pump gas compression

[brettco]

 I have a .020 over 428 with what is apparently 28cc dished kb pistons and 70cc iron heads already cut .010. What can I do to get the compression correct for 91 pump gas.  It also has a cj cam. Thanks

[Falcon67]

Without knowing your deck clearance, you're already between 9 and 9.5 from what I figure roughly: 

4.15x3.98, 68cc chamber, .041 x 4.4 Felpro gasket, .005 deck, 28 cc piston = 430.7 CDI @ 9.22:1

Should be plenty low for 91 depending on timing and cam timing/cylinder pressure. 

[FrozenMerc]

With the CJ cam, a proper tune, and 9.5:1 or under, you should be able to run bar pour 87 all day with out any issue.  My 352 with aluminum Edel heads, and 9:5:1 was tuned and dynoed on 87, with no sign of detonation.

If my memory is worth a shit, the CJ cam has rather late intake timing, driving the need for more compression.

[brettco]

 the heads that came off were 73 cc and I thought deck was .016 which gave me 8.8ish compression. I'm thinking I need more than that- my new heads are now 70cc ind I think I'm shooting for 9.5-9.6 to one. my cracrancranking compression is155-160.
 should I cut the heads to 66cc? any thoughts? Thankyou!

[Falcon67]

I would not go much past 160 on pump fuel.  Even 91. I don't know about your area, but pump fuel here is crap.  Gummy intake ports, 10% ethanol.  Works fine in everything but won't handle much compression.  The 351C in the Falcon has 10.5:1 with 65cc iron heads.  I used to run it on a mix of 1 gallon VP 110 + 4 gallons pump 91.  Now I run it on straight 110 because I see "pepper" on the plugs even at a 50:50 mix ratio.  Cranking compression is 160~170. 

[Faron]

I use a very safe formula that I suggest , especially with a small or stock cam poor quench and cast Iron heads take the octane you want to use and move the decimal one point to the right 91 gas = 9-1 , with good quench and the correct timing curve and more efficient combustion chambers as in modern heads you can run more compression , but my formula is safe