Does anyone know to calculate fuel octane required to avoid detonation? I've googled around a bit and honestly haven't found anything conclusive.

Curious at what point does "high" compression warrant leaving 87/89/91 octane pump gas (R+M2) for 96/98/100 octane (or even higher)?

Thank you,

Good question, and there is an easy answer.... if you don't get any detonation under any
load, rpm and temperature conditions, then you have enough octane. The chart below gives
one generalised answer, but there are many factors involved and no-one can give you a specific
answer to the question "how much octane do I need for my engine ?".



Of course if your engine is factory stock the manufacturers handbook will tell you what octane
rating you should use. But for a modified engine the octane required depends on a number of
factors such as engine design, effective compression ratio, ignition timing and fuel mixture.

Given that light detonation may not be audible when driving, but is still capable of causing
engine damage, you can either rely on your engines knock detection system to keep you safe
or you can book a dyno session. Check and adjust the tune if necessary before testing. Make
sure to use a competent operator with decent gear for listening for knock, run the engine
thru its full operating range. If knock is detected you can either retard the ignition slightly
and see if it goes away. If not, fill up with a higher octane gas and back to the dyno.

That is the only sure way if you don't have a working knock detection system connected to
your ECU to retard the ignition timing when knock is detected.

Cheers... jondee86

Thanks Jondee,

Will be getting on the dyno but I was trying to ballpark and thought there might be a (at least somewhat) accurate way to figure out what would be safe. I'll be somewhere between 11.5:1 and 12:1 but I've still to calculate everything.

The Toyota 86 (Scion) has a static compression ratio of 12.5:1 and runs on 98RON with a modern
ECU with knock control. On this basis alone I would suggest that (if available) you run 93AKI.



However, that is not the full story. To better estimate your octane requirements you need to
consider the Effective Compression Ratio rather than the Static Compression Ratio. You
can use this Wallace Racing calculator to estimate your ECR for various intake valve closing
settings (vernier cam pullies required).... http://www.wallaceracing.com/eff-cr.php.

The ECR together with ignition timing largely explains why some high static compression engines
will run quite happily on 91AKI pump gas without having detonation problems. So if 93 octane
is not readily available in your area you could tune for 91 octane on the dyno. So long as your
tuner uses good knock detection while tuning, that may very work for you.

Cheers... jondee86

Thanks again Jon,

Seems I'm on the right track, I had tried calculating the effective C/R earlier but stumbling through it. Once the shop gets started I'll have the actual calcs but this is a good rough idea.

What is the VE% value on the calculator?

Volumetric Efficiency (VE) is a measure of how efficient your engine is at drawing air in
to the cylinders on the intake stroke. If the volume of air drawn in on each intake stroke
is equal to the calculated swept volume of the cylinder, the VE is 100%. However, as the
intake system will have some resistance and cylinder scavenging is poor at low rpm, the
VE varies with engine speed.



Modern 4-valve engines will usually achieve close to 100% VE at high rpm and can be as
low as 45-50% VE at low rpm, depending on the design of the intake system. I would
suggest that you use a value of 85% VE for calculating ECR.

Cheers... jondee86

just to add a little...

1) Chamber shape/condition can either minimize, or exaggerate detonation. Anything "sharp" in the chamber will promote detonation

2) Squish gap - distance piston is away from head - tighter tends to minimize detonation