slaquer5

OK thanks for the info. on the Procharger. Since I should not put a Prochagrer on my 7.4 could anyone tell how do get more speed without puting a new motor in the boat


DMH

Pat McPherson

I was where you are last year. I corresponded with many knowledgeable people.
What I was told and what I have now experienced first hand is that the GM fuel injection system will only flow enough air to make 375HP at 4600-4700rpm.
I changed the cam and exhaust to get there. The engine does make a good bit more low end torque now too.
My boat went from 65-66mph @ 4300-4400rpm to 68-69mph @ 4600-4700rpm spinning the same prop...
If you would like a bigger gain than that, you will need to ditch the GM fuel injection.
Good Luck!

robyw1

Respectfully I disagree. I can work over the GM fuel injection system to be as good or better than Mercs. You first need to extrude hone the intake because it is rough in there. while you have it off gasket match the intake to the heads. We are talking about Vortech heads here so this can be done. I replaced my injectors with the RCI disc and am looking to AS&M for a throttle body. The bad thing in the stock 7.4MPI is the silencer gasket that is used in this engine. Look down the throttle bore and notice how it blocks more than a third of the opening. If need be I will build my own rails but I think the stock one will suffice for the undisclosed horsepower that I am looking for. I have already successfully completed most of these mods and I have to tell you I feel as though I have gained good numbers. Best of all the engine looks stock except the wild hissing sound coming from my TB. I will post more when I am done

Roby

Pat McPherson

Hey Roby,
Are you an engine builder?
I have an extra intake I would gladly donate to your development.
When you come up with a good combo I would definitely be interested!
My engine is a Volvo Penta version so I don't have the gasket behind the TB that I believe you are referring to. I don't have the elbow between the TB and the intake plenum either. Is this better for flow?
I changed the L-29 cam out for a 454/502HO cam, changed the valves and springs, and installed Stainless Marine exhaust. Had and ECM re-calibrated 3 different times by AZ Speed, but the stock Volvo calibration still works the best
This off season I was planning to change the cam out for a ZZ502cam.
I am thinking I have to ditch the MPI for a carb to realize the gains from the larger cam. Would be great if I could retain it! I also have another set of Vortec heads that could be machined and ported...
Roby, I hope you can do what others say can't be done.

DonMan

This all sounds great if cost were not a factor. For me it is. I have already ditched the MPI for carburetion and I`m extremely satisfied. I eventually replaced this 7.4 for a stroker big block, displacing 489 ci. Now I have POWER.

PowerboatsNW

Personally, save your money and do not mess with the 7.4L motor. This motor only has cast pistons and crankshaft. Spinning it more rpms will result in a broken crankshaft. This motor I tell everyone is what it is. Small heads low compression truck motor. Messing with props will get you more out of your boat than spending $2000-$4000 dollars for 50hp. However that is only my opionion.

Revd Up

I have to agree, I pulled my 7.4 and dropped in a 502. My 7.4 ran hard, but the 502's a blast. It's a stronger engine. Roby1 keep up the good work, get that scarab rippin.
Steve

slaquer5

Once again thanks to all of you for helping me out...

DMH