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Discussion Starter · #1 ·
I will be installing an adjustable fuel pressure regulator soon and was hoping someone could give me a little insight into it`s installation. I have a 7.4 MPI (310 hp) Mercruiser (1998 model).
I have been told that I need to pull the intake manifold and stand on my head to get this done...True? Also, what is the factory PSI? About what should I adjust it to? I have added Stainless Marine Hi-Torque exhaust manifolds and large diameter tailpipes/tips, along with a K&N flame arrestor. Otherwise this is a stock set-up.
I am somewhat disappointed in what little improvement these modifications have given. Will bumping up the fuel psi unlock the horsepower that the exhaust should have given?
I`m a little frustrated right now and the wife isn`t too happy about the money I spent for such a small gain (if any gain...)
Can you guys enlighten me?
THANKS!
 

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Donnie,what size boat are you running? I could be wrong,but I dont think a fuel regulator will do much.What was your gain after the exhaust up-grade ?What rpm are you running and what kind of prop?
 

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Discussion Starter · #3 ·
I have a Baja Hammer (22', 3300 lbs) I am running a 23P Mirage Plus. Before mods, was pulling 4650 rpm @ 68 mph (speedo).
I took it out Friday (very windy, extremely choppy) just for a short burst, I hit 4600 at about 67 mph. Didn`t really get to play with trim much and the little Baja was sure bouncing around, so I didn`t try anymore. I just felt like I would easily see 70 or so......
 

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I think you should do more testing under better conditions.The exhaust alone could possible get an extra 2mph. I gained almost 3mph and had to go up another prop size just after installing my Gils. Keep us informed on your results.
 

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Don, you need to be very aware of what your plusgs look like. You have increased the breathing of the engine without increasing fuel flow and I suspect you are running lean-this is bad. This would explain why you lost rpms and speed. Upping the fuel pressure may help a little but a reprogram on the computer is probably what you need. The stock 7.4 mpi does not really need any help breathing until you make other mods and since the efi does not have a feedback circuit it doesn't know there is more air in there-it is still putting the same amount of fuel through the engine. The best course of action is to decide all the mods you are going to make and then get with a company that can reprogram your ecu to work with all of them.
 

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Discussion Starter · #6 ·
Thanks for all your advice Boatnut. I am going to try to take it out again first thing in the morning, weather permitting. I was told by a couple different sources that I would be needing to up my fuel once I increased my intake/exhaust flow. That`s why I was asking about the reg.
Stainless Marine verbally guaranteed me at least 2 mph and 150 rpm. The water conditions were pretty awful the day I did my testing, so I may as well throw that out the window and try again.
Will let you know what happens when I get some good test runs in....
THANKS!
 

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I agree with insptech,ck your plugs.I would suggest AC Delco rapid fire #1. I don't think the exhaust change warrants a update on your computer.My buddy runs a 7.4 with Gil and did not change computer settings,and has NO problems.

insptech hit the nail on the head,get all your mods done,re-mapping a computer is costly.
 

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Discussion Starter · #8 ·
insptech said:
Don, you need to be very aware of what your plusgs look like. You have increased the breathing of the engine without increasing fuel flow and I suspect you are running lean-this is bad. This would explain why you lost rpms and speed. Upping the fuel pressure may help a little but a reprogram on the computer is probably what you need. The stock 7.4 mpi does not really need any help breathing until you make other mods and since the efi does not have a feedback circuit it doesn't know there is more air in there-it is still putting the same amount of fuel through the engine. The best course of action is to decide all the mods you are going to make and then get with a company that can reprogram your ecu to work with all of them.
I appreciate your advice Ted. I have a full day off work tomorrow, so I plan on tinkering with it all day. I will read the plugs first thing. I know a lean condition is not good. Do you think if I up my fuel, that this will help me, without recalibrating the ECU?
I was planning a cam and head swap next winter, but not until then.
 

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It will certainly not hurt and it could give you enough enrichment to make a little more power. But also be aware that it could lead to the infamous black transom at idle due to the richer mixture. Play with it, watch your plugs and listen for pinging;)
 

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Discussion Starter · #10 ·
Here`s the latest....

I took the boat out again yesterday morning. Very good water with a slight chop, calm winds, temps in the 60`s. Top speed 68 mph @ 4650 rpm on Farria gauges. This matches my best speed, but I still think it should have done better. The plugs looked good with a light tan color all over...Probably a little on the lean side, but not in danger zone. I went home and called several marine techs and a couple of exhaust manufacters. The exhaust people told me that not everyone sees an increase in top speed, but most all report an increase in low to mid-range power, and I will agree my boat pulls harder now. The techs all advised to get an actual fuel pressure reading before jumping out and buying a regulator. But most agreed that I will probably need it anyway.
I talked with a guy at Arizona Speed & Marine. He was informative and quite helpful. He suggested to up the fuel pressure to 39 psi (if not already there) and try that. His thought was that I would benefit most from recalibrating my ECM ( $350 ). But his main suggestion was to have my prop blueprinted since I was near the redline of my 7.4 MPI ( 4700 rpm ). I`ve been thinking about doing that anyway. A shop in Houston blueprints for $175 and guarantees +2 mph or your money back. Can`t beat a deal like that!
Here`s my plan: Check/adjust fuel pressure, blueprint prop and go with that this summer. Over the winter, install larger cam and possibly a pair of Merlin heads and send ECM to Arizona for recalibration. After all that I will realize I have too much power for this little hull. Wait a minute, there is no such thing as too much power!
 

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well guys that 310 hp motor has a limiter on the rpm's at around 4650 to 4700 so i thank you need to blueprint the prop or go up in pitch, then you will see a improvement, with the increase in H.P.
 

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Discussion Starter · #12 ·
LATEST UPDATE:

I am working with a prop shop to blueprint my prop for more pitch. Should be a good improvement....
Will let you know.
 
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