Lowering compression..suggestions please

[mc_hawkings]

Hi

now then now then

What i've dont is 1.4 block, 1.2 head, 1.3 inlet with 1.2 gasket. Now i've highered the compression with the gasket.

I want to lower it again should i

a: use gasket thicker than 1.6 custom sbd maybe

b:change back to 1.4 head....not sure whether to as it has smaller ports less flow, 1.2 head i dont think it raises CR as it has bigger bigger ports to aid flow just like u would if you had mild porting.

c: skim piston tops to lower compression.

[Stuart]

fit a 1.6 head. bigger valves and lowers the CR enough for a turbo lol

[mc_hawkings]

Dude which 1.6 head , carbed sv head or gte head?

cheers


You dont know of any good way of checking CR to be exact or is it just math.

[mikeyredtop]

gte gsi head, plus ive heard a lot of gud feedback off fitting a skimmed c14se found in loads of astras, mk3, and nova sri's

[mc_hawkings]

hmmmm i dont have one to hand have everything else though.

Just work out the CR would be 8.75 bit low but if i use a gasket the thickness of 1.2, SBD item might get away with it. But depends what inlet to use hmmm

Cheers dude

[mc_hawkings]

I just had a look at carbed 1600 head & the 1600 gte head. The gte has round ports & carbed one has square. So gte it is, will look at the 1.3 inlet when i get it, see how big the ports are on it.

[S_Gault]

I never rely on the std chamber volume figures..

I always burrete each head bespoke, that way its matched to the block and correct.. not an inspired guess.

Just out of interest.. anyone know the std figures for all the chamber volumes of the heads..?

Steven

[mikeyredtop]

http://www.geocities.com/MotorCity/5353/cylhead.htm

usefull link

[MC]

I'm confused with where this post is going. Why do you want to lower your compression? Is it not running right with your raised ratio or do you want to lower it for a turbo?

High CR of about 11:1 will result in more power, lowering to 8.75 will be a back step.

The 1.2 head plays a major role in your increased compression ratio, more so than the gasket. Fitting a 1.6 head to the 1.4 will drop the ratio too low unless you drastically skim it. If you plan to fit a 1400 head, use the Sri head, not an SR head. It will need matching to the 1.3 manifold you are using.

MC

[mc_hawkings]

I should have said .........yes i want to turbo it..... for fun & build on it.
Just doing a bit of research before going ahead

[MC]

right, I'm clear now, yes use a 1600mpi head, although 8.75:1 is still alittle high for a decent turbo set up. Modern VAG engines run high compression engines with turbo's but they are very strong and have good management.

Don't be tempted to do the double gasket and sandwhich plate as I've never seen a turbo 1600 that uses that technique run quicker than a well sorted NA setup.

Your best bet is to use a 1300 block and 1600 head, or as previously decribed, enlarge the combustion chamber (quite easy) or machine the pistons.

MC

[mc_hawkings]

I'll only do it with the 1.4 as its what i have to hand & have only just fitted it. Looking at tech data cav turbos run 9.0 CR so if they can run that & be fine i think 8.75 would be good thats with normal gasket size but i'll see if i can get a metal sbd item.

Regarding the head will that be fine just opening it up for the inlet will this up CR atall?

Cheers

[Snowface]

cav turbo blocks and pistons have much stronger parts than the standard 1600/1400 N/A block. If u try at 8.75 most likely you'll mel a piston or blow the engine.

[mc_hawkings]

hmmmm melty melty oh balls didnt think about stress.

[S_Gault]

no on.. it depends on the volumetric efficiency of the engine, if its very effiecnent then it needs less timing.. so it may not necessarily melt.

Ive built WRC escort engines.. they run 9.5:1 compression . Yes thats not a typo! They also run about 1.5bar of boost, and never give hassle.

Steven