5.7 Liter Vortec replacing a MAG 350 MPI

kennydalglishkennydalglish Royal Oak, MI.Member Posts: 87 ✭✭
Hello,  For many reasons I am replacing my current Mercruiser Mag 350 MPI with the following block.


Its the 5.7 Vortec long block.  Having it profesionally done.  Just wondering if anyone has ever done this and did they lose much power on the water ??  I know the MAG 350 MPI is rated at 300 HP.  Just wondering if I can trust the website HP rating for the Vortec ?



  • SeaHareSeaHare Member Posts: 183 ✭✭✭
    Im pretty sure what makes it a 350 mag is all the bolt-on's. Like the MPI injection etc. Im pretty sure the long block itself is just a 5.7 block with vortec heads.

    If you get this block and reuse your intake and injection system, it will basically be the same engine.

    Not positive on this so someone else might want to chime in.
    01 FV 310, 5.7s carbed, B3s
  • 212rowboat212rowboat Member Posts: 2,576 ✭✭✭✭✭
    Small block is a small block.  Vortec refer to the heads.  MPI is the intake manifold and fuel management.  

    Long block is block+rotational assembly+heads. A short block lacks heads.  A vortec head can be ran carbed or throttle body injection or multi port injection... 

    There is no telling what the true rating of the engine will be.  It'll be able to handle 300hp, though. The long block will arrive with the heads quenched and the camshaft clocked, but will likely lack ignition.  You'd be better served buying a new ignition, and checking the timing of the camshaft.  You'd be well served by performing both compression test as well as leak down test.  You'd be well served by rotating the engine by hand with a clock on the timing and bouncing the dynamic compression off the chart for the camshaft... this is likely in your builders chores, but it's up to you to make sure they're done.  

    If something was tired from your old engine, it'll be tired on the new core too.  i.e. sloppy ignition... hopefully you're bolting new exhaust manifolds to it... hopefully the valves will be adjusted on a warm engine with the engine running... dirty job yeah but only way to know it's right... hopefully new water pump and timing chain, oil pump and... anything consumable.. 

    You get the idea.  

    The only engine I ever was fully satisfied with in my many swaps was a merc 357mag.. which is a merc rebuild 350 bored to 7 more cid of displacement.  Ot was a carbed engine with a 750 four barrel.  I probably should have gone the MPI route, but a good dialed carb isn't that far off from MPI performance wise- it's WAY off in economy and longevity,  though, and won't last as long as the fuel trim can be the life or death of an engine, it often is, and a mpi with downwind sensors reconciling fuel trim makes for a clean and happy engine.  Merc knows what they're doing with the build.  More than I do... by far... and all my "hot little engines" were a memory with an engine more than capable to move the boat and stone cold reliable. 

    A turn key engine, which this was, is about twice the cost of a long block.  But... you're new engine is planted and ready to go in a day, and comes with a warranty- as in every part of it from water pump to dizzy to the freakin neutral cut off switch in the linkage... 

    If you've not pulled the trigger yet, circle back and get the turn key engine.  It's worth the upcharge.  

  • 212rowboat212rowboat Member Posts: 2,576 ✭✭✭✭✭
    And. Fuel pump... if yours is electric and you swap it over it'll pretty much require you to run MPI, and.... I'd get a new one.  I think merc rates the difference between carbed and mechanical fuel pump and carbed with electric fuel pump at 25hp differential, and between carbed+electric to MPI (gotta be electric) at a full 25hp more. 

    That's a 250hp, a 275hp, and a full 300hp... everything in the rotational assembly and block is the same.  I think the same engine with monitored exhaust is another 25hp making that same basic long block with vortec heads 325hp... all reliable power.  That 325 will outrun and outlast any of the others, though, odd enough. 
  • 212rowboat212rowboat Member Posts: 2,576 ✭✭✭✭✭
    And... an aside because I'm bored... 

    There was a series of seadoo jetskis once... a 130hp, a 155hp, a 210hp, and a 255hp. 

    Top speed on the 255 was around 75.  On the 155 it was close to 70.. on the 130 it was 55 or so.  

    The engine was a rotec maybe? 

    So a 130 was base of the engines... a larger throttle body and tuning made thar engine a 155.  If you were to go the supercharged route, the smaller throttle body like on the 130 with the supercharger and tuned was 210... the larger throttle body and tuned was 255... all based on same engine with no more mechanical displacement (boost accounting for more volumetric flow). 

    There are differences in the hulls of these skis.  A jetski, no matter the make or model, will top out around 75mph.  How fast they get there is another game, but top speed is limited by design.  There is an agreement between makers and coast guards..... however... if you were to swap throttle body's and supercharge the engine of a 130 model and tune it properly (as the 255 is), you'd have a ski north of 90mph.  Why you'd want that I don't know, but the hull would no longer limit you. 

    Then, there is Yamaha who have since corrected the issue, but at one time a computer swap on a 225 hdpi two stroke would be a 300... the only difference was tuning and the sticker on the cowl...  It got so bad you'd have to send your computer to Yamaha before they'd provide a new one- and you'd get back a new one rated as the one you sent.  Same everything down to injectors and all. 

    How either figured they'd slide that past the hot rod bunch is beyond me.  
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