daqcraig

I have a 98 Baja 342 with twin 7.4L MPIs. Hours are 220. This boat will go 58mph at about 4200 RPM with 3 blade 23 pitch mirages.

I am told these motors are relatively low compression and good candidates for 5 PSI boost with the Prochargers.

I really don't care anything about a hole shot or anything like that. It would be nice just to be able to get more top end and cruise across the lake at 50 Mph with less RPM. So it sounds like the power curve of the centrifigal supercharger would work for me.

Here is the problem. I was told by prochargers own tech that I should beef up my head gaskets. Then, prochargers largest dealer told me I shouldn't beef up the head gaskets. He said it was better to blow them versus the alternative. In addition, the mechanics of a guy who has the same exact boat as I, told me he keeps floating the heads and was going to go another route in his pursuit of extra power/torque.

So, I don't know what to believe or think. My local marina mechanic told me to leave the motors alone and be happy. Procharger says the 7.4 MPI is an excellent motor to supercharge, but only 5 PSI.

Can anyone tell me the truth? Also, if I do add prochargers, do I need to add boost and fuel pressure gauges?

Dock Holiday

Build it right and DON'T blow anything.

I would definately change the head gaskets.

Pat McPherson

If the engines are L-29s with the 100cc vortec heads they have 9:1 compression so maybe with 3-4 lbs of boost they will live OK.
An easy way to tell is if they have the GM MPI or Merc MPI on top. If they have the Merc MPI them they have 119cc round port heads and 8:1 compression.
Either way, you should probably consider rebuilding with Forged pistons and good valves before supercharging.

daqcraig

These are Mercruiser 7.4 L MPI Gen VI. Compression is listed as 8:1.

Does that change your opinion about the pistons and valves?

They say this motor will take on 5 PSI boost with no problems. True?

ThruHull

Do a search on 7.4L MPIs in the search box, there is a ton of information on them. I explored the same route and I read more bad things about adding a charger to this engine than good things. Bottom line, I don't think you get alot of HP per dollar spent doing this. Alot of things limit your benefits such as fuel system/intake and cast internals. I decided just to pull the engine and replace with 502, kept the 7.4 for a "back up" motor.

Pat McPherson

No, forged pistons and good quality valves are important if you want the engine to last with a supercharger. The stock cast pistons will melt and grab rings or burn a hole in the top sooner than later. The stock exhaust valves may also melt with the added heat.

From what I have read the cast crank and 3/8 bolt rods are good for 450 to 500HP MAX and it is recommended to not spin more than 5200rpm.

My guess is that 5psi boost added to a stock 330HP engine will net 430 to 450HP.

IMO, before supercharging, rebuilt the short blocks with new pistons, rings, bearings, oil pumps. Rebuild the heads with at least new exhaust valves. And yes, good quality gaskets...

Other things to consider are fuel system upgrades as well as ECU mods...

jaw111

You need to get cometic head gaskets.best head gaskets for blower,nitrous high horsepower engines.They come in different thickness,you can get a thicker gasket to lower your compression.did this on my blown with electronic fuel injected 509i went from a .035 thickness to .085.lower my compression 3/4 from 9-1 to 8.1/4-1

mike11

Are they Mag mpis? If so I would deffinatly go over the valvetrain, check head and deck surfaces before installation, for good gasket sealing before assembly.The 5 PSI kit is max without reflashing PCM or upgrading injectors. Even doing 5 psi you may see high 60's mph. If they are not mags I do not suggest superchargers. FP and boost gauges are a good thing to have.Both gauges allow you to make sure your pres regulator is working properly and things don't go boom. If you have any other Q's feel free to call. We are a Pro Charger Marine Dealer if you need anything. Thanks Mike

Roccard Marine Engines
570-956-2840
www.roccardmarineengines.com
[email protected]

RBeyer

There have been a lot of great replies to this thread in regard to modifications, why we should or should not. I have had a head gasket expieience with a ProCharged motor.
Kometic gasgets were the answer.
I agree that if you are going for performance that you should do some planning have the Jing $$$, my guess is by the time you are done it will be $20,000.00 this includes the rebuild after about a year, the Pro Charger and a definite Drive upgrade.
I'm not saying don't do it. I love the fact that I went down this road but be prepared. Do your homework regarding fuel systems and computer remapping. You can have a great turn key piece if you pay attention to the details.
Bob

Outdrive1

On a non mag motor I would stroke it and put a carb on it with some forged internals and a decent cam. Maybe even do a little port work on the heads and you should easilly see 500 to 550 hp for less money than a complete Procharger set-up. Then you'll know it will be built right and not blow up any time soon. If it was a mag motor I would say a low boost procharger would be easy hp, but IMO your motor with cast internals won't make it the first season and you'll be rebuilding them anyways. Supercharger kits usually wont include remapping your computer and an upgraded fuel system(pump, regulator and bigger line). Those seem to be the most important elements to motor life with a supercharger, and you can add an extra 1500 to 2000 for those little extras.