Hi,
We are considering a boat with a pair of 420 hp Merc 502s. I'm pretty sure they're MPIs that have been rebuilt (the owner claims 150 hrs. ago). A few questions:
1. Any issues to be aware of in these engines?
2. How do we determine what year they are?
3. Will it be a problem to rebuild them down the line since they've already been rebuilt?
4. Does anyone have experience supercharging these? Results?
Thanks,
David

mpi means multi port injection... so expect them to be injected

if so, hooking up a scanner gives you the total time on the motors

early versions have whats known as a VST fuel system
which causes people some grief with hard starting hot.

later versions have " cool fuel " system which is a somewhat better system.

get the serial numbers off the valve cover to determine the precise year.

i have later ones which have been bullet proof mechanically but be aware that they were built originally with very light ring tensions and will use oil. if rebuilt, i would want to know what rings were used. also, the intake manifolds will rot away internally at the back if the motors were run in salt water and not effectively flushed.

like any motor thats been " rebuilt" you really don't know what you are in for unless it was done by someone reputable that you can question directly about what was done.

oh... and before you do anything, you really need to do all the basic stuff like compression tests get the motors leaked down and the like.

Are they 502 mpi's which are 415hp or 420's which are carbed 454's???????

They are 502s, not 454s. So, are all 415 hp 502s MPIs? That's good to know about the rings - I'd heard these engines use oil. Do they use any less fuel than carbureted engines.
Thanks,
David

Good engine to supercharge if rebuilt well.
Should be able to be rebuilt again when needed
1996 was the switch to Gen 6 with the roller cam setup Merc engine # OF802600 and up is Gen 6. To be sure, the timing cover has 6 bolts.
It is all about the rebuild, if done well you are in good shape, if not who knows. Did they used good forged internals like the original?
Look for the VST on top rear of the engine, some have vapor lock trouble with these, some don't.
They all use a lot of fuel, carbed of injected.
Some report oil usage but I never have.
Great motor overall. Strong, minor issues.

Do you really have a WalMart inflatable..That's great.

Thanks for all of the advice. I am hoping to upgrade my vessel significantly sometime soon

I have twin 502mpi's. I used to use a qt of oil per engine every 8-10hrs with regular oil. I switched to Mobil 1 15-50 and now I use qt per engine about every 15hrs.
They are more efficient/better on fuel than carbed engines. Its hard to compare apples to apples though. You would need identical boats for a fair comparison. Probably 10% or so better on fuel economy.

I have 502 mpi's. A Merc service center owner/ mechanic said if you dont get at least 800 hours out of them your doing something wrong.

You could probably double that figure if you take it easy.

i got my two 1999 versions w/ 150 hours on them in " as new " condition. i ran them very very hard for another 200 hrs or so using mobile 1 delvac. at that poit i got some water in one exhaust port and rusted a valve in a guide and the next time i started it it bent that one . when i took it apart the piston tops had a ton of oil on them and the bores were all glazed and while the compression ring seal was perfect, ( as were all the other structural bits ) the oil rings had absolutely no useful sealing at all. when i spoke to merc and a variety of ring manufacturers about this they all universally agreed that the low tension oil rings used from oem while supposedly for performance had proved to be a bad idea just from the oil contamination in the chambers stand point. i did a quick freshening of mine using std tension oil and compression rings from total seal and got much much better results both in sealing and power than original. i agree that from a structural standpoint the motors will run essentially forever, but ill bet you at 300 hours you are both using some significant oil and are giving up some significant power and efficiency from the lack of oil sealing in the bores. from a milage standpoint... my boat had carb 454's in it when i got it. it went 54 mph and got less than 1 mpg at wot. with the 502 mag mpi's it went 65 mph and got essentially double that mileage. after freshening and with no other changes from stock, it goes 67 w/ the milage about the same. the injection has had problems twice and both times the issues were solved quickly with the scanner and factory manual. bad injector once ( $ 120 ) and bad water temp sensor once ( $ 45 )