502 MAG MPI - upgrade heads or stay stock???
 

Friends need advise on the following...

I recently purchased a pair of stock 502 MAG MPI's. Except for the Stainless Marine Exhaust everything else is stock.

Before purchasing these engines I was in the process of rebuilding my 454's (short blocks currently for sale on eBay) which I purchased a brand new set of Merlin Rectangular Port Iron heads as part of the build.

These are the specs;

310cc Rectangle port intake runners
119cc Open chambers
2.300" Stainless Intake valve
1.800" Inconel Exhaust valve
1.500" Dual valve springs 150lbs seat pressure approx. 400lbs open pressure
10' steel locks and retainers

I'm debating whether or not to install these heads on my 502's or sell them.

My questions are the following:

1) what gains can I expect in horsepower & torque?

2) would I have to make any adjustments to the ECU?

3) can I swap the stock heads with these heads with no further modifications?

Reality is my boat has been out of the water for 2 + years. Reason I purchased these 502's was because I could just drop them in. But before I do asking myself if it's worth it to install these heads or not.

Your thoughts!

Thanks!

IMO, not worth it. Merlin heads are not that much better than GM rect ports.
Any mod that gives any real increase in hp will need the ECM tuned.

I'd just drop the engines in the boat.

Griff makes a valid point here but you can bump those engines 100HP each w/ some head work, cam, and ECM tune. We had Tyler Crockett do that for us in a 454MAG years ago (also w/ Stainless Marine exhaust) and it really woke up the motor. To Griff's point and to address your question he modified the existing heads.

And in this process you may move your peak torque and HP up 1000 RPM. Would you be happy with that? And are you prepared to fine tune the prop selection to make use of it?

Landfish,

Can you please elaborate?

To my understanding these engines will turn 5k rpm's. Would these heads add another 1k rpm's? Whats that in HP and Torque?

Griff / SecondWind...thanks for your responses. My 502's our ready to go as they are. The thought of any disassembly makes me cringe! Lol!

From research my boat (Pantera 28') will be at the 78-80 mph range with twin 502's as is. Fast enough for me just trying to take advantage of engines being out.

Any other quick mods any of you suggest?

I have 2 sets of Crane Roller Rockers from my 454 build. I understand thats an easy install. Worth it?

The cam is teeny weeney on these motors. Thus why heads by themselves won't do much. Then from there the intake + heads could use major updating after/during cam upgrade..

Change rockers? No need. One thing to consider. Steal the new valve springs off the new heads. Everybody who runs above 5000 rpm needs to replace their OEM valve springs. Ok, is valvesprings one word or 2? Now, if you did spring upgrade, a pair of ZZ502 cams for 200 bucks each would wake them up a bit. THEN you can put on the new rockers.

Leave them be. They'll be the most reliable engines you'll ever have. When they get worn out, wake them up at rebuild time.

I would leave them 100% stock.

My stock 502mpi's have around 700hrs on them and never had a valve cover off.