higgsy91

Ok so it's not so much a game.. more a question with an educated guess..
I have a zetec blacktop with JE low comp pistons (9:1 CR on JE website based on standard ZX3 engine ST170 equivalent) and I have a lightly skimmed head and 3 layer MLS focus RS head gasket.

Any ideas?
Good CR?

Cheers!

higgsy91

Any ideas roughly.. debating the 4 layer gasket now. But would it make much difference to CR anyway with a light skim?

jamie's

how long is a piece of string?
nobody's gonna be able to say without it being a total guess
without knowing actual(not stated on the box) comp ratio before head work
then a light skim could be anything, and can you be 100% sure the head hadn't been touched before?
and is dish/bowl in piston 100% original?
is the combustion chamber in the head 100% original etc
the slightest amount of skimming or work to head or bowl would make a biggish differance to the comp ratio
and the head gaskets can be differant sizes when compressed (differant makes etc)
so there is no way in the world that anybody could possibly guess the comp ratio without having some accurate numbers to work with

higgsy91

I agree it's a complete stab in the dark, was more hoping there was someone running a similar spec.
The pistons are brand new JE pistons made for the US Spec Zetec. Which would give you a 9:1 CR. Difference with mine is it has the FRS head gasket and a light skim.

rhinopower

If its in pieces measure it correctly and don't guess! Gonna be a chocolate teapot if the head has been skimmed enough to bring it up to a standard c/r.

higgsy91

Dont suppose you know a good thread on how to do this do you? Guy at the garage was going on about filling the chamber up with liquid and all sorts. It's just i have these pistons, which don't make measuring very simple..

jamie's

need to agree about just doing it right
measure it up properly, just google it and it'll show you how to measure to area and calculate the volumes against each other and work out the comp ratio
do you actualy need to know the static comp ratio? or are you just wanting to know what is?

higgsy91

Cheers, basically i've just been reading up and i basically didn't want to run the risk or too much compression.
I mean it's designed for US spec engines which actually normal have a 0.6mm head gasket anyway, so the FRS 1.2mm gasket (IIRC) should be more than sufficient in replacing the lost material from the skim

higgsy91

It's a mystery, in one place i read the RS uses a 1.2mm gasket, another says it's the same 0.6mm gasket from the N/A engines. I just don't wan't do go much higher than 9:1 CR. Is it worth going for the ST170 4 MLS gasket..

nevsrevs

focus rs ran two hg, 1.3mm and the revised 1.8mm, I would opt for the st170 4 layer 1.0mm

higgsy91

So you're saying go with the hg with the least thickness out of the three? Everyone has a different number lol.. I checked Ford's system but it wasn't much help tbh. In fact useless! Anyway I can find out from my Victor reins packaging on the hg what one I have?
Cheers!

higgsy91

Excuse the poor English.. I'm on the underground and you have to rush to finish a sentence from station to station with the free WiFi!

jamie's

around 9:1 shouldn't be a major prob as long as its mapped accordingly

Mk4 Rick

Sounds very interesting this.If my 2 litre zetec blacktop cr was 10.0.1 and I fitted a 4 layer st170 head gasket what would I get cr wise.To my knowledge the engine has been untouched.

Mk4 Rick

Im sure my Volvo s40 T4 was 9:1 and the engine was a 1.9 as well which made 200bhp at 13psi from factory

jamie's

9:1 is fine for a modern turbo engine, fitting a thicker head gasket will drop the comp ratio slighly,
but make sure your getting the correct info
as some manufacturers give a out the pakage thickness
and others give a compressed thickness
just dont mix up the 2

higgsy91

Some good info here guys thanks. I think I'll stick with the rs gasket. I'm gonna see if I can find out the thickness of it and maybe take a crack at working out the CR too. I mean how much difference can a head gasket make anyway (here comes the attacks)

higgsy91

Lower compression = more boost
Higher compression = less boost but same power
Is this a fair comment to make?

jamie's

kinda sumfin like that, but its also to do with ignition tables
higher CR will make the car more responsive off boost and be less laggy
so more modern set ups usualy go in favour of that as it makes the car nicer to use around town etc

higgsy91

wah that makes sense.. although I really do want that punch from my turbo. Can't have it all though haha.

Turbo Zetec

iTrader: (1)

depending on whether you have the 3 or 4 layer Reinz gasket, the CR will vary by roughly around 0.5

higgsy91

So I'll be running roughly 9.5:1 seems a little high, guess I'll have to cut back on the boost a bit! I fitted the head last night and noticed with those pistons (when I was timing it 180° out somehow??) That the valve cut outs seem to have made it a non interference fit. Result!

rsmark86

iTrader: (3)

Take it too an engine builder and pay for them to cc and test it to be 100% that you have the correct cr

No point building it first and finding out later it's wrong

I bet they would only charge an hours labour too

If it is high you could as they do on the karlos g budget route and fit upto a 1.8mm frs head gasket

But having not built it up any decent engine builder can machine your pistons and recheck the clearence while there

Wayne at wpe (wacky performance engines) did this for me once as I had an engine that the compression was way too high but had accrulites in it

He charged £200 to machine it from 13-0-1 too 11-5-1 and built the engine and redone the head chambers and valve pockets

But I had too supply gaskets and bolts