i am in the process of rebuilding my 502mpi mag,1994.in the process of r/r of the heads the machine shop has found a crack in one of the heads.it is in the crossover port.bad spot they say and cannot be repaired.so ,here is my ??s.i have a set of merlin,castiron,square port,320 runners.i was told not to use the merlin heads on the efi motor because they are not stock heads and the efi motors run on such a narrow perineter that any small change to the motor upsets the tune-up,and decreases the realiabltey.what do you guys think.my machine shop says that the head will only flow what the intake will let it.thanks.

If you change the exhaust also you need to get the ecm tweaked. Just changing the heads I would expect the exhaust to keep your flow down. Some one else will chime in soon.

sounds to me like your machine shop is uneducated in what can and can't be repaired these days. There is a place in Ohio that can weld your head up good as new. Probably cost more in freight that it would to do the repair. You'll probably have to replace the seats if you go that route. On teh hother hand, if you have a set of Darts, put 'em on and be done with it. The exhaust side will flow much better than the GM head and make more power. Not sure if you would need to reprogram the ECM or not. Others might chime in on that one.

Check the casting number on that head. If it ends in "990", I have a good one you can have cheap. The computer can adjust for the subtle change the aftermarket head flow will cause.

I have to agree with Griswald about the probability of a repair.

the casting # for the head is 14097088.1993-94?the boat is a 94.putting a head on the motor that flows more exhaust will that make the motor lean?i am hopping just to r/r the heads and do nothing to the ecm.

gen VI head....

I have 2 sets of 500 efi heads that will bolt right up, one set is fresh w/ brand new valves. the other set has about 60 hours or so on them.

If you don't change the cam, you don't need to recal the computer.

gen 5 heads.everything stock except breather.

Gen V blocks/heads had some issues anyway. (Actually with the coolant port size/shape on the block deck) If you have not had any head/gasket problems with the Gen V engine, you have been fortunate. That is why Gen VI engines came out so quickly after the gen V's were introduced.
I recomend two things.#1 Put on the aftermarket heads (That have been designed to work on Gen IV, V & VI decks) and you will end up with a more reliable, better running engine. (The ECM will be fine. They run too fat from the factory anyway) or #2 Find a replacment casting and put it back to original.(Use the correct Gen V Fel-Pro, Marine head gaskets)

Good luck, Curtis