Advice on MPI's fuel systems
 

I'm planning to buy another engine either a used Gen VI 454 0r 502. I want to bore the block about 30 over, put in some forged pistions and have the heads re-worked and larger valves added new cam and exhaust. My question is, what can be done with the stock fuel injection system ? Will raising the fuel pressure be enough. I really don't want a blower, because of cost but most of all reliabilty.

Rick,

You are asking the same question as I did yesterday. Look at my thread titled Modifying a 502 EFI. I thought I would get several replies but the topic is quickly dropping to the bottom of the page. This is the most common engine in performance boats and I figured lots of people modifyed them but I guess not.

Good luck and post what you find out.

Ron

Rick, as you know from my email, the MPI can be a source of limitation on a performance engine. The big question here is: Which MPI system are you using? The 7.4 MPI or the 454 Mag system? They are totally different. I will relay all that I have found out. I have spoke with Arizona Speed & Marine, Vortec and a few others. The consensus is you can modify either system with more psi at the injectors, recalibrate your ECM and even swap to bigger injectors (depending on your horsepower level). The biggest factor seems to be CFM flow through the throttle body. If you look at a throttle body on a 7.4 MPI and compare it to one from a 454 Mag, you will see what I`m talking about. Also, by comparing the two intake manifolds, you can easily see that the 454 Mag has a superior style design for pushing air. It is a big, tall intake compared to the 7.4 MPI intake. As I stated in my email to you, I am planning on swapping to a "Mag" style fuel injection. Later down the road, I want to get a lot more serious with my engine, and I truly feel like my current fuel injection will not support more than about 375-400 hp.

Thanks fellows.
Don: you answered my question. I have the 7.4 MPI system. I'm glad to know it can be modified. I'm trying to get everything lined out before I get started and hopefully stop as many "surprises" as possible.

7.4 MPI

lets see ..Scat crank and rods ... blower pistons .... Edelbrock ported oval port heads .... custom roller turbo cam .... 7.4 MPI intake .... 2 turbo chargers ..... missing Electomotive engine managment system and one laptop. Will I get to it this next season. building three other engines right now and guy down the street wants more from his oval track car just lucky to get out on the water right now .... turning to page one of oval track rule book........

Rick, What type of mods are you planning? According to ASM, the best mods for the 7.4 MPI would be: High-flow flame-arrestor, adjustable fuel pressure regulator, good exhaust system, high-perf camshaft and a recalibrated ECM. Anything more than this, they didn`t think you could get the required CFM from your current MPI system. I`m talking about things like: aftermarket heads, "big" camshaft and so on...

Gentlemen....... your attention Please !!

ANY time you change to an adjustible fuel regulator the new regulator MUST be mounted as close to the fuel rail as possible or you cannot maintain pressure. If a rail mounted pressure gauge oscillates, you have a serious problem and do not use a oil/water dampened gauge. I have proof of that!!
Increasing the pressure only allows more fuel in during the interval the INJECTORS are open (interval is controlled by ECM).
Mercruiser MPI & Mag's are BULK injection. That is ALL injectors are open at the same time....Adding pressure is not an upgrade per se, but will be required to obtain gains from a cam or blower and have the engine live.
NOTE: if a Aeromotive fuel pump is used, be sure it was built or upgraded to the mechanically attached magnets in the pump. The old epoxy design was still used until MAY 2002. Our fuels/gasoline has alcohol & additives that destroy the epoxy in the old style pumps. Those epoxy particles the stop up your injectors & viola!!! fried engine!! Trust me on this! Call Aeromotive... they are very helpful.

no adjusting the psi is NOT the only thing you will have to do!...502 mpi can make 550hp NO PROBLEM...and have done it....make your life much easier....put a carbonmaker on it!

I'm really nowhere near qualified to comment on this (as if that stops any of us) but seems common sense if the computer don't tell the injectors to squirt more, then how could you ever maintain the correct air/fuel ratio if you don't re-calibrate the ecm after the engine mods?

you can but an injector in tested and calibrated to flow X at Y...the EEC tell it time and durration...not how much!...one again you should play if your not for sure...like i said...bolt on a carbonmaker:D

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

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

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 :D

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

Fuel pressure pulsations
 

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

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.

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

"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.

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 :D

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

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.

DonMan,
actually I did see a fuel economy increase from a base 300hp 454 to a full-roller 454 -450hp block with Procharger and Nickerson flowed carb. AWESOME combo and never broke anything!!
The economy increased in cruise and in planing performance. WOT was where ya' would lose it!! This was on a '93 252SS Formula.... not a light boat!!

I tend to agree with the ECU/ECM-EFI problem....but that goes for us know-nothings that do not have the knowledge or the TIME (most of all) to really learn some of this stuff. We are more comfortible with carb's because we feel we know them. And in many ways a carb is better/easier....you cannot lean out one cylinder and melt one piston with a carb getting stopped up, the entire engine lays down....... you really can't feel it or hear it in a twin eng config. until it's too late ( my experience this summer). Now mine is fixed and still EFI. If more problems arise, I too will probably do the carb thing even though I don't want to..:D :confused: :rolleyes:

Jdelo- Not really familiar with the FAST system, but I'll assume it's sequential [or batch],a little more tuning resolution,and still a "speed density" type control system...No ego here, but I think it's the "tuned" OEM runners that make it hard to deviate from the intended use. Also, did anyone experience any seat of the pants diffrence when they went from a MEFI-1 to aMEFI-3?....In the real world, it's probably not discernable...my point is, while the MEFI3 was in controller terms apretty nice improvement, it still wasa micro improvement...this is the way we will feel about the new sequentials coming to us now...another micro improvement...they do add up over the years ,though. Put the FAST system on a suitable [for your RPM and hp range] manifold and I'll bet it's pretty sweet.

