Head gasket information
 

i am talking with cometic head gaskets and the offer the svx head gasket in three sizes .065, .066 and .075. they will make any thickness i require up to .140. does anyone have any idea or know the math behind how these sizes will affect the compression ratio, or will it be to small of a change to notice?

I believe the standard size is .066. thanks guys

Re: Head gasket information
 

Cometic gaskets for the EG33 come in 3 bore sizes. 98mm, 100mm and 102mm. they are the BORE sizes. The stock engine takes the 98mm bore.


All three gaskets are the same thickness at .066".

Difference between the .066" and the .140" is .074" (or 1.88mm)




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Re: Head gasket information
 

John, I believe cometic sells the headgaskets in 3 different bore sizes, and 3 different thickness measurements on their website. 66mm is the correct size for our stock HG, though.

Re: Head gasket information
 

Their website shows 3 bore sizes, but all the same thickness.

Part numbers are different, but the prices are all the same.

Stock is still 98mm x .066"

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Re: Head gasket information
 

If you talk with them, you should be able to get a lot more options. The stock thickness should be used unless the machine shop needs to take off a lot of material when the mating surfaces are refinished to get them just right. The stock bore should be used unless you're having the cylinders bored out.

Re: Head gasket information
 

my bad guys i should have given some more specifics. I am talking to cometic through e-mail. I understand they come in three different bore sizes, I am gonna order a set of head gaskets in the stock bore size. They offer three different thickness' of head gasket in the stock bore size and that is .065, .066, and .075 however they make a gasket in any thickness up to .140

I am wondering if anyone knows the math, or some way to see how these different thickness' of head gasket will affect the compression ratio.

In Example
A stock engine with a standard thickness of .066 = 10:1 compression ratio.

A stock engine with a thickness of .075 = 9.9:1

A stock engine with a thickness of .140 = 9.5:1 etc. etc. I am just trying to see how the different thickness of the head gasket will affect the c.r. or will it be to small of a difference to even notice.

any help will be appreciated

Re: Head gasket information
 

It is my understanding that on a boxer engine, changing the headgasket thickness more than just to compensate for the machine work will affect the timing. I am not 100% sure on any details, but I would look into that before making a decision.

That said, it could be estimated mathematically, by calculating the cylinder volume both at TDC and VDC assuming the stock headgasket thickness and compression ratio, then factoring in the additional volume from the headgasket. All you need is a couple of formulas from geometry class, and the bore/stroke.

Compression ratio 10
Bore, mm 96.9 (headgasket bore is 98mm for a little bit of extra space, I guess - for example if the cylinders are honed)
Stroke, mm 75

I can work it out later if you want.

Re: Head gasket information
 

First we have to calculate the cylinder volume at TDC.

The CR is 10:1, the stroke is 75mm, the bore is 96.9. (If any of these values are wrong, please let us all know.)

If the CR is 10:1, then that means that the volume at BDC (bottom dead center) is 10 times the volume at TDC (top dead center).

The difference between the volumes (BDC - TDC), is given by the volume of a cylinder of height = stroke, and diameter = bore: V=pi r^2 h = pi x 48.45^2 x 75 =

10 x TDC = TDC + 553093.68 mm^3
...algebra...
TDC vol = 61454.853 mm^3
...more algebra...
and BDC vol = 6145485.33 mm^3

Any thickness added or subracted to the headgasket corresponds to volume added or subtracted to the TDC and BDC volumes. Lets say that we're considering a .140" headgasket and the stock is .066". (remember, when talking headgasket thickness, we are always talking about "compressed height", the height when it's all assembled and torqued to factory spec). The difference is .074", which is 1.8796 in mm.

This volume comes out to 13861.26 mm^3.

Add that to TDC and BDC, and we have 628409.80 mm^3 and 6159346.60 mm^3, respectively.

The compression ratio comes out to: BDC/TDC = 9.801:1.

Still pretty high, but you might be able to get away with 87 octane. Not exactly turbo friendly though.

There ya go. God, I'm such a nerd.

Re: Head gasket information
 

Nice work icingdeath88. ha ha I know enough about math to count money and that's pretty much it and by counting money I mean I can count to 10. None-the-less thanks for figuring that out. I was thinking if it would come out to around 9.6-9.7ish that might be good enough to run 87 octane, but I am not sure about the ignition timing, it may be to far advanced or to far retarded, or maybe it won't have an effect. thanks again.

Re: Head gasket information
 

So stock is 98 bore and I should go with .066 thickness? I found this thread searching around and I think it answers some of my questions. I tried to read all of the math on those posts, but it was hard to read plus it is late. :cool: Any befit to trying anything other than stock, larger or smaller (both in bore and thickness)?