Can someone tell me what the max HP the stock fuel pump will support. This is the engine mounted pump. I'm planning on 550 hp. The info I have found shows 120 GPH, I would think that is fine.

Your gonna be real close to the limit and may lose pressure and flow at a long full throttle run. I would call Teague marine and buy a fuel pump manifold and add a small electric pump as a supplement to the mechanical pump. I had to do this when I put a Whipple on a stock 454 Mag. It ran out of fuel at anything over 3/4 throttle. You need to safe guard against going lean or you will fry the pistons in a matter of seconds.

The 120 gph is a free flow #. When you put a fuel hose on the pump, choke it down and raise the pressure, the flow goes down considerably. Almost all pumps are rated as free flow. That is why you need so much more pump than you think you need. I always go at least twice the flow that I need. For example, if the engine will use 100 gph, then I use a pump that has at least 200 gph. That is the minimum.
Smitty had a great idea. You could also just change the pump section of your existing fuel pump. This will save some money by not having to replace the entire pump. Good luck with it.
Eddie

How do i change the pump section? Do you sell the parts to do this?

It is just a straight bolt on replacement. It only takes a few minutes to do. I do sell them, but you could just as easily call CP Performance (1-800-225-9871) and speak with Mike (ext. 17). Tell him I sent you and he will fix you up.
Eddie

Didnt the merc 525SC, 500HP, and others use a stock mechanical pump? My buddie had some hopped up 500HP motors that made around 600hp, stock merc sea pump mounted pump. 100+ hours on that setup. Was he just lucky?

Thanks Eddie!

I ran the stock pump on my hopped up 525SC with about 560hp with no issues. Even following Eddies recommendation, the stock pump should be enough.
A 550hp engine will burn about 50gph at WOT.

Thanks Griff, thats even better news.

I run the carb 500 HP stock fuel system with motors that dynoed at 605 hp, maybe 575 hp in the boat. No issues, 6-8 psig at the carb all the time. Been that way for 6 years.