Swapping out a 305 for a 383.

[DaNinja]

I'm in the initial phases for swapping my cracked 305 (5.0) for a 383 (6.3?). My mechanic is trying to dissuade me, but I think it should be a direct swap. I may upgrade the intake while I'm at it and throw a four barrel into the swap.

Besides the carb upgrade, is there any reason a 383 couldn't use the bolt-ons from a 305?

ETA...Same year motor from Jasper.

 

[Bondo]

Re: Swapping out a 305 for a 383.

Besides the carb upgrade, is there any reason a 383 couldn't use the bolt-ons from a 305?

Nope,... All the holes are in the Same places....

 

[DaNinja]

Re: Swapping out a 305 for a 383.

Nope,... All the holes are in the Same places....

That's what I'm thinking, at least mount wise. A 350 is a bored 305. A 383 is a stroked 350.

I guess I'm wondering if the bolt ons or the rigging won't work. My mechanic is really good, but I'm not seeing the reason for his reluctance. The only thing I'm slightly concerned about is over-powering a Alpha drive.

 

[dukeofwsu]

Re: Swapping out a 305 for a 383.

I just completed a swap from 305 2bbl to 350 FI, flywheel hp around 300. That's about all the alpha drive should be expected to handle, based on my research beforehand. I was lucky to have a 1.47 gear ratio, too. Based on much of my homework, I wouldn't try it with a much taller ratio than that. It seems like 1.47 and 1.62 are the common ratios for the 305.

With my jump in ponies, I'll be adding about 4" of pitch, and I plan on being nice to it while getting up on step, rather than dumping on the gas and trying to grenade my drive.

 

[proshadetree]

Re: Swapping out a 305 for a 383.

Alpha will handel anything a 383 will throw at it. Great swap if you need to replace a mill

 

[DaNinja]

Re: Swapping out a 305 for a 383.

I just completed a swap from 305 2bbl to 350 FI, flywheel hp around 300. That's about all the alpha drive should be expected to handle, based on my research beforehand. I was lucky to have a 1.47 gear ratio, too. Based on much of my homework, I wouldn't try it with a much taller ratio than that. It seems like 1.47 and 1.62 are the common ratios for the 305.

With my jump in ponies, I'll be adding about 4" of pitch, and I plan on being nice to it while getting up on step, rather than dumping on the gas and trying to grenade my drive.

So I should stop at 350?

 

[DaNinja]

Re: Swapping out a 305 for a 383.

Alpha 1 will handel anything a 383 will throw at it. Great swap if you need to replace a mill

So a 400hp 383 won't frag an Alpha?
I think early Pachanga owners have all replaced their drives
after big power was put to a Alpha.

 

[oops!]

Re: Swapping out a 305 for a 383.

i am making the same swap.

that is the only problem.......overpowering the drive.....but i dont think you are going full all out on the 383?.....if you are around 350 ponies....and treat the drive with resp[ect....it should last you.

it also will depend on the gear ratio of the drive.....a lower ratio like 1.9 will spin fast and you could over rev....but a 1,5 will take more ponies to turn it.
so be prepaired to play with your props.

one thing tho.....be thinking of a bravo in the future. so look around for doner boats.

on a side note........when the send you the new block......make sure you know EXACTALLY WHAT YEAR AND WHAT IT IS......if they send you an older 2 part main....you will need a different flywheel and coupler

 

[TilliamWe]

Re: Swapping out a 305 for a 383.

That's what I'm thinking, at least mount wise. A 350 is a bored 305. A 383 is a stroked 350.

I guess I'm wondering if the bolt ons or the rigging won't work. My mechanic is really good, but I'm not seeing the reason for his reluctance. The only thing I'm slightly concerned about is over-powering a Alpha drive.

A 383 is a BORED AND STROKED 350. (the 6.2L is 377 cid, and that is a stroked 5.7).
If you know how to drive your boat, the Alpha will handle the hp and TORQUE of the 383. Just don't hammer the throttle from a standstill, and don't get airborne jumping wakes and waves.

 

[TilliamWe]

Re: Swapping out a 305 for a 383.

...
on a side note........when the send you the new block......make sure you know EXACTALLY WHAT YEAR AND WHAT IT IS......if they send you an older 2 part main....you will need a different flywheel and coupler

If you two are buying the 383MAG MPI from Mercruiser. they are using later blocks will one piece rear main seals. My dad installed one in place of a 454 in my sister's 1990 Chaparral 2350 cuddy. It does a nice job. ( it is coupled to a Bravo, though, since the 454 was the original engine). It also isn't anywhere near 400hp, more like 325, with Merc advertising the newer ones at 350. But not with stock exhaust manifolds.

 

[proshadetree]

Re: Swapping out a 305 for a 383.

With stock exhaust and a factory 4 barrel you can drop the hammer all you want on a 383 and not sweat it. You couldn't make 400 ponies if you wanted too. Well if you wanted to be most will not spend the cash. Jasper states 385 with headers and a 750 carb. Marine manifolds are defiantly not headers. If a cast 4 barrel intake is used it will choke that 383 over the performer rpm they used. So should be close to 325hp rather than 385 according to desktop dyno. Heck they even used them behind big blocks for a while and some are still in great shape.

