Mercruiser 350 MPI rebuild
 

Hello, everyone, I have some engine building questions to ask, it seems like there is more of an engine building crowd here.

One of the two 5.0 MPIs in my new to me boat turned out to be seized up due to water getting into the cylinders. I went ahead and picked up a 98 5.7 vortec truck motor to use as a core (will get another tomorrow). I will trade my 5.0 ECMs for 350 MPI ECMs. I do have a couple of questions though before I order parts and such. Just to be clear, I want to build bone stock 350 MPIs. The most important thing in the build by far is for the stock ECM555 controller and tune to play nice with the engine. Here are my questions:

1. Is a compression height of 1.56, piston volume of 12cc, and squish gap of 0.040 correct?
2. Summit pistons ok for this purpose or are they a no no? Any other brand?
3. Is there a difference between the 811658 and 866022T cams? Difference in ECM calibration? Or are the interchangeable with the same ECM?
4. Where should I check for cracks in the block?
5. If I decide to make a 6.2 out if it, will the 811658 cam that I have screw up the 6.2 ECM? or is it interchangeable with the 5.0?

I am sure I will have more questions as I go along
Thanks!

1. Don't sweat those details as far as the tune is considered. Get the compression in the ballpark and go from there.

2. Get a good forged piston. I have no experience with the Summit branded pistons. OEM were hypereutectic. I use SRP forged.

3. I did not find cam specs but both cams are used in a metric **** ton of Merc small blocks. Kind of like the GM "395" hyd roller, very common.

4. Where ever the magnaflux detects them. Not trying to be a smart ass. FWIW my original 1995 got minor cracks between the #5 and #7 lifter galleys at about 725 hrs. Was discovered on a rebuild but not the cause of the rebuild if that makes sense. I did not reuse the block.

5. The motor is not going to screw up the ECM but the tune can screw up the motor. With a stock motor/cam/heads going from a 300 to 350 or 377/383 will require minor tuning in the fuel tables. I will lay money down that a bone stock 350 tune will start and run a 377/383 right out of the barrel (I have done it). It won't be perfect (in fact it will be closer than you think) but it will run. HOWEVER you would NEED to run wideband and see what the AFRs are doing. Tuning the 555 is going to be the biggest issue.

Ok, so I am at a little bit of a dilemma with pistons again. Talked to the machine shop and they said hyper was fine. I plan on selling the boat soon too so no need for forged IMHO. Basically I have two options, neither of which I like but I have to pick one. Both set the compression to 9.4-9.5

Option 1 is to run a dished 1.55 compression height speed pro H815DCP. I like this option because I get 50 thou quench with this setup. Also they are in stock at rockauto and I can get them right away for peanuts. What I don't like is that I will have to use a 20 thou shim gasket on an undecked block and heads to make this work. Not sure if that is a really bad thing, as the heads and block are really nice and flat.

Option 2 is to run a flat top silvolite 3437HC with a 50 thou head gasket. My machine shop recommended this method. The problem here is that I end up with about 75 thou on quench, not sure if this is a concern at 9.5 compression. Also the availability is kind of spotty.

There is also a silvolite "marine" piston (3457H) which is kind of in between these two options.
Let me know what you would run and why.

Thanks

I just went through this on my son's 305 MPI motor. Turns out that the casting for the intake manifold is very thin where the intake runner meets the thermostat housing. I discovered a pinhole leak which opened up with heat. Buy serial number the correct intake manifold is nla. Any of the Mercury MPI intake manifolds will work you just may have to use an adapter for the injector o-rings, that is the difference between the part numbers. The bore on the available intake manifold for the injector is larger and there is a commonly available oversized o-ring to compensate for that.

For me it was not the intake, I found rust in one of the exhaust manifolds. I did get two complete new dressed intakes though that I found on FB marketplace with injectors and all.

You already know to get the injectors flow tested, your ECM does not know if one is flowing incorrectly and could cause a lean hole. Articfriends on the board here does a very good job at a reasonable cost. Our tell was rust in the exhaust manifold, #1 was the intact tract where the leak was but showed rust in the exhaust manifold as well. For anyone else running across the NLA MPI intake PN but other is available the O-ring you need is available from several sources including fleabay and Amazon.
https://www.lingenfelter.com/product/L700025305.html
or
https://www.ebay.com/itm/32531781763...3Avlp_homepage

v,
1. Hypereutectic pistons are fine. They are in 90% of the engines out there including most Mercruiser small blocks. Use a name brand. Not sure who makes Summits.
2. Stop worrying about the quench/squish. It simply is not as much of an issue as people make it out to be. If you are really worried about it you can deck the blocks .010" or .015" and get almost to where the factory pistons are. I have never worried about it.
3. Dont use a shim head gasket. They will leak eventually and you will regret it. Use the regular composition Fel Pro Marine head gasket. Stop worrying about the quench/squish :)

Why not let your machine shop get you the parts ?? I can usually get a complete kit and actually save customers money over buying everything seperately.

