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Does Higher Octane Fuel Burn Cooler?

Discussion in 'Terminator Talk' started by NEWSVTGUY, May 9, 2009.

  1. NEWSVTGUY

    NEWSVTGUY New Member Established Member

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    I know it takes longer to burn but does it burn cooler or hotter than a lower octane?

    As in will it cause higher temperatures in the cylinder?

    Thanks
     
  2. NEWSVTGUY

    NEWSVTGUY New Member Established Member

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    Hmmm ok that's what I thought, I was just arguing about it with somebody who disagreed and said it ran hotter.

    Also I know nitrous is a cool (temperature wise) gas but does it increase temps in the cylinder? I figured it would cool things down but my buddy was insisting that it increased temps which is what causes motors to not last as long running N02, I can see it increasing cylinder pressure but not temps, it's just increasing the amount of oxygen in the motor basically right?

    I was insising on this but he continued to questino me which is making me second guess myself, and google isn't seeming to come up with much info for me.
     
  3. Smokin04

    Smokin04 Pump Gas KING! Established Member

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    The higher the octane, the harder it is for the fuel to ignite. The only time you will see a cylinder temp. drop is when you run a richer mixture or have a better than stock cooling system. In general terms...the more fuel you put in it, the cooler it runs, NOT the more OCTANE. The stoich level of race fuel is also a little different though. NA (naturally aspirated) stoich levels of say 110 octane is around 13.7 (depending on brand) instead of the industry typical 14.7 for 93 pump. So it can be confused with running cooler because the race fuel inherently burns at a richer (cooler) level than pump gas with no computer changes. FI (forced induction) cars have to run richer to keep detonation at bay at higher pressures. Pressure creates heat, heat adds to "hot spots" which if left unchecked, can turn red hot. That will cause pre-ignition (much more destructive than detonation).

    N20 is a cool air charge, but does increase cylinder pressure. It chemically introduces more O2 molecules where as blowers and turbos do it mechanically. Again, more pressure = more heat...but ALL N20 applications require additional FUEL to burn right. If not you're just introducing additional oxygen without additional fuel which will lean your mixture. The leaner the A/F...the hotter the chambers are. Also why running colder spark plugs helps ward off detonation.
     
    Last edited: May 10, 2009
  4. NEWSVTGUY

    NEWSVTGUY New Member Established Member

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    Ok so this is what I am understanding:

    87 or 110 the temps will remain the same in the combustion chamber. But wouldn't 100 octane have more burnable material in it opposed to 93 causing it to be cooler? I'm assuming the answer to this is no.

    11.5:1 is going to burn cooler than 12.0:1 AFR

    I though stoich was like idle/cruise afr, and that can be maniplated in the tune? Bot by octance rating, so what you are saying is that 100 octane may have a different afr than 93, bc my wideband doesn't show a difference whether I runn 100 or pump.

    So what's colder about a spark plug? The temp the spark is?

    So basically more pressure = higher temps
    Higher octane means nothing in realation to temps just that you can run a more agressive tune, which does not effect the cylinder temps just is more efficient.

    Runner more timing doesn't create more heat though? I thought it did.
     
  5. Great post :thumbsup:
     
  6. Well he said it will lower the temp but not because the combustion process itself is lower in temp but because the nature of the higher octane fuel creates a rich condition if the tune is not recalibrated.
    Stoich is the point at which all of the energy in the fuel has been combusted, but the ratio at which this happens is different depending on octane.
    It has to do with the plug's ability to dissipate heat, not the spark itself. Hotter plugs stay cleaner but have more of a tendency to cause predetonation.
     
  7. NEWSVTGUY

    NEWSVTGUY New Member Established Member

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    Thanks for the info!

    So why wouldn't everyone run the coldest plug possible?
     
  8. NEWSVTGUY

    NEWSVTGUY New Member Established Member

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    Hmmm interesting, thanks for the info!

    I just noticed that my afr at idle is at like 16 or so with the ac on but when i turn it off the afr drop back down to 15, any ideas?

    Thanks!!
     
  9. Mystic03

    Mystic03 Active Member Established Member

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    good post only part i tought was not right was that pre-ignition. it is not worse than detonation pre ignition is still a burn when detonation is an explosion that smacks the top of the pistion which you do not want at all, all higher octane is is that it can handle more timing when you talk about or wanting cooler burns in example nitrometh burns cooler and slower but you need double the fuel supply to make the same power if not more
     
    Last edited: May 16, 2009
  10. Smokin04

    Smokin04 Pump Gas KING! Established Member

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    You're close...

    Pre-ignition is a burn NOT caused by the spark plug. Pre-ignition happens when a piston or chambers area becomes hot enough (red hot) to ignite the mixture in the chamber which can happen when the piston is on the up-stroke. So naturally, ignited fuel mixture colliding with a piston on the upstoke = broken rods/crank/pistons.

    Under normal conditions, the combusting air and fuel mixture inside the combustion chamber ignites in a controlled manner. The mixture is ignited by the spark, normally in the center of the cylinder, and a flame front moves from the spark towards the outside of the cylinder in a contolled burn. Detonation occurs when air and fuel that is ahead of the flame front ignites before the flame front arrives because it becomes overheated. Under these conditions, the combustion becomes uncontrolled and sporadic and often produces a pinging noise, or a "knock" noise when the conditions become worse. Detonation is a timing related issue (spark based event) usually happening at or near TDC when the piston is close or already on it's way down the bore...so it's more forgiving because it's not two forces colliding like with pre-ignition, instead it's a spark at the wrong time. A motor that is pinging can still run, but is just way down on power due to non-optimal spark event.