 

[telstar1]

Re: Swapping out a 305 for a 383.

Interesting point Bubba but Im not so sure about that. Getting out of the hole you are definitely using every bit iof torque available at that rpm say 2500.Cruising along at 4000,the motor is not using all the torque available at that rpm. If it was, that would be your top speed because the prop has a way steeper curve than the engine and any attempt to get more speed would be stymied by the props greater demand for torque and power than the engines ability to supply it.I am by no means an expert on this but I do believe that the torque on a drive is higher at hole shot than cruise. Not sure about wot though.

 

[Friscoboater]

Re: Swapping out a 305 for a 383.

I put a pretty hot 350 in my Glastron and she is holding up so far. I think I read somewhere that the Alpha was rated for 300 HP and if you go any higher, you should be in a bravo. I am not sure where my 350 is sitting, but according to the desktop dyno I am over 300.

 

[DaNinja]

Re: Swapping out a 305 for a 383.

Good replies and advise.
I definitely have some question to ask my mechanic.

 

[GLENN M]

Re: Swapping out a 305 for a 383.

first off a 5.7L is a 350,put a 400cid crank in a 350 thats bored .030 you get 383.put a 350 crank with spacers in a 400 block you get 377cid.if your running 305 intake on 383 you will achieve max laminer air flow at a lower rpm,just like big trucks,smaller ports faster air moves creates more tourque at lower rpms.gas motors make max power at 400 fps air flow after that it becomes turbulant and you lose flow.bigger ports will kill low end tourque,will increase top end power.tourque curve and hp will always intersect at 5252rpm. when building a motor you must know what rpm you want to run at then figure airflow to be 400 fps at that rpm and use right parts to match,ie; heads,port volume,carb,exhaust.it will only be as good as your smallest restriction.no good head flows200cfms and intake only 160cfms.383 will work nice but build it for rpm less bottom tourque.it will already have much more than 305.that way you can run it out for more speed and less strain on alpha.

 

[TilliamWe]

Re: Swapping out a 305 for a 383.

I honestly have never understood this advice. Look at the torque curve of any marine engine and you'll see that it's lowest torque (force on the drive) is at the bottom of the RPM range and maximum torque is reached around 3500- 4000 RPM depending on cam grind. Hammering the throttle (WOT) from a stand still puts far less strain on the drive line than running WOT at the engines peak torque.

Bubba, static friction takes more force to overcome than kinetic friction. And for every force there is an equal and opposite force in the opposite direction. If you understand those two concepts, then you'll realize why hammering the throttle from a standstill is so hard on an outdrive.

Glenn, sorry, you are not correct. A 400 has a 4.125 in bore with a 3.75in stroke. If you reduce that stroke to 3.48 (350) you are left with a 372. A 383 has a 4.03 in bore with a 3.75 in stroke. Using the formula for volume of a cylinder "Pi times Radius squared times height", you'll see how it all works out that a 350 (4in bore 3.48in stroke) is 359.67 cid, a 6.2 (4in bore 3.75in stroke) is 376.8 cid, a 383 is 382.47cid, and a 400 is 400.7 cid.

 

[TilliamWe]

Re: Swapping out a 305 for a 383.

... Heck they even used them behind big blocks for a while and some are still in great shape.

You are referring to the Alpha Ones in 1987 and 1988, and you are absolutely correct. And those were Alpha Ones, not Alpha One Gen 2 like the original poster has. I don't know if any of you have ever copmared an Alpha One to a Gen 2, but the Gen 2 is quite a bit beefier. Close to a Bravo, actually. So yes, I'd use a 383 in front of an Alpha One Gen 2 every day of the week, and ALL DAY LONG on Sundays!

 

[DaNinja]

Re: Swapping out a 305 for a 383.

You are referring to the Alpha Ones in 1987 and 1988, and you are absolutely correct. And those were Alpha Ones, not Alpha One Gen 2 like the original poster has. I don't know if any of you have ever copmared an Alpha One to a Gen 2, but the Gen 2 is quite a bit beefier. Close to a Bravo, actually. So yes, I'd use a 383 in front of an Alpha One Gen 2 every day of the week, and ALL DAY LONG on Sundays!

Hmm..looking at my manual, it doesn't look like Merc used an Alpha I (even Gen II) past a 260hp 5.7.

The 383 I'm looking at is available up to 400hp.

 

[proshadetree]

Re: Swapping out a 305 for a 383.

TilliamWe my point exactly. I did not notice he said gen two but if he has that its icing on the cake. Irresponsible boaters will tear up an bravo behind a 4 banger A good Alpha will handle a 383 all day long as you stated.

 

[DaNinja]

Re: Swapping out a 305 for a 383.

TilliamWe my point exactly. I did not notice he said gen two but if he has that its icing on the cake. Irresponsible boaters will tear up an bravo behind a 4 banger A good Alpha will handle a 383 all day long as you stated.

I think the year of the boat makes it a Gen II unless I had done a retro replacement. I could be wrong though..