Picking up pair of low hour Merc 502 EFIs. They have the stock merc manifolds and am curious what I would lose in HP versus using the Gils I own?

Plan is to upgrade CAMs, intake to Victor Jr, and go Carb. Goal is 500-550 reliable HP and good torque numbers for a big cruiser. Most cruise will be 3500-4000 rpm with short blasts to WOT. NOT a poker run boat by any means.

Should I reuse the EFI stock manifolds or use my Gil exhausts off my 454s?

What do I lose or gain? These are closed cooled motors.

As always - thanks for the advice Guys!

Happy 2017

OE is fIne for a stock motor but I wouldn't trust them even then (mine failed). I'm no builder by any means but from what Iv'e read you will be limited with your cam selection... I would use your Gil's.

Once you upgrade the cams the stock manifolds will revert, fwiw

I don't see any downside to using the Gils. They are better flowing and have a dry joint from the manifold to riser.

thanks - what I thought.
Appreciate inputs.

741 CAM okay with Gils?

distance from engines to exhaust throughulls is 4 feet and there is about 10-inches of drop. Water dumps into the Gil riser and I dont know if it has the "ridges" inside them that I have seen described in other posts. Am I okay?

O

Dont forget some good valvesprings . The cast iron manifolds with a cam upgrade would not be good. I would still keep a eye on it with the aluminum gils , at least initially just to make sure your safe. you may have to filddle with special riser plates water flow idle speed ect with a zesty cam but you should be able to run with it

Thanks Guys for inputs but I can't "Fiddle" with things - just dont have the skill set.

If there are spacers I should use to keep reversion from happening I can install them. If I need to have the water dumps moved down stream I can get that done too. Only looking for 500-550HP and low RPM torgue so hoping the CAM wont be a water sucker.

If you have a pair of low hour 502MPI engines with no problems, you should seriously consider leaving them alone. They will run forever and by design, they are high torque at lower RPM engines. A 741 cam is very near the largest cam most would recommend in a marine performance 502 and swapping the MPI intake for a standard intake and carb certainly is not going to boost lower RPM torque. I think you could save yourself a ton of money and aggravation leaving them stock or doing a proven cam upgrade with ECM remap.

Solid idea..

I agree.I've owned my boat for 11 years with twin 1997 502mpi mags that now have a 1000 hours on them. They run as good as when I bought the boat with 240 hours. The engines have never been apart.