explain headgasket sizing to me

[archaeic_bloke]

hey,

was just looking at this: http://www.rhdjapan.com/tomei-metal-...rb26dett-13436

and it occured to me i don't entirely understand how to choose a headgasket.
i know you need to account for compression, piston size, bore etc... so could someone enlighten me on how you go about choosing the appropriate sized head gasket for say my motor (stock bore, piston etc)

also if you could further elaborate on the type of gasket... like which one is best for what application? according to the site you can get "Head Gasket Structure (Bead Type,Grommet type,Super Grommet Type)"

[BenC]

Size really depends on what compression your shooting for. I went with a 88mm,1.2mm for my 87mm bore and thats suppose to yield 8.5:1 according to Tomei's charts. I would go for 8.7:1 if I were to do it again

Found this from HKS regarding the gasket design

The 'stopper' type is most effective in sealing because it incorporates a stopper instead of a grommet or bead which can be weak over a period of time. Stopper gaskets consist of the most metal layers for the best possible seal. The 'stopper' type gasket is the most expensive of the three.

The 'bead' type is the most popular gasket, due to its simplicity of construction. A raised bead around each combustion chamber ensures a tight seal for high-compression as well as high-boost applications. This gasket is very effective for sealing because of its flexibility. The beaded type gasket is the least expensive of the three.

The 'grommet' type gasket is a very effective type of gasket due to the metal grommet that surrounds each combustion chamber. This metal grommet insures a tight seal between the cylinder head and the engine block. The application of grommet gaskets is limited due to the high cost of production.

[archaeic_bloke]

wow thats actually a super informative, precise answer! much appreciated bud!

[Skym]

What is the purpose of engine (road, racetrack, dragstrip), boost level you want to run with what turbo's????

Tomei explain and show via diagrams (headgasket types, how to work out compression, etc) -

http://www.tomei-p.co.jp/_2003web-catalogue/e102_gasket.html

[NismoS-tune]

9.0:1 or higher ftw. Forget lower than 8.5 if you're nothing running 40 pounds of boost.

[nelsonmxmarc]

Hmm for my application i decided on a 1.5mil gasket to lower compression.. in my case my im running the stock head and bottom end. I want to be running around 20 psi but i may regulate down to 17 or so for safety and reliability until i have my forged motor put in..

Any idea?

Tomei strangely enough depicts bore for the RB20 as 80.5? Any reason for this? Stock is obviously 78mil..

http://www.tomei-p.co.jp/_2003web-catalogue/e105_RB20_gasket.html

Stock compression ratio for the RB20 is 8.5:1.. which isnt very exciting.. 1.5 mil would bring it down to around 8.2/8.15:1..

Would it be worth lowering my compression ratio by that much to allow the added boost pressure for safety precautions? OR should i continue to use the OEM 1.2 mil gasket and compression ratio and limit my boost pressure to not destroy my top end? (bottom end is good for 400 crank..)

Target hp range was 360 wheel; with my setup i will easily see that, but knowing that it could cause a midsummer catastrophy i'd be happy with 320 reliably and call it a day

edit; sorry for the hijack josh - this is for breaking my car last night (due to your presence..)

[cobrAA]

9.0:1 or higher ftw. Forget lower than 8.5 if you're nothing running 40 pounds of boost.

Funny, on the tomei head gasket chart. the higher you can get is 8.7.

[Skym]

Nelsonmxmarc. From what I understand, it's 80.5mm on metal headgasket because headgasket compresses, spreads out when head is torqued down.

For street use you want more offboost performance, so 1.2mm is perfect. But for racetrack you probably would run a 1.5 to 2.0mm (I think HKS used to or maybe still sell 2.0mm) due to higher boost levels (20psi+), higher revs (engine sucks in more air). Seen one street/drag driven RB20DET run into 7's with compresssion, stock bottomend (I think was the 2.0mm HKS headgasket).

[cortexx]

The advantage of dropping compression ratios slightly on a turbo charged engine is that you can actually force more fuel air mixture into the combustion chamber under boost , using a thicker head gasket has the effect of raising the cc of the combustion chamber slightly which equates to more fuel / air mixtures available for the same boost level reducing risk of detonation. Although the compression ratio is lower , you actually make more power under boost because of the extra fuel air burn't.

However if for what ever reason your turbo decides to shed its exhaust turbine (see standard gtr turbo with ceramic wheel) and you have no boost , your engine will feel like a smartcar towing a dumptruck !

[archaeic_bloke]

yep... basically:

thicker head gasket = lower compression ratio = less 'off boost' power but more 'on boost' safety
thinner head gasket = higher compression ratio = better response while off boost, but can't safely run as much boost.

its why honda's have really high naturally aspirated compression #'s

[cobrAA]

so I want thick :P. Dowshift and make the turbos scream

[allwheeldrifter]

im also looking to buy a headgasket, but dont know wich one.... i got stock bottom end with gt2860r-5, getting cams, and probably some port&polish......... should i put an oem size one, or go bigger?

[cortexx]

Really i would go with a oem thickness but get one of the metal gaskets. and replace the head studs with arp ones.

You want a good balance of boost performance and engine responsiveness across the rev range. not just while its in boost.

Depends on what you want to do with your car ..

[jessem]

But dont forget if you do a metal head gasket I would hope you are decking your head and/or block so some material will be lost in that process.

I would check with your machine shop and see how much they ended up taking off then use that number to determine compression #'s.

[allwheeldrifter]

i have different options in bore and thickness when i choose my gasket, would you guys know the oem specs?