Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board A65 - Piston Replace or Not Replace

It does little for them, as you say the head is good for a hemi anyway, high plug, shallow dome etc.

I suppose talking 'Pool' petrol would make the use of 9-1 'high compression'.
I wasn't aware we were still at war.................................. If thats's the case i'll
just nip out and buy a gallon of TEL eh?

with fuel since the late 60's people have run 12-1 and higher on race tracks and
some cars including the old 60's v8 rovers were 10-1+.

As for squishing the heads and getting massive turbulence in the combustion chamber,
you can look at the 'Mays' head that Jag did for the V12 as a strange example.

In the context of British 650s, 9:1 is high compression.

Look at the range of compression ratios used as standard on the bikes they sold for road use. Where is 9:1 placed in that range?

Need to look for similar cylinder head layouts really.
SS norton, Norton Commando, triumphs were 8.5-1 but head was very domed.
Matchless 650gsr around 8.9-1. Enfield (late intecepter)
So i stand by my comment 9-1 wasn't exceptional given that the head was better than most at the time.

First step on any tuners ladder was to use higher compression normally.
You could buy 11-1's for triumphs, beezers, nortons etc.

Back in the seventies, the people I knew and rode with, with 650 and 750 BSA and Triumph twins, went around in a questionable state of tune half the time, but no one suffered from pinking as I recall. But that was when you could get 95 octane leaded ethanol-free fuel at the pump. So at that time, 9:1 pistons was considered "normal", or not problematic. I do recall though, a friend with a Camaro Z28 with 11:1 or 12:1 pistons talking about having to seek exotic fuel.

Consumer Magazine did an article here about the weed killer we are being sold ... They had it tested and reckoned that the so called 91 was actually more like 85