How to achieve a desired compression ratio?

[Howieau]

Im rebuilding a 3sgte and am aiming for a CR of 9:1 roughly (stock being about 8.5:1 I think), but I have no idea as to how to go about achieving it. Im purchasing forged pistons, metal head gasket, having block/head machined too. I understand that decking the block/head will raise the CR, and also a thinner head gasket will raise it aswell. The pistons ive got my eye on are 8.2 - 8.6 CR. How can I accurately get the CR I want with all these given factors? This is my first engine rebuild and Im stumped as to how to acheive the CR im after.

Thanks in advance.

[Screamn_Sleeka]

Buy pistons that are 9:1 instead of ones that at 8.2 - 8.6 which sound like factory replacements. - yes they will cost more.

Dont mill the head or use a thin gasket on a turbo engine please.

[MZ20_drifter]

how much boost are you going to be running?
you could go for a thin gasket? i generally like my cars with really low cr and upping the boost more

[TC1600]

If you machine too much material off the head and/or block, or use a thinner head gasket, it will affect your cam timing, as the tensioner will only take up the slack on one side, requiring the use of variable cam gears to compensate. The best way is to use the correct pistons to start with.

[hemi twofifteen turbo]

Why not mill the head? You can change comp by a number of ways.

1. Oversize pistons/bore will increase CR slighly.
2. Mill head (decrease chamber cc's)
3. Different pistons (differnt dish cc, deck height etc)
4. Mill block (same as increasing deck height of piston)

Use some maths to work it out, ie

bore X stroke + head chamber volume / chamber volume

oh remember to include headgasket thickness too in above eqn.

if you have the head off use a big syringe to CC the chambers to see exactly where your at, then go from there. Hope this helps.

[thechuckster]

i'd avoid #2 as you can: stuff with carefully designed squish areas, affect swirl and possible make the thickness between chamber and water jackets too small.

[Roundy]

1. Oversize pistons/bore will increase CR slighly.

Use some maths to work it out, ie

bore X stroke + head chamber volume / chamber volume

how much does the Oversize bore etc change it? it isn't a huge amount.

also if you know the current CR you can (roughly) work out the chamber volume of the head, on a 3S-GTE it is ~66cc's

increasing the bore by 0.5mm gives ~0.15:1CR increase, so not a lot, and probably less then that as the chamber volume is slightly increased as well

Cheers
Nathan

[Andrew162]

my last 3S-GTE was O/S by 1mm (40k") so was effectively 87x86 bore/stroke. based on what roundy's suggested above, mine had a 0.3:1 C/R increase making it about 8.8:1. however I'd be strongly in favour of decompressed pistons (like 8.0:1) and cranking up teh boost....so those factors combined with decking the block/head will quite likely put you up around the 9.0:1 area.

my question is why do you want 9.0:1 on a turbo 3S?

[danny2]

on my old cordia turbo i shaved the head with copper head gasket .. wasn't a good move, as she would ping a lot easier under boost .. mind you it could have been other things but i reckon its not worth the trouble unless you want low boost and the engine to be a little more responsive off boost.

[oldcorollas]

whats wrong with 9:1 on turbo 3S??

on 2 valve heads you have more space to adjust the compression by changing combustion chamber volume.
on a 4 valve head, you can change the CC slightly by grinding it out, OR you can ge the valve seats cut a bit deeper (you can work out the change from valve area x depth change), but there is a limit to that, and this also changes the spring seat height etc.. and may need valve tips ground to suit.
you might need to do this to adjust the chamber depending on pistons...

millinghead/block have their own issues, but ...

going from 8.5 to 9...
you want roughly 62cc CC volume for the stock pistons.
or.. you need roughly 530cc per pot swept instead of 500
which means roughly 88.5mm pistons.. = 100 thou oversize
or... a change of 5.8cc per mm of head/block/gasket change.. which means shaving/milling 0.7mm

so...
just for kicks...
assume you get 40 thou over pistons
so bore and stroke is now 87x86
new swept volume = 511cc

old combustion chamber + gasket = 66cc.
of which 0.8mm was gasket = 4.65
so actual chamber was 61.35 cc (including piston dish/dome)

new gasket (87mm dia and 0.8mm) gives 4.75cc

so just with stock 40 thou over and stock thickness gasket
511+61.35+4.75 / (4.75+61.35) = 577.1 / 66.1
8.73 CR

to get that to 9:1, you want a chamber with 63.8cc total
would need to remove 2.3cc

each mm (with 87mm bore) = 5.944cc

so you need to remove 0.38mm.. which is 15thou...

so.. in the end... order stock 8.5:1 40 thou over pistons (assuming the dish/dome is the same volume), use a stock 0.8mm thickness gasket, get the deck skimmed 5 thou (0.127mm) for flatness, get the head skimmed 10 thou (0.254mm) for flatness...

and you have 9:1 Cr....

[Howieau]

Just reading over this thread again as im close to ordering pistons and such.

If you bored a cylinder 2mm over, do you have to get a special head gasket that also has the 2mm extra so there is no weird shape in the combustion chamber where the head gasket sticks out over the cylinder?

Like this as if it were 88mm bore, 86mm head gasket.


Would that be an issue, or wouldn't it matter at all?

[Andrew162]

question - why do you want to have an overbore of 2mm? I guess an overbore of 1mm wouldn't make that much difference, at least it never did on mine. 1mm is all you'd want to overbore it, surely? with mine I guess I had a HG overhang of about 0.5mm per cyl, so bugger all really.

[BeRad]

How much can you bore them out before you get to close to water/oil galleries etc and run into overheating issues.

[Howieau]

I meant more in terms of theory, I dont plan on doing it. Ill take it as its not a good thing then haha. Still learning here

[oldcorollas]

it is better not to have the gasket sticking into the chamber at all.

often, gaskets are a little oversize to allow for a bit of boring to standard OS specs. some more than others.

for 2mm, you would ned to look at a different gasket if it protrudes past the bore edge.. imho.. but i don;t really know