Building a 350

[Josh in Pa]

I recently bought my first boat, and shortly afterwards discovered it had a cracked block. I've decided to fix it, and found a block out of a truck of the same year. (1992) The motor in the boat was a 350 magnum. What makes it a magnum? I would like to build my new motor to the same level as the old one. I see engine kits listed on ebay that seem pretty complete. They are for a marine 350. Will these work as long as the year is correct, or does the magnum have special parts? I thought I had read that the 350 magnum had 9.4 to 1 compression ratio, this seems pretty high. Is this standard for the marine 350? I've also found a used set of vortec heads and am considering putting them on the engine. I am aware that I will need new intake. They will also raise my compression, and I am concerned that if stock is 9.4, the vortec heads will raise it too high. Any experience here? Basically, I have experience rebuilding motors, but never a 350 or a marine engine. What is the most economical way to acheive similar or better performance than the 350 magnum that I had? Thanks for any help,
Josh

 

[Fishermark]

Re: Building a 350

While I can't tell you for sure what makes a "Magnum" engine, I can tell you that you will be okay with the vortec heads and intake. That makes a nice combo for a boat. Just be sure to use the brass core plugs and switch all the externals over from the original marine engine - the starter, alt, distributor, etc.

 

[John_S]

Re: Building a 350

A '92 350 Mag would have a high-rise dual-plane cast-iron intake and possibly a different ignition module than the same year 5.7LX. It might have a roller cam vs flat hyd, but no power difference.

Your '92 probably has the dished pistons. If so, no problem with vortec head compression ratio.

Take the heads and intake off your 350 mag to ID exactly what you have. Even if you don't reuse them, ebay them.

A '92 stock 350 mag is 250hp at prop. If you go with vortec heads/intake, about 280Hp at prop.

 

[Mkos1980]

Re: Building a 350

Vortecs wont raise compressions as the heads you have on there now are probebly the 217 or 083 heads with 64cc's. Vortecs are 64cc as well. As John said you probebly have the 4 eyebow pistons. I personally would probebly give it a tad bigger cam as well. Comp makes a nice marine roller cam that will give better performance then the one you have now. It works well with the stock exhaust too.