What do people mean when they say it's better to build a 350 than to rebuild a 305?

[Nutcase76]

Have been doing a ton of research about rebuilding my 305 or putting in a 350. Almost every post says it's cheaper/better to install a 350 if you're already pulling a 305 but I can't find exactly what they mean by that. Is that just if you have major internal damage to the 305? I have a 1992 21' Corona CC with the 305 with Thunderbolt ignition. I am pulling the motor to replace the flooring. 2 years ago I could each 51mph but last year it was down to about 43. Found a hole in the rear exhaust valve and a burned spark plug from it. I tore it down and just replaced the 1 bad valve, new head gaskets, and used it the rest of the season. Compression was a little low on a couple cylinders but it has 600 hours, so expected. So now that I'm pulling the engine anyway, should I just do a standard rebuild on the 305 or find a 350 to put in? It's only like $450 for a marine master rebuild kit. I have access to several 350's as I have a mechanic friend that has a ton laying around. Should I just freshen her up and throw it back in? More HP is always good but with my 305 being in pretty good shape?? Thanks for any tips.

 

[NHGuy]

Do a 350. Use all the marine stuff on the exterior. Use the marine distributor, alternator, starter, exhaust and fuel pump. You need a carb for a 350 so either rejet what you have or buy one that's correct for the bigger engine. Everything else is just bolt up and go. Just use a standard 350, not something with a high performance cam.

 

[charleso]

The 350 is simply a superior engine, even though they share parts. It will be way easier locating a new block, etc than rebuilding a tired 305.

 

[Nutcase76]

I have a 4 barrel quadrajet carb on the 305. That should be sufficient, correct?

 

[Scott Danforth]

Depends. A 350 will have more power from larger bores, and parts like pistons are a bit cheaper. Crank and rods are the same

If you have to machine the crank, its cheaper to buy new. Same goes for rods. At that point i would be building a 383 stroker as the cost is now the same

For what its worth, you can rebuild a 350 for less than a 3.0

 

[khe]

The boat was able to do 51 MPH with the stock engine in good working order, I would think a 350 would be overpowered and a 383 would be way overpowered for a 21 foot boat. More HP is not always better, it can sometimes be dangerous.

The price to machine the valves would be the same for the 305/350 and bearings would be the same. I doubt the crank would need to be machined unless a bearing was spun. The engine needs to be torn down to asses the components. Make sure to mark all the pistons, rods, etc. as to what cylinder they came from. 600 hours is not a lot of hours in my opinion.

 

[Scott06]

The best technical answer I was given for why the 5.7 related to hotter exhaust gas temps in the 5.0 leading to shorter life of the engine vs a 5.7. From a practical standpoint they are the same cost size weight and the 5.7 makes more power.

Ultimately I think it comes down to the following

How happy were you with your existing performance when it was running well?
If you don't need more power for watersports or heavy loads why upgrade?
What does your existing engine need when you tear it down? Possible all you need is a ring bearing and valve job this would be cheapest. What's your budget
If you change to a 5.7 you either need to repitch prop or change outdrive gears. Most 5.0 have a 1.62 ratio vs 1.47 for 5.7.

I bought a 21 ft with a cracked 4.3 and went back in with a 5.0 vs a 5.7 because I didn't want to pony up for a new drive. Was able to keep the 1.81 V6 drive gears and go from a 19" pitch to a 21" -23" prop depending on what I'm doing. If I had done a 5.7 I would have run out of prop pitches. Ultimately I wish I had gotten a 5.7 cause you can always use the power, but adding the cost of the drive at the time was pushing me over my cash limit

 

[Nutcase76]

No Title

I appreciate all the fast responses. I think I'm leaning towards just getting a 350. I was happy with performance before but dammit, I'm a guy. A couple more mph won't hurt my feelings at all! I get quite a few comments already with people surprised how fast it is. It surprises them because it's so quiet without a thru hull exhaust that people associate with speed. I believe I have a Bravo drive on it but have been working on a couple friends boats lately so I forget what has what. Most of my friends and family have boats so I have multiple different size props that I can swap around and find the best performance. 80% of my time is in Lake Erie with the balance in smaller lakes around Michigan. That's why I'm gutting basically the whole interior because I really just like the boat. If I go any bigger, I can't get onto some of the smaller lakes that I play on. Doing new floor, upholstery, and brand new seats. Money wise, it's not like I'm a strict budget but am dropping quite a bit on all of the new interior plus stereo system. I definitely dont want to spend the money on a new or rebuilt engine though. I enjoy doing the work myself and with friends. If I understand the research that I've done, I just need an engine from a 1988-1995 truck with the 350? Everything from my 305 will swap over? I'll know more tonight or tomorrow once I pull it out and tear it down to see what I'm dealing with.

 

[bruceb58]

If I understand the research that I've done, I just need an engine from a 1988-1995 truck with the 350?

If you go with a newer 350, you will get one with Vortec heads. You will have to change the intake manifold and possibly have to modify the throttle linkage but way more power.

 

[Nutcase76]

If you go with a newer 350, you will get one with Vortec heads. You will have to change the intake manifold and possibly have to modify the throttle linkage but way more power.

Wouldn't I also need to switch to an electric fuel pump if I did that?

 

[boatman37]

The exterior of a 305 and 350 are identical so all external parts will bolt on (starter, alternator, brackets, etc.). The only time you might have issues is changing years. Most years will still interchange but make sure first. Something else most people don't think about when comparing similar engines with different bores, like the 305 vs 350. When you have a smaller bore the intake valve is 'shrouded' by the cylinder wall because it is too close, meaning less air flows past the valve because of impedance. Do a search for valve shrouding for more info but the power difference isn't just because the bore is slightly larger, but more because it unshrouds the valve to free up airflow.

 

[bruceb58]

Wouldn't I also need to switch to an electric fuel pump if I did that?

Correct...you would.

 

[Scott Danforth]

Get the vortec heads new (they are cheap).and get a 1988-1995 block from a car (has roller cam where trucks and boats didnt) or get a crate motor. Crate motor blocks have the fuel pump boss machined

Then you dont need electric fuel pump

Then run a comp cams XM270HR cam

 

[Lou C]

Also if your pre-vortec engine has a Quadrajet 4bbl, and you switch to a vortec 350 there is no exhaust crossover on the vortec manifold to heat the choke spring. So you'd have to convert the carb to use an electric choke or use a carb that already comes that way.

 

[Nutcase76]

Well it looks like I'm gonna be going to a 350. Gonna spend a few days doing some more research on which direction to go with it. Looks like someone didn't winterize it properly years ago and there's 3 different spots with jb weld on them so there's no way in hell I'm gonna dump any money into this block. I appreciate the help and ideas and will take a few days to do more research and decide which way to go. It's a balancing act of how much more work and money I want to spend versus I don't necessarily need the extra HP. I plan to buy a 25-26 foot boat next year just for my lake Erie time but still going to keep this one for playing on the small lakes.

 

[Nutcase76]

Thanks for all the help again guys. Had to get put on hold for a couple days because I had a carrier bearing come apart in the rear diff of my tryck monday morning so was working on that. A buddy has a 350 vortec from a 97 Silverado that I'm gonna get from him. Going this weekend to tear it down and make sure all is good. Will get new head gaskets. Looks like I'll order the Edelbrock 2116 intake so that I can still use my Quadrajet 4bbl carb. They're only about $250. I was mistaken about the outdrive. I have the Alpha Gen 2 on it. Haven't had time to research choke issue yet but will as soon as I get a bit of time. If anyone can think of anything else I need to know, please feel free. At least I can get the engine project started and get intake ordered. Thanks again.