Anyone useing two head gaskets.

[4WheelDrifter]

Lou, oversize pistons will also raise the CR, but by a very small amount.

I decided to use one head gasket with my build, but skimmed my pistons to drop the CR to about 8.5:1 (I have the exact figure noted down somewhere). I figured if I wanted to drop the CR further I could always add another gasket. But I used to drive a car with 7.9:1 CR, which was great on boost, but not so good off boost. So I thought I'd go for a compromise this time.

You really need to get the machining done that needs to be done, then measure what you have to calculate your expected CR, then go from there. It may not be as bad as you think.

[s2martyn]

so by using this modded gasket then what kind of cr drop do we get?

[JP]

8.7-8.8:1

[A80Avant]

I have pistons that on OEM spec would be 8.9:1.

i wish to raise the compression ratio to OEM spec. How much skimming of the head would i have to do? Can that be calculated? Also, how much would it be allowed to avoid the valves going very close to the pistons when using OEM cams (ADU)?

[s2martyn]

i thought standard pistons were 9.1:1,are yours skimmed vas?

[A80Avant]

i dont have standard pistons. I use Mahle ones from PK motorsport which are 8.9:1. But the current cyl head was skimmed to increase the compression ratio.

The next cyl head will have the same done, and maybe increase the ratio back to stock to allow for better driveability off boost and less lag. I just wanted to know what measurements are made to know how much to skim off.

[Billman]

I doubt that anyone would feel the 0.1 difference on the CR.

[A80Avant]

i agree, but the from 8.9 to 9.3 there is a difference

[220bhp]

How can I find the orginal from the factory measurement through the bolt holes, or the reason why 118.1MM is stated by Audi as the minimum,what happens beyond this measurement.are the head bolts likely to bottom out,will the valves get to close to the pistons,if I could find the answer to this it would help to decide whether to lower the CR or not.
When I stripped the engine down I noticed the carbon deposits on the valves EM ports and the hot side of the turbo where white,I wondered if it could of been caused by to much compression raiseing the EGTs, are these white deposits a good or a bad thing or is it normal.
I would be very greatful if someone could aswer these questions, I have spent an awful lot of money getting the the new parts and the machiening done,I dont want to get it wrong now.

[A80Avant]

white usually means leaner mixture.

when you say 118.1mm height, do you mean the total cyl head height?


edit: just read the manual, it says measure through a cyl head bolt hole. And yes, it doesnt say why not any minimum than that

[JP]

The I5 head is an interference head which basically means the valves can hit if the clearances are wrong.

As for the white that is usually burned oil whilst running lean. So you are suffering 2 issues, lean running and oil burning. So you need to confirm whether the deposits where on the face of the valve (rings) or back of the valves (guides).

[Billman]

i agree, but the from 8.9 to 9.3 there is a difference

stock RS2 CR is 9:1

[220bhp]

Thanks for the quick replys, much appreciated.RWT19T the white deposits where on the face of the valves.

[A80Avant]

i dont think you are right.

ADU/ABY is 9.3:1 stock compression if i remember correctly. Not 9:1

[A80Avant]

the valves hitting the pistons also have to do with the cams as well, isnt that right? as so many cams with higher lift are used without problems.

is that 118.1mm height for the purpose of the valves hitting the pistons or something else?