Skimmed head - what increase do you get

[james_mk]

As im having the head gasket , cambelt etc done...im also having the head skimmed...the garage that are doing said that they could skim it by 4mm as it will increase the combusion ratio or something like that.... any ideas what gains id get
mines a 2L8v

 

[m0rk]

why are you getting it skimmed?

but don't get much taken off of you may have issues

 

[Cupra_power]

Improved volumetric efficency, improved mid range torque depending on the increase in compression ratio you might have to start using high octane fuel.. 98 RON or 99 RON

 

[cordobabrendy]

4 mm you mean 4 thou? dont bother its no better than std and if you ever have the ehad gasket go youll have less room for repairs.

 

[james_mk]

yeah i know there wil be less room if the head goes again..but its not likely.

I run my car on BP's super unleaded anyway..whichever RON it is...

 

[cordobabrendy]

yeah i know there wil be less room if the head goes again..but its not likely.

I run my car on BP's super unleaded anyway..whichever RON it is...

mk2 heads can go at any time, mine went twice in 8k. and octane rating means diddley.

 

[hopkinsgm]

...octane rating means diddley...

Pardon? If compression ration is increased significantly, then you'd need to run higher octane fuel to reduce the tendency towards pre-ignition (which seems to be fashionably referred to as "detonation"). A skim on 4mm would increase compression ration significantly. A skim of 4 thou wouldn't.

If the head gasket has gone, then the head may require a skim if it's warped a bit, but it's not always necessary. If all the valve gear is getting stripped out for the head to be skimmed, it's probably worth having it crack tested too... (it's also a convenient excuse to have the head ported while you're at it...)

 

[ibizacupra]

4mm!

surely not

 

[Cupra_power]

4mm is perfectly fine, you just have to start using diesel instead .

 

[james_mk]

what are you on about diesel dont confuse me!! so your saying im going to need to use higher octane fuel , right? What sort of work is involved to have the head ported? a lot more?

 

[hopkinsgm]

I think Cupra_power may have been kidding...

If you know the swept volume (easy enough - divide engine capacity by no. of cylinders, so for a 4 cyl VAG 2.0 it's 1984cc/4 = 496cc) and the original compression ratio, you can work out the size of the combustion chamber before the skim. Compression ratio is (swept volume + combustion volume)/combustion volume. If you then measure the size of the combustion chamber after the head has been skimmed and rebuilt, then you can work out the new compression ratio as swept volume won't change.

(edit) to answer the question about octane, no, you probably won't need to change from whatever fuel you're using at the moment so long as the compression ratio doesn't change from say 10:1 to 12:1. Check with your engine man about how much is going to be skimmed off. 4mm sounds ridiculous (was half asleep when I posted earlier...)

 

[Cupra_power]

what are you on about diesel dont confuse me!! so your saying im going to need to use higher octane fuel , right? What sort of work is involved to have the head ported? a lot more?

Pulling your leg , if you skim enough off the head, the compression ratio will be high enough to ignite the fuel without the spark plug and Viola you have a TDI

 

[ibiza_95]

it does do a bit ive heard, but i had mine done when my head gaset went and i cant honestly say its noticeable, its only 20 pounds to have it skimmed anyway so if your gaskets gone just get it done anyway for the sake of it.
if its not gone... 20 skim plus 20 gasket and bolts, still only 40 quid, you never know

 

[ibizacupra]

Consider also the implecations of cam timing and relative PDC position when its been skimmed. 4mm sounds waaayyy tooooo much, and I would also check for valve to piston clearance to see what was actually available.

Was it 4mm? or 0.4mm?

 

[cordobabrendy]

Pardon? If compression ration is increased significantly, then you'd need to run higher octane fuel to reduce the tendency towards pre-ignition (which seems to be fashionably referred to as "detonation"). A skim on 4mm would increase compression ration significantly. A skim of 4 thou wouldn't.

slight misunderstanding here G, I meant that on an 8v otherwise std, changing ron wont make the car noticeably faster, i had my head skimmed the first time it went and it was exactly the same as before hand, ie same speed at road markers etc.

 

[james_mk]

Im gona have to check avec mon mechanic, im sure he said its 2mm ml for a normal skim on a h/g but he could do it 4mm.. ibiza95..dunno where you got this 40quid from?? the whole jobs costing me £375....thats for a new head gasket,skim,and cambelt.

 

[hopkinsgm]

A skim on its' own will probably cost about £20. Cam belt isn't expensive, and a set of gaskets won't cost the earth either. Most of the £375 you've been quoted (or estimated? That's always worth clarifying...) will be labour charges. Labour ain't cheap these days. And by time you've removed the head, stripped the valve gear, re-assembled the whole lot, that's quite a few hours work.

 

[ibiza_95]

i did mine myself and it cost 40 pounds without cambelt change. mybe your having significantly more work done than i did

 

[H8SV8S]

I'd only skim a head enough to flatten it off for sealing purposes... if you want to up the CR noticeably, get some new pistons