Does anybody know what the GPH flow rating is for the 2004 and earlier Mercruiser 496HO cool fuel, fuel pumps (low pressure pump 861155A3 + high pressure pump 861156A1)?

Not sure of the ratings on those pumps, so I am probably no help, but the engine uses around 35 GPH wide open.

Would also like to know. Are the pumps in the late model gen 3 cool fuel box the same as the earlier pumps?
Rage, are you still running the stock pumps on your setup?

Put system at 14 volts (don't run engine) and measure fuel out of the return. If this is doable of course.

Yes, I am still running the stock 2004 496HO fuel pumps. I may have been lucky to get away with that because of the 915' elevation and atmospherics I run at. Diacom calculated/reported 37 - 40 GPH @43.5psi or 39.7 - 42 GPH actual for 50&48 psi test fuel pressures @5200 rpm on Dyno test of 2008 build engine (AFR 12- 12.4). The Dyno flow meter how ever recorded 10% higher GPH @ the 5200 rpm. This was fuel for an actual 514cshp @5200 with full wet exhaust at test elevation and atmospherics, which would correct to dyno standard 594cshp @ 5200 rpm with wet exhaust which would require ~16% increase in fuel flow.
After ECU recal'd to correct out of balance AFR of three cylinders in a lake test Diacom calculated/reported 38.5 GPH @ 43.5 psi or 40.4 GPH actual at 5800 rpm for 48 psi fuel pressure (12.8 AFR). Boat test elevation and atmospherics at time of test indicate engine was producing ~567cshp @5800 rpm at 40.4 GPH Diacom recording or 44.4 GPH projected actual flow at 48 psi fuel pressure. These GPM's all assume the fuel pump is keeping up and holding the fuel pressure at set point.
Upgraded engine has much better VE but needs about 46 GPH for same test conditions or ~50 GPH at standard dyno test conditions. Replacing 37 lb Delphi injectors with 52 lb Bosch so fuel pump can run at 43.5 psi fuel pressure for better GPH and reasonable injector duty cycles. I do not want to replace current cool fuel system unless it is inadequate for current engine.

Interesting suggestion. Engine is currently apart.

Actually great Idea which I will attempt.

I agree with this


www.marathon-boat.com

I finally tested the GPH flow rate for the 2004 and earlier Mercruiser 496HO cool fuel, fuel pumps (low pressure pump 861155A3 + high pressure pump 861156A1).
Result was 58.1 GPH @ 13.4 volts with 43 psi Mercruiser OEM fuel pressure regulator. Test set up pictures attached. Jumper cables to running vehicle to maintain 13.4 volts is not shown.

Test fuel was E10 which weighed at 5.94 lb./gal. At 5800 rpm engine is projected to require 337 lb./hr of E10 at standard dyno test conditions of 60F air and 29.92 "hg atmospheric pressure which would be 56.7 GPH. So Mercruiser fuel pumps are adequate for my engine.

Plan B was a 67 GPH @45 psi Holley 12-920 fuel pump rated for 700hp that could in theory be adapted to be a direct replacement for the high pressure Mercruiser pump 861156A1. The tricky part would be to drill out and ream the female pipe threads on pump out put to accept the Mercruiser oring'd elbow fitting with out contaminating the pump internals. https://www.holley.com/products/fuel...s/parts/12-920

Good info. Seems the stock 496 fuel pump setup is pretty stout.