Liquid Lounge,

FAST is just one of the aftermarket fuel management systems available. HALTECH,HOLLEY AND ELECTROMOTIVE are other vendors I've been researching. And yes the FAST can be configured to be Batch or Seq. . The point I'm trying to make is that you yourself can not change or modify and parameters in the MEFI 1 or MEFI 3. The software is just not available. Plus it gets very expensive everytime you make a modification. Add headers , flame arrestor and fuel reg. 350.00 . Make a cam change and head work another 350.00 on and on and on.
Here's another issue getting it right!! The only way to tune the ECU, in my opinion, is in the boat on the water under changing loads. other wise it's just a crap shoot.

JDELO

This is a question that has been in my mind for a long time. I came very close to do modifications to my motor. After discussing it here on this board, I decided to leave the motor alone. I have been running this boat for 2 years and so far, I can say that other than upgrading that $*itty stock exhaust, I have not done anything more than change the oil and plugs. As was mentioned above, these injected motors run great at the level that Merc. sets them up for. Anything else and you will be rolling the dice as to how long it WILL take for a meltdown. Personally, I would rather run the boat every weekend than be pulling the motor out every other weekend to drop in new pistons.
The way I decided to get a little more power for next season is to look for a used HP500EFI to drop in.

Blown Formula,
While I agree that moving the regulator as close as possible to the fuel rail will help reduce the bounce you are talking about I think you may be overlooking the cause. The bounce is caused by insufficient line/rail volume after the regulator. The injectors are draining all of the supply and pressure from the rails. When you see the bounce what is actually happening is fuel is rushing back to fill the empty rails. Moving the regulator as close as possible reduces the bounce by giving the fuel less distance (volume) to fill suddenly. This occurs most often in batch fire EFI but I have seen it happen quite a bit in SEFI as well. No matter what volume or pressure pump you are using if the fuel rails cant hold sufficient volume you will still see this happen. The gauge might not show a bounce but it is still happening. The rails need to hold a reserve or you run a lean condition. This isn’t new to me at all since I dealt with it years ago when building high hp EFI fords. We put bigger pumps in the cars, bigger injectors, and moved the regulator right onto the rail. Guess what? The gauge stopped bouncing but we still were toasting pistons. This was on a sequential system requiring only 8 42lb/hr injectors. The problem was solved when we changed to larger fuel rails and installed larger lines from the regulator to the rail. In fact we ended up with an overly rich condition. If you already lost an engine due to insufficient volume and think that moving the regulator close means problem solved I would rethink that. It sounds like you have a volume issue with the fuel rails. Do yourself a favor and have the engine hooked to a good air/fuel meter just to make sure. Change out the rails for a larger volume set, make sure all the fittings are as large as possible, and use the largest lines you can after the regulator. – 10 lines usually do the trick.

My 1999 7.4 MPI has the dampener attached to the stock fuel rail. I just went and pulled it off the shelf to make sure. The dampener just compensates for the surge of fuel by cushioning the impact. Its sole purpose is to smooth out pressure irregularities seen in batch fire EFI systems. This reduces shock to the regulators diaphragm and allows it to function properly. My engine currently has a Whipple on it. That kit replaces all of the stock manifold, fuel injectors, and rails. The kit includes 6 higher flowing injectors on a custom rail. It only has 6 injectors total now. The rail provided in the kit is very large volume. The only changes to the stock fuel system are; #1 modifying the stock regulator to an adjustable, #2 the rail and 8 injector to 6 injector swap, #3 setting fuel pressure to 45-50 psi. The kit also uses a Whipple provided dampener which sits on the rail. I NEVER see any pressure bounce no matter what RPM. At WOT I am still running too rich.

Rick,
The 7.4 is a batch fire EFI that fires 4 injectors at a time, left bank then right bank. Modifying the fuel system on one of these is very limited unless you want to yank it out and start from scratch. The cool fuel box integrates the regulator so if you want to move it you need change the pump assembly to a more conventional design. As long as you have a properly functioning pump and don’t exceed the volume limits of the 7.4 common fuel rail you should be fine by using one of ASMs adjuster kits. Most likely you wont need to raise fuel pressure unless you get extreme. The stock system on these engines are usually too rich and reducing pressure works better but I would have the ECU reprogrammed before I messed with fuel pressure. Some little things I did to get a little more punch before I supercharged really helped. First throw away the stock flame arrestor and get a K&N, while you are in there trim the throttlebody gasket so that the passage is no longer blocked. This makes the intake air a little noisier but you probably wont hear it with the hatch closed. You should notice better acceleration after doing this. Make sure your base timing is set properly; I found that my engine was based at 4 degrees when I checked. I am pretty sure its supposed to be set to 10 degrees (the whipple requires 10 base). Open up the exhaust, with mine I still use the stock manifolds but I replaced the tips with a free flow un-muffled set and picked up 3 mph(before putting on the blower). A set of headers should do more for you but then you start spending more. I plan on keeping the stock headers(manifolds) until they rust out. With the blower I am already 22mph faster than I was stock (with room to go) and better headers don’t seem that necessary now :)

I'm with JDELO, 'the only way to check is in the boat on the water'. But you need proper instruments. Wide band O2 sensor and readout at minimum. That is why I'm working on the On-Board-Dyno so that with the adjustable ECU's you can change the timing and injector pulse width at different rpm's to get the max Torque. Or also with a carb to adjust the jets.

What he Said,, altho I woulda said 60 hp :cool: