MPI Vers. EFI - Std. Vers. Mag

[TXDONZI]

What are... if any... pros and cons realted to the different engine packages? Is one more or less desiralbe when it come to simple performance mods? If so what issues arise when trying to perform mods. such as cams and exhuast? Are ECM re-flashs or chips required for these types of mods. if so how do you obtain them?

Tt

[The Hedgehog]

on 502's the 502 EFI and MPI are basically the same. Magnum denotes forged internals and some higher performance goodies. I am not sure that that they made one that was non-mag. The 500 EFI is another story. It is a 502 MAG long block with the cam, heads, pleneum and exhaust done over by Mercury racing. The 500 EFI has about 75 more hp than the 502 mag. It is built to run hard. It is one of those pretty blue motors.

454's have the non-mag and the Mag MPI's. You definately want the Mag. You will pick up 40 more hp or so and forged internals.

When it comes to mods you can take a 502 MPI Mag and make it into a 500 EFI by doing a cam swap, punching out the heads, and a higher performance exhaust. This requires a reprogram.

When doing any serious mods it is best to reprogram. You can do the exhaust without doing that and you might get 2-3 mph (on a lighter boat). Beyond that the computer needs to know what is happening.

Marc Boos at PMEFI.com has written a number of programs for EFI systems with a variety of combos. I used him to reprogram mine after installing the Procharger. It ran even better than before.

Eddy Young of Youngs's Performance Marine has spent some considerable time on a dyno programming EFIs as well. I am using him to build a serious motor for me this winter using a 502 mag as a base. I will ask him if he has any programs available if you like.

[TXDONZI]

I am sorry I had ment to ask this question pertaining the 7.4L MPI std. vers. magnum engine package. I would like to know if a cam change and ssm or eddie exh. can be added to it without any major induction system and/or ECM changes to the 7.4L MPI std. package. Is it possible to make a std. perform closely to or better than the mag package.

[The Hedgehog]

I dunno why not. If you want the regular MPI to perform like a Mag the cheapest solution would probably be to sell the MPI and buy the Mag. You will then also have forged internals. You should be able to take the Mag up to around 425 hp or so with upgrades if you want.

A better question would be how fast you want to go. If you have a mag you can keep upping the ante. Your switch to a mag will get you around 40 or so. You can then easily take that another 50 with heads, cam and exhaust. With forged internals you can Procharge it and really fly with 550 - 600 hp.

[handfulz28]

I "think" the difference between the "7.4" and the "454MAG" includes different intake style and different heads (round/oval ports on 7.4 vs rectangle on MAGs); non-forged/light duty rotating assembly; and possibly smaller valves.
So you're going to have a lower operating RPM, and less "room for advancement" if the valves are smaller. But that doesn't mean it won't respond to a cam and exhaust upgrade, possibly roller rockers, and of course an ECM reflash.
I'd bet that Precision Marine (pmefi.com) has done some mods on these motors and could tell you exactly what to expect. I'm guessing that a cam will require a reflash and possibly a change in the fuel system ("hopefully" a different fuel pressure regulator would be all); just an exhaust upgrade usually doesn't require a reflash. Your current motor is probably rated at 350hp; I'd say an increase to 385 (10% increase = 454MAG) is easily supportable with leaving everything else alone. They'll probably tell you can/will need to spin that motor to 5000rpm.

[Pismo]

The MPI mags were introduced in 1996 replacing the EFI mags. The 1996 MPIs were the first year to start using the Gen VI block with roller cam setup. It probably yielded a little more hp but merc kept the ratings the same.