LS Conversion From 400

[Matthew]

Hi everyone, I'm fairly new here. This time last year, I bought a 1976 Chevrolet K20 Silverado Camper Special. The truck was completely original, with 86,000 miles and a total time capsule. Well, I've had it for a year, and I've decided to repaint the truck (original Yuba Gold of course), and lightly restore it (hardly no bodywork needed, no rust, SoCal desert truck ). As of now, it has the original 400ci small block that is #'s matching with the truck, original auto trans, NP203 transfer case, and 14 bolt rear and Dana 44 front with 4.10 gears. Here's where I get stuck and undecided. I'm wondering whether or not to refresh the 400 and pep it up a bit(new gaskets, heads, camshaft, etc.), along with put in a 700R4 as it has an overdrive... OR... Purchase an LS series engine (looking at 6.2L's), along with transmission. Regardless of the combination, I want to replace the NP203 full time chain driven t-case with an NP205 part time gear driven t-case. The 400 and TH350 are relatively issue-free (they both seep fluids here and there, not bad for 40 years old, but they aren't perfect), and the NP203 makes a rattling noise as if the chain is skipping or out of adjustment(only on part throttle, 1/2 throttle or more stops it). I would love to hear some positives and negatives along with possibly some cost figures (as I'm up for anything reasonable)... Thank you so much!

 

[1987 GMC Jimmy]

Welcome! We'd love to see some pictures of what you have going on. It sounds like a great truck and great powertrain setup. You've got a beefy drivetrain, too. I'd like to disclaim that I'm giving my personal opinions that may or may not conflict with others' opinions, but everyone's opinionated so it happens. First of all, a little bit of it has to do with your plans for the truck. Are you gonna haul, tow, or use it off-road? I totally agree with your position on the NP205. It'll modernize your 4x4 setup and be tough as nails doing it. My position on the motor is rebuild it and spruce it up some. It's low mileage so you may have some time left before it needs something. It's cool that you have a 400. Relatively few people do since they were only made for a few years. It's cheaper, and you can do it yourself more expediently with fewer headaches. There are a lot of great square bodies and older GM cars and trucks in general built with the LS as the powerplant. It's a good motor, but I think you have something neat going on already, and you should use the same mantra for the motor as you are the paint job. You can read on here about the adaptations that are required for an LS and transmission to fit/work correctly, and it's quite a checklist, especially considering your vehicle is fully OBDII when you get done so all the electronics have to be working to get a tuneable motor. @dougbert has a very thorough build thread you can look at. Again, I think you'd come out cheaper and completed sooner with fewer headaches and a strong, dependable motor keeping what you have. Personally, I don't subscribe to the LS craze because it's done a lot, and I don't like to do what other people are doing, not because it's bad but just because I like to keep things as original as they can be practically maintained. Plus, I don't trust the GM (or really any automaker) of the last 20 years so I rebel by supporting their powerplant of yesteryear. The transmission maybe another story. It sounds like you're wanting highway use in hauling and/or towing, and you want to quell the fuel demands of your short gear ratio. I understand that, and the TH400 is a fantastic, ridiculously strong transmission, but it doesn't meet those requirements. As far as a 700R4 goes, I'm thinking no. Maybe a late model one that's rebuilt with a shift kit, but I don't think it's the best idea with a built 400, and I don't think it should even be an idea with an LS. If it was a G/B Body or a Class 1 truck, I would be all for it, but you have a beefed up Class 2 truck with a lot of cubes and off-road capability so I question the longevity of even a built one. If it were me, I'd consider taking the money I saved on the LS swap and putting it into a fresh 4L80E and a standalone control module. There's your overdrive, and there's your rigidity. It's practically as tried and true as the TH400. The aspects that don't usually translate in transmission swaps are driveshaft lengths, spline count, and stuff of that nature. I don't know for certain what it would entail going from a TH400/203 to a 4L80E/205, but there are people who frequent the forum more experienced and older than me. I think for what you have, what you'd potentially want to do, and what I assume you want the truck to feel like when you're done, a SBC 400/4L80E/NP205 setup would really bring your truck to life and keep it as simple as it is now. You could consider whether or not your engine needs a full on rebuild or if you have like say a rear main seal going out or a bad gasket somewhere. I'll venture a guess that the motor is mint on the inside, especially if you have good oil pressure and you don't see any oil blow-by. If that's the case, you might could bypass dealing with the bottom end and focus on the heads, finding a nice dual plane intake that fits your carb or the carb you want to get, picking a good fitting set of headers, and selecting a cam that'll really mesh with whatever voracity of head work you want to do and what you want it to perform like ( @rich weyand ). You're probably looking at four hundred dollars max for the cam/lifters, intake, and headers. The headwork is up to you, but at a minimum, I would do a spring and valve seal job at home just to ensure they're fresh and ready to take on the cam you choose. Be cheap, though. Hunt for deals on eBay or Craigslist. Use the same strategy for a transmission core or rebuilt unit, the standalone module, and the transfer case.

