gaket thickness and compression ratio

[e8_pqck]

what is the formula for gasket thickness and compression ratio?

for instance std compression on the VTS is ....

so using a thicker gasket will give it...

gasket thickness of 1mm = compression ratio of

and so on.

all based on a standard head i.e. not skimmed and std bore size.

[yr51ocw]

pretty easy to work out, crack out the calculator and google.

If you give us what the std h/g thickness is, and what h/g you want to use then I can work out your new (approximate) compression ratio.

[blackie_2k5]

thats not the proper way to do it, youll need your head skimming when removed so you need this done first as you dont know how much will need to be removed(this will alter the figures, then you need to measure the combustion chambers and work it out from there, then you work out what compression ratio you need and pick a gasket size to achieve this, or if you know the head is sweet, you can work out what you want to run, find out a gasket size that will help you achieve this and get th head skimmed to the measurements youve worked out

[e8_pqck]

found the calculator.

thanks blackie, its a simple case to subtract the skimmed thickness off the gasket.

so what compression ratio for turbo conversion on these engines? My celica ran 9:1 on CP forged pistons. However i want to have a go at reducing compression via gasket first before i go forged route.

[e8_pqck]

all things equal:

82 x 78.5 stroke and bore
78.5 gasket diameter
0.3 gasket thickness
39 cc volume
0 piston volume
0 deck height

gives 10.8:1 compression which i think is standard.

changing gasket thickness only, sees a compression ratio of 9:1 with a 2.1mm gasket, so i need 1.8mm thicker than a standard gasket?

does this sound about right?

[e8_pqck]

easier way is to change gasket value to 0, then adjust cc until you get standard compression then alter gasket thickness till i see required compression. it comes out at 1.8. so that would be 1.8+standard gasket thickness to achieve 9:1....

thats if 9:1 is desirable.

9:1 on the celica was running 400hp but im not aiming for that on this!! maybe 240 with about 10psi? dont want a dog off boost.

[e8_pqck]

http://www.csgnetwork.com/compcalc.html

[atspeedracing]

allow me to save you some of the hard work....

TU5 16v Decompression Packages (http://atspeedracing.co.uk/products/index.php?main_page=product_info&cPath=12_63_64&products_id=198)

also available for other TU engines, incl J4, JP4 & 1.4 and 1.6 8v.

regards

colin

[e8_pqck]

Thanks, i see the price but no information. Could you provide:

plate thickness ?? + gasket thickness ?? = 8.6:1
plate thickness ?? + gasket thickness ?? = 9:1
plate thickness ?? + gasket thickness ?? = 9.5:1

[blackie_2k5]

wow, expensive!

[yr51ocw]

e8 - the figures you have put seem sensible

Blackie - you dont always need to skim the head once removed, you need to CHECK if it needs skimming first. Often they dont, or if they do its very little. I skimmed my head 6 thou just to clean the surface, which will not affect the compression ratio by an appreciable amount.

However, I do agree the best way to measure the CR is to volume check the combustion chamber, as there can be a wide variation in cylinder just from tolerancing alone.

[blackie_2k5]

e8 - the figures you have put seem sensible

Blackie - you dont always need to skim the head once removed, you need to CHECK if it needs skimming first. Often they dont, or if they do its very little. I skimmed my head 6 thou just to clean the surface, which will not affect the compression ratio by an appreciable amount.

However, I do agree the best way to measure the CR is to volume check the combustion chamber, as there can be a wide variation in cylinder just from tolerancing alone.

if running boost its most sensible to get the head skimmed, 6 thou will change the compression, i got mine skimmed a few weeks ago which was slighlty warped and only needed 3 thou removed

[e8_pqck]

6 thou = 0.152mm

this alters the compression ratio by:

.17:1 (appx)

guess you could use the next size up but that would lower the compression ratio on a standard engine.

[e8_pqck]

is the standard gasket 1.5mm?

[16vax]

volume checking the combustion chamber and piston dish is the most acurate way to do it then work out the size of gasket needed and i think when i done that with std pistons i got 2.0mm head gasket
im still running std pistons with a 1.9mm gasket cr about 10:1 i think cant remember .5 bar had a good few track days with no problems
colin

[axsaxoman]

all things equal:

82 x 78.5 stroke and bore
78.5 gasket diameter
0.3 gasket thickness
39 cc volume
0 piston volume
0 deck height

gives 10.8:1 compression which i think is standard.

changing gasket thickness only, sees a compression ratio of 9:1 with a 2.1mm gasket, so i need 1.8mm thicker than a standard gasket?

does this sound about right?

your numbers are not corect for either the j4 or the jp4 engine . (0.3mm gasket --no such thing--gasket size 78,5mm --wrong) and that is presuming everything is std . but if not you must measure to be sure .

I am guessing you are wanting to turbo ?

decomp plates --i personal don,.t like them , they are expensive and see no advantage of using them over the use of mutiple MLS gaskets --
you know the right answer is to fit forged pistons as std cast pistons will eventually crack as crown is not as thick and ring lands are thinner and so are the thickness of the rings .

you want gaskets see our site

http://www.gmcmotorsport.co.uk/index.php?main_page=index&cPath=21_91&zenid=30a1303bb81fd08389656bf9cf484c23

[16vax]

as john says pistons will prob eventualy crack. thats why i will be changing my pistons to lo comps.but it has had some abuse on the std ones.
colin

[yr51ocw]

if running boost its most sensible to get the head skimmed, 6 thou will change the compression, i got mine skimmed a few weeks ago which was slighlty warped and only needed 3 thou removed


Read "by an appreciable amount"

If you have ever volume checked a combustion chamber then you will realise that there is a larger difference than this between the same engines because of manufacturing tolerances.

[yr51ocw]

6 thou = 0.152mm

this alters the compression ratio by:

.17:1 (appx)

guess you could use the next size up but that would lower the compression ratio on a standard engine.


VERY approximate, infact it will be less than that as the combustion chamber in the head is not a perfect cylinder which is what you have presumed it to be.