BI 305 to 350 engine swap

[snowbuzzard]

Just lunched the 305 in my 87 4x4 1/2 ton, had a quick question. I have heard that when swapping from a 305 to a 350 TBI that the injectors and everything are OK, just need to swap the computer. I have also heard that these old OBD1 TBI's don't require a computer swap, that all is ok.

I'm thinking of swapping in a long block 350.


Looks like the timing turned and advanced it so far t shook #3 piston apart. There's water in the cylinder also, so it's most likely pooched.

Anyway, thanks in advance for any help, been reading the forums for awhile, so not quite a newbie for the first post.

[Prerunner1982]

i am curious on this as well, for when my 305 goes I have thought about dropping in a 350. Wondering about the mpg difference as well..

Also, please do me/us a favor.. please remember to keep the language family friendly.

[Russell]

When switching a 305 TBI truck to a 350 TBI truck, there are 3 things that have to be changed, and 1 thing that is optional, but best changed.

First three include the knock sensor (located down by the starter the coolant drain hole), the injectors (305 injectors will not support a 350 at higher RPMS), and the chip in the ECM (Computer)

The final thing that is optional is the ESC module (big flat square just to the left of the throttle body). It controls your timing, and there are minor differences in the frequencies from the knock sensor that it responds to between the two engine displacements.

A properly running 350 with overdrive, and a good rear end gear can easily get 18 - 22 mpg highway, while I found my 305 typically got between 16 - 20 mpg highway.

I think the difference is that the 305 has to unlock the torque converter to make it up hills all the time. 350 gets better mileage in the city if you accelerate the same, as the 350 doesn't have to work as hard to get the vehicle moving, but anyone who is here looking for performance is probally gonna push that 350 as hard as they can, lol

[snowbuzzard]

Thanks for the answer, I suspected there was more to it than the injectors.

This truck has a 400thm in it, no overdrive, will that make it very computer specific w/o the lockup convertor?

[Russell]

Nope, just tie the lockup stuff up out of the way and forget about it. The computer doesn't recieve any feedback when the tranny is locked up, so not having it hooked up will not cause a code to be set.

[Prerunner1982]

I get 16.5mpg consistantly from my 87 Gmc, 305/700r4 in the city. Havent figured highway yet.. but thought it to be about 18-20..With this truck I would rather have a few more mpg, than a few more horses...

[snowbuzzard]

The 305 seems to be underpowered for this truck. I got terrible mileage, I'm sure a higher hrosepower engine will give me better mileage, especially towing.