Just my worthless opinion :)

Very good advice on letting the machine shop source the parts. As long as they are familiar with Marine Build engines he will set things up based on experience and more likely to stand behind work if they know the parts he put in instead of a mix mash of parts. Some things just work better together.

Where are you? If you ask you certainly will get recommendations on shops familiar with Marine builds.

Just my opinion again but if you are going to use the Vortec heads, the flat top pistons you mentioned will be too much compression. With todays fuel availability I Iike to keep anything with iron heads down around 9 to 1. I would use the H815 and deck the blocks .010 or so. That with a normal head gasket will work fine.

This is what I wanted to do at first. But then the owner of the machine shop told me to avoid decking at all costs as it creates some kind of alignment issues. After that he suggested that I go with the flat top silvolite with a 50 thou head gasket, apparently he said he did many like this and it works great. My only two requirements are 9.4 compression and that the motor doesn't ping. I can't really mess with the compression ratio due to the tuning in the ECM555.

I am just north of Toronto. The machine shop recommended the flat top silvolite piston with the 50 thou head gasket.

Most of those flat top pistons with 4 valve reliefs are about a 5-6 cc "dish" for the valve reliefs. Unless I am doing my math totally wrong with a 1.560 compression height the piston will be about .020 down in the cylinder. That combined with a .050" gasket and the 64cc Vortec heads gives you a compression ratio of just about 9.5 to 1 ? I guess as long as you are ok with 93 octane fuel then it should work.

Machine shop said that would run 87... Stock mercruiser 350 compression is 9.4, but it has 50 quench to help. With this flat top option the quench is out the door.

Many years ago, I had "260 Mercruiser (350 non Vortec GM) in my 23 ft Wellcraft. I removed the engine around 250 hours and installed brand new GM Vortec heads, different valve springs, a mild flat tappet Comp Cam around 212 / 218 @ 0.050. New Edelbrock intake, roller rockers, timing chain, HiPo Mercruiser exhaust manifolds, and kept the stock flat top pistons with 4 valve reliefs and never even removed them. I calculated around 9.3 compression and was concerned as well, since I think I used the stock Mercruiser head gasket. I started at 32* total timing and 89 mid grade. It gained 9 mph and I switched from a 21 Mirage to a 23 Mirage and it turned that prop to 5100 rpm. Later, I put the timing back to 30* total and ran a few tanks of 87 octane and could not detect any detonation or pinging. I went back to 89 for safety and ran the boat like that 8 years, no issues, sold it to a friend and he has had it with no issues for 5 years.

Personally I am a firm believer in the benefits proper quench. The NA and boosted engines I have built with propper chamber/piston matching and clearance have run better and been less detonation prone. Every mercruiser/indmar 350 & 383 has full dish pistons and non-existent quench so they will run fine on pump gas without it, just not ideal.

The last two 350 mags I built were

96 carbed
14096217 'D chamber' 64cc heads
speed pro H815DCP30 12.3cc
balanced factory rods
Zero deck
Felpro marine .039 head gaskets
9.6:1 compression

Pistons aren't an ideal match to the D shaped chamber but have a much wider band around the perimeter to provide some quench. Pistons were all within a gram and ran great on 91 and 89 octane for well over 500 hours mostly pulling with up to 1800lbs of ballast and people in the boat

2004 350 mag fi
​​​​​062 vortec heads - 64cc
KB hyper 12cc Pistons (vortec mirrored dish)
balanced factory rods
Pistons .001 out of the hole .039 gaskets

9.67:1 compression with the heads resurfaced

This one ran even better imo on 91 & 89. Also had 87 mixed in once or twice on the water with no noticeable detonation.

All merc/indmar 350 mags/monsoons will have cam pn 14097395 which is the standard rv/ram jet 196/206 @.50 109 lsa grind.

The 383 hammerhead pulled off 9.7:1 compression with aluminum heads and called for 89 octane. EDIT - looks like they ran a larger 222/230 @.500 112lsa cam to drop the dynamic compression a bit

I would spend the extra money for forged pistons. The 350 EFI's seem to be detonation prone, and many have head gasket failures, leading to burned pistons. Forged may save the engine if this were to happen.