Comression ratio advice, 6 cylinder motor build Options

[team914]

Hi,
I am planning to build a 6 cylinder street motor for my 914/6 factory replica. My original 84mm barrels were worn, so I elected to machine the barrels to 84.5mm and have bought a set of 84.5mm Wossner pistons. With a 70.4mm crank, the compression ratio is stated at 10.11:1 with the pistons out of the box. I intend on using Weber carbs, E cams and an E distributor/123 Distributor. My machine shop engineer has recommended reducing the compression ratio from the 10.11:1 to a more suitable figure. The petrol quality in Ireland is poor, typically 95 octane, and we now have a compulsory addition of 10% Ethanol added (E10)!
Any advice greatly appreciated

[ogdougy]

I would also add that some piston manufactures will provide on their website or, if you ask, what values they use to calculate their advertised compression ratio.

It helps give a rough idea with minimal work. For example, I ordered a set of pistons from Arias for my 4age. Advertised with 12:1 comp ratio. But their calculations used a combustion chamber size larger than any 4age head had. So i knew i was getting more compression than advertised which is what i wanted.

[Krieger]

I would also add that some piston manufactures will provide on their website or, if you ask, what values they use to calculate their advertised compression ratio.

It helps give a rough idea with minimal work. For example, I ordered a set of pistons from Arias for my 4age. Advertised with 12:1 comp ratio. But their calculations used a combustion chamber size larger than any 4age head had. So i knew i was getting more compression than advertised which is what i wanted.

It's called a spec sheet. I would expect Wossner to supply one with your pistons. Dome volume is one of the specs you need. This is a complex engine to build correctly.