Metal Head Gaskets

[Sneezer]

A good number of you have fitted metal head gaskets in varying thickness to your RB25DET engines, 1.2mm seems quite poular. I know this allows you to run higher boost pressure from the turbo.

What does the head gasket take the compression ratio down to?

Why not just fit lower compression pistons when rebuilding the engine?

Or is this done as metal gasket give slightly better sealing characteristics?

Cheers

[Steve]

I am using a 1.6mm HKS stopper type head gasket. It takes the CR down to 8.6:1

Why didnt I use pistons? I wasnt doing a full rebuild, just wanted to lower the CR.

AFAIK the HKS gaskets do seal alot better than just a stock gasket.

[GTRman1992]

I had the head ported and decompressed the cylinders slightly while having them cleaned up. now run 20psi. Still using standard head gasket. Been like this for 2 years no problems.

[benm]

Slightly off topic but someone might like to help out a n00b.

Can someone explain what it does (head gasket) and what lowering the compression ratio is and does? Also where in the list of mods would 1 replace the standard gasket with a HKS Metal gasket and why ?

[B-Man]

When you joint two surfaces together you use a gasket as a sealer - - you have gaskets all over your engine - where ever you want a tight seal - eg : Exhaust Gasket, Water pump Gasket, and Head Gasket, etc, etc, etc.

The head gasket seals the cylinder head to the engine block.

Using a thicker head gasket increases the gap between the cylinder head and the block. This reduces the compression ratio - lower compression ratio means it is easer / less destructive to run more boost. Ie: On the compression stroke the boosted air does not compress as much , so in turn does not get as hot as if the compression ratio was higher. Very hot combustion can cause knock or pinging. - We all know the advantages & disadvamtages of running more boost - ie more air flow through the head and into the cylinder - combined with the right amount of fuel , a bit of spark and viola - you have a stonger more powerful combustion stroke.

Hope that helps - it's B-Man's laymen explanation - anyone more techncial feel free to contribute.

BTW - I have fitted an HKS Composite hed gasket - 1.6mm - same as Steve - to reduce my compression ration to 8.5-8.6:1 - Hopefully this will allow me to run 4.0 bar bro - fulsik !

[T0nyGTSt]

Yeah see I went the other way... stock N1 gasket but N1 pistons which runs a similar CR.

Many ways to skin a cat.

T.

[N I B]

The best head gasket you can buy is still the factory Nissan one. I've seen 700hp race motors blowen to bits with the factory gasket still intact but have changed many leaking metal gaskets.

[rb25]

factory gasket is all good.....

[GTRman1992]

I agree, I've run 20psi for 2 years or so now with the standard gasket and faultless. I think a decompress with head porting is the way to go.

[Sydneykid]

I'm with Steve-SST, I don't like thicker head gaskets, standard factory gasket is good for me. On the larger output engines we o'ring the block, never blown a head gasket then. Thicker head gaskets are a band aid, I simply believe there are better ways to correct the compression ratio.

Plus with the current state of turbo technology there is simply no need to run the massive boost that used to be the case 5 to 10 years ago. We can get 70 lbs per minute of air flow (enough for 700 bhp) without going over 1 bar if we set the engine and support systems up that way.

Hope that adds to this interesting thread

Merry Xmas to all