You don't need it!

 

Now I know it's not as straight forward & simple as this but for the most part my next statement is true.

Why would you run more timing & slower burning fuel(high test) when a couple degrees less advance & a faster burning fuel (low test) will give you the same results cheaper

 

I didn't think that was true (it didn't make sense to me chemically), so I searched a bit and found this www.theserviceadvisor.com/octane/html (and other sources, this was the most concise)

Higher octane has more resistance to igniting spontaneously due to heat or compression (meaning it won't burn until you want it to, EI when your spark plug sparks). If we have our timing advanced a lot there is obviously more of a chance of pre-ignition causing damage, so would have to prevent it as much as possible.

I'm not trying to build a case for using expensive gas when not needed, quite the opposite actually, but there is more to the answer than "use the gas your owner's manual recommends". When the mfr tells you that, especially with modern cars with electronic everything, they assume nothing has been changed regarding fuel mapping, timing, compression, boost, etc. As soon as you modify some of that stuff you have to add things to the octane equation (like "chipping" a car might allow it to run twice as much timing advance, boost, etc as it ever would with factory settings).

When are the petroleum engineers and hard-core powertrain guys gonna get in on this thread?

 

Star 79 is correct, octane has absolutely nothing to do with how fast the fuel burns and exactly what he said about what the octane reading actually is, just resistance to detonation. As far as what octane for what car , there's too many variables to say all the lower compression ratio cars on here can run 87 becouse I guarantee they all can't and have maximum performance. the reason why is there are two different kinds of compression, static and dynamic. one has to do with the mechanical compression ratio which is the stat we all give for our engines and the other has to do with actual compression pressure. As we all should know with pressure comes more heat, the more pressure the more heat and with enough heat we have detonation, the reason why blown motors with very low compression need high octane fuel. The cam profile plays a big part in this as well, this is the reason why some high compression motors get away with lower octane fuels and some won't. With enough cam overlap, cumbustion pressures are held at a low enough level to prevent detonation. Timing curves also have a lot of bearing on octane requirements becouse of cumbustion pressures. So my personal recomendation is to run what the owners manual says and adapt from there depending on modifications.


Pat Kunz

 

and run the cheaper fuel. But high compression cars will require higher grades to maximize power via tuning. What they said about low compression cars and high test, slower burning high octane. Basically, octane detunes fuel to where you can compress it more before igniting it. Then the dynamite is in a smaller package and gives you more intense bang. Cars that run without pinging on low test fuels are either low compression or not optimized on timing. Pinging is really premature ignition of the fuel, meaning that it is exploding before the piston is at top dead center, so it starts exploding and pushing down on the piston before it reaches top dead center and therefore is: 1, fighting your other cylinders on power stroke and 2, creating a lot more heat and pressure on your engine parts twisting the crank and hammering the top of the pistons. Pinging is hell on a motor.