Which new Vortec cyl. heads for my 350 Mag MPI motor?

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
I have a 2006 350 Mag MPI Horizon engine with 1,700 hours on it. Boat is a heavy 24' SF boat. I pulled the heads at 1250 hours, did a valve job with two new exhaust valves. Also changed the timing chain at that time. My boat is only used in salt water.

Engine has no blowby, but has become very rough at idle, etc. 99% sure some exhaust valves are not sealing well. I have not done a compression test yet, but I can hear strange raspy noises at the air intake. I would like to put brand new heads on it, or do a valve job with all new valves. Any recommendations on which Vortex heads to buy? I usually run at about 3,300 to 3,600 for an hour at a time. Fish in an area for the day, then run back at the same speed.

Any recommendation on quality Vortec heads for a heavy boat used in saltwater? Thanks for the replies.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
Unless your wanting to rebuild and build more power stick with what you have (standard GM cast iron heads). At 1250 plus hours on a raw salt water cooled I'm thinking your at or real close to the point your going to need another long block.

How many hours is on it now?
 

Fun Times

Moderator
Staff member
Joined
May 16, 2009
Messages
8,776
Being a horizon engine model...Is your cooling system closed cooling which contains antifreeze running through the engine? It's a 50/50 if you have antifreeze or not is the reason I ask.

Personally I'm with AllDodge as to trying to stay as near stock as possible to help keep fitment of all working components as simple as possible.

Your engine serial number would help here better but I believe your cylinder head part number might be 938-803860T Which seems to be no longer available but it looks like sierra marine has a part number replacement to the OEM number as seen here for example...., https://www.ebay.com/itm/Sierra-18-...h=item56b5710c3c:g:FBYAAOSwdPtax5Zw:rk:1:pf:0

Or, here too, https://www.ebay.com/itm/MerCruiser...ly-938-803860T-with-906-casting-/172284037991

Of course this would be just one way of many other avenues of buying new GM heads that fit too but figuring for a starting point of knowing the OEM part numbers would help get you heading in some sort of direction.
 

aimlow

Petty Officer 2nd Class
Joined
Nov 7, 2018
Messages
180
There's basically only 1 GM vortec head for a 350. They are all thinwall castings, and cracks are common.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Pull the valve cover on one head and look for the casting number. Find new heads with that casting number. As a Horizon engine, you have closed cooling. 1,700 hours is not a lot. If you are having exhaust valve problems, look at your prop(s)... Are you overpropped? I suspect so... What RPM are you getting to at full throttle? (should be 4600-5000).

If it was a raw water cooled engine, I would say the same thing, 12 years is not a long time for those blocks. The hours you're putting on it is a good thing, not bad.... It's not USING a boat that kills the engine, it's NOT using it. (or overpropping)

Chris.........
 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
It is Horizon, so it is a full system FWC. Only the exhaust elbows see saltwater. It is a relatively late model engine, s#OM675521

I have the least pitch prop I could find, and it will spin 4,600. Boat goes 35 mph at that speed. However when I have done a speed test, the engine always runs rough afterwards, but the next day runs better. I believe the exhaust valves are getting overheated. I wish I had less pitch, I could try having the diameter cut down.

I have seen some ads for heads with different valve technology, I will find those and post the info. If I rebuild the heads, does anyone have recommendations on the best exhaust valves to use?
 
Last edited:

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
Best exhaust valves are Iconel

However when I have done a speed test, the engine always runs rough afterwards, but the next day runs better.

WOT at 4600 rpm is on the low side but within range (4600-5000). Might have a timing or fuel issue

Run at WOT and when it acts up check some spark plugs. If they are white, then your running lean. Also use an accurate tachometer, because the boats may be correct but not always.

Do a compression and leak down test before pulling things apart to see where it may be leaking
 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
I did look at a plug two weeks ago, and was surprised at how clean it looked. Engine timing is computer controlled, but I will check it.

