22 replies to this topic

[blacktulip]

I found that with a +60 overbore and flush decked flat top pistons I needed 25cc chambers on my old 12g295 head to make 9.5cr

[Echan42]

Found a fresh thread thanking about 295's!

So I just picked up this 12G295 for super cheap from my local craigslist alternative

Incredibly it's remained untouched for 40 years.

I'm planning of skimming 80thou (2mm?) from it since that's what other threads here mentioned, my goal is to fit it to my 998 engine, my pistons are most probably dished since the engine has never been serviced.

 

Can anyone confirm skimming 2mm is what I need for 10:1 compression?

What book/site covers this stuff? I want to fit it with correct valves and springs to get this head the love and care it deserves.

[Cooperman]

It's not as simple as saying 'skim xxx to achieve xx comp ratio.

 

You must first measure the relative positions of the piston crowns to the block deck level at TDC and confirm what the piston dish volume is or whether the pistons are flat-top type.

 

From there you can calculate the volume in the bores at TDC. Decide on the comp ratio you desire, then do the CR calculation. How to do this is already on here.

 

The result will be a figure for the required combustion chamber volume and the head must be accurately skimmed to give this volume in each chamber.

 

For a 998 you should allow 2.7 cc for gasket volume.

[DeadSquare]

Found a fresh thread thanking about 295's!

So I just picked up this 12G295 for super cheap from my local craigslist alternative

Incredibly it's remained untouched for 40 years.

I'm planning of skimming 80thou (2mm?) from it since that's what other threads here mentioned, my goal is to fit it to my 998 engine, my pistons are most probably dished since the engine has never been serviced.

 

Can anyone confirm skimming 2mm is what I need for 10:1 compression?

What book/site covers this stuff? I want to fit it with correct valves and springs to get this head the love and care it deserves.

 

 

What Octane fuel do you have in Spain that will benefit from a 10:1 compression ?

 

I'd take advice from which ever business you get to do the head modification, because the quality of fuel is sadly not as good as it was when David Vizard wrote his books.

[Echan42]

I'd take advice from which ever business you get to do the head modification, because the quality of fuel is sadly not as good as it was when David Vizard wrote his books.

I'm not sure the machine shops in my area have any knowledge outside of operating machinery and working metal, Spain only has 95 and 98 octane on pump, I haden't thought of how gas quality has changed over the years, but you're definitely right.

[Echan42]

While we're on topic I guess I'd have to look into a new carb setup for my new 12G295. 

Would this twin HS2 setup I found local get the job done? 

im.currently running a stock singe HS4 setup

[Echan42]

Sorry my phone double posted

Edited by Echan42, 09 August 2019 - 06:25 PM.

[Dusky]

Just stick with Shell Vpower 98. Atleast in belgium Thats ethanol free fuel.