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Is the 11:1 Compression ratio an issue when it comes to a 65 Skylark andmodern fuels?

Discussion in 'Small Block Tech' started by Spicoli, Aug 18, 2015.

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  1. Spicoli

    Spicoli Member

    Hi, I'm considering purchasing a clean 65 Skylark convertible that came with the 300 ci motor and the 4 barrel/11:1 Compression ratio option. Question (and apologies if this is a FAQ): Will the 11:1 compression ratio be an issue with regard to modern gasoline and engine knock? Or will I be fine as long as I go with the high octane gas? I'll be running mostly at above average elevation (>4,000 ft amsl), which, if I recall, makes a difference too. The seller, who seems like a straight shooter, says knock/ping has not been an issue for this car, and he's running it at lower elevations (near the coast).

    Thanks much.
     
    Spicoli, Aug 18, 2015
    #1
  2. Aaron65

    Aaron65 Well-Known Member

    Re: Is the 11:1 Compression ratio an issue when it comes to a 65 Skylark andmodern fu

    You should be fine with premium pump gas. First, the '65 was actually rated at 10.25 compression (the 11:1 is a common misprint based on the '64). Second, I doubt the actual compression ratio is even that high. I run mine on pump 93, and I have for 12 years.
     
    Aaron65, Aug 18, 2015
    #2
  3. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    Re: Is the 11:1 Compression ratio an issue when it comes to a 65 Skylark andmodern fu

    Yes, engine would need to be blueprinted to factory specs to be the actual listed compression rating. As we have discussed in depth on here a number of times the sbb engines usually have extra deck height from the factory, the older blocks around .030" while newer blocks can have as much as .070" extra deck height.

    If the head gaskets have ever been replaced in its 50 year life, odds are they have the thicker Fel-Pro .040" thick head gaskets installed instead of the factory steel shim head gaskets that were about half the thickness. And if the engine has been rebuilt the pistons can be the "economy" rebuild pistons that would make the C/R MUCH lower closer to 8:1 or even lower.

    If the engine was blueprinted and the deck machined to the 9.543" listed spec and a factory style head gasket was used when re-assembled with the factory pistons, then you'll need to run premium at least 91 octane with 93 being a better choice. If it is an untouched from the factory engine you can probably get away with a 50/50 mix of 87/93 or 89/91 depending what fuel is available where you're at. GL



    Derek
     
    300sbb_overkill, Aug 18, 2015
    #3

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