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
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
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