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Ignition timing with modified combustion chambers

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I’m interested in how much it is possible to raise the knock limit with combustion chamber work (but near standard compression). Any experience or opinions that can be shared would be much appreciated.

My rb26 has had the combustion chambers modified to raise the knock limit. Almost all the ignition maps I’ve seen for this engine are around low to mid 20’s at 15psi & 6000rpm. On 98 octane fuel this engine has taken 36deg of timing at the same point before passing the knock threshold & distinct indication on the audio (Plex equipment). This is about 10deg more advance with a safe tune.

Is this sort of gain in knock limit even possible? Opinions on this for any engine would be useful.

Cheers guys,


The short answer is no, that sort of increase in the knock limit would not be typical unless there was something fundamentally wrong with the stock chambers. I've tuned a lot of RB26s over the years and I'd agree that low to mid 20's would be a fairly typical timing map on pump gas at 15 psi. In most instances at this sort of boost however you can achieve MBT timing or at least quite close to it without knock. I'd be very surprised if the engine actually wants 36 degrees at 15 psi boost for optimal torque - Have you tuned this on a dyno to confirm?

What modifications have been done to the combustion chambers? A tidy up and polish of the combustion chamber to remove sharp edges is one thing, however it's common for some head porters to remove the squish pads on the RB26 head which will have a reasonable affect on the compression ratio too.

Hi Andre,

Thanks for the reply, very useful to hear your input.

I have recently found that my CAS was 4 degrees retarded … knock at 32 degrees sounds less ridiculous but still excessive. Timing is a bit unstable on the timing light and logs, so in the process of setting up a crank trigger. Very interesting to hear about rb26’s not usually being knock limited on such boost. I have only road tuned and yet to have it on a dyno, but have just organised a session.

I have road tuned another rb26 and when over advanced it knocked obviously from inside the car (understand it was knocking prior to being audible). This rb26 however I have never heard knock from inside the car including runs on 26psi & easily 400awkw. Once prior to having knock equipment I gave it a run at 40deg at 15psi 5000 to 6000rpm … Nothing. After this I bought equipment.

Unfortunately, I don’t know what has been done to this head as it was performed prior to me purchasing the car. I do have photos of the combustion chambers and ones of a standard head so will compare … potentially the compression ratio has been lowered. Will measure chamber cc next time the head is off, thanks for drawing my attention to this.