9 replies to this topic

[mighty_mini]

ok here goes...

I have a std metro engine, with +020 pistons to make it 1293, assuming the stroke is the same and the block has not been skimmed, and it has a std 12g940 head... what should its approximate compression be?

now for what im after... how much would i have 2 take out of the head (increase the combustion chamber size) to reduce my compression to an appoximate 8:1


failing that, how do i work out CR from the volume of the bore, etc

[Bluemini]

Metro Compression Ratios Try this for the comp ratio.

You'll like this one Dave Compression Ratio Calculator

[siggy]

To confuse the matter which 12G940 head is it?

That is a generic number for heads, going back to 11 stud Cooper 'S', 1275GT heads and up to the latest metro motors including turbos.

Siggy

[mighty_mini]

mg-metro non-turbo.. late type

[Turbo Phil]

All A+ 1275 heads have the same chamber volume, 21.4cc or so.
Are your pistons dished ? if so how big ?

Phil. :grin:

[Turbo Phil]

To calculate the compression we need to know the swept volume + the volume above the piston.
OK here's an example build -

Swept Volume. {V}
1275 with + 0.20 overbore on standard stroke crank {81.3mm} = 1293cc
1293 divided by 4 = 323.25cc, this is the swept volume

Volume Above The Piston. {C}
You need to know the following- Ring land volume, piston dish, volume above the piston & below deck, volume in head gasket & volume in head chamber.
So for example- RLV=0.80CC
PD= 6cc
VAP+BD= We'll say the pitons come within 10thou of the block face, in which case the volume is= 1.01cc
VIHG= Average gasket 4cc
VIHC= Standard head typically around 21.4cc

Add all these figures together 0.80cc +6cc + 1.01cc + 4cc + 21.4cc = 33.21cc

0K so we have swept volume of 323.25 {V} & volume above piston of 33.21 {C}.
To work it out V+C divided by C = 10.73

This is the compression ratio 10.7

As you can see you really need to have the exact figures to get it spot on, but this gives you a guide.

If you wanted a compression ratio of 8-1 you would need a volume of about 46cc above the piston.

Hope this helps.
Phil. :grin:

[mighty_mini]

i worked it out to be 9.87

after taking the head off and finding all the values...


i take it u cant really skim dished pistons to lower the CR, for the obvious reason of it not being flat?

ive read up about a few people who have got a metal shim made up from steel so they ran 2 head gaskets, if i did this i can easily accomplish my 8:1 CR, but is there any reason why this isnt safe?? i could see it as being weaker...

on head work alone i need a combustion volume of 35, and im sure i cant get that safely.

[Turbo Phil]

Material can be removed from the piston tops to lower the compression yes, this is not ideal though. I take it this is for a Turbo/supercharged application ?
In this case you really want as thick a piston crown as you can, due to the extreme loads & temperatures seen.

I've also heard of the metal shim idea, though i've yet to find anybody who can prove it's reliability. It does sound like it could be a weak area ....

It is possible to create 35cc chambers in the head, though this seems to be about the biggest possible.

Phil. :grin:

[mighty_mini]

this is for a turbo conversion yes.... and i have also found out something useful.

my pistons are AE 21251 +020

They are the 8.8:1 low comp pistons, this makes getting my desired 8:1 alot easier, rite?

they have an 8.4cc dish dont they :grin:

[siggy]

Head gaskets can be a weak point on Turbos and surley to add a second can only increase the problem.

Siggy