How did you get such high compression in an A15? I sruggled to get mine to just under 12:1, before the valve cutouts in the pistons were as thin as I wanted to go. What chamber shape head and what pistons do you have?
Mine runs sweet at 11.75:1 on avgas.

We had a similar thing here too. When Challenge opened, they were getting petrol from singapore as well. But then shell bought them, and now they use the rubbish made at the marsden point refinery. Pump gas here is complete and utter sh!t. Some companys sell so called 98 octane, but it's not availble in the south Island yet.
The fine for using leaded gas on the street is ten grand, and it's not cheap either.

(sorry to go off topic, but this is a sore spot for me. When we lost leaded gas, cars that ran sweet turned into pigs overnight)

I am facing a similar dilemma..... My car has been running AVGAS, but that is about to be phased too.....

I had an interesting conversation with someone yesterday who is pretty well connected in the racing world. He was telling me that in the past, Woolworths were importing their petrol from Singapore, and the premium was actually 102 octane, he has been running it in his race car. However, He also believed that Woolworths had done a deal with Caltex recently, and so this situation might have changed.

I have no idea of how accurate this is, but it is something I am going to investigate further.....I need the octane too!

I faced a similar problem a few years ago when I was no longer allowed to run Methol Benzene in my race engine. I rebuilt it to be 10.5:1 and it runs ok on 98 octane, just be careful about your advance curves because they can be critical.
But 10.5:1 should be what you have to aim for, any higher than that on 98 octane and you are going to have problems

According to Ricardo's testing, for 98 ron 10.6 : 1 would be top CR

This is figure is available from a table in the book "Design and Tuning of competition engines" by Phil Smith.

I would say 10:1 would be safe.
If you want to REALLY push it you can go to 11:1.
Most motorcycles run 11:1 comp ratio, BUT the newer motorcycles have knock sensors to prevent pinging.
Maybe, maybe even 10,5:1 would also work.

Hi.

Just interested in everybodies experience with maximum compression ratios.

I have bought a 1000 race car with a hot A15 which needs a rebuild. It is currently running a 55/85 cam with massive port job on head and twin 45's. The current c/r is very high and runs fine. Idles under 1000rpm when warm and goes like stink down the straights.

To make matters more interesting, the race fuel it has been running is being phased out, so I need to set it up for premium unleaded (98 octane). I need some definate figures for what works and what doesn't !! Can anyone help me ??

QIK1000