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
A Mathamatical question for you all....
Started by
mighty_mini
, May 10 2004 07:09 PM
9 replies to this topic
#1
Posted 10 May 2004 - 07:09 PM
#2
Posted 10 May 2004 - 08:26 PM
Metro Compression Ratios Try this for the comp ratio.
You'll like this one Dave Compression Ratio Calculator
You'll like this one Dave Compression Ratio Calculator
#3
Posted 10 May 2004 - 09:42 PM
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
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
#4
Posted 14 May 2004 - 02:11 PM
mg-metro non-turbo.. late type
#5
Posted 14 May 2004 - 02:34 PM
All A+ 1275 heads have the same chamber volume, 21.4cc or so.
Are your pistons dished ? if so how big ?
Phil. :grin:
Are your pistons dished ? if so how big ?
Phil. :grin:
#6
Posted 14 May 2004 - 03:13 PM
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:
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:
#7
Posted 14 May 2004 - 03:40 PM
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.
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.
#8
Posted 16 May 2004 - 09:38 AM
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:
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:
#9
Posted 16 May 2004 - 12:22 PM
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:
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:
#10
Posted 16 May 2004 - 08:23 PM
Head gaskets can be a weak point on Turbos and surley to add a second can only increase the problem.
Siggy
Siggy
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