Jcomeau0

I have a Mag 350 MPI. I’ve got the engine torn down. Is it worth stroking to 383?
Would I see any noticeable speed gains?
If I were to stroke it to 383, would I be able to keep my current MEFI 3 controller and just have it reprogrammed?
How much would that programming cost?

SB

What boat and how fast with current power ?
will you or someone else he able to program it in the boat on the water ?

SabrToothSqrl

seem very unlikely. even if you go from the 5.7 to the 6.2, you gain maybe 2-5 mph top end.
I have 2001 Merc 350s MEFI3s (new base engines in 2021)

top speed on my formula 292 with twin 350 is 68 mph.
twin 6.2s is like 72...

so... meh. I run WOT maybe 1% of the time?

Jcomeau0

it’s a 1969 Formula 233. I’m restoring g it so never had it run with this power. I’d have to have someone else do the programming. I’ve never done anything like that.

articfriends

The problem your facing with the mefi programming is no one including mefi burn has ever figured out the unlock codes for the factory 5.7 ecm so any tuning done would be starting over from scratch. Since theres not alot of NA 5.7's with mefi3 hopped up, I highly doubt anyone has a "canned" tune either that would be remotely right. Not saying it cant be done but I 100% would say it needs to be at a minimum done on water WITH 02 feedback from exhaust and really/ideally done on a dyno. I do both, Im in michigan, dont know where your from but I tune mefis all the time, Smitty

Jcomeau0

thank you. I’m located in Massachusetts.

Trash

Yes you would be able to keep your current MEFI 3 controller. Yes you would be able to have it reprogrammed, but there are some catches. The password to unlock the MEFI 3 needs to be acquired IF you want to modify the original tune. I was under the impression that Bob at MEFIBurn had some codes for MEFI 3, but I could be wrong on that account. The other option is to wipe the original tune and start from scratch but this can be highly problematic if you don't know what you are doing. The most realistic option is to get a tune for a MEFI 3 for a 454 and transpose to a 350/383. Basically you would need to scale the fuel tables for the injector size and fuel pressure.

There is another option that may cause some here to have a stroke at what I'm about to suggest. Assuming you are only going from a 350 to a 383 and not extensively changing a bunch of other stuff then begin by running the new motor with the original factory tune. YOU MUST DO SO WITH WIDEBAND installed and monitored. I say again YOU MUST DO SO WITH WIDEBAND! The factory 350 tune will successfully start and idle a 383. The only major parameters you alter in MEFI tuning are primarily 1) fuel tables (BPW vs MAP vs RPM) and 2) spark tables. Theres a whole bunch of other stuff in the MEFI tables that in most cases does not need to be tweaked, at least initially. Now I am NOT saying just run it as is a 'send it', but rather do incremental tuning explorations under no load or mild load and see what the AFRs are doing. I would start this on a test stand which can be done fairly easily at your home or shop. You might find that there is enough slop in the stock tune to handle a 383 for the parameters you boat in. I cannot guarantee that but it is possible. The factory tunes are slightly rich and it is possible the AFRs would still be reasonable. There is not a major gap or change is basic small block timing between the two motors either. Both are going to have about 8 deg BTDC base timing at idle and roll up to 32-34 deg max for WOT.

Let me give you a real world, first hand account example. I had a bone stock 350 Mag/EFI MP motor (1995 MEFI1) that got turned into a 377, with bigger cam and heads. Started up and idled first try on a bone stock factory tune. Idle AFR was about 12.4-12.8. From there I kept it simple and made one change at a time and checked the results.

The cost of tuning can vary quite a bit. The most cost effective method is to get MEFIBurn and do it yourself on your time table. That's what I did. It will be intimidating and possibly overwhelming at first but if you immerse yourself in the simple basics it can definitely be done.

I think its worth it to stroke the motor since its already torn down. You will get some extra cubes and torque, all of which boats tend to appreciate. Speed gains will depend on how amped up the 383 is. I think 2-4 mph is a reasonable gain for a mild build.

boostbros

can also throw an intake and carb on it old school and it will run great many fishing boats have been retroed because a carb(eldebrock 1409) almost always idles better for 6 to 8 hrs also you can make tiny throttle adjustments to get the 1.6 - 1.8 mph troll speed as you head into the wind or downwind idle rpm is most often 500-650 rpm range a few go below 500 but i dont like to i try to keep 600 or above to keep things circulating the problem with the 6.2 in the fishing world is you have to really wind them up to get the boat going they make power at higher rpm at least in the inboard world

articfriends

Bob has told me they have never spent time to crack the 350 5.7 ecm codes when I asked fwiw

Trash

This would be a great alternate plan.