 

[Matthew]

Thanks for the input! I didn't really consider any of those options. I'll definitely be looking into it in depth as I start to sort and part things out. As of now, the usage of the truck will really be just a driver here and there, with no real intentions as a tow/haul/off-road vehicle as I have other vehicles for that purpose. It'll end up being restored and treated well, just taking it on some nice rides and to some shows. By the way, I'll upload some pictures in a separate post so I don't clog up this one. You are right when it comes to the LS Swap, most people practically LS Swap anything and everything, so going against the grain and restoring the 400 would be neato. The engine is practically perfect. There's just some normal oil consumption (leaking and burning haha, but oil is cheap enough), I get really good oil pressure, and as far as I can tell there's no blow-by. I'm considering a decent rebuild just so I don't have to do it again down the road, but I probably wouldn't completely redo it (especially the bottom end as you said). The only issue I've ever had is the darn Quadrajet, which always refuses to work when I hit a speedbump really hard or hit a bad pothole (typical Pennsylvania roads), and it'll be replaced. If I were to redo the 400, I was already considering a new carb, intake, headers, cam, and heads. Of course I have to make sure those siamese-ports are correct for the 400. And the super weird thing about my 400 is that the darn thing runs ice cold (which 400's famously run warm). During 100 degree days at sustained 65mph (4.10 rearend with 1.00 final trans ratio) the thing wouldn't even get up to the middle of the temperature gauge, it just sat at it's normal 1/4 mark. In the cold, it doesn't even wanna reach the 1/4 mark... I only ever get it to the operating temperature 1/2 mark if i idle it for a significant period with low airflow. Other than the carb, there's no issues! The truck has factory working AC which I'm seeking to preserve, and I'm trying to find an AC Delco Cruisemaster (factory cruise control), it's just a preserved time capsule. Thanks for replying once again!

 

[Matthew]

Here's some pictures:

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[1987 GMC Jimmy]

No problem! Sounds like a great automobile, and with what you've said being said. I have a few more items to throw at you. Have you replaced your thermostat? It may be partially sticking open causing it to run cool, and there's always the possibility of a malfunctioning gauge and sending unit. I'd verify with a non contact thermometer just to be sure, and thermostats are like seven dollars(ish). Let me say this as well. If you do decide to keep the 400, you may find that with a professional rebuild, that carburetor would be the best one in the world. Quadrajets are a sophisticated design, still arguably the most sophisticated available. It works efficiently by only taking in as much air as the motor needs instead of operating on a fixed CFM like other carburetors, and no matter how much you build that motor, the carburetor can handle it. Most were up to 750 CFM, but the RV ones were 800 CFM. They use a spread bore instead of a square one and small primaries, which helps with economy and practicality. They're sophistication makes it less convenient to setup because the idle mixture screws are right on the base, and the curb idle screw is tucked away, but I got a carb tuning screwdriver from O'Reilly, a vacuum gauge, and a small quarter inch wrench, and it's just fine. I have an E4ME Quadrajet (the computer controlled one), and it's supposedly the worst Q-Jet made. However, it went 245,000 miles without a rebuild and was still doing well. It was the most pitiful looking carburetor I've ever seen, but it just kept on going. I sent it to Mountain Man Carburetor in Arkansas, and they rebuilt it and bushed the throttle shaft. It looks and works great. I also know that @Quadrajet Power offers the same service. He builds them in three stages for OEM to mild built to extensively built motors, and he knows them well. Overall, they were designed for practical, everyday vehicles, which is exactly what yours is, and they can do anything you want them to do more efficiently than a Edelbrock or Holley would or could. Again, that's just my opinion, and you can process it as you wish, but I would give the old carb a fair shake before you make the decision of round filing it. I promise you with that Q-Jet properly tuned and rebuilt you can outrun, outpull, outhaul, and outlast anyone's Holley or Eddy. And yep, one of the best looking squares I've ever seen.

