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Help With Compression!!! updated with specs.
https://www.ausmini.com/forums/viewtopic.php?f=2&t=64550		 Page 1 of 1
Author:  NG [ Wed Jul 21, 2010 5:02 pm ]
Post subject:  Help With Compression!!! updated with specs.
Bore 2.8inches

stroke 3.2inches

cc dish in pistons 8cc

deck height 35 thou

Piston deck height 35though .035inch

combustion chambers is 21.4 cc

need to get down to 8.5 to 1

can anyone help me with the cc's needed to take out of the cylinder head please!!!!!
Author:  VulcanBB18 [ Wed Jul 21, 2010 6:01 pm ]
Post subject: 
Mate you have all the info you need already.

I calculate you have roughly 322cc per cylinder.

you want 8.5:1

322 / 8.5 = 37.9

So you need a total squish area of 38cc or so, which you know you have already 29.4cc in head and pistons alone. Head gasket+ring lands+down the bore is harder to estimate because it depends on so many factors. If you "guesstimate" 3 or 4 cc then you still need to find another 4 or 5 cc...

why only 8.5:1?

cheers

Jacob
Author:  NG [ Wed Jul 21, 2010 6:11 pm ]
Post subject: 
it is for a super charger set up.

so to get down from my current compression?

how many cc's would i need to put on the head?

are you saying around 7cc's ?

i just have no idea how to work it out to be honest with you, was hoping some one can help me as im getting conflicting cc's

im running 9.2 to 1 currently
Author:  miniobsessed [ Wed Jul 21, 2010 6:35 pm ]
Post subject: 
I calculate your current CR to be 9.5:1 given your figures.

To get to 8.5:1 your total chamber volume (ie everything above the piston at TDC) needs to be 43cc. Given your 8cc dish, 35 thou deck (3.5cc), HG (~4cc) and the area above the top ring around the piston (~1cc) - this totals 16.5cc - you will need 26.5cc in your head :wink:

It'd be a good idea to actually measure everything properly before you go carving out your head though... For example, head's are rarely actually 21.4cc to start with.
Author:  NG [ Wed Jul 21, 2010 7:02 pm ]
Post subject: 
ok cool. thank you.

im getting conflicting things again.

that is the measurements my mechanic gave me.

and said im running 9.2 to 1.

sorry my Piston deck height 35though .035inch

does that it different at all?

the person doing my head, said i will need 27.5cc's, now my mechanic thinks he is wrong.

i put my own numbers in see pic below but im not sure what that is telling me.

http://www.accralite.com/accralite/Sear ... ulator.asp

[img][img]http://img22.imageshack.us/img22/2308/screenshot20100721at708.png[/img]

Uploaded with ImageShack.us[/img]
Author:  miniobsessed [ Wed Jul 21, 2010 7:09 pm ]
Post subject: 
yeah I took the 35thou into account...

If you want to work it out yourself...

CR= Swept Volume (1/2 bore * 1/2 bore * pi * stroke) plus 'chamber' volume (everything above the piston at TDC) divided by the 'chamber' volume.

CR = (SV+C) / C

To go the other way it's...

'Chamber' volume required is the Swept Volume divided by your desired CR minus 1

C = SV / (CR-1)

Easy! :)

And if your figures are correct then so are mine ;)
Author:  NG [ Wed Jul 21, 2010 7:14 pm ]
Post subject: 
hehe a bit closer

this what phil said.

standard stroke is 3.2" or 81.28mm
Your bore size is 2.8", so as standard bore is 2.780" yours must be +0.020 giving 1293cc

V+C divided by C = compression ratio.

V= Swept volume of 1 cylinder, so 1293 divided by 4= 323.24 {V}
C= Volume above the piston, so:

Pistons: 8cc
Head gasket: 3cc
Piston to deck volume with 0.035": 4.4cc
Head: 27.5cc

Total= 42.9cc {C}

V+C divided by {C}= 8.53 compression ratio.

I hope that makes it clear.
Regards
Phil.
Author:  simon k [ Sat Aug 14, 2010 2:28 pm ]
Post subject: 
out of curiosity I just went and attempted to measure my C/R...

I put #1 at TDC, jacked the nose up as high as I could and squirted jack oil down the spark plug hole until it puddled in the hole - I put in 35.5cc

I have a BK450 head gasket, and +.040" Hypatecs which are meant to be 9.75:1, but I've heard that they give a bit more than that. My pistons were flush with the top of the block when I installed them...

I calculated my ratio at 10.23:1. If I wanted 8.5:1 I'd need to take 8cc out of the head.

Where do we start grinding?
Author:  drmini in aust [ Sat Aug 14, 2010 3:02 pm ]
Post subject: 
Simon your calcs sound about right. Hypatecs give around 10.0:1 with an undecked, stock head.
I'm running the 1412 at 10.7:1, with RE13 cam, 1.46 rockers & 98 unleaded, no problems.

If you want/need more volume (why? you will just lose torque & HP) I'd take it out of the heart shape side first.
Author:  simon k [ Sat Aug 14, 2010 3:22 pm ]
Post subject: 
drmini in aust wrote:
If you want/need more volume (why? you will just lose torque & HP) I'd take it out of the heart shape side first.


just hypothetical... ;)

edit:

hang on, which bit do you mean by 'heart shaped' - the whole thing is heart shaped.... do you mean the beak side, or the rounded side?
Author:  GT mowog [ Sat Aug 14, 2010 3:30 pm ]
Post subject: 
simon k wrote:
drmini in aust wrote:
If you want/need more volume (why? you will just lose torque & HP) I'd take it out of the heart shape side first.


just hypothetical... ;)

edit:

hang on, which bit do you mean by 'heart shaped' - the whole thing is heart shaped.... do you mean the beak side, or the rounded side?


Nah, he just means you gotta cut your heart out :lol:
Author:  drmini in aust [ Sat Aug 14, 2010 6:40 pm ]
Post subject: 
simon k wrote:
drmini in aust wrote:
If you want/need more volume (why? you will just lose torque & HP) I'd take it out of the heart shape side first.


just hypothetical... ;)

edit:

hang on, which bit do you mean by 'heart shaped' - the whole thing is heart shaped.... do you mean the beak side, or the rounded side?

The beak side, it's what most tuners do when fitting a turbo or huffer.
I did it on my 1310S when it was running flat top Mahle pistons on the street...
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