would like to change an 5.0 mpi Mercruiser with 555 ecm to a 5.7 .What would need to be done to insure it's not running lean ...Other than buy a 5.7 ecm (hoping to not have to do that).Motor froze so it needs to be longblocked anyway...

your outdrive gear ratio will be different between the 5.7 engine and the 5.0 engine. Changing props could get you in the recommended rpm range but that will move your torque curve. To correctly follow what your asking, you want to replace the 5.0 engine with a 5.7 engine and use the 5.0 ecm 555 on the 5.7 engine.

You will be running lean on that change over. The air fuel ratio's are different between the ecm calibrations. You need to have the ecm remapped or buy the correct 5.7 ECM controller or stick with another 5.0.longblock & heads.

Who can remap the ecm

Dustin Whipple. He's authorized by mercruiser, since he's selling his product for Merc engines with ECM 555

Motor, just a thought...might not be much more to upgrade to a 377 or 383 (6.2) over the 350.
With the right cam and maybe heads it should not be hard to reach 330-350 prop H.P.

Go big or go home . There is 575 SCI for sale in Vegas on CL. good price on it also.

That is not necessarily true that the new motor would be lean. The only way to tell is run it with a wide band and see where you are at. The 5.7 should start and run with the 5.0 ECM and tune. It will likely not be optimized, but it would run. Problem is the 555 tuning is limited to a few individuals. Weigh the cost of getting that ECM calibrated with a whole sale change over to a MEFI-4 and see what works best. The EFI stuff can be daunting but it is very do-able.

If you are going from a 5.0 to 5.7, as others have said you might as well go all the way to a 377/383.

This is actually a friends boat ..A 2004 240 sundancer that he bought with a 5.0 Bravo,not knowing any better .He let the motor freeze and i am going to put in long block for him ..We did talk about a 6.2 instead of a 5.7.I own a 6.2 i would like to sell him(just a longblock)I just don't want to get him in to more cost than reasonable ,considering it's never going to be much on performance ...Are the injectors the same on the 5.0-5.7 and 6.2...Any idea what it cost to remap an ecm...Thanks

I can order you a 383 stroker thru Mercruiser reman it is a complete drop in and dropped shipped to your door.

next anytime CID = cubic inch displacement gets bigger the need for more air to enter & exit the engine. More air always = more fuel. Of course it will start and run with any computer installed on it but you want it to run properly and looking for an upgrade in performance. Correct ?

Next the 5.0 and the 5.7 do not share the same cylinder heads nor the same intake valve. 5.7 has an intake valve that is slightly larger - larger intake valve means more air & fuel.

Also the bore of the 5.7 is 4.00 inches compared to 5.0 bore is 3.74 inches. Motor 101 tells me the stock 5.7 compared (apples to apples) to the 5.0 is the 5.7 needs more fuel as it is a heavier breather than the 5.0. Again more air always means more fuel to achieve proper air / fuel ratio's.

Next you are going backwards by using a MEFI system on your app as the ECM 555 is calibrated for a shift switch starting around the 2003 / 2004 year model. Most MEFI systems on the 5.0 and 5.7 were set up for TBI injection on the marine end plus the engine harness is completely different from MEFI to ECM 555..

ECM and PCM 555 started for Mercruiser late 2000 model year 2001 with MPI. fuel injection. Also if you have smartcraft you need ECM 555 controller because MEFI will not work for any smart craft nor DTS.

DTS = digital throttle & shift only works for ECM or PCM 555 or the current 09 mercruiser computer controllers. Also the distributer & ignition set up is totally different from MEFI to ECM 555 for the jist of things not to make a story book.out of this.

Also the injectors are all the same for the 4.3 L MPI thru the 6.2 L MPI. Its the computer that controls the air fuel ratios along with the sensor inputs. ie: CTS, MAP, TPS and so on. Also timing is controlled by the ECM as all timing on a ECM 555 & PCM 555 is non adjustable.

I would opt for the 383 complete drop in and again your gear ratio is the difference along with prop change..

Thank you .What I get from what you wrote is that I need a 6.2 555 ecm which I would have to buy used ,to make sense .But thanks for offering to Sell me an engine on this do it yourself forum