305 TBI on a carbed 350?

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87redsquare

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Barnesville, MN
First Name
Ellis
Truck Year
1987
Truck Model
C10
Engine Size
305
I have an 87 c10 and we are going to do a body swap to a 4wd chassis. The 4wd chassis has a 350 and I would like to keep the TBI from the 305, except have it on the 350. Will it be hard to switch the throttle body?
 

1987 GMC Jimmy

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Mississippi
First Name
Jesse
Truck Year
1987
Truck Model
V1500 Jimmy
Engine Size
350
I have an 87 c10 and we are going to do a body swap to a 4wd chassis. The 4wd chassis has a 350 and I would like to keep the TBI from the 305, except have it on the 350. Will it be hard to switch the throttle body?
I would say it depends on how adept you are as a mechanic and how resourceful you are in finding parts. If you're good at both, it should be pretty straightforward. If you're relatively new, it may be a little bit challenging considering it's a nice little to do list in order to turn a 305 TBI system into a 350 one on a 1986 or older 350. Here's what you'll need assuming your cab has all the wiring ready to go:

You'll need a way to mount the actual TBI unit. You could get a adapter that conforms to the style of intake on the 350 (i.e. Quadrajet [spread bore] to TBI), or you can swap the TBI intake over. The bolt angling on the center four bolts of the TBI intake doesn't correspond to the old style heads. They have to be drilled/machined to correct this. I don't know the specs, but plenty of people have posted about it on the web. I see more problems in ease of the swap with the intake adapter because of sensor mounting that takes place on the intake. If you have the intake that already has provisions for those sensors, which you do, it will look better and go faster from there. I'm referring specifically to the CTS and the carrier that holds the ESC module, EGR solenoid, and MAP sensor. You could probably manage the CTS on the carbed intake with a CTS ported water neck, but you may have to improvise on the carrier. You'll need to swap in the TBI distributor. As far as sensors go, you'll need a bung to thread in your O2 sensor, and you'll need to buy a 350 knock sensor and thread it into the coolant drain plug that corresponds to the side the pigtail's on. I want to say passenger. If the carb manifold doesn't have an EGR valve provision, I'd forget about it and make the TBI one work for you because you'll need that. You'll need to source a 350 ESC module to work in conjunction with the aforementioned 350 knock sensor. You'll need to swap over the oil pressure safety switch that works with the fuel pump relay. I don't know what the provisions are on the older motor for more than one oil pressure port, but I'm assuming it'll have it for factory electronic choke purposes. You're good on the ECM, but the PROM chips are different. You'll need to source one from a 1987-1991 square body or GMT400 that came with a 350 and also a 700R4 if you're running a 700R4 or TH350C in yours so you can have TC lockup. You'll need new (or known good) 350 injectors because the output of the 350 ones is slightly higher. You'll need to probably swap your tank or tanks over so you can have the in-tank electric pump, and you won't have to fool with a bunch of nonsense with trying to adapt the different fuel systems. You'll need a little block off plate for where the fuel pump pushrod lives, and you'll need to pull that out of there, too. That's all I can really think of. It sounds like a lot, but any job worth doing is worth doing right. And plus, you have the big stuff. All you need that you don't have is a knock sensor, an ESC module, 350 injectors, PROM chip, fuel pump block off plate, and an intake adapter if you decide on that route. All of that (minus the knock sensor) can be scored in junkyards, Craigslist part outs, and eBay for cheap. I think that about covers it, though.
 

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