Does high octane fuel reduce combustion temperature?

Discussion in 'General Questions' started by AussieSteve, Dec 6, 2009.

  1. AussieSteve

    AussieSteve Active Member

    I started to post this in '2-Stroke Engines', but it's probably relevant to both types of engine.
    This isn't so much a problem as a general theory question.
    A couple of weeks ago, I fitted a high compression head, so went up to about 98 octane fuel. Good combustion, no pinging, plug a light tan colour.
    Temporarily, I've re-fitted the stock head, but I'm still running the same fuel.
    The plug runs much colder, (black), than it did before the hi-comp head was fitted. It used to be tan, the same as with the high-comp head. (In fact, with the hi-comp head, an NGK B6HS burns pretty nicely.) The only change with the stock head is the fuel. (I added 8ml/litre Nulon Pro Octane Booster to standard unleaded, 91 octane in this country.)
    My plug is an NGK B5HS - usually hot enough for these engines.
    As I said, it's not really a problem because in a week or two I'll have the hi-comp, (Rock Solid Engines billet aluminium), head back on.
    I'm just interested.

    ... Steve

  2. give me vtec

    give me vtec Active Member

    I dont think so...
  3. Al.Fisherman

    Al.Fisherman Active Member

    When you have higher compression ratios, you will have higher combustion chamber pressures. When you have higher combustion chamber pressures you will have higher heat. The more heat the higher octane you need. You want to ignite the fuel at the proper time (timing) and not that of the heat. The higher the octane the more restraint it is to prematurely ignite from heat. Now being resistant to prematurely igniting it might ignite slower, not burning all the fuel and the end result is a cooler cylinder temperatures.

    Using a low octane with high compression ratios will trash a engine....preignition. Real quick in these. On the other hand it is a waste of money to use a high octane fuel in a non modified engine.

    I went to mid grade as I changed my compression by not only removing the two head gaskets but using a thin gasket.
    Last edited: Dec 6, 2009
  4. AussieSteve

    AussieSteve Active Member

    Gee, I was sure I replied to this...
    Thanks, Al,
    That's pretty well what I thought. Higher octane = less volatile so lower temp.
    ... Steve