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Thread: 4age bigport optimized for 91 octane fuel

  1. #1
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    4age bigport optimized for 91 octane fuel

    I am currently getting my engine together for a rebuild and was hoping to get it optimized for 91 octane fuel as Shell sells no ethanol in that grade.

    Anyone know what set up I should do? Compression ratio? Will the oem engine management be able to adapt to high compression?

    Just trying to see if it is possible to get more power/economy/fun out of the higher octane sine i'll be using it regardless.

  2. #2
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    South Lyon Michigan
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    optimize for 91 octane

    If you are looking to optimize the 4AGE you should shot for quench in the range of .030" to .032". [ .032" to .034" if looking for the safe side] Assuming the pistons are only a few thou below the deck, you should be able to get a Cometic head gasket that will get you in the ball park. For increasing the CR, every .004" cut will reduce the head volume by about .5cc. davew

  3. #3
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    I can tell you the OEM engine management has no real adaptability features, and there's no knock sensor either to help out with that. It keeps the spark on time and the AFR's in tune at cruise.

  4. #4
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    I have seen some references to the quench at just a hair bigger than 0.75 mm (converts to your number of 0.030"). Good to know the amount to reduce combustion chamber volume by 0.5 cc. I just need to find the proper #'s for engine compression ratio calculator, going through a bunch of posts and info online I get differing info that doesn't seem to get the right numbers for the stock compression ratio. been punching them in there and got it somewhere close to stock with these numbers:

    Cylinder Bore Size: 81 mm
    Piston Stroke Length: 77 mm
    Head Gasket Bore Diameter: 82 mm
    Compressed Head Gasket Thickness: 1.2 mm
    Combustion Chamber Volume In CCs: 37.75 cc (probably wrong, first try doing this so will definitely be doing a few more tries)
    Piston Dome Volume In CCs Negative For Dished Pistons (Use '-'): -3.06
    Piston Deck Clearance Negative If ABOVE Deck (Use '-') : 0 (also some people say it is a few thou below deck so probably have to see after getting the block cleaned up and assembled.)

  5. #5
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    Quote Originally Posted by Sparkie View Post
    I can tell you the OEM engine management has no real adaptability features, and there's no knock sensor either to help out with that. It keeps the spark on time and the AFR's in tune at cruise.
    so new project for you to remap the oem for 91 octane lol? or would I be able to make it work with a piggyback system?

  6. #6
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    I have rebuilt three 4AGE motors and the lowest piston was .004" below the deck, highest .001" below.
    A standard head volume is 36.cc un-cut head is 116mm
    chamber volumes will depend on the gasket thickness, deck to piston and head mill. but could be as high as 38.8cc
    Probably a 30 yr old head has been milled and is closer to your 37.7 to 38.cc range plus it depending on the type of gasket you will be using. Some can compress up to .010" when torqued down.
    To be right on the money you need to measure everything. davew

  7. #7
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    Quote Originally Posted by bongmuon View Post
    so new project for you to remap the oem for 91 octane lol? or would I be able to make it work with a piggyback system?
    well the factory ecu is pretty low powered, but if you had timing adjustments I have a rats nest of logic... somewhere around here... that can implement it.

    never did fab up a system like I did with the V8. Ultimately standalones are too cheap and too powerful not to use in most cases. Think sweet sweet coil on plug ignition!

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