10 replies to this topic

[jn1702]

Can anyone tell me the standard height or thickness of a 998 A+ head? I've just picked one up that measures 2,700 thou it's been skimmed but by how much? Thanks

[tommy13]

I believe the original thickness to be 2.750" so you have had .050" skimmed off (about 1.25mm) As you wouldn't skim this much off normally just to bring the head flat it has probably been done to raise the compression ratio, or the result of 3 or 4 previous skims.

Edited by tommy13, 21 April 2010 - 07:17 PM.

[bmcecosse]

That's right -ALL heads start at 2.750". If you use it with flat top pistons - the CR will be too high, unless of course you grind metal out of the chambers to unmask the valves.

[mini-geek]

don't forget it's common practice for the top of the head to be skimmed a bit too, just to give it the 'new' look.

[bmcecosse]

Aye - but only a touch usually.

[samsfern]

50 thou shouldnt raise the comp too much, on an exrace head me and my mate decided to skim for his domino thing, we only ouched it with the mill and started to just cut through oilways! he runs a 230-240psi per cylinder so you can probably guess how much has been shaved, interesting to know the standard height, is that for the early and late type 12g940? ill measure with a vernier when we next have it off...

Edited by samsernie, 21 April 2010 - 07:32 PM.

[bmcecosse]

It's the same for ALL A series and A+ heads. If that 998 engine has flat pistons - it would be 10:1 to start with - that 50 thou will make it ~ 11.5 : 1. That's a LOT with todays crappy petrol.

Edited by bmcecosse, 21 April 2010 - 07:34 PM.

[samsfern]

true, unless you used a real good octane booster, to get like a 102 octane petrol! probably make the starter struggle a bit too, ive got a 11.5;1 comp ratio on my 1293, and the starter struggles a bit in the morning....

[jn1702]

Thanks for that, thinking about compression ratio is 11.5 to 1 about the highest you can go or is anyone using any higher?

[bmcecosse]

It's really TOO high for a road car.

[samsfern]

my mate runs 12.5:1, but thats a full on race engine, i run about 11.5:1, but would say an 11:1 is the maximum you want if you use the car everyday...

Edited by samsernie, 21 April 2010 - 08:54 PM.