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Octane vs safe AFRs & knock detection

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I’ve found it’s pretty common for a lot of tuners to target 12.5:1 for NA as “safe best power”. This is what the tuner that tuned my car chose. However the car makes noticeably more power 13:3 - 13.5, which is often recommended for Hondas like mine.

Is octane and fuel quality the main safety risk regarding this decision? In my area (California) the powers that be have long since decided 91 octane is all anyone really needs. There are some niche stations here and there where I can get better but it’s uncommon.

I’m trying to decide how risky 13:5 is. How bad would a bad tank of gas be?

My car has no built in knock detection (that is of any sort of value). I am considering buying a Plex because aside from having good knock detection for tuning it also has indicator lights so that if I did stray from the 12.5 for the improved power I’d have a bad gas indicator that blinks if I got a bad tank (in theory).

Is that sensible?

You'll have to remind us if your car is normally aspirated or forced induction. I see no problem running 13.5 AFR (.92 La) for NA cars, as long as your ignition tuning was done on a dyno with that AFR and is knock safe.

The reason I say ignition timing done on the dyno, is that the dyno can help you identify the minimum timing that really doesn't lose more than a couple of percent of power. You'll never find that street tuning.

I’m totally with you on the dyno tuned ignition.

You think the bit about using the plex as a bad gas detector makes any sense since my ecu has no knock protection strategy? (Of course would also have it for tuning as well).

In my experience 12.8-12.2 is indeed safe AFR that I have found in a few MMC stock NA engines at high load and high revs. I believe it is done for two reasons: 1) to keep stock catalytic converter at certain temperature where it is most efficient 2) prevent combustion chamber overheating under high load during long time such as going uphill or speeding at high gears. If the car is not meant to see this kind of load it would be absolutely reasonable to use leaner AFR such as 13.8-13.5 - this is exactly what we are using in our drag racing Honda since the engine is under the load for just about 12 seconds...

@David naturally aspirated b18c with some more aggressive cams.

We usually reply within 12hrs (often sooner)

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