Britbike forum

Classic British SparesKlempf British PartsBaxter CycleThe Bonneville ShopLowbrow CustomsGirling Classic MotorcycleLucas Classic MotorcycleHepolite PistonsIndustrial tec supplyJob Cycle

Upgrade your membership to Premium Membership or Gold Membership or even Benefactor


New Sponsor post
10% Off Girling at The Bonneville Shop 2/3-2/9/23
by The Bonneville Shop - 02/03/23 6:14 pm
New FAQ post
News & Announcements
Premium members! 🌟
by Morgan aka admin - 02/08/23 12:00 pm
Buy BritBike staff a coffee ☕️ or pint 🍺
by Morgan aka admin - 01/15/23 9:29 am
How to guides - Technical articles
ShoutChat
Comment Guidelines: Do post respectful and insightful comments. Don't flame, hate, spam.
Member Spotlight
Gordon Gray
Gordon Gray
North Carolina
Posts: 6,345
Joined: August 2001
Top Posters(30 Days)
Lannis 90
DavidP 87
kevin 56
Top Likes Received (30 Days)
Kev. 32
Lannis 26
DavidP 26
Newest Members
Igloo, patrickblue, j-bud, Uneasy Rider, Mally
12,395 Registered Users
Forum Statistics
Forums35
Topics76,566
Posts784,685
Members12,395
Most Online204
Jul 10th, 2022
Random Gallery photo
Photo posting tutorial

Previous Thread
Next Thread
Print Thread
Page 2 of 2 1 2
Joined: Jan 2014
Posts: 2,410
Likes: 451
Britbike forum member
Offline
Britbike forum member
Joined: Jan 2014
Posts: 2,410
Likes: 451
Originally Posted by kevin
i run 9.5 to 1 in a 72 T120, which has a lousy head compared to the A65, i think.

its been twin plugged and i pour in 87 r+m/2 gasoline with no difficulty.

twin plugging is harder to do on an A65 but there are people on this forum who have done it.


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

British motorcycles on eBay
Joined: Jan 2014
Posts: 2,410
Likes: 451
Britbike forum member
Offline
Britbike forum member
Joined: Jan 2014
Posts: 2,410
Likes: 451
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.

Joined: Aug 2001
Posts: 9,283
Likes: 226
Britbike forum member
Offline
Britbike forum member
Joined: Aug 2001
Posts: 9,283
Likes: 226
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?


Amateur Loctite enthusiast.
Joined: Jan 2014
Posts: 2,410
Likes: 451
Britbike forum member
Offline
Britbike forum member
Joined: Jan 2014
Posts: 2,410
Likes: 451
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.

Last edited by NickL; 01/10/23 12:37 am.
Joined: Aug 2001
Posts: 6,112
Likes: 150
Britbike forum member
Offline
Britbike forum member
Joined: Aug 2001
Posts: 6,112
Likes: 150
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.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
2007 Triumph Bonneville Black
Joined: Oct 2007
Posts: 2,021
Likes: 75
Britbike forum member
Offline
Britbike forum member
Joined: Oct 2007
Posts: 2,021
Likes: 75
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


"There's the way it ought to be and there's the way it is" (Sgt Barnes)
Page 2 of 2 1 2

Moderated by  Allan G, Jon W. Whitley 

Link Copied to Clipboard
British Cycle SupplyMorries PlaceKlempf British PartsBSA Unit SinglesPodtronicVintage MagazineBritBike SponsorBritish Tools & FastenersBritBike Sponsor






© 1996-2023 britbike.com
Powered by UBB.threads™ PHP Forum Software 7.7.5