High Compression ratio? 95 octane vs 93 octane

[NEW]

This is obviously not aimed at all those "lower" persons living near sea level. Good luck to you, the polar caps are melting.

As I understand, 95 octane is only really needed on the Highveld for high compression ratio engines.

What is considered to be a high compression ratio?

[grips]

I would say above 10.5 :1. But most modern engines do close to 11 and some even higher.

[NEW]

Ok, so a weirder question...


When to start using only 95 octane?

[Paul#25]

We found with testing on the race cars that below about 11.5 : 1 95 octane was all that was needed. I'm with Grips on the 10.5 : 1 for 95 instead of 93.

[NEW]

So how about a 2.4L with a compression of 10:1 ? 93 or 95?

[Paul#25]

It should run equally well on either. It should be fine tuned for the specific fuel on a dyno to get optimum performance.

[Woolf]

On my Jimny there is a sticker in the petrol flap stating that 95 must be used. Have used 93 on occasion with no noticeable difference in performance. However fuel consumption is less with 95 than 93. This is up in Jhb

[Michael]

I know that people will argue with me, but on every car I've tried (and it's quite a few), I have got better fuel economy on 95.

Fuel economy aside though, I personally believe the higher the octane, the better. Higher octane fuels are better at preventing detonation. Detonation/pre-ignition/knock/pinking/pinging occurs when the fuel ignites prematurely (ie. before the spark plug fires) which is equivalent to taking a good swing at your piston with a hammer. There are only so many hits it can take before it fails. In my opinion, the 26c a litre is nothing compared to the cost of a new engine.

Most new cars are optimised for a higher octane fuel so will run better, smoother and more economically on it. In Japan they have 100 octane fuel at the pump and in Europe they have 98/99.

Also remember that temperature and air pressure can all make a difference to the chance of detonation. So running 93 might be fine on a cool day, but on a hot day, the car may detonate.