machining pistons to lower comrpession ratio

[dub-disaster]

  whilst im having this work done on my 1341 engine with accralites im considering getting the pistons machined down slightly as i think its been said on here before that witch accralites you end up wih more like 9.4 or 9.5:1 and im after between 8.8 to 9:1 exactly. i understand that to work this all out accuratly you have to measure the volumes of everything and do the math and work out the exact compression ratio and exactly how much you have to remove from the pistons.
   my question is though is skimming the pisonts less than ideal ( does it affect the quench/squish area )? does it have any draw backs or cause any problems  ? or is it completely safe and fine ? if it is ill get on and do the maths and work out how much needs skimming of all the pistons to give me between 8.5-9:1

[Robin]

this is proper lol!

why are you after a certain CP???

[hayesey]

the accralites give 9:1 dont they?

[dub-disaster]

Robin- I was just seeing if I could keep it a bit lower to see if it would mean my engine wouldn't be so det prone on boost, then I mite not have to run water/meth injection. Why's it so funny ?

Haysey- according to one of jez's posts when actualy measured the cr ends up being a fair bit higher more like 9.5:1, in the same post yoof said he worked his out to be 9.3:1 using and nick s was around 9:5 aswell all using accralites  

Been Reading up bout compression ratios and it seems that 9.5:1 is pretty high for an 8v engine with 20psi of boost I know robin runs it ok but I was just trying to future proof my eninge a little more incase I want to change the turbo in the future and want a slightly lower cr. Aswell as trying to get awY without using water/meth injection

[GR40]

If you want to lower the compression why not trying a thicker flange? Machining the pistons will weaken them as they have a lair of hardened material on the top! If you machine them you will need to harden them and that is expensive! They will also have to be weighted to achieve similar weights and that is up to the machine shop skills!

[dub-disaster]

Asked accralite about this when I ordered new rings from them the crowns are hardened at all or coated in anyway.

[Nick_S]

You get the high CR with Accralites and the mk4 polo metal head gasket. Get the vernier caliper out and measure a new OEM g40 gasket cylinder bore diameter. The mk4 gasket definatley accommadates up to 77mm, but i think the OEM G40 gasket (which is thicker) only allows up to 76mm IIRC.

[dub-disaster]

I'll have a measure up tonight when i get home thanks for the advise nick do you run a metal head gasket then ? Think I will get my engine shop to measure my cr as accuralty as possible then if it is to high then I'll either get some je pistons or skim kine depends whlicj works out cheaper skimming mine probably.
   Victor reinz standard head gasket is 76.5 mm measured with a caliper

[Andy]

Both Robin and I have run genuine VAG stock headgaskets with 77mm 1341cc Accralite pistons without any issues. With that it's circa 9:1 CR, though I'm currently running a 6N gasket taking it to around 9.3:1 CR.

Have never been arsed to measure it all up accurately though with crushed gaskets etc. and seen what it really is.

[dub-disaster]

How come you swapped to 6n gasket ?? Wich engine code is the gasket for isit aex?

[Andy]

Only swapped the gasket because I swapped to a BV head (something like 5 years ago), and the 6N gasket was easily available from GSF. Had the arseache with my local VW dealer at the time, so was boycotting them as much as possible!

Since then I've swapped heads a few times (long story that involves PSD chocolate valve guides and shite machine shops) and always used the 6N gasket because it was cheaper and has worked okay so far.

If you're building a K03 turbo'd motor and already have the 1341cc bottom end with Accralites I'd leave it all alone - unless you're dead set on going for well over 200bhp with a big turbo.

[dub-disaster]

so baisicaly it doesn't realy matter wich one of the gaskets you use then, does the metal one have any advantages like being stronger over the standard one ? Doesn't it leak oil slightly think I remember haysey saying that. Does robin run metal 6n jobbie or standard?

    I have got a 1341 bottom end with accralites but it has been stripped and is being rebuilt as it was a wank engine full of swarf and the bores where too big and one was off centre !!so I'm only saying this now as if I replace the pistons with say wossners now I can get a set for £450 and the cost of aquamist is £300ish so if running 8:1 is aqaumist still neccesary, or is it just benificail to running higher boost levels. In previous posts I've seen a few propel including yoof say that of he was building his engine all again he would of used wossners think you said it some time back aswell didn't you?

    I don't intend on going massive turbo or anything I do intend in after getting it running to swap to a gt17 and running around 20 psi from that. I will be coming to you for the gt17 maps so wich ever compression ratio you suggest is what I'll run !!   

[Yoof]

I'd use the Wossners again as mine is track focused, so I could compensate the lower VE with a little more boost.

If your eventual plan is a GT17 and circa 20psi then the accralites are fine- 9.3:1 requires careful mapping, but you can get the power out of them without suffering from det problems providing your intercooling and ignition system is suitable. You'll get better off boost drivability, better bsfc and better spool up with a higher CR.

If you're measuring components up, remember to use the height of a squished haed gasket- not a new one.

Nick- stock G40 gasket have enough clearance to allow 1341cc without problems, I know of various conversions running this set-up (Robin's and Andy's included)

[dub-disaster]

Thanks for the input, I'll stick with them as they are. although I shall measure it all up as I'd like to know what the cr actualy is.
    Oh and what is bsfc ?
 

[hayesey]

Brake specific fuel consumption I assume