7.4L Mag MPI Performance Issues

[dcg9381]

Re: 7.4L Mag MPI Performance Issues

A 2nd opinion is good. No issue there, but let me take a 2nd or 3rd run at you too. I would ask where does this fuel go if it is being pumped out of the tank at correct WOT rates? Assuming you have good compression, no big broked parts, nobody with a vice grip on any of your drive parts and no fuel leaks, unless timing is off, the only place for low power is an intake air restriction or some sort of fuel supply problem. In those last two cases then you would have low WOT fuel rates or I guess crap fuel . . .

In a boat, with a non-circulating fuel system, it sits in the fuel line until it's let out of the injector(s).

I agree with you here. I suspect if I do have a problem it's in the injector(s). Seeing as I've had one injector go "bad" - which is a very odd thing to happen in my experience, I need to figure out if it was a clogged injector or truely a bad injector. A single clogged injector might indicate that it's worth pulling and having all 8 professionally cleaned.

As these have non-circulating fuel systems, I'd like an opinion on how "common" injector issues are.

Ahhhh, but you're forgetting about "absolute pressure", you have to count Atmospheric pressure too, zero is Gauge not absolute . . . that's how it knows how to adjust for altitude which is another good question . . .
Edit: I see your 1 ATM there now, but I am not sure why this is relevant?

The vehicle I have available for experiemental tuning is turbo'd - so I can't really test the WOT theory. I can say that tuning naturally aspirated things, using an electronic gauge that read off of the MAP sensor, at WOT I'd see near zero manifold pressure, if I recall correctly (which I may not be recalling correctly)... and it would not deviate if air/fuel mixture was off. I have logging that can provide information to the same accuracy level that a MAP sensor can read...

O2 sensors are there to provide perfect stoichiometric burn ratios so that you don't foul a catalyst. We are talking teeny weeny tiny variations of rich/lean ratios if the ECU is mapped properly and the MAP sensor is good . . .

I'm not sure that they're just on there so we don't burn out a catalyst. Modern turbo vehicles are configured from the factory to run pig rich (safe) under boost - I think it would take a ton of fuel - enough to really make the vehicle run badly to foul a catalyist.

I do believe they are on there for the sake of fuel ecomony. They measure stoich, which is leaner than peak power. Closed loop, vehicles adjust to 14.7:1 at cruise... Leaner than that, you can feel it. Many modern vehicles also have a "learning" capability to shift the entire fuel map up / down based on 02 feedback.

 

[mcleaves]

Re: 7.4L Mag MPI Performance Issues

I think at the performance levels you are running at and are seeking, small differences in prop cupping, diameter and pitch could yield the discrepancy. You can put several props on there to give you the same rpms and you will be at a variety of speeds.

I am with bond-o. The engine sounds like it's running well. This may just be what it is with your setup. How clean is the bottom? Is the hull waterlogged or have a high moisture content? 5-10 gallons of water wicked up in the wrong areas can have big weight ramifications, especially the further back you move it (I realize this latter is a stretch, but there is a point to bringing it up.) Battery location? Battery sizes? Your boat and your buddys boat may not be the same.

BTW where is that RPM reading coming from? If it's the dash get a real tach on it and run it again. You might be surprised to see you are running a couple hundred lower than you think..

M

 

[f_inscreenname]

Re: 7.4L Mag MPI Performance Issues

GPS has crushed many dreams.
I installed a Gen IV 454 (MIE340) totally rebuilt to original spec's in my 19' in the avatar last spring. Volvo Penta (with trim), my best prop L 14.25X24 SS = 54.9mph @ 5000rpm's and I am in the wicked high teens when it comes to prop slip.
In a bigger boat (got a head on that thing?) and a smaller prop you are doing this?
Results now:
4700-4800 rpm, 61-62 mph, 23 pitch Mirage Plus prop. Bravo-I outdrive.
Calculated prop slip is 11%. This is good performance, but not to spec.

You're not doing to bad and I'm re-building the top end this spring.

 

[dcg9381]

Re: 7.4L Mag MPI Performance Issues

Port-o-head, yes.

The hull of mine is very very clean. Land-lubber kept, gel coat almost looks new.

I went through a few problems when I bought this thing - I was told that he speedo was off (not unusual), but I didn't bring GPS and I knew that the boat should tach out to near 5k with a 23 pitch prop, so I thought it was fine. What I didn't know is that someone had tweeked the tach to read high.. :-(

I eventually went in and recalibrated the tach using an inductive timing light which does RPM.

I'll double check it next time I'm out on the lake for high rpm inductive vs actual tach.

Speed was via GPS.


If I am actually getting 4600-4700, 11% slip, that's about all I can realistically expect out of current tune. More RPM = less hp / torque (guessing).

I've also found a local marine dealer that has a dyno - that's probably the easiest way to answer my question...

Busy changing the raw water pump, impeller, and water circulating pump this weekend... Not quite ready to unwinterize, even in Texas.