Big valve intake conversion

[chipis]

Hello,
would like to get any information regarding putting bigger valves into c1j turbo engine head. how much we can increase valve diameter and from which car to use it? what kinf of problems i can face during conversion or any tips and tricks are welcome regarding head performance upgrades.

[Alastair]

You can slightly increase the inlet valve using (i think) megane or similar valves, but there is not much point. Just up the boost to overcome any shortfalls in flow. It is also very tricky to get right. I have seen some appalling heads with bigger valves where the valves don't even seat properly, and the original valve seats were still used. It is easily doable if you have a big cheque book and a tame machinist though. If i were you i'd get a half decent ported and flowed head, with shorter guides and a decent squish area shape, then get it matched to your fanimolds and not worry about it.

[Tony Walker]

plus the head cracks between the valves all the time on std size.

[chipis]

what is the advantage of shortened guides?

[Tony Walker]

the bottom of the guide goes into the port quite alot disturbing the inlet flow.