ursus

Actually Merc fires 1/2 the injectors at one time they are banked in 2 groups of 4 at least on the Mags, not sure about the 7.4 MPIs. I am also not sure why the adjustable regulator needs to be closer than the current one. In the cool fuel system the stock fixed regulator is on the pump (actually the regulator is not fixed as it veries preasure with vacum from a vac line connected to the plenum) . An adjustable regulator merely replaces this regulator adjusting preasure to say 46 0r 48 psi from the stock 43 to add 5-10% more fuel.

If the preasure is going down a lot and fluctuating seems the problem would be the stock pump could not keep up with the increased demand for fuel not the regulator position. I guess it depends how much extra power you are adding from stock, +50 hp is prob ok with the stock positioned regulator, but for 600hp you need to start from scratch with fuel pumps/regulators, injectors ecu programing etc

blown formula

Ursus,
The problem comes when the regulator is moved farther away from the fuel rail is: the injectors opening and closing so rapidly cause a "hammering" effect between the regulator and the fuel rail. My pump is the Aeromotive A1000- well able to handle over 60# pressure. I installed a fuel pressure gage at the fuel rail a\nd the regulator on the engine bay firewall about 30" away. The pressure needle oscillated rapidly from 20#-60# and was never steady. There was not enough fuel to even idle the engine for over 40 seconds.
We moved the regulator to the Merc ECM box and pressure (as the regulator had been set was well below 30#. Per my Merc manual the minimum pressure is 37#...... I keep hearing other guys give other pressures but mine will NOT run below 36-37#. Anyway, we reset the pressure to 37# and it idled fine. We finally set it for 39# as a safety measure ( I had melted a piston!) .
The reason Merc may get away with the location is because they return the by-pass fuel to the cooler (t's into the fuel line) and not to the tank. Maybe that makes a difference. All I can do is advise what "was in the real world" when changing things and creating problems. This one ran me, Aeromotive & Procharger crazy 'til we found the fix (3 weekends!!).

And yes, I am probably close to that 600 hp with the Prochargers running on the Mag MPI.......
Ain't speed FUN!!

ursus

I wasn't trying to dispute your problem / solution was just trying to point out, if you have the stock merc fuel system and just want to up the fuel presure a few lbs to compensate for exhaust/intake mods then you don't need to relocate the regulator just cause you switched to an adjustable one.

If you do more radical mods than this then yes you need to go to bigger injectors, bigger pump, in short redesign the fuel system as you did

P.S. The different fuel preasure #'s you here may also be due to merc changing (increasing) it with the 1999 model

Dennis Moore

Blown Formula is on to something here! The cool fuel EFI engines have a second fuel pressure regulator on the fuel rail. It is not connected to engine vacuum nor does it control fuel pressure. Mercruiser uses this regulators diaphragm and check ball to control fuel pulsations.
Sincerely
Dennis Moore

DonMan

Not that this applies to any of the stuff already mentioned, but the 7.4 MPI uses a "GM" designed injection system which probably fires the injectors with the spark plug firing order (that`s my guess). All other Mercruiser big blocks use a "Mercruiser" designed system. On my 7.4 MPI, the regulator is located with the cool fuel system. I had 32# stock (at the rail) and had to turn it up to 43# before I had a good idle and throttle response. This after adding exhaust and K&N.

ursus

only on the 1998 cool fuel engines, and the 1997 and earlier VST , Merc dropped this on 1999 and newer models which use a smaller injector and higher preasure (43 psi) than the 1998 and earlier models

liquid lounge

"550 HP, No Problem" Well, I'll take your comments out of context to make a point; you will have nothing but problems on your way to making 550hp w/ that set up...AND as an added bonus: You find that your buddies carb motor makes 550HP with LESS cam AND HAS BETTER DRIVABILITY and even runs cleaner??!!!WOW! It took me a few years, a lot of dough, and some serious down time to finally admit that a carb is superior in almost every way to F/I in a high perf. marine application. I just want to HURL[sp? i.e.:throw up] every time I read on this site: "put in a bigger cam, exhaust, then send your ECM in for re-programming" .....O.K...can someone tell me what this does? It never did anything for me. At best you are trying to band-aid the effects of intake reversion making the MAP sensor drunk,and no, not even JIM can control this with a skyscraper full of reprogramming equipment[as some would have you believe] Any more overlap introduced than a 502 mpi cam starts to degrade your closed loop perf. Save your cash or be happy with a 70hp max increase, if you want more, you will be happier with a carb.

blown formula

Hey guys,
The two types of FI I understand are sequential and bulk.
Bulk is what Merc uses, all injectors fire at once/per side. You can confirm that by looking at a wiring diagram and all injectors are wired in parallel on each bank & have the same colored wires on each side.
Ursus- I'm sorry, I did not take your remarks that way. I was trying only to cleify myself. It is too easy to be misunderstood when having only a limited exchage such as this. Truthfully, I had not even considered exhaust only mod's as requiring more fuel pressure, but it sure could cause a lean condition, just not as bad as other mods.
But fuel pressure ALONE will not in itself create any appreciable difference in hp. More fuel & no more air is not the answer!!

Thanks Dennis, I hope you got my PM with the suggestion for an article

jdelo

Rick252,

when modifying Merc EFI systems, it always boils down to making changes to the Merc ECU.
Unless you can get your hands on the software or have a friend that works for Merc or GM powertrain, your at the mercy of a select group of people. So with that said, the first MOD I would make is change out the MERC EFI to a good after market fuel management system such as the F.A.S.T. system

JDELO

DonMan

I would love to chunk my MPI and install an RPM intake with a 830 Holley. I truly believe this alone would wake up my engine (after a high$$$ exhaust did nothing). My big concern would be fuel consumption increasing alot. Can any of you guys give me a personal experience of swapping from injection to a carb? I am ready to do it, but am very afraid of seeing the gas gauge move faster.