Compression Ratio and The Pontiac Engine

Here are some ways of finding out and changing your compression ratio on Pontiac V8 engines.

On a Pontiac engine the compression ratio's were change by using different heads. Below are some popular heads and chamber sizes (always be sure to have yours checked before any milling).

cc	year	casting	disp	comments	valve size
71cc	'68	R96A	400	RAII R-PORT	2.11/1.77
71cc	'69	722	400	RAIV	2.11/1.77
71cc	'70	614	400	RAIV	2.11/1.77
72cc	'69	48	350/400	HO/RAIII	2.11/1.77
72cc	'70	12	400	HO/RAIII	2.11/1.77
72cc	'67	670	400	GTO	2.11/1.77
72cc	'67	97/997	400	Ram Air	2.11/1.77
72cc	'68	62	400	std	2.11/1.77
72cc	'68	16	400	GTO	2.11/1.77
72cc	'68	16	428	HP	2.11/1.77
72cc	'68	31	400	RA	2.11/1.77
72cc	'69	16	400/428	Std	2.11/1.77
72cc	'69	62	400	Std	2.11/1.77
72cc	'70	12	400	RAIII	2.11/1.77
72cc	'70	13	400	GP/F-bird	2.11/1.77
75cc	'68	62	428	Std	2.11/1.77
87cc	'70	15	455	B-body	2.11/1.66
87cc	'70	64	455	GTO/GP	2.11/1.77
92cc	'78/'79	6x-#4	400	F-bird	2.11/1.66
96cc	'71	96	400	GTO/F-bird	2.11/1.77
96cc	'72	7K3	400	A-body/F-bird	2.11/1.77
98cc/114	'73-74	4X	400	Std	2.11/1.66
101cc	'75	5c	400	Std	2.11/1.66
101cc	'76-79	6X	350-400	Std	2.11/1.66
111cc	'71	197	455	HO	2.11/1.77
111cc	''72	7F6	455	HO	2.11/1.77
111cc	'73-74	16(orX)	455	SD	2.11/1.77
114cc	'71	66	455	Std	2.11/1.77
114cc	'72	7M5	455	Std	2.11/1.77
98/114cc	'73-74	4X	455	Std	2.11/1.66
124.51cc	'75	51	455	Std	2.11/1.77
124.51cc	'75-76	6H	455	T/A	2.11/1.66(?)
124.51cc	'75	6S	455	T/A	2.11/1.66(?)

Now that you know what it started out as you can subtract 1cc for every .005 milled off the head. It is not recommended to go farther than .060 off the head and for anything over .015 you must mill the intake side (your machine shop will have to determine how much).

Be sure to have your heads cc'ed and do not assume yours to be this size.

HERE IS A COMPRESSION RATIO CALCULATOR

To get the compression ratio here are the formulas:

CR= Cylinder swept volume + Compression area at TDC: Compression area at TDC
Deck height (cc) = bore x bore x .785 x deck height x 16.387

Cylinder swept volume= bore x bore x .785 x stroke x 16.387

*Pontiac 400 Compression area=25.45 + head cc (this is assuming 6cc for valve relief, 9.7cc for head gasket, 2cc ring area, and 7.75cc deck height volume.)

*Pontiac 455 compression area = 25.57 + head cc (assuming 6cc for valve relief, 9.7cc for head gasket, 2cc ring area, and 7.87 for deck height volume)

*blocks are assumed to have a .030 over bore and heads cut .010.

78blackbird@thepontiactransampage.com
Feel free to drop me a note. * I am looking for some good articles for the third and fourth gen birds. If you have any that you would like to see on this site send them to me and I will post them on my site.

The information in here is just an outline.
The author claims no responsibility for the info contained in these pages.
If unsure of any tip or modification seek a professional for the job.
In some states this may violate pollution laws
Check with your state on the laws.
Not affiliated with Pontiac or General Motors