I have a longer term plan to build a 383 stroker, but in the interim I have picked up a solid L31 Vortec 350 from a 2000 GMC 2500 Van - I am going to clean it up and do marine gaskets in it.

I am thinking of repacing the stock truck roller cam with an LT4 Hot Cam. Has anyone done this? If so, what was the result?

Thanks.

I just bought a pair of 355 Marine Vortecs for my 255 Formula and considered the LT4 cam. I went instead with a Comp Cams Extreme Marine Roller with Comp Needle Bearing rockers. I will get the cam cards out and post the numbers. These engines were run in on the dyno and made 365 horse power at 5000 RPM...390 horsepower at 5600 RPM. Torque was 425 Lb ft at 3200 RPM. These numbers were attained running a Holley 750 CFM Marine Double Pumper on an Edelbrock Volvo/Penta High Rise/Dual Plane Marine aluminum intake with brass lined water passages. Ignition is Delco Voayger with 32 deg total advance. The heads are stock Vortec with Comp springs and retainers, port matching and mild cleaning of burrs in the intake passages. The valves are stainless with a 3: angle valve job...nothing exotic. These engines will run most of the time in the 3000~4500 rpm range. I was advised against the LT4 for marine use due to the probability of water reversion even with through hull exhaust.

Another benefit of the Comp Roller was that it has the Fuel Pump lobe on it in case you elect to run a Mechanical Fuel Pump.

Don't forget to put brass freeze plugs in...the steel plugs will not make a season in salt water and maybe 2~3 if you are lucky in fresh. Go to the marine cam...the truck cam doesn't like to run much past 4000 RPM.

I'll post results as I move through the install.

I have the lt4 hot cam, ran it all last year with no issues. My setup isn't quite where I want it but I believe the carb needs some adjusting to get all the power out of it. I can tell you this, it's a hard and fast revving SOB.
Also showing no signs of reversion with stock exhaust.
The lt4 also will allow you to run a mechanical fuel pump but the block you have won't. I used the block from a 1997 Chevrolet truck and they did away with place to mount the mechanical pump in the later years. I'm running a Mallory marine electric pump and have had no trouble.

Here Is a link to my build:
http://www.offshoreonly.com/forums/d...under-way.html

87 Mag...this is good to know I thanks for sharing your build experience. My cams are not that strong...my lift is .488/.495
and duration is 212/218 with 112 deg lobe separation.

These are going in a 5700 lb boat so I should have a strong runner. I'm looking at 750~800 horsepower and 850 lbs ft of torque in the water so she should zip right along.

I think I am working through the same coupler /flywheel issues you did. My boat is a '79 and so are my transom plates...pre Alpha. I plan to use that 12 3/4" one piece main seal style fly wheel and matching coupler. This is so I can keep my smaller Bell Housing to match up to the old style bolt pattern on the transom plate....sound familiar?

I am interested in your performance numbers, what carb and props you ended up with and the size/weight of your boat.

Thanks in advance

This is all good stuff...

I will be running the stock thru-prop exhaust, so one of my concerns is/was reversion. However, looking at the LT4 cams specs, it is not much different to the Comp Cams XM270HR - slightly more duration at .050, 219/228 vs. 218/224. LSA and intake centerline are virtually the same.

I will have to use an electric fuel pump. I also have to remember to replace the drive gear on my distrubutor (or buy a new on) as it is coming from a non-roller/cast cam engine.

I did the LT4 Hot Cam kit (cam, springs, retainers, locks & 1.6 roller rockers) on an LT1 a number of years back - it made great power and would pull right to our rev-limiter at 6k - although that was in a 2500# car, not a 3500# boat. I never dyno'd the engine, so I have no idea what the HP & TQ numbers were like.

On a good day I can get mine spinning 5400 rpm @ about 63-64 mph,
Im running a box stock 650 demon carb which I believe is a little o. The rich side. I'm go an start messing with the jets this spring and see what gives. My goal is a pretty consistent 65 mph boat

Easier to spin up to 6k in a light car...

I was going to give original Q-Jet a try - although I will have to do an electric choke conversion on it. Any thoughts on this?

I'm pretty new to the world of engine building so Im dumb as a post about carbs. There are a few other guys on here that have used this cam with great success and can probably help you with the carb. My personal opinion is to ditch the quadrajet and get a Holley/demon type carb. I also went with mechanical secondary's over vacuum, which makes for killer acceleration. The stock jets in my demon are a 72 primary and 80 secondary, but as I mentioned before I think it's way rich but I'm not experienced enuff yet to know for sure.

One other thing, the lt4 cam has to much lift for a stock set of springs and you have machine the spring seats on the vortec heads to get the lt4 springs to work. You will see what I mean once you get it torn down, but it's a must to run that cam with vortec heads. And use the truck valve covers, the 1.6 roller rockers do clear them inside with no issue

Trying hitting up mr fixall, haxbyspeed, or tomz. These guys can help you with the fine tuning details.

87 Magnum...did you have the old style vs new style bell hosing/flywheel/coupler issue to deal with? If so...how did that work out?

That is one other thing I will have to do - I have the old style crank bolt mounted coupler and will have to go with the newer flywheel mounted one. Are the bell housing different between 1 piece vs 2 piece crank seal?