Skimmed pistons

[S2central.net]

Funny you should ask that Paul, as I have noticed that part of the pistons is higher than the rest and slightly protrudes above the block deck, well it does on mine (I had the barest minimum taken of off the deck). I've used Marcos pic to show where. Presumably this is for squish and forces the charge into the bowl and chamber areas of the pistons and head?
I've checked the standard pistons and these too are higher at this point.

Cheers,

Mark.

Oh thats a bit surprising - is gonna make some harder guesstimations then - fancy buretting one of them - at say exactly 2cm from TDC - with compression rings fitted and a wadge of grease in bore to seal it ?

Hows the build coming along anyway - despite all this last minute flap on Comp Ratios ?

[auditude]

Sal - thats a damn good question. I always understood that all the 20V I5 turbo engines had a CR of 9.3 but somebody (Mihnea or Marco) told us that when the heavier duty metal gasket was introduced it effectively reduced CR to 9.0 by nature of being much harder to compress than the no longer available paper gasket. The theory seems to hang well in practise as its known that the metal gasketted cars can handle more ignition advance and boost than paper gasketted ones. This is a positive side effect of having lower CR. Only way to prove this is to measure the dimensions of used gaskets (assuming they don't relax much after being removed to skew the results).

Here's at least one of the posts. I couldn't find one that elaborated, altho' I thought I remembered reading more discussion about it.

head gasket thread

[Mark Halligan]

Just remembered Mark - you buretted your chambers didn't you - what were the results before and after head work ?


Paul

Paul,

I burretted the chambers and they luckily all turned out to be within 0.2ml/cc of 45ml/cc's close enough for me.
I borrowed the Burrete from someone and I doubt I'll be able to get it in time to do another measurement on the pistons before it all goes together .
I will try to measure how far above the deck that part of the piston protudes though, give me a day or two as I'm busy all day/night tomorrow, I'll see if I can take a close up pic too.

Cheers,

Mark.

[sal_s2]

Has anyone ever replaced a head gasket on an rs2 and if so was it a metal one from factory?
I've always heard the metal one is an rs2 item which would mean no change in cr...

My 96 aby had a paper one.

[Pisobiker]

As Mark say the part of piston crown among intake valves and valve pockets is protruding out about 0.8mm compared the rest of the crown, corresponding to the flat part of the combustion chamber. And as a matter of fact, as I sayed before , this is probably the only part of the piston really involved in squish effect, which might be wise leaving as it is.

Mihnea pointed out the difference in CR provided by the metal headgasket compared to the paper one, and as Paul says, there are practical results that confirm this.

@ Tom:

Unless you really want to know exactly how your CR is, for which you will need to precisely measure combustion chambers, piston dome volumes and thickness of compressed headgasket, the compression ratio estimation isn't that hard to calculate.

In my case I actually didn't care too much the real CR value. All I know is right now I'm running 1.7 bar boost with slight signs of knock sometimes, so I belive I'll be happy enaugh with a 0.3 reduction in CR being able to run some 2.0-2.2 bar boost peak.
No big deal if starting from 9.3:1 or 9.0:1 or whatever it might be.

In your case, assuming you want 0.3 reduction in CR you will need 2.1 cc taken off the piston crown somewhere + 0.515 cc for each 0.1mm skimmed off the head and block deck (on the std 81mm bore).


Marco

[diesel des]

Marco, what are your thoughts on shimming the head and leaving the stock pistons alone? Using the layers from a second steel gaket would allow this to be done. I have done this on diesel engines before with no gasket blowing problems. Obviously this will affect squish too, but its reversable. And as the steel gasket mod to paper gasket cars seems to allow more advance them maybe shimming the head doesent hurt squish that much? Thanks,des.

[Pisobiker]

It might be a solution, of course.
The 0.3 points CR on the 81mm bore are only about 2.1 cc which mean only slightly more than 0.04mm, which probably won't hurt significantly the squish.
So if for example metal headgasket (compressed) is 1.0mm and it has 5 metal layers (just giving numbers, whitout having actually checked), adding 2 layers coming from a second gasket will do the job.....
Might be worth trying.

