Merc 4.3 CR and Quench

[adam619]

So I have a freshly rebuilt running 4.3 (1993) Balance shaft/4bbl with non vortec head paired to alpha one gen2, and I am terribly confused. I had it machine .030 over and put in speed pro h345dcp hypereutectic flat top pistons. Figured it could use the extra compression with 76cc heads so went with flat tops, they have 4 valve reliefs and -6.8cc. At time of purchase I was told the compression distance on those was 1.560 , which would of been same as stock. But I am now finding out that apparently that was a mistake and they are 1.548. My deck was not machined down. Now when I run the compression calculator trying to figure out the stock config, I cannot come to the advertised CR of 9.3:1. The only way I make it to 9.28:1 is with the following config.
4.00 x 3.48 , 76cc head , 0 deck height , +2.5cc dish pistons , and a .039 gasket.

That being said the dish pistons certainly have more than 2.5cc of negative volume and the deck is most likely 0.025 -+2. So at a conservative +5cc and .025 deck I get 8.6:1.
I assumed stock gasket a .039 , this is what my fel pro is.
Now adding another .012" (1.560'' - 1.548'') to deck height and +6.8cc gets me a lousy 8.3:1. I neglected to count in the additional bore to 4.030 above which has very little impact.

I am assuming stock quench(or lack of) at ~.064 (.025deck + .039 gasket) which has now worsened another .012 to .076". I think at 8.3:1 even on 87 octane I need not to worry about detonation but it just seems my math is off somewhere. Obviously not looking for a rocket or I have put a 350 in it but still want something decent and don't mind running 91 octane, I was planning vortec head and ellderbrock intake eventually.

Anyone else running the h345dcp in a 5.7 or 4.3 with std deck height?? Where does the stock 9.3:1 come from?

The engine has only run on a stand as the boat rebuild is not done so I cant say how it feels power wise,

Thanks in advance

Adam

 

[adam619]

I did not actually measure head volume an I am now thinking the non vortec may be 64cc as well (is this possible? I am certain I had read that non vortec were 76cc?). |The engine number is 0F118521. This would mean stock CR at 9.3:1 at .025 deck and -9.2cc dish volume 4.000 bore .039 gasket. This would mean that .030 over adding .012 to deck and -6.8 piston i would be a 9.4:1 . But I guess the question now is is there any quench effect at .064" stock piston to head clearance , and if so would would this new clearance on .076" mean for this build. From what I understand a marine engine with a 142F thermostat would have more detonation resistance than automotive with lower operating temps. Should I post these engine questions in other area?

Thanks

 

[alldodge]

What's your head casting number?
Hey @Scott Danforth

 

[adam619]

Cannot see the casting number as everything is assembled and it looks like i may have to remove the exhaust riser to remove the valve covers. 12 Bolt manifold and definitely NOT the kidney shaped chamber. 4.3LX without a doubt the non vortec.

 

[alldodge]

Being a few different non-V heads, only way to know the volume is with a casting number, outside of measuring with the head removed

 

[Scott Danforth]

first, no way you have 76cc heads. should be between 64cc and 68cc depending on what the casting number is.

you have to know what you are starting with

LB4 Heads (1987-1995??) These heads flow the worst
10144103

L35 Heads (1992-1995) These heads flow better than LB4's
10238181
10077626
14099064
10240209

"Vortec" Heads (96+) These flow the best
10235772
*there are other #'s)


from some light reading: https://forums.iboats.com/threads/complete-history-of-the-4-3-liter-with-casting-numbers.693935/

The 10077626, 14099064, 10240209 or 10238181 castings were used, but be sure to check them over carefully because there are two versions of the 10238181 and 10240209 castings with an important difference. In 92 and 93, they came with an 8 top angle on the intake seat and a 75 throat, but that was changed to a 30° top angle with an 80° throat in 94. Separate the 8 heads from the 30 heads and use them only on the 92 and 93 engines.

 

[adam619]

@Scott Danforth Thanks for the insight will read up on that , lets assume for the purpose of discussion that I have 64cc heads, and a stock quench of .064 @ 9.3:1 with dish tops. This motor has been historically reliable on regular gas. ..At a new .076 quench and 9.4:1 should I worry about detonation or the previous 0.064 piston to head clearance already had no quench effect. Has anybody else used these piston on stock block height?