View Full Version : Merc 502 MAG MPI to 500HP Conversion questions?

03-25-2013, 02:14 PM
I am considering upgrading my 502 with some HP500 Heads and a custom marine conetics cam and ecu tune. Just curious what other parts would be needed to convert a 502 to an HP500? I already have the HP500 Gill exhaust. I assume I would need to change the intake as well? If anyone has done this before, I would really appreciate some advice. Thanks!

03-26-2013, 07:29 PM
Intake certainly, cam, heads, gil exhaust of better (check for leaks), ecm flash. You got it covered. Throttle body.

03-27-2013, 08:21 AM
I think glasshole did this to his old Blackhawk.

I'd research the fuel injectors as it's a pretty decent jump in power so the stock ones might be undersized.

If it were me I'd port the intake while everything is apart. I have to assume the 500EFI intake is different because it flows more so anything you can get out of the 415 version to flow better seems like it would be worth it.

As for a parts list to consider in case you haven't thought of it... lifters, valve springs, roller rockers, valves, pushrods...

joseph m. hahnl
03-27-2013, 06:05 PM
Bump the compression up a notch ,use aluminum small chamber heads with a longer duration cam,and 1.6 rockers:propeller:

Pat McPherson
03-27-2013, 08:08 PM
The 500EFI uses a Crane roller cam that is basically the "741" with a 114deg lobe separation. You want that cam or something similar along with roller rockers. The heads you have are the same castings but the 500s have the larger 2.25dia intake vales. The stock Merc MPI will feed the beast with a program change; I suggest Mark @ Precision in New Orleans. The bottom end of your engine is the same as a 500EFI.
I just sold a buch of parts I had collected to do just this conversion on my last boat's 502MagMPI. I do still have a set of Gen VI HP500 heads if someone wants them; $500.

04-16-2013, 11:51 AM
Thanks for the responses. One more question, Sometime in the near future I am goign to need to pull my engine and refresh. I am starting to loose a little bit of compression in #8. It is at 115 now with 145-150 everywhere else. So, I'm planning on changing the cam and lifters as well as having the heads machined/ ported and upgrade to Inconnel valves/ upgrade the springs and rollor rocker conversion. My question is, should I re-machine and port the stock heads that are on the boat, or would it be worth it to upgrade to some HP500 heads like Pat's and have those re-built and upgraded? I would rather not buy new expensive heads if possible since I am not looking to pull huge amounts of power out of the motor. I would hope to be around 500-525hp or so which I dont think will be a problem with an ECU tune. I do already have HP500 exhaust manifolds and risers on the engine. Anyways, any advise would be appreciated.

Pat McPherson
04-17-2013, 08:18 PM
The difference between the 502mag heads and HP500 heads are larger intake valves, stronger spring for the bigger cam, and studs for adjustable rockers. The castings are the same if they are from the same generation; IV, V, or VI... You can turn your heads into HP500 heads but it may cost more than buying a low hour set with the larger valves already in them.