[71 T/A]

Sometimes I have seen factory heads, mostly in the 1968-70 400 heads, which were stated by Pontiac to be 72 cc's, really be 75-80 cc's. Did the factory state numbers differently to "boost" compression higher than it really was?

[Kenth]

What is the source of Pontiac stating 72cc on those heads?

[steve25]

Yes, it was a gift by the factory to the racing folks building motors so to allow for those smaller chamber CCs to be legal,and get greater compression without breaking racing organizations rules.
As it turns out very few of the high compression era bread & Butter heads with screw in studs had only 72 CCs.
Most where 74 to 78 CCs and most times only heads used on manual trans RA motors got the 72 CC or less heads.

Most 68 to 70 400 motors stated as having a 10.5 to 1 compression in the Pontiac sales literature and in the Chilton’s books where really only 9.5 to at best 9.8 to 1 in compression ratio when you run the numbers!

[scld1354]

Originally Posted by steve25

Most 68 to 70 400 motors stated as having a 10.5 to 1 compression in the Pontiac sales literature and in the Chilton’s books where really only 9.5 to at best 9.8 to 1 in compression ratio when you run the numbers!

There's no way 5 or 6 cc's would drop a full point. What else was different in reality? Head gasket thickness?

Sent from my SM-A505U using Tapatalk

[dataway]

My original YS 68 #16 heads are 72cc right on the money, about 0.5cc variation between chambers. Although they have been surfaced .005". Which I think is about 1 cc.

[steve25]

Yes, different thickness head gaskets where used also and in conjunction with slight changes in the milling of the block deck and the effects that makes on deck height.

There where also many motors 4 bbl motors rated by the factory as having 10.25 comp too!

[Half-Inch Stud]

my 48s read 78cc after chamber grindings and tulip valves. Heads date coded C189, from a Judge ~82-83

[shermanator2]

My unmolested #48s with factory valves and A099 and A139 date codes measured 66cc.

[AG]

My 62 heads from a 1969 YS were 78 cc.

[71 T/A]

Originally Posted by AG

My 62 heads from a 1969 YS were 78 cc.

I had 69 62 heads that were 80 cc's.

[gtojeff1967]

Something to think about my comp cams CC kit seams to be 3 cc off when i used something else to measure it with.

[steve25]

Jeff, is your Buret plastic or glass?
This seems to be the case with plastic ones.

I busted my first glass one a few years ago, then got a plastic one which was off, and now I am back to a glass one.

A Syringe is the way to test how accurate both are!
Vet offices are a good source for these ,

[shermanator2]

Originally Posted by steve25

Jeff, is your Buret plastic or glass?
A Syringe is the way to test how accurate both are!

Is this true? Is a syringe more accurate than a buret? I use a 50cc glass buret with 0.1cc resolution. Is there a syringe more accurate than this? I have a triple beam balance. I guess I could use some distilled water and see if the buret and balance match each other.

[Tom Vaught]

I use the same glass Burets, that Ford Research has used for years in their QC Labs and how no reason to question the numbers.

My head plastic plate is 2 inches thick, perfectly flat, and I know what the "Transfer Channels" volumes are.

I read the Head AND THE HEAD GASKET, AS A UNIT vs assuming the volume of the gasket or measuring it separately.

I will take my numbers to the bank, as they rig matches the Research numbers exactly.

Tom V.

I have a bunch more of the 2 inch plastic is someone wanted to make a checking rig like mine.

[steve25]

I would hope that a medical syringe would have a tighter tolerance then the average China made plastic buret that can be had in compression ratio testing kits, no?

[unruhjonny]

Since it seems that a part of the question was somewhat missed, in that it seems to have not been directly addressed:

Originally Posted by 71 T/A

... Did the factory state numbers differently to "boost" compression higher than it really was?

Yes.

I wholly agree with earlier posts that if you calculate the real world compression these purported "10:1+" engines all come in as not quite hitting the mark of 10:1.

Case and point to illustrate this (My area of most knowledge is 1970):
The 1970 WT, WS, and YZ all used the same heads, same head gaskets and same pistons.
With all the mechanical similarities between these three engines, the compression ratio (according to the 1970 'Performance catalogue') differs a quarter point between WT (400-4bbl) & WS/YZ (Ram Air III);
Yet more confusing is that if you look at any Lemans literature, that same 400-4bbl (WT) engine looses another quarter point of compression vs the GTO engine;
So on just one platform (Tempest-Lemans-GTO), there is a full half point compression swing, with no mechanical differences.
Lemans 400-4bbl = 10.0:1
GTO 400-4bbl = 10.25:1
GTO Ram Air III = 10.5:1

The 1970 'Performance Catalogue' lists the 400-4bbl engine as having 75.7cc chamber heads, and the RAIII having 66.27cc chamber heads - but neglects to mention that the heads are shared between engines.
It is worth noting here that most knowledgeable people in this community seem to assert that the #13 head (400-4bbl/auto) came in somewhere around 78cc, while the #12 (400-4bbl & RAIII) came in at 67cc and the difference seems to have been more or less split between the two for both of those engines nominal cylinder head cc rating.

