head compression vs fuel octane

[tsrt]

Can anyone share some info:

head compression (psi) = what octane ?

[Mark75H]

There is no strict formula. Many variables go into octane demand. The effectiveness of scavenging, shape of combustion chamber, location of spark plug, shape of piston, rpm, carb size, effectiveness of exhaust tuning, bore size - just to name a few. Changing any one of these with the same psi can dramatically change the necessary octane.

[Fast Fred]

150psi is the cut off for pump gas, that is to say at 150psi 94oct or less just prelights and burns down. 150- 180psi you can run 50-50 LL100 and high test,
at 180psi + just LL100 or 102.

[Fast Fred]

timing is key, plays a big part.

[Mark75H]

150psi is the cut off for pump gas, that is to say at 150psi 94oct or less just prelights and burns down. 150- 180psi you can run 50-50 LL100 and high test,
at 180psi + just LL100 or 102.

That would be for a particular motor, not all motors

[Roy Hodges]

There is no strict formula. Many variables go into octane demand. The effectiveness of scavenging, shape of combustion chamber, location of spark plug, shape of piston, rpm, carb size, effectiveness of exhaust tuning, bore size - just to name a few. Changing any one of these with the same psi can dramatically change the necessary octane.

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It seems to me ....that octane,in itself, only determines how fast or how slow the combustion occurs. Higher octane does not add power ,by itself. In fact , in a given engine , lets say a mercMK55 ,which only needed 72 octane(itwas stated so on the gas tank , )you try 115/145 av gas in it (a totally stock fish motor) -I would bet you lose power ! It has such high octane that it burns so slow that it has very incomplete combustion. the JETTING in the carb should also affect the octane requirement .

[Mark75H]

Octane rating is not combustion speed , it specifically is resistance to an abnormal flame pattern with 2 colliding flame fronts in an uncontrolled manner.

There are fuels with differing flame speeds and temperatures, but even these have no direct relation to octane/knock performance. 2 different fuels that have the same flame speed can have vastly different octane performance.

You are correct that carb jetting affects octane requirement ... its one of the many things that I did not mention; there are still more. You are also correct that octane rating of a fuel does not increase a motor's power; it is other things in the motor that increase the power, higher octane gas is just a secondary thing needed to go with the things that actually increase the power.

It could be thought of this way ... a bigger carb may require a different mounting pattern because the base is bigger. The mounting studs or holes would need to be farther apart, but the stud location by itself does not increase the power in the motor, even though they are necessary for the larger carb that does increase the power.

[Fast Fred]

That would be for a particular motor, not all motors

timing becomes a factor, you could run pump gas at 150psi, you got to take out most of the timein, then it makes no Beef.

ya maybe not all motors, just outboard loopers

[russhill]

I think there’s a little confusion here and I’d like to shed a little light, if I may. Quit now if you’re easily bored.

First octane: It doesn’t really exist, now, it’s a fabricated number. Originally, when gasoline was refined, there were three products that went into gasoline: Sextane, septane and octane. The better gases had more “octane” and less of the other stuff. The higher the octane, the better the performance of the gasoline. Bragging numbers even got to 100 Octane, which by this definition is as high as you can get.

Octane’s primary purpose was to slow combustion. In later years, tetraethyl lead was added to achieve the same purpose, and therefore, the effective octane could be greater than 100. Now, in the absence of tetraethyl lead, they use smoke and mirrors in the refining process to increase octane.

Like with other government numbers, they have changed the method of reporting—look next time at your pump. There is a number which is the average of two numbers called “Motor” octane and “Research” octane. On the pumps you’ll see R + M / 2.

Remember now, “octane,” real or artificial is used to slow combustion or flame propagation in the combustion chamber. (Advertised ans “Anti-knock”) Don’t confuse flash point with flame propagation speed. Slower flame propagation speeds tend to provide combustions further down the stroke, and therefore are more desirable.

Within the engine, there are lots of things that can impact flame propagation. The primary thing is combustion chamber temperature. I’m speculating here, but I believe compression pressures do not directly impact flame propagations, however compression does increase temperature and therefore indirectly accelerate flame propagating. Other things which impact flame propagation are combustion chamber shape. A baffle on the piston certainly does not encourage propagation over the entire cross sectional area of the piston.

Spark timing, in my opinion, does not really impact what I’ve said above; however, recognizing that such octanes, temperatures, etc. do impact flame propagation, spark timing should be set to optimize performance by being delivered sufficiently ahead of top center to assure the flame has properly propagated through the combustion chamber and is ready to do its job. (That’s a pretty dumb statement—set your spark right!!)

Now is that more than you ever wanted to know or what???

[Mark75H]

Russ, by definition "knock resistance" or "octane rating" is not related to flame speed, but only the suppression of a secondary flame front formed after normal combustion is started by the spark plug.

Any other characteristic of the fuel or flame is not directly related to octane.