350 to 305 downgrade swap

I have a 1991 1500 with a 350 tbi in it . i have a rebuilt 89 305 carb on hand . I don’t really mind the downgrade mostly because it’s a work truck and mpg is better with 305 . I’ve heard that it should be out and in . But I took it to a mechanic and he said it’s gonna require “special” pieces to run . Is this true ? I don’t know if I should continue with the swap. Other than the fuel system , what is different? Shouldn’t it be plug and play? Thanks

Andy913 said:

I have a 1991 1500 with a 350 tbi in it . i have a rebuilt 89 305 carb on hand . I don’t really mind the downgrade mostly because it’s a work truck and mpg is better with 305 . I’ve heard that it should be out and in . But I took it to a mechanic and he said it’s gonna require “special” pieces to run . Is this true ? I don’t know if I should continue with the swap. Other than the fuel system , what is different? Shouldn’t it be plug and play? Thanks

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I'm not sure on the swap details, but a carbed 305 being better on fuel than a fuel injected 350 sounds questionable to me. The 350 is a larger engine, but it has a more efficient fuel system.

What's wrong with the current 350? If it's not worn out, I'd make sure it was running it's best.

454cid said:

I'm not sure on the swap details, but a carbed 305 being better on fuel than a fuel injected 350 sounds questionable to me. The 350 is a larger engine, but it has a more efficient fuel system.

What's wrong with the current 350? If it's not worn out, I'd make sure it was running it's best.

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My carbureted 305 was getting 17+ mpg in a poor state of tune. I worked on the Q-Jet, bolted a performer rpm spreadbore manifold and headers on it then put a small cam in it. I messed with the carb jetting and timing curve, I regularly saw 21-23 mpg on the highway. 700r4 and 3.08 rear gear. The 305 was a good bit more powerful than the stock L05 that replaced it as well. The 350 was not exactly stock either, as it had a 3704 manifold, a modified TBI unit and the headers on it.

I do however question the 1989 305 being a carbureted engine to start with unless it was a boat engine. In US production, every small block was TBI by 1989. In limited models the 305 was carbureted in 1988. Monte Carlo and the big B-body cars being the last carbureted 305s.

Andy913 said:

I have a 1991 1500 with a 350 tbi in it . i have a rebuilt 89 305 carb on hand ... ...Other than the fuel system , what is different? Shouldn’t it be plug and play? Thanks

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Are you keeping the TBI or the carb on the 305? Big difference.

Schurkey said:

Are you keeping the TBI or the carb on the 305? Big difference.

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I’m keeping the tbi, system already on truck

Andy913 said:

1991 1500 with a 350 tbi in it . i have a rebuilt 89 305 carb on hand... ...I took it to a mechanic and he said it’s gonna require “special” pieces to run . Is this true ? I don’t know if I should continue with the swap. Other than the fuel system , what is different?

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Andy913 said:

I’m keeping the tbi, system already on truck

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305 injectors, 305 PROM, 305 knock sensor. Does the 305 have cylinder heads with the goofy bolt angle on the middle two bolts? If so, the TBI intake manifold will bolt right up. If not, you'll need to adapt. Maybe more than that, but those for sure.

Andy913 said:

I’m keeping the tbi, system already on truck

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I assumed you meant going with the carb on the 305, since you mentioned it.

I'd think you would need 305 programing, however that's achieved. I have no idea how close the mechanicals of the 305 you have would be to a factory tbi 305. If it's modified or from a boat, you might have quite different heads, cam, and compression ratio. This is beyond my knowledge so I say all this subject to review by the others in this thread.

Schurkey said:

305 injectors, 305 PROM, 305 knock sensor. Does the 305 have cylinder heads with the goofy bolt angle on the middle two bolts? If so, the TBI intake manifold will bolt right up. If not, you'll need to adapt. Maybe more than that, but those for sure.

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305 ESC module is also specific to the 305. As is the smaller calpak module in the ECM.