[Diego]

Does anyone know why Pontiac ever-so-slightly cut the compression of its performance engines for 1970?

And did this affect performance at all?

[SteveGB21]

There are two reasons why:

1) Insurance rates in the late 60's/early 70's were increasing premiums.
2) The emissions (particularly California) made it necessary to cut down on leaded gas, and was introducing the low-lead engines (eventually unleaded).

[Diego]

You're thinking of 1971, not 1970.

1970 was still a high-compression year among all brands.

[Lloyd-TX]

Originally Posted by Diego

Does anyone know why Pontiac ever-so-slightly cut the compression of its performance engines for 1970?

And did this affect performance at all?

Diego - in my opinion the advertised CR "reduction" was on paper only. No real change, just closer to the real CR rather than 0.5 higher than actual.

[Diego]

So then why did they bother making new heads for 1970?

[Lloyd-TX]

Dunno? Nominal chamber volumes were similar if not identical between the 1969 "722" castings and the 1970 "614" castings.

Why did they bother making new cylinder heads for the 1972 455HO? As you know the 1971 HO engines used the "197" castings, and the 1972 HOs wore the "7F6" castings.

[Diego]

Originally Posted by Lloyd-TX

Why did they bother making new cylinder heads for the 1972 455HO? As you know the 1971 HO engines used the "197" castings, and the 1972 HOs wore the "7F6" castings.

Actually, I *don't* know this stuff, as you guys are into parts numbers, I'm into production numbers!

Anyway, my main interest is why the change for 1970.

['ol Pinion head]

Diego, if you look at '69 Dport big valve heads, there were the following as virgin condition production cyl heads:
-slightly smaller chamber 350 HO app 48's... 67-68 cc (?) chambers
-the more common 400 WT and RAIII app 48's... normally 72cc chambers
-the extremely common 400 350 hp auto 62's @ 75cc chambers

for '70, there were the following:
- no more 350 HO's...just 40 years later on eBay
-400 WT and RAIII app 12's... again, 72 cc chambers
-extremely common 400 350 horse auto 13's @ 78-80cc chambers.
Some old books like to list the 13's @ 75cc chambers, but the virgin heads I've
cc'ed came in the high '70's.

Anyone's educated guess is as good as another on the lowering of the C/R slightly on these automatic applicationheads, but with the growing popularity of heavy automatic a/c cars, with higher gear ratio's, my guess is slight towering of the compression ratio had to with combatting detonation.

[Diego]

But if you look at Buick, Olds, and Chevrolet, they went all-out with their engines, for the most part. It seems Pontiac's experience for 1970 was about compromise and anticipated performance options that ended up being cancelled.

Seems terribly strange to me that Pontiac was so concerned about detonation in a year where most brands produced their most powerful cars, no?

['ol Pinion head]

Diego, looking at the performance lineup for Pontiac for '70, I'm betting we can agree
-the RAIV was again tops,
-the RAIII came in behind it,
-the 455 big valve was further back
(Pontiac basically offered a 7.5 liter engine, as other GM divisions did as well).
-Next, there was the base 350 horse 400's which the VAST majority of GTO owners and Formula owners bought. With the large majority of 400 350 horse engines being YS's, and many of them being laden with AC, the 400 auto base horse cars weren't designed to be cutting edge performance wise, just to sell to the masses, and be relatively easy, warranty wise on Pontiac.

[Lloyd-TX]

Originally Posted by 'ol Pinion head

Diego, if you look at '69 Dport big valve heads, there were the following as virgin condition production cyl heads:
-slightly smaller chamber 350 HO app 48's... 67-68 cc (?) chambers
-the more common 400 WT and RAIII app 48's... normally 72cc chambers
-the extremely common 400 350 hp auto 62's @ 75cc chambers

for '70, there were the following:
- no more 350 HO's...just 40 years later on eBay
-400 WT and RAIII app 12's... again, 72 cc chambers
-extremely common 400 350 horse auto 13's @ 78-80cc chambers.
Some old books like to list the 13's @ 75cc chambers, but the virgin heads I've
cc'ed came in the high '70's.

Anyone's educated guess is as good as another on the lowering of the C/R slightly on these automatic applicationheads, but with the growing popularity of heavy automatic a/c cars, with higher gear ratio's, my guess is slight towering of the compression ratio had to with combatting detonation.

oPh has got it down, Diego!

[Diego]

Perhaps I should be clearer:

Why the marginal changes to the RAIV - changes that seemed to compromise horsepower - when everyone else ramped up their efforts? It seems the RAIII also experienced the same fate.

[Lloyd-TX]

And I still say that there was VERY little if any torque and horsepower difference between the 1969-1970 RA3 engines, and the 1969-1970 RA4 engines.

"Paperwork" changes to the "advertised" compression ratios only.

The cars DID get heavier and more option laden, however! That affected performance more than any negligible changes to the engines (if any), for certain!

['ol Pinion head]

maybe advertised C/R ratios were down slightly for '70 model Pontiacs, but in all honesty, those C/R ratios had been slightly fudged higher than they actually were and they were still slightly higher than if you tore down, measured, and calculated the actual C/R. With the chamber sizes being the same, the head gaskets being the same, the deck heights being the same, the only difference is some number in a brochure...

[Lloyd-TX]

Originally Posted by 'ol Pinion head

maybe advertised C/R ratios were down slightly for '70 model Pontiacs, but in all honesty, those C/R ratios had been slightly fudged higher than they actually were and they were still slightly higher than if you tore down, measured, and calculated the actual C/R. With the chamber sizes being the same, the head gaskets being the same, the deck heights being the same, the only difference is some number in a brochure...

Very well put, Roger, and I concur . . .

[Kenth]

This may be of some interest.

#12 heads are around 66 cc´s, as are the #48´s.

This is EASY to see when you have the heads, chamber side, in front of you.