over. Basically a 402 and turbo 350 and 4.10 gears. I'm averaging between 10 an10 to 13 mpg depending on what I'm doing.

[HO455]

What exactly are you wanting to know? If you want to run what the majority of the members running then you will be running a carbureted 350. But it seems you have a bias against carburetors so that is out. My 2wd Burban with it's Q-jeted 350 and T400 gets 11.7 around town and a best of 14.9 on the highway with 3.73's
You didn't mention what model truck you are looking to change up?
The subject of what engine combination is a varied as the colors of our trucks. I-6, V-6, V8, gasoline, diesel, propane, Chevy, Cummins, Cadillac, etc they all have proponents and haters. And their owners all have valid reasons for their opinions.
It sounds as if your current engine has too much compression to run on pump regular. I'm not sure as to why the no ethanol restriction from your engine builder.
If MPG is the only concern then a late model diesel engine and transmission swap while costing more will net the largest gains. Using the engine transmission package with the factory controllers is where these gains are realized. That's why the 700r4 with its primitive electronics was discontinued. It just isn't that efficient of a transmission.

[special-K]

I was thinking about this. Is ethanol even a greater issue on the island out in the Pacific? I know the marinas on the Chesapeake Bay sell non-ethanol. Out on the road all you find is ethanol. I have no choice.

I was also thinking too bad you didn't build a 307. Those are much easier on gas and do fine in K/20s with no taller gearing than 4.10

Did your machinist recommend non-ethanol for a carburetor and you want FI so you can run ethanol with less worries?

[68 TT]

Originally Posted by HO455

...If MPG is the only concern then a late model diesel engine and transmission swap while costing more will net the largest gains. Using the engine transmission package with the factory controllers is where these gains are realized. That's why the 700r4 with its primitive electronics was discontinued. It just isn't that efficient of a transmission.

The 700R4 also isn't up to the task of living in a 3/4-ton truck hauling a load of gravel. GM knew that and always put a T-400 or 4L80E into them when a 700R4 was in the 1/2-ton version of the same rig.

I have broken enough of them in 3rd & 4th gen F-bodies to know their weak points personally. If you want to run one in a truck that gets used hauling heavy loads and survive for long periods of time it can be done but it isn't going to be cheap. In the end you are still pushing many of the internal parts to their breaking point. They just weren't designed to handle that much load.

You are better off going with the 4L80E to start with. In stock form with just typical high performance rebuild kit parts and a shift kit they are up to pretty much anything a street truck will ever throw at it.

Another option is to convert it to a manual transmission and run the NV4500 in there. Less expensive than either auto trans option if you can find one complete at the wrecking yard and grab the whole bell housing / trans assembly. Manual trans pedal assemblies and tunnels are plentiful for our trucks so it won't be a problem finding those pieces. I had one in my 95 C3500 and it was a great shifting trans with a nice overdrive 5th gear and the granny low for when you really loaded it down.

[special-K]

This all goes back to what do you want to spend money on, gas over the long run or a whole lot all at once to do these changes to save a little on gas? You have to figure where you'll break even and start saving with the latter. You can also just drive with a lighter foot for free starting right now

When you get a minute let us know what you think so far

[57taskforce]

You also need to remember that putting your 205 behind any of those transmissions is going to cost you. You can’t run the late model cases because the front axle drop is on the wrong side. Honestly if it was me I’d keep what you have. I looked extensively at putting a 6l90 behind my 6.0 in my ls swap, ultimately the 6l90 is going to go in my Camaro when it gets boost. My K20 is keeping the sm465 it was born with because I couldn’t justify the cost to make the 90 work. The 4l80 really isn’t the cheapest option either especially if you don’t have a th400 np205. A non th400 np205 requires Machine work to accept the bigger input bearing that the 400/4l80 requires. With the 4l80e you’ve also got to worry about adding tone rings and speed sensors in the rear output of the np205 so the tranny will shift correctly.