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305 to 350 swap questions

nhproxr

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nhproxr Enthusiast

nhproxr

Posted
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Found a '97 Sierra for short money that seems to be in great shape, only problem is it has a 305 and I want (need?) a 350. Truck will just be mainly a winter commuter/errand runner and occasional snowmobile hauler on weekends. My concern is when I tow my sled trailer. It's probably 2800-2900 lbs with 2 sleds and 3500 lbs with 3 in it. My '98 Yukon with 350 and 3.42 gears struggles to maintain speed towing this trailer up steep grades, this truck has 305 with 3.42s and I can only assume even though it is lighter then suv it will struggle more.

 

My old man has had 305s in past and they were dogs towing....I have never known anyone with a vortec 305...are they any better?

 

What would be involved with throwing a 350 long block in it? Does 350 computer have different programming or is it same? Does 350 have different size throttle body? I have a '79 4-bolt 350 in shop....can I rebuild lower end, throw some vortec heads on it then put on all accessories/electronics, throttle body, etc. from 305 or is there any issues??? I know my '79 block has 2 piece rear main, but think I've seen a conversion kit to make 2-piece engines a 1-piece seal engine....will this be compatable with the '97 drivetrain???

 

I know it sounds like a hassle, but don't usually find trucks in this condition this cheap.....might be worht the effort.

95Sierra2500 Senior Enthusiast

95Sierra2500

Posted
Posted

Shouldn't have a problem, I think the vortec heads off the 305 will fit...You'll be running premium all the time though, as you'd be upping the compression quite a bit.

mcreyaufmiller Senior Enthusiast

mcreyaufmiller

Posted
Posted

The 305 heads are different from the 350 heads. If you use the 305 heads, they have a smaller combustion chambers and will increase the compression ratio. As far as the computer, the 305 and 350 are interchangable, if you would like to reprogram the computer wait 4 me performance can make changes for n ot too much money.

 

The manifolds and accessories will fit between the 2 blocks, most of the sensors are external and connect to covers, but I do not knwo what blocks are usable with the Vortec heads.

nhproxr Enthusiast

nhproxr

Posted
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Thanks guys, glad to hear computer is compatiable. I would probably buy new vortec heads for the 350...ones on 305 would need rebuild anyways and brand new vortec heads from GM are pretty cheap. I know all the acessories would be interchangeable, but what about the throttle body, do 305 and 350 take the same one?

 

Also does anyone have an answer about the 2 piece rear main seal. I don't know if a 2-piece seal motor can bolt right up to the tranny. :smash:

 

Maybe its just as cheap to just buy a complete 350 vortec long block and swap over all the accessories.

nhproxr Enthusiast

nhproxr

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Shouldn't have a problem between the 1-piece and te 2-piece...the bellhousing bolt pattern should be the same.

 

 

 

 

What about the flexplate? Isn't there a different one for each motor? I guess you can just use the flexplate/torque converter combo for what ever engine you are using? Either one should work with the 4L60E I assume?

95Sierra2500 Senior Enthusiast

95Sierra2500

Posted
Posted
Shouldn't have a problem between the 1-piece and te 2-piece...the bellhousing bolt pattern should be the same.

 

 

 

 

What about the flexplate? Isn't there a different one for each motor? I guess you can just use the flexplate/torque converter combo for what ever engine you are using? Either one should work with the 4L60E I assume?

 

 

 

 

 

It shouldn't make a difference, as I believe GM kept MOST of that stuff the same. The only real difference is that the one-piece seal is just that, a one-piece seal. The flexplate and converter should all bolt right up.

nhproxr Enthusiast

nhproxr

Posted
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It shouldn't make a difference, as I believe GM kept MOST of that stuff the same. The only real difference is that the one-piece seal is just that, a one-piece seal. The flexplate and converter should all bolt right up.

 

 

 

 

That's what I was hoping to hear. I know GM is pretty good at making stuff compatiable, just don't want to get a nasty surprise halfway through a project.

OUMike Senior Enthusiast

OUMike

Posted
Posted

Hey guys. This might sound like the obvious answer, but I think it might be more cost effective just to find a 350 in a yard and rebuild it. Hell, could probably buy one from GM-Parts Direct for not to much money.

 

Tha'd probably be what I'd do.

mcreyaufmiller Senior Enthusiast

mcreyaufmiller

Posted
Posted

For a stock motor, I would go to a dealer and get a long block for $1600, the manifold is the same for both motors as well as everything else. (except the exhaust, the 305 is a 2 into 1, and the 350 is a 2 into 2 with a cross over.

nhproxr Enthusiast

nhproxr

Posted
  • Author
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Hey guys.  This might sound like the obvious answer, but I think it might be more cost effective just to find a 350 in a yard and rebuild it.  Hell, could probably buy one from GM-Parts Direct for not to much money.

 

Tha'd probably be what I'd do.

 

 

 

 

I have a tired 4-bolt 350 on the shop floor right now, so I don't need to buy one. Question is (beyond what it takes to swap from 305 to 350) is it possible, and cost effective to rebuild a '79 350 to drop it in a '97 sierra or just buy a new 350 long block from gm that'll drop right in and be brand new. Of course the pride associated with building your own engine goes a long way. :)

mcreyaufmiller Senior Enthusiast

mcreyaufmiller

Posted
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I would go for the crate motor in the long run. Too many hitches to hit building your own and if you are like me, you will go overboard and spend more than you budget on cool parts.

 

For my current project, I opted to have a professional build the enging and had it balanced, blueprinted, and all of the fun stuff to go with it. Let the proplr who know do the work.

 

Mike

  • 3 months later...
forgottenmemory Enthusiast

forgottenmemory

Posted
Posted

i am not any expert on any means, but check into the fact that the computer indeed is compatible as well as the PROM chip that it may contain. That memory was all preset at the factory for the 305 engine. Just make sure their won't be any issues with fuel distribution or anything that is computer controlled. I am just saying from a past experience.

Nytemare Senior Enthusiast

Nytemare

Posted
Posted
For a stock motor, I would go to a dealer and get a long block for $1600, the manifold is the same for both motors as well as everything else. (except the exhaust, the 305 is a 2 into 1, and the 350 is a 2 into 2 with a cross over.

 

you cant use an old block in a new truck if you want to keep the factory fuel injection and computer you have. Something to do with not having places for the sensors. Cant recall at the moment.

 

You'd be better off if you want to go that route to just rebuild it get an adjustable fuel pressure regulator and run a carbed setup.

 

However, why not just ad some power goodies to the 305. I have heard of guys with 300hp 300ft-lbs 305's for relativly little money. start with a tune. throw in a decant cam, and roller rockers. that should be a good start.

d57gmcbell Enthusiast

d57gmcbell

Posted
Posted

The older 2 piece rear seal blocks don't have a spot on the block for the knock senor. Also the 2 piece rear seal use a different bolt pattern for the flexplate than the 1 piece rear seal cranks. GM has a 383 stroker crate motor that they say is a drop in replacement with no need to retune the computer. It is the HT383, it has a larger camshaft than a stock 350 vortec. With that said i would think that you should be able to find a hydralic cam for a older block (vortec uses a roller cam) that would run with out needing a tune. Too much difference form stock and the motor won't run correctly. :banghead::banghead:

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