Head skimming and porting?

[Mieran]

I've got a rough idea on what this means but its slightly confusing so thats why I'm asking.

From what I've read skimming the head means that the head is machined to make it flat before doing a head gasket change, but why is this needed? Wouldn't the head be flat anyway so why take metal off?

Porting - whats that all about?

Compression - how does that all work? Why do people use a thicker gasket?

Cheers :D

[ldavies]

by porting are you refering to the throttle body

[williamsvts]

if your car over heats, like when HG is fucked, it can get soo hot it warps the head. so if you fit a new HG with a warped head it wont seal, skimming it will make it flat again so it seals properly.

porting is when you remove metal and change the chape of the ports to improve the flow of the gasses/fuel mixture.

using a thicker head gasket lowers the compression. trying to think how to explain what compression is/does. for example, say when your piston goes down from TDC to BDC it draws in say 100cc of air/fuel, its a ration between this and the volume of the combustion chamber, say 10cc, that would give a compression ratio of 10:1 i think haha, may have worked it out wrong though :geek:

[ldavies]

very well described mate:y:

[Mieran]

Williams = legend! I wish I knew as much as you lol

The reason I asked in the first place is cos I've signed up on "mig web" recently and almost everyone doing a XE conversion is rebuilding the engine before it goes in and the skim they head etc.

The people over there are very knowledgeable and I didn't want to get laughed at so thats why I posted here :p

[williamsvts]

lol, no prob. vauxsport is a good site to go on mate. so really clever people on there. that chip seems to know his shit like.

[Mieran]

I'm on there as well. Both sites are interesting and I got quite a lot of info from them.

Its funny how I like Vauxhalls so much now, before I had my car I didn't even care about them lol and reading all these projects on mig web makes me want to start a project too, but not sure if I want to take the XE'd Corsa route or flat arched 16V Saxo route (probably niether cos I'm still a n00b and don't trust myself).

[williamsvts]

lol, i was into vauxs first, had 2 corsa's before the saxo (and 1.1 AX), then the nova and vectra tehn back to my old saxo.
few of my mates are into vauxhalls, 1 has a 2ltr corsa XE on TB's with hi comp pistons, cams, PP head etc + nos, nova XE on bike carbs and my old cams, another 2ltr corsa. all in 1 lockup, cant say where it is though haha

[Mieran]

Speaking of Vauxhalls and GM have you heard anyone ever do the 2.0 turbo Saab conversion?

I've seen a 2.3 turbo Saab engine in an Astra van but not sure about the 2.0 lump.

There is a Saab 900 with a rear end damage for £200 advertised locally and if it fits then I might buy it. Will work out much cheaper than a C20LET.

Vauxhalls ftw. You can get power for much cheaper. lol

[swampy]

Skimming the head will alter the size of the head pockets.
Makeing them smaller and in return increasing the compression ratio which in turn increases bhp.
A thicker headgasket can be used to restore standerd ratio

[williamsvts]

Speaking of Vauxhalls and GM have you heard anyone ever do the 2.0 turbo Saab conversion?

I've seen a 2.3 turbo Saab engine in an Astra van but not sure about the 2.0 lump.

There is a Saab 900 with a rear end damage for £200 advertised locally and if it fits then I might buy it. Will work out much cheaper than a C20LET.

Vauxhalls ftw. You can get power for much cheaper. lol

they use the same mounts and fit the vaux gearboxs iirc

[Toad]

Yeah, Williams has it correct, as usual... ;)

Skimming will only reduce the size of the combustion chamber, or head pockets as Swampy has stated. When ignition takes place, moments before TDC, the mixture will be at a greater pressure, so more force will generated from the explosion. There's only so far you can go however, mainly due to the increase on volatility of the mixture, causing detonation.