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compression, octane, and boost

Discussion in 'Engine Building' started by rjbaker, Jan 6, 2004.

  1. rjbaker

    rjbaker Junior Member

    I am considering buying a stroked, bored, B16 motor that has some crazy piston/head setup that make the static compression ratio 13.1:1 or 13.4:1.
    I'm pretty sure as the static compression ratio increases, power increases across all rpms, so this motor should definitely be more powerful than your average B16.

    Here are my questions:

    1. What is the maximum compression I can run on pump gas (93 octane)? I don't want to buy racing fuel or add a quart of something every time I fuel up.

    2. Can I supercharge a motor with this high of a compression?

    3. What exactly is the difference (in power, fuel use, anything else you can think of) between:

    a) a NA motor with high compression (eg. 13:1 compression)


    b) a supercharged motor with a lower compression (eg. 9:1 w/Jackson Racing supercharger)

    I've been researching this for about a month now...if anyone has links to the topics above (or a damn good explanation of static & effective compression ratio and the effects on fuel required, boost allowed, and power output), I would appreciate it if you posted those links. Thanks for your help.
  2. phunky.buddha

    phunky.buddha Admin with a big stick Admin

    1. I ran 12.6:1 compression on 92 octane with really rich A/F ratios, and then 12.4:1 later on 93 with another build, using a not too rich A/F ratio of 12.5:1 to 13.5:1. You should be able to get 13.4:1 to behave on 93 octane pump gas, but you'll probably have to pull the timing back a good amount. You might want to consider lowering your compression ratio with a thicker head gasket. If your compression ratio is 13.4:1 (and your engine geometry all follows stock B16 specs for bore/stroke) with a stock thickness (0.029in) head gasket, a custom 0.050in thickness gasket will get you down to 12.4:1 compression. That will allow you to tune easily for 93 octane and still have a nice power curve.

    2. Yes, but since you're worried about running pump gas already, I wouldn't recommend it. You'll almost definitely have to run non-pump grades of gas to run a supercharger on top of static compression like that.

    3. I don't know any links to the information above off the top of my head, but I may be writing articles explaining some of it later. Think of it this way though... an NA engine with high static compression will make power because it can breathe (head work) and because the high static compression ratio "loads" the air/fuel charge up more, causing it to release more energy upon combustion. The supercharged engine makes more power because it's forcing more air into the engine, giving more opportunity to burn more fuel and make more power. Typically, higher effective compression ratios will yield more power given the same potential energy stored in the fuel that is to be combusted- but you can't really use that to directly compare (easily anyway) a high compression NA engine and a low static compression engine under boost. Just look at some dyno charts.

  3. pissedoffsol

    pissedoffsol RETIRED

    why not just start with a gsr? the hell with stroking it.
  4. JDMilan

    JDMilan Senior Member

  5. phunky.buddha

    phunky.buddha Admin with a big stick Admin


    That's what he's considering buying though... oh well. He can answer for himself if he comes back and sees this at all.

    :lol: :lol: :lol:
  6. pissedoffsol

    pissedoffsol RETIRED

    both of these are in our articles section.... have a look. page 4 or 5 i believe
  7. rjbaker

    rjbaker Junior Member

    Well, this motor has the same stroke as the GSR, but it is resleeved and bored to (I believe) 86mm - total displacement a little over 2 liters. I didn't do an in-depth price analysis yet, but I suppose that the prices would be about the same, the prebuilt motor vs. buying a GSR and boring it out.
    But I will look into the GSR as another option...thanks for the comments, all.

    - RJ

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