Of course in my case, where I'll need 5.5 cc more then what I got, I would need two full metal gaskets.....


Doesn't sound that good to me......


Marco

Marco, what are your thoughts on shimming the head and leaving the stock pistons alone? Using the layers from a second steel gaket would allow this to be done. I have done this on diesel engines before with no gasket blowing problems. Obviously this will affect squish too, but its reversable. And as the steel gasket mod to paper gasket cars seems to allow more advance them maybe shimming the head doesent hurt squish that much? Thanks,des.

[Pisobiker]

I meant that 2.1 cc on the 81mm bore are 0.4mm, not 0.04.

How come I cannot see the "edit" option on my posts?

[Mark Halligan]

part of piston crown among intake valves and valve pockets is protruding out about 0.8mm compared the rest of the crown, corresponding to the flat part of the combustion chamber.Marco

Here's a picture, a bit out of focus, but it shows how that part of the piston protrudes about 0.8mm, as Marco says, into the space between the head gasket and cylinder head. Again as Marco says this corresponds with the flat part of the combustion chamber, presumably so that the charge is pushed/squished into the piston bowl and combustion chamber.
Skimming the whole piston crown would definately change this characteristic, although I dont know what disadvantage there would be if any?

Cheers,

Mark.

[Pisobiker]

Well, from what I can remember, the "squish" effect was originally introduced in two stroke engines, where time for combustion is very short. On those engines the squish area is on the combustion chamber (pistons crowns are only slightly rounded) all around the outer part (spark plug is located in the middle).
This way charge is strongly pushed towards the center, so that at TDC actual combustion chamber is very compact and flame doesn't need to travel all the way to the cylinder wall to fully burn the charge, therefore increasing efficiency and power output at higher rpm.

In our case the eventual squish effect, which part of the piston and head seem to use, should do something similar while, among other advantages, also reducing the risk of knock. As a matter of fact knock is a pretty slow phenomenon which needs time to take place (this is one of the reasons why while increasing rpm knock effects are reduced). The faster the combustion the less probability knock will take place. Therefore the more compact the combustion chamber, the faster the combustion.


Marco

Here's a picture, a bit out of focus, but it shows how that part of the piston protrudes about 0.8mm, as Marco says, into the space between the head gasket and cylinder head. Again as Marco says this corresponds with the flat part of the combustion chamber, presumably so that the charge is pushed/squished into the piston bowl and combustion chamber.
Skimming the whole piston crown would definately change this characteristic, although I dont know what disadvantage there would be if any?

Cheers,

Mark.

[Mark Halligan]

Top answer Marco

Mark.

[audiman]

I spoke to a top audi tuner this morning with reference to this. Basically a car that they built 4-5 years ago is running with std pistons skimmed level with the dish. This outputs over 600 bhp with 2.5 bar boost. Cant see there being much problem with this then on my car. He said that its best to have either std squish or no squish, not an inbetween.

We will have to wait and see what its like wont we. Im going for it.

PS Mark, dont dispose of your old pistons yet

[S2central.net]

Tom - you are the guinea pig on this. I for one don't think the reduced 'squish' will be a noticeable issue - you are increasing knock resilience anyway by reducing CR by this tactic.

It would be superb if you could burette the pistons before and after (in the bores at a set depth) having them machined. Then we will actually be able to quantify the CR differences accurately. I've got a good book on this stuff - I can show you next weekend if you like...


Paul

[audiman]

Yeak Ok Paul. sounds good. Little bro just finished secondary school, so he cant nick a burette for me now

Does someone have a small one for testing this out? I could use a seringe(nice spelling!)

[Billman]

So, how does the Skimmed pistons work on heavy tuned engines? Do you have it with the Valve pockets remaining?

What instructions should someone give the Machining workshop to do the pistons? Sorry but i cant follow the conversations above!