 

[1987 GMC Jimmy]

Before and After of Mine:

 

[Matthew]

I'll consider what you said! Of course I wasn't gonna throw the Quadrajet out of the question, as it really isn't a bad carb at all (cough cough thermoquad's take the throne in my opinion), and I'd definitely be capable of getting it rebuilt and cleaning it up, as it probably hasn't taken well to the unleaded fuel of today (I used to run this additive I got from JEGS that made the darn truck run ultra good and cleaned out that Q-Jet nicely)... and I absolutely adore the fact that the primaries are super conservative and then when I go full throttle those secondaries open up to gallons per mile... Thanks for all the input!

 

[1987 GMC Jimmy]

You're welcome, buddy.

 

[Georgeb]

I gotta agree with 1987 GMC Jimmy on this one. That is a sweet old truck and that 400 will most likely outlive any of us. The thermostat and and a good carb rebuild will go a long ways. Paying some attention to the distributer advance curve, vacuum advance to manifold and initial timing could also wake it up some.
I would drive the literal **** out of that truck. In fact my wife would feel cheated and neglected if I owned it.

 

[dougbert]

if you want to know your quadrajet get Cliff Ruggles book

https://www.amazon.com/Rebuild-Modi...UTF8&qid=1478149496&sr=8-1&keywords=quadrajet

Helped me on my QJ

 

[Green79Scottsdale]

Your situation is eerily similar to mine. '79 Camper Special with a 400 that is not going to get driven all the time. Here is my .02 on top of the good advice already given...

The 400... Built with some thought and consideration, this will be a great torque orientated motor that you will not leave you regretting not going LS. Vortec heads and a nice cam and you are in business. Rebuilding the QJ is a good direction, or maybe look at going to a TBI style EFI unit. My 400 runs cold too, even after putting in a 190* thermostat. Heavy duty cooling system with the CS package. Overheating with a 400 is more a myth than anything IMO.

Overdrive... How many miles are you actually going to put on this truck in a year? How much gas are you really going to save and what is the pay-off time for doing a swap? Everyone has a different situation, and I am keeping my TH400, even though I only get 10mpg. I don't drive in enough to justify the cost and effort. And it stays stock...

I would be looking to keep that truck as stock as possible. Maybe add some choice upgrades along the way, but overall keep stuff stock, drivetrain and body included. Your whole drivetrain is bulletproof as is, especially if it's just going to be a fun driver. Your addition of cruise control is a good one. I added a tach to mine and my dad upgraded to an AM/FM radio back in the day. Upgrades like that are nice, but they don't change the fundamentals of what the truck is.

 

[dougbert]

My first engine rebuild is in my FJ60 where I took a 1973 4-bolt main 350 and rebuilt it with retro-roller cam, Vortec II heads from GMPP, etc

Spent way too much money on it - tuition I call it. Next was a LS swap into a 1984 K1500 and a future will be a LS into a 1983 Caprice. Links in my signature

 

[Matthew]

Thanks @Green79Scottsdale and @dougbert! Really, I'm not seeking into putting an overdrive in for fuel savings to balance the cost, I'm just seeking to do so simply to let the engine take a break because I don't like running it at those higher rpms when I'm even going 55mph. It's more of a preference to keep everything relatively stress free haha! It'll be seeing relatively low miles for the foreseeable future (at least 5,000 maximum a year), as it'll be a cruiser, I have plenty of other vehicles to drive. I get around 8 miles per gallon, which could be improved if it wasn't for my exhaust sounding so good! Actually, I'm not adding cruise control, I'm simply trying to fix the original system found on the truck. It has factory vehicle speed control, which includes the AC Delco Cruisemaster system mounted on the driver side inner fender with all of the lines going to the carburetor, and a cruise control button turn signal stalk (seen above in my dashboard picture!!). The vacuum diaphragm is busted on it, and I'm sorting out a replacement. The more I'm thinking and researching, the more it makes me want to go old school and make the 400 a torque beast. Thanks y'all for the input, I'd still like to hear some suggestions and comments!!

 

[86 C10 Shortbed]

My 305 was shot so I went the LS route.Fits nicely and the power difference and mileage are amazing. Engine and transmission came out of a 1999 Chevy Truck. 5.3L and 4L60E. But I do love a 400!

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[foamypirate]

My 305 was shot so I went the LS route.Fits nicely and the power difference and mileage are amazing. Engine and transmission came out of a 1999 Chevy Truck. 5.3L and 4L60E. But I do love a 400!

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Very nice! Are those the Dakota VHX gauges?

In any case, I echo your sentiments. Having swapped to a 6.0L/4L80E, the difference in power and driveability is amazing. Hop in, turn the key, and it fires up within a second, EVERY time, no fuss, and has tons of power. I've pulled down as good as 19mpg in my 3/4 ton Crewcab with 4.10s (A/C off at 65mph for a long highway drive). Averages around 15-16mpg though.