    Burning cooler is kind of a way to describe nitromethane. Again you're close, but burning is still burning. But to "burn cooler" is kind of an oxymoron.

    The high temperature of vaporization of nitromethane means that it will absorb substantial engine heat as it vaporizes, providing an invaluable cooling mechanism. The laminar flame speed and combustion temperature are higher than gasoline at 0.5 m/s and 2400 °C respectively. Power output can be increased by using very rich air fuel mixtures which also increases the cooling affect. This is also something that helps prevent pre-ignition, something that is usually a problem when using nitromethane. Due to the relatively slow burn rate of nitromethane, very rich fuel mixtures are often not fully ignited and some remaining nitromethane can escape from the exhaust pipe and ignite on contact with atmospheric oxygen, burning with a characteristic yellow flame. Additionally, after sufficient fuel has been combusted to consume all available oxygen, nitromethane can combust in the absence of atmospheric oxygen, producing hydrogen, which can often be seen burning from the exhaust pipes at night as a bright white flame.
     
    Last edited: May 16, 2009
  11. NEWSVTGUY

    NEWSVTGUY New Member Established Member

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    So is a degree of timing in refrence to an angle?

    Any idea about my afr?
     
  12. Smokin04

    Smokin04 Pump Gas KING! Established Member

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    Perfectly fine. A/F at idle and cruise should fluctuate from rich to lean constantly, it your O2 sensors dialing in optimum fuel consumption.

    Yes 11.5 will produce lower chamber temps than 12.0. "Burnable material" just refers to volume, not octane. So you're right the answer is no.

    Think of the term "stoich" as optimal. AFR can be altered by the tune, but is not related to the fuel you are burning. AFR is a product of fuel volume and incoming air which is controlled by the PCM. What octane you're running has no effect on the PCM (unless programmed to know it's running a higher octane). That is why you SHOULD re-tune your PCM when running race fuel because the stoich (optimal) required volume is different than pump gas. But simply adding race fuel, wont hurt anything, but you won't pick up any HP either; unless you add boost or timing.

    Clear as mud?
     
    Last edited: May 16, 2009
  13. NEWSVTGUY

    NEWSVTGUY New Member Established Member

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    Great info man, I appreciate your willingness to help/inform!

    Any idea about the timing?
     
  14. Smokin04

    Smokin04 Pump Gas KING! Established Member

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    Timing is essentially when the plug fires in relation to crankshaft rotational degree...IE 25 degrees BTDC = 25 degrees of crankshaft rotation (of a total 360 degree crankshaft revolution) before that piston reaches top dead center (TDC).

    What about your AFR?
     
  15. Mystic03

    Mystic03 Active Member Established Member

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    yes the preignition is not caused by the spark plug but by hotspots in the cylinder such as carbon which is still a burn rather than an explosive blast on top of the piston which you know causes alot of problems, now detonation is caused by overheating in the cylinder usually caused by a lean condition we can go in circles about which is worse and which will cause more enigne damage but what is seen doing more damage is detonation, now the oxymoron of it burning *cooler* which it actually does and reason why it takes 2x for the fuel supply thats why they are able to run very high on timing. either way we both are on same track just explained differently :beer:
     
    Last edited: May 16, 2009
  16. NEWSVTGUY

    NEWSVTGUY New Member Established Member

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    You already answered the afr question.

    So I know people run anywhere from 15-25+* of timing but what would be the result of running too little timing?

    Thanks again!!
     
  17. Mystic03

    Mystic03 Active Member Established Member

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  18. Smokin04

    Smokin04 Pump Gas KING! Established Member

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    Despite nitromethane having a much lower energy density (11.2 MJ/kg) than either gasoline (44 MJ/kg) or methanol (22.7 MJ/kg), its addition to the fuel mixture has the net effect of increasing engine output by around 2.3 times compared to gasoline for the same mass of air -- 14.6 kg of air is required to burn one kilogram of gasoline, but only 1.7 kg of air for one kilogram of nitromethane. Since an engine’s cylinder can only contain a limited amount of air on each stroke, 8.7 times more nitromethane than gasoline can be burned in one stroke. So the fact the nitromethane takes "2x" the supply has nothing to do with timing or burn temp. Please understand, I'm not trying to start:poke:but nitromethane can't even be compared to traditional pump gas. The reason it takes more volume (and it's more like 30-40x) is due to the fact that it's a methanol based fuel which requires more volume to get the same energy as traditional oil based fuels (read first sentence again), and the fact that they're producing 7-8000 HP. That alone requires enormous amounts of fuel. Traditionally it takes around 30% more methanol to make equal hp (HP) to regular fuel. Also it's much harder to ignite (which is why you see two distributors on a TF car...two plugs per cylinder) because nitromethane when mixed with methanol changes the effective octane . Higher octane, higher resistance to ignition/detonation. In fact the actual numerical octane of nitromethane isn't really measured due to it's mixture variations, but it has nothing to do with temperature.
     
    Last edited: May 16, 2009

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