Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Pontiac engine question
View Single Post
  #13 (permalink)  
Old 01-24-2013, 02:51 PM
cobalt327's Avatar
cobalt327 cobalt327 is offline
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,036
Wiki Edits: 1616

Thanks: 128
Thanked 598 Times in 547 Posts
Quote:
Originally Posted by jeffs66 View Post
Hiya, I have a 66 Catalina.It has the 389 2v engine.well recently I put on #62 heads w/ I beleive the 72 cc combustion chamber.Heres my ?.I know Pontiac used a basic flat top piston and would change compression with head swaps.Well does any body have a guess on what compression I have now,or how I may find out without tearing the motor back down?I know its up there cuz its got a bad case of the rattles if I dont burn premium,even when I back the timing down.plus the little extra power.
The difference in head chamber volume between the heads you removed and the replacements is not that great either way, unless they've been milled excessively. So it should exhibit the same tendancies it did before the head swap.

The exact CR is hard to say as has been said already. Using a quench distance of 0.055" (0.040" head gasket and the piston about 0.015" in the hole at TDC), a piston valve relief volume of 6cc and stock 389 bore and stoke, you're near 9.7:1- which is stretching the limits of pump gas detonation resistance w/an iron head Pontiac if you're using best power total timing- as you've found out.
Reply With Quote