Compression Ratio for 425 Engine

pennst8er

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Oct 26, 2021
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Hey All, I have a 1966 Delta 88 with a 425 engine. It is still completely stock (60k original miles) with a 2 barrel intake/carburetor and single exhaust.

Did all 425s come with the same heads, hence compression ratio or did they come with different heads/compression ratios? I want to get the most horsepower out of it as I can so of course plan to put dual exhausts on and a 4bbl intake/carb setup.

I have been trying to research this answer as best I can but can't get a straight answer. I read somewhere that there were 425s with 9 to 1 and some with 10.5 to 1 ratios. I also read that all came with 10.5 to 1 ratio combustion chambers.

I figured that if there were experts anywhere on Olds 425 engines, they would be on here. Thanks in advance for your answers.
 

Rocketman269V

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Feb 26, 2020
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South Central Florida
The 425s were available as low compression 2-bbl & 4-bbl AND high compression 2-bbl & 4-bbl. This was accomplished with different piston dome configurations. There were actually three different compression ratios: low, high and ultra high. The tip off from the factory was the air cleaner color; black for low compression and red/orange for high compression. The heads also had either small or big valves. Confused?
 

pennst8er

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Oct 26, 2021
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The 425s were available as low compression 2-bbl & 4-bbl AND high compression 2-bbl & 4-bbl. This was accomplished with different piston dome configurations. There were actually three different compression ratios: low, high and ultra high. The tip off from the factory was the air cleaner color; black for low compression and red/orange for high compression. The heads also had either small or big valves. Confused?
Haha, yes, somewhat confused. Thank you for your answer. So my air cleaner is red/orange so I have high compression. Is there a way to tell if it has ultra-high compression? Also, is there a way to tell if it had small or big valves?
 

Rocketman269V

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Feb 26, 2020
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South Central Florida
I'm pretty sure that they didn't make any 2-bbl 425s with the ultra high compression pistons, as it was more of a performance option. That being said, Olds did build some "Turnpike Cruiser" models from '68-'70. These were ultra high compression, 2-bbl equipped A-body cars. They were geared toward traveling salesmen and came with a 2.56 positraction rear end. It was supposed to give you better fuel economy on the highway; however, I question their logic, as a 4-bbl should get better mileage at a steady cruising speed.
 
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pennst8er

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Oct 26, 2021
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This attached photo is a 425 from a 1967 Toronado. Does this have the big valves?
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