Jump to content
SAU Community

Recommended Posts

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

Link to comment
https://www.sau.com.au/forums/topic/30289-metal-head-gaskets/
Share on other sites

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 ?

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 ! :P

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

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • By popular demand.. it was a coil. Got my hands on 1 new OEM coil, replaced with the one that made the less noise difference when I unplugged it while the car was running and started the car up. No stutter and the engine light was gone. I guess I’ll buy the other 5 they have lol
    • No, code 21 is very straightforward. It can only be the things described in that diagnostic flow. In fact it has no way of knowing that the spark plug resistance is out of spec.
    • Hi, SteveL Thank you very much for your reply, you seem to be the only person on the net who has come up with a definitive answer for which I am grateful. The "Leak" was more by way of wet bubbles when the pedal was depressed hard by a buddy while trying to gey a decent pedal when bleeding the system having fitted the rebuilt BM50 back in the car, which now makes perfect sense. A bit of a shame having just rebuilt my BM50, I did not touch the proportioning valve side of things, the BM50 was leaking from the primary piston seal and fluid was running down the the Brake booster hence the need to rebuild, I had never noticed any fluid leaking from that hole previously it only started when I refitted it to the car. The brake lines in the photo are "Kunifer" which is a Copper/Nickel alloy brake pipe, but are only the ones I use to bench bleed Master cylinders, they are perfectly legal to use on vehicles here in the UK, however the lines on the car are PVF coated steel. Thanks again for clearing this up for me, a purchase of a new BMC appears to be on the cards, I have been looking at various options in case my BM50 was not repairable and have looked at the HFM BM57 which I understand is manufactured in Australia.  
    • Well the install is officially done. Filled with fluid and bled it today, but didn't get a chance to take it on a test drive. I'll throw some final pics of the lines and whatnot but you can definitely install a DMAX rack in an R33 with pretty minor mods. I think the only other thing I had to do that isn't documented here is grind a bit of the larger banjo fitting to get it to clear since the banjos are grouped much tighter on the DMAX rack. Also the dust boots from a R33 do not fit either fyi, so if you end up doing this install for whatever reason you'll need to grab those too. One caveat with buying the S15 dust boots however is that the clamps are too small to fit on the R33 inner tie rod since they're much thicker so keep the old clamps around. The boots also twist a bit when adjusting toe but it's not a big deal. No issues or leaks so far, steering feels good and it looks like there's a bit more lock now than I had before. Getting an alignment on Saturday so I'll see how it feels then but seems like it'll be good to go       
×
×
  • Create New...