My belief is that Pontiac was streamlining manufacturing, and although it was great for guys competing at NHRA sanctioned events, it was just simpler for PMD to make the hotter car look on paper to be a little more than it's lesser siblings, and in the case of automatic vs manual engine differences, it was probably again just easier for PMD to split the difference between motors and use that as the nominal compression ratio - why the RAIII carried over the 400-4bbl(L78) cylinder head cc rating is anyone's guess...

[PontiacJim1959]

Pontiac used different thickness on their head gaskets. I see 2 part numbers used, 9792995 - .028"-.033", and 9790910 - no thickness listed. "995" is used by 1968 F-8/T-8 400 4 Bbl, 1968 P-8 HO, 1969-70 All 400 4 Bbl except XV/XZ engines, 1969 P-8 428 & HO.

I also read that the chambers will measure a few CC's larger due to the fact that the leaded gas would carbon up both the combustion chambers and tops of the pistons reducing CC's, so Pontiac took this into account.

And, advertised compression ratio's were marketing ploy's so as to measure up to competitors in an effort to get sales - just as cubic inch sizes were comparable.

[unruhjonny]

Originally Posted by PontiacJim1959

Pontiac used different thickness on their head gaskets. I see 2 part numbers used, 9792995 - .028"-.033", and 9790910 - no thickness listed. "995" is used by 1968 F-8/T-8 400 4 Bbl, 1968 P-8 HO, 1969-70 All 400 4 Bbl except XV/XZ engines, 1969 P-8 428 & HO.

I also read that the chambers will measure a few CC's larger due to the fact that the leaded gas would carbon up both the combustion chambers and tops of the pistons reducing CC's, so Pontiac took this into account.

And, advertised compression ratio's were marketing ploy's so as to measure up to competitors in an effort to get sales - just as cubic inch sizes were comparable.

The trouble is that some part numbers were superseded by a part number which was an exact duplicate, while some other numbers appear to have superseded old part numbers to more or less combine multiple similar/comparable parts together.
With that in mind, this might help:

Originally Posted by SD455DJ

Here are all the factory Pontiac head gaskets (in no particular order) that Dan has saved. There are probably a few missing from post-1966, but we haven't investigated it.

9790910: (0.038”) 68-70 400/428 H.C.; 69 400 HO/RAIII/RAIV
9790911: (0.048”) 68 400/428 Sm.Va.; 70 400 2V; 71-73 400
9785654: (0.038”) 67 400/428 H.C./HO
545055: (0.038”) 70 455 All
489029: (0.042”) 73-74 400 All
488575: (0.042”) 71-76 455 All

[PontiacJim1959]

Originally Posted by unruhjonny

The trouble is that some part numbers were superseded by a part number which was an exact duplicate, while some other numbers appear to have superseded old part numbers to more or less combine multiple similar/comparable parts together.
With that in mind, this might help:

Agreed, some part numbers will remain while some will be superceded - and may have been due to a different supplier and means to ID?

I do have all the same head gasket numbers, but they did not have all the thicknesses. The different compression thicknesses would be cause for the difference in some of the advertised compressions, no?

[Kenth]

This is what i´ve found in 1967, 1970 and 1971 MPC´s:

1965-66 389/421 HO uses gasket #9772289 (.028"/.033" thick).
1965-66 389/421 non-HO uses gasket #542973 (.040"/.045" thick).

1967 T/8 400 or P/8 HO uses gasket #9788026 (.028/.033" thick).
1967 F/8-T/8 400 std. and P/8 400 exc. G.P.-4B.C. w/1st type cylinder head and P/8 428 H.O. uses gasket #9785654 (.040"/.045" thick).
1967 P/8 400 G.P. 4 B.C and P/8 400 G.P.-4B.C. w/2nd type cylinder heads and P/8 428 exc. H.O. uses gasket #9787642.

1968 F/8-T/8 400 4bbl/428 HO uses gasket #9792995 (.028/.033" thick).
1968 T/8-P/8-400-exc. G.P.-4B.C. uses gasket #9790910 (.040/.045" thick).
1968 P/8 400 G.P.-4B.C. uses gasket #9790911 (.055"/.060" thick).

1969 All 400 (2B.C.) and F/8-T/8 Ram Air-III, IV or H.O. and P/8/GP/8 428 uses gasket #9790910.
1969-70 All 400 4B.C./428&H.O. uses gasket #9792995.

1970 All V/8 400 (exc. 8.6 ratio) uses gasket #9790910.
1970 455 (YH) uses gasket #545055 (.040"/.045" thick).
1970 All 455 (exc. YH) uses gasket #9792995.

1971-72 All 400 gasket #9790911.
1971-72 All 455 gasket #484351.

1973-76 455 uses gasket #488575 (.050"/.055" thick).

FWIW