Jkrum10238

Long story short, I've got a little snafu with my motor project. I'm building a 355 with iron heads for my 21' scarab trailered to and from the lake so 93 octane is not a major issue. I'm coming up with a static CR of 10.3:1 and a quench in the .060.065 range with a .039 gasket and a 8.31:1 dynamic CR. My question is should I be getting a thinner gasket for more quench, or stay where I am, or go for a thicker one to lower the CR? I'm working on this with a drag racing background I'm new to marine motors.

mcollinstn

I'm gonna guess that you haven't tried running this yet?
My prediction is that you can't run over 30/32 degrees timing on 92 octane without detonation unless your cam is already too big.
(Unless you have steel bottom end with good fasteners and plan on spinning 6000+, which is possible on a light hull but probably not the best idea on your 21 Scarab).

At .065" piston to head, you have NO QUENCH.
Rule of thumb is that .060" and larger do NOT provide the quench benefits.
On a smallblock I would shoot for .030" to .035" on a steel rod, under 5500rpm motor.
Minimum would be .045" to get benefits of a quench motor.

So, right now you are "no quench" iron head 10.3 CR.
BUT if you deck the block and end up with a .035" squish, then you lose more than 6ccs which puts you at 11.2-11.3.

Going to a .045 squish (about as fat as I'd be comfortable with), then you will be at 11.0:1 which will still require a nasty cam that isn't "Scarab friendly".

To get functional quench benefits, you need to be under .040" squish. With iron heads on a <5500 rpm marine engine on pump gas, 10.3:1 is the upper ceiling.
If you can make it perform up to your expectations, then go for it.

If it doesn't deliver, you will be looking at optimizing your combination with modern closed combustion chambers, .035" squish clearance, and as close to flat-top pistons as you can run to get 10.2:1 with a fast-ramp cam in the [email protected] duration neighborhood.

dereknkathy

You already have pistons paid for and in the block? I assume small domes? What heads? What chamber sizes?

SB

iTrader: (1)

Any parts you can return ?

ezstriper

just did a upgrade for a friend, stock 260 short block, vortec heads, custom ground comp roller cam, performer RPM intake w/600 edelbrock carb and large flame arrester. We run the timing @ 28 max, runs very strong, engine set to turn 5500, but stock ignition is stopping it @ 4800, running this on 87, only thing available at marina. This should be in the mid 9-1 area

getrdunn

iTrader: (7)

I assume machine work is complete and parts are in hand? So it looks like your roughly .020 in the hole. A little more than ideal but not a huge deal in my opinion but I would think you would want a thicker head gasket to get the SC down around 9.5:1. As you know you can lose some cc's in your chambers in addition to a thicker gasket. 10.3:1 in the street no biggy but in a boat with constant load it would like result in a bad day at some point.

Like others have asked what are you running for heads and Pistons?