Octane plugs

[Joner1983]

Apologies if this subject has been covered elsewhere but I've just noticed my 20seh has a brown 98 octane plug but I'm not using super unleaded. Should I switch to a yellow 95 octane plug or is there benefits of keeping the 98 plug and running super?

Also I've just started using super in my C25XE. This has the A plug at mo. Do I need to change this to see some benefit?

Cheers, AJ

[chrisp]

My C25XE V6 also has the brown A octane plug and I've started using Esso Supreme 98 octane petrol - mainly because it is claimed to be free of ethanol. I haven't noticed any difference in performance over standard 95 octane fuel but I too would be interested in any views on whether changing the octane plug would be of benefit.

[James McGrath]

This is quite a complicated subject and often miss-understood.
Firstly, the plugs themselves are very simple. They are just resistors.
The ECU reads the resistance level and adjusts the engine accordingly.

All octane/RON plugs are double sided. Each side has a different resistance value. This corresponds to two different settings stored in the ECU.

Your brown one in the 20SEH will have 98 on one side and 95 on the other.
So just unplug it and turn it over if you are using 95 octane fuel.

The only thing that the brown plug does on the 20SEH is adjust the engine timing. on the 98 setting it is slightly advanced compared to the 95 setting.
This, in theory, should give you a bit more torque.

It is important to note however, that the plugs have different effects depending on what engine they are used in.
As the plugs are just resistors and the important information is in the engine ECU itself.
You can't take a brown 98 plug from a 20SEH and put it in a C20NE and expect it to make use of 98 octane fuel. This is because the C20NE doesn't have a 98 octane fuel setting built into its ECU.

More info can be found here:
viewtopic.php?f=83&t=16784&p=169911#p169911


As for the V6, it's the same part but is used for something completely different.
The V6 (along with the redtop, ecotec 2.0 and 1.6i e-drive engines) has automatic knock control, so will adjust engine timing depending on the quality of fuel you are using.
They instead use the resistance values of the plugs to adjust the engine noise.
Little in known about what actually changes in the engine when the plug is turned over, but it's supposed to be for countries that have stricter controls over engine noise. Just keep it on side A

I've experimented wit it on my V6 and it doesn't seem to make any difference.

In short, swap the brown plug over on your 20SEH and yes, your V6 can take advantage of high octane fuel, it does this automatically.

[Joner1983]

Thanks so much for this, that's really useful info. I'll turn my 20seh plug around when I get back and see how I get on using v-power in the V6