Question on 350 sbc omc 800 build

[Priesman64]

Hey all new here first post, have an 83 sportcraft 250 with the omc 800 mechanical. Boat came with 305 was junk. Currently in the process of building 350 to replace it but running into some questions before I order parts. Heads on the 350 are shot and were automotive truck heads anyways. Running stock pistons engine came out of pickup so compression ratio should be around 8.5 to 1. Issue I’m having is planning to use the 305 marine heads casting 416 on the 350. Yes I know I won’t gain anything but my compression ratio but using 305 heads in a car is different than boat. Having issues picking cam to be safe with current setup. Looking at the comp cams xm256h or the zm262h marine cam. Have been told with the tighter compression ratio a cam with more duration can drop the possibility of spark knock. Just wondering on people’s opinions. Compression should be around 10-10.5 to 1. 58cc chambers on the 305 heads.

Thank you

Nick

[RicardoMarine]

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Hey all new here first post, have an 83 sportcraft 250 with the omc 800 mechanical.
Nick, all due respect, be prepared for the issues that come along with the Stringer Drive!

Boat came with 305 was junk. Currently in the process of building 350 to replace it but running into some questions before I order parts. Heads on the 350 are shot and were automotive truck heads anyways.
You will want the correct cylinder head combustion chamber volume that compliments the piston dish volume.
(see ** below)
Also, look for the heavy castings, and avoid the light castings. You can do a search for the SBC cylinder head casting numbers and learn which ones to use and to avoid.

Running stock pistons engine came out of pickup so compression ratio should be around 8.5 to 1.
Stock pistons will most likely be the dreaded GM Full Dished.
These were used in both the auto and marine version SBC.



Issue I’m having is planning to use the 305 marine heads casting 416 on the 350.
From what I see, the 416 combustion chamber volume is only 58cc.
Valves are small at 1.84/1.5.

Yes I know I won’t gain anything but my compression ratio but using 305 heads in a car is different than boat.
** Car or boat engine, along with your 3.480" stroke, the piston dish volume and cylinder head combustion chamber volume will determine the Static Compression Ratio.

Having issues picking cam to be safe with current setup. Looking at the comp cams xm256h or the zm262h marine cam. Have been told with the tighter compression ratio a cam with more duration can drop the possibility of spark knock.
The concern will be "marine engine load/ignition induced detonation" that the GM F/D piston is prone to causing.

Just wondering on people’s opinions. Compression should be around 10-10.5 to 1. 58cc chambers on the 305 heads.
Nick, I would NOT do this with the GM Full Dished pistons!

Give this thread a read starting at post #7.
http://www.marineengine.com/boat-for...V8-engine-swap

Thank you

Nick

[Priesman64]

Rick thank you for the reply. I’ve actually got 2 set of pistons that I can use on this block. The set that was in it was the full dish piston. The ones I have available are flattop with 4 reliefs in them. Replacement for mercruiser I had bought awhile ago. As for the omc I am well aware of the issues they can bring. Since I’ve posted this I’ve came across some mercruiser heads and will be running those. They are the 624 castings which I know have a bad rap for cracking but the gm part number is different from the mercruiser ones and from what I can tell the castings are different as well so I’m gonna try them. 76cc chambers with 2.02 and 1.6. Should be much better than the 58cc heads. I am leaning a different direction on cams now because I’ve read some horror stories about comp.

[RicardoMarine]

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Rick thank you for the reply. I’ve actually got 2 set of pistons that I can use on this block. The set that was in it was the full dish piston. The ones I have available are flattop with 4 reliefs in them.
Flat tops with the 76cc chambers will give you a suitable static compression ratio. Better yet, single valve relief flat tops.
Bring your quench dimension down to approx .038".

Replacement for mercruiser I had bought awhile ago. As for the omc I am well aware of the issues they can bring. Since I’ve posted this I’ve came across some mercruiser heads and will be running those. They are the 624 castings which I know have a bad rap for cracking but the gm part number is different from the mercruiser ones and from what I can tell the castings are different as well so I’m gonna try them. 76cc chambers with 2.02 and 1.6. Should be much better than the 58cc heads.
Yes, the 58cc chambers would bring the static c/r up higher than what would work well for the Marine version 5.7L.


Here is an easy way to determine Light -vs- Heavy castings:


I am leaning a different direction on cams now because I’ve read some horror stories about comp.