Building 350
 

Another guy is putting together a mild 350 build and think he may be getting some bad advice so here is the issue, and wants to run 87 octane but would go to 89
Its a 1984 350 and he plans to bore .030 over and Deck at plus 0.015, 0.025 gasket, LCQ piston 0.012 in the hole w/ a 7cc piston volume.
Comp Cam XM270HR 12-417-8 - Xtreme Marine? -Designed to maximize torque, acceleration and throttle response while providing excellent high RPM horsepower. A faster intake valve opening increases engine vacuum and enhances throttle response. Special intake closing ram

See any issues?

What heads? What do you mean by deck at plus .015, is the piston .015 or .012 in the hole ? What head gasket, if he is using the thun one to up comression, i would probaly run the standard one and deck the block a little more. If he is using the original heads they arent very good.

x 2. Need more and more complete info.

Vortex heads and the block is machined down to where the piston is .015 below the machined surface of the block. He is using a .025 gasket to get a .040 quench (.015 + .025).

With those numbers and vortec heads the compression is around 10:1 which is a bit high.
I believe stock 350 vortec pistons are about 13cc. That would put you around 9.5ish.
also max lift on stock vortec is about .48 inch before the retainer hits the valve seal but this can be modified.

flat top 4 valve relief 5CC pistons and vortec heads with .044 gasket and zero deck is 9.75:1 id personally do .008 deck or less and a std .040 head gasket and you yeild 9.6:1 which is about max for 87 octane in a boat..

the XM 270 is a desecent cam for the mid 3xx hp level and id do screw in studs taller valves and taller springs to maximize the.475- 500 lift range which is the PEAK flow area of a vortec head and she will be about as reliable as they come with approx 180 PSI cranking

I to like the flat top my self after reading ICDEDPPL write up in the AFR, EGT thread. Have any idea what the guy can expect if he sticks with the original plan?

9.7-10:1 depending on how much hair cut the heads have had and what they actually CC at ... it will work but id run 89 in it at a minimum ..

not really a fan of deep decks rather set quench with gaskets than the deck itself.. but thats just me

Question....can you even find marine head gaskets that thin for a SBC?

My build was very similar.....speedpro pistons, P bearings, pinned studs, valve job, Fel-Pro Marine gasket set, Indmar 340 cam (same as 300Mag) and double roller timing set. My compression comes it right about 9.6-1. I plan on running 91 Ethanol free, since it's available at most marinas.