Best exhaust valves are Iconel



WOT at 4600 rpm is on the low side but within range (4600-5000). Might have a timing or fuel issue

Run at WOT and when it acts up check some spark plugs. If they are white, then your running lean. Also use an accurate tachometer, because the boats may be correct but not always.

Do a compression and leak down test before pulling things apart to see where it may be leaking
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
If "clean" means white and not tan in color then your running lean and this can warp valves. If it gets lean enough then detonation starts and burns holes in pistons.

Your timing is computer controlled and gets info from the crank sensor. If you start running lean and get a knock the PCM will retard the timing. If the PCM does do this then there would be an alarm and code set.

Your fuel pressure should be 43 psi at 1800 rpm and should stay close to that at WOT
 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
I have the Rinda software on a laptop, and it shows no faults. I should check the fuel pressure, have not done that in a few years.

I was very surprised at how white the plugs were, but these were new platinum plugs with only about 24 hours of run time. When I checked the plugs, this was after running the last 5 minutes to the slip at 1,000 rpm. Power has been down for the entire season, and fuel economy has gone from 1.5 mpg to 1.1 mpg.
 

aimlow

Petty Officer 2nd Class
Joined
Nov 7, 2018
Messages
180
If you have a true vortec motor, you have the dreaded flat topped distributor. The distributor is adjustable, but ONLY for positioning it properly, It MUST be set with a scan tool. Improperly adjusted, you will get massive spark scatter at high rpms. If the knock sensor takes over, it will retard the timing, and power loss will result.

With this distributor, 0.038" is the recommended plug gap.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
It MUST be set with a scan tool

Incorrect statement
Distributor is installed manually and only adjustment is a slight movement of the distributor left or right if there is a no start or miss. There is also a clocking adjustment with motors having issues with emissions and alarm
 

aimlow

Petty Officer 2nd Class
Joined
Nov 7, 2018
Messages
180
Incorrect statement
Distributor is installed manually and only adjustment is a slight movement of the distributor left or right if there is a no start or miss. There is also a clocking adjustment with motors having issues with emissions and alarm

I've got hours and hours on my distributor machine with that crappy distributor. Got cut away caps, rotors. I just posted in Volvo section about those. Marine isn't as bad as street, because the street ptogram allows up to 45 degrees advance under certain conditions. 45 degrees advance is 1/2 way to the next cylinder, so theoretically the spark can jump to either terminal. In practice, the firing cylinder is under much greater pressure, so the firing voltage requirements are much higher. This higher voltage is much more likely to jump to the next cylinder, or crossfire within the cap.

The cutaway cap is an excellent idea. Line up the rotor and cap at 15 degrees ignition advance (retard the distributor base 15 degrees) and you should be good to go.

"cam retard" was used to tell the computer which 360 degree crank rotation the engine was on, so proper sequential injection could initiated.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
I've got hours and hours on my distributor machine with that crappy distributor. Got cut away caps, rotors. I just posted in Volvo section about those. Marine isn't as bad as street, because the street program allows up to 45 degrees advance under certain conditions. 45 degrees advance is 1/2 way to the next cylinder, so theoretically the spark can jump to either terminal. In practice, the firing cylinder is under much greater pressure, so the firing voltage requirements are much higher. This higher voltage is much more likely to jump to the next cylinder, or crossfire within the cap.

The cutaway cap is an excellent idea. Line up the rotor and cap at 15 degrees ignition advance (retard the distributor base 15 degrees) and you should be good to go.

"cam retard" was used to tell the computer which 360 degree crank rotation the engine was on, so proper sequential injection could initiated.

Nope, AllDodge has it dead right. Setting up is PURELY done manually... The ECU deals with all the timing after that. These 'distributors' correct name is HVS, High Voltage Switch, as called out in the Merc manuals. I just finished putting a 350MPI back together, and no adjustment to the HVS after the assembly was required. It started up and ran perfectly.. Please feel free to PM me if you'd like a copy of the relevant pages...

Chris......
 
Top