If your dead set on keeping the merc efi, i would talk to tyler crockett in michigan. He' did a buddy of mines setup. 540CI, custom build, with a merc 525EFI on top of it. A few bugs along the way getting it tuned properly, but it made some real nice numbers on the dyno.

My thoughts are though, if the reason you are wanting to keep the efi, is because of turn key type performance, then leave it stock. Almost everyone i talk to who has modified their merc efi engines, and played with different programming etc, fuel psi, etc, has had issues getting it to run properly. It can be a royal PITA to have to keep sending your ecu box halfway across the county, wait a couple weeks, get it back, and hope it runs properly.

EFI systems are a dream when they work right, and a nightmare when something goes wrong. For example, you get a custom ECU program on a modified engine. One day you're out boating, and boat just aint runnin right. Something is wrong. Could be a TPS sensor, map sensor, ignition issue, knock sensor, maybe even bad fuel. If you yourself are not a wrench, who you gonna have service your boat to find the problem? Local merc dealer prob wont wanna touch it, and if they do it'll prob cost a ton to have them try and sort it all out. Before ya know it you'll be towing the boat several hundred miles to get it dialed in.

A good old carb engine might be a dinosaur setup, but when somethin aint right, its usually nothing you and a buddy cant straighten out at the dock with over a couple beers. Not too mention the massive amount of options you have to with your new power. Guy's like Mel Riggs, Eddie Young, Dave Wesseldyke, and a few dozen others, can build you a turn key carb motor that will idle like EFI, and provide many years of service. Eddie Young also does a very nice 598CI 750HP turn key EFI package.

Keep this thread on track...:boat::bigbird: