Compresion - pistons!?

Hi,
how many mm should be the head gasket if you put the 7.8.1 compression pistons.? ...Galant VR 4 comes whit 7.8 factory compression...right?...
My GTI comes whit factory 9.8 ....and i'm scare to drive whit big boost.....

 

[galant344]

a mitsubishi 4g63 gasket is .051. I don't know about your other vehicle.

 

...051...??? /ubbthreads/images/graemlins/dunno.gif ..what is that..?..
sorry ..for that question..

 

[johnnyRacer]

That's the thickness of the head gasket in mm (millimeters).

 

Wrong. .051 is in inches. Which is 1.29 mm

 

ok...so Galant vr4 comes whit 1.29 mm stock head gasket and 7.8.1 compression pistons...is thet right?
If i use 1.4 mm head gasket and 7.8.1 compression pistons ..is that a problem??

 

[GSX_TC]

No it's not a problem. Your compression will just be slightly lower that's all.

 

i will put 7.8.1 pistons .... for low compression...
the stock VR4 comes whit the same compression - 7.8.1 ..is that correct?

 

[GSX_TC]

Yes that is correct

 

Do I have to change the head gasket with MLS head gasket in situations that i put pistons 7.8.1 .??

 

[Jesh]

Yes I would strongly recommend a mls headgasket, especially since you are wanting to run alot of boost. I'm pretty sure a regular old head gasket wouldn't last very long.

 

Ok....so i can put metal gasket 1.4mm and 7.8.1 pustons...and i am ready to boost /ubbthreads/images/graemlins/devil.gif

 

Has anyone used such pistons??
click

 

[fuel]

Quoting Stefan:
Ok....so i can put metal gasket 1.4mm and 7.8.1 pustons...and i am ready to boost /ubbthreads/images/graemlins/devil.gif



Why do you want such low compression? To be honest you're more than ready to boost at 7.8:1 pistons and the factory 1.2mm head gasket. Many people increase their compression ratio up to about 8.5-9:1 rather than decrease it even further.

What are your power expectations here exactly?

 

[GSX_TC]

Quoting Jesh:
Yes I would strongly recommend a mls headgasket, especially since you are wanting to run alot of boost. I'm pretty sure a regular old head gasket wouldn't last very long.



normal composite headgasket can and will last along time man. i ran one with 20 pounds of boost for like 4 years without any problems.

 

I know that factory VR4 have a 7.8.1 compression,that's why I want to put these pistons whit that low compression .... but still i cant understand that compression 7.8 is god or bad for my car.Now the compression is 9.8 ... and I am afraid to drive a big boost....now is 0.5 bar pressure. ...
I want at least 1 bar ....

 

[fuel]

1 bar is not big boost and 7.8:1 CR will be absolutely fine, even perhaps slightly on the low side still. I would not want to go any less than 8.0:1 CR for just 1 bar of boost.. 2-3 bar of boost maybe but certainly not just 1 bar.

Is the quality of fuel over there really that bad? What's the highest octane pump gas you can purchase?

 

So I picked up more than 1 bar boost if I put 7.8.1 pistons ... it sounds good .. /ubbthreads/images/graemlins/applause.gif
Is it dangerous if i put so low compression and I have 1 bar of boost???....buy the way this is the pistons which i wanna take it
click

Gasoline here in Bulgaria is very bad .... high octane at 100...

 

[fuel]

Those pistons are standard as per fitted to the E39A VR-4 - these pistons with a standard 1.29mm thick head gasket will result in a 7.8:1 compression ratio which would be fine for 1bar of boost. It's not dangerous to have lower compression, in fact it's quite the opposite - but you will basically create an engine which will have poor performance when off-boost. Anything in the 7.8-8.5:1 region is completely safe for 1-2bar, especially at 100 octane providing your tune is good. You can get detonation and catastrophic failure on 7.0:1 compression if the engine is running too lean or has too advanced ignition, and yet 9.0:1 could be perfectly safe with enough fuel and spot on ignition timing.

Heck I would be inclined to try run 1bar of boost on your 9.8:1 motor with a good tune and see how long it takes to pop.

 

/FUEL/-Thank you for the info bro.So i can use 7.8 compression but whit 1.29mm head gasket...that combination will be fine for 1 till 2 bar boost..
Is it necessarily to use a MLS head gasket or standart graphite head gasket....?
WHat pistons and head gasket you use?....