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| Reducing chamber volume on 12G940 https://www.ausmini.com/forums/viewtopic.php?f=5&t=35213 |
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| Author: | smac [ Tue Aug 28, 2007 6:07 am ] |
| Post subject: | Reducing chamber volume on 12G940 |
Right, I know how to properly measure chamber volume and how to physically calculate the amount to remove to get a particular volume....however, I was wondering if anyone knows roughly how much volume is lost on a standard 12G940 head for each thou shaved off the face? I guess where I'm going is how small a chamber can you get before you can't shave any more? Starts out at 21.4cc....where can you get to? |
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| Author: | drmini in aust [ Tue Aug 28, 2007 7:46 am ] |
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You can shave a 940 head by 4mm or more, they are thicker deck than a smallbore head. I had one at 19.5cc and it only had about .090" off. How much reduction per mm off depends a lot on how much the chamber shape has been changed.. for example my current head on the 1360 has had the heart shape totally removed, looks more like a Chrysler Hemi head now! 
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| Author: | smac [ Tue Aug 28, 2007 8:56 am ] |
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Ya so every thou' you take off will mean more volume lost than a standard head....but hten you have more to start with... The situation I'm in is I'm getting some head work done but I can't be shagged taking the bottom end apart again. SO I'm stuck with pistons 1.5mm down the bore and about an 11cc dish. All adds up to piss poor compression, even with 1380cc. Long term I'll toss the pistons and get something decent, I was just curious how far I could bump the compression by skimming as a short term compromise. Once I redo the bottom end I can hog the chambers out to gain the volume back. At what point does valve lift become an issue? |
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| Author: | mason [ Tue Aug 28, 2007 9:04 am ] |
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Whats your C/R at the mo stu |
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| Author: | drmini in aust [ Tue Aug 28, 2007 9:06 am ] |
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I had no valve to piston clearance problems running that .090" shaved head with a Camtech VP3 cam and 1.5 rockers. Bear in mind that at full lift, the pistons are (or should be) around 1/2 way down the bores... 
Sounds like your pistons are the real problem- I like to set them up with deck height of no more than .020". You could deck the block heaps, but that will stuff you up in the future.
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| Author: | smac [ Tue Aug 28, 2007 10:20 am ] |
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With standard chamber and the details above I work it out to be around 9.2 I think. Like I said I'm not opening up the bottom end at the moment or buying pistons.....so I'll just get a bit carved off the head and be done with it. |
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| Author: | mason [ Tue Aug 28, 2007 12:34 pm ] |
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I would do a swep volume check of cylinder and cc check it at tdc including head gasket then maybe start with .75mm on cylinder head.Do a cc check of head after that and see what you have got. Easy to take head back to shop and get some more milled off if needed.How many times has your head been refaced already? |
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| Author: | ken@miniauto [ Tue Aug 28, 2007 9:33 pm ] |
| Post subject: | Reducing chamber volume on 12G940 |
For a standard 12G940 head .012" taken off reduces volume by 1cc hope this helps |
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| Author: | smac [ Wed Aug 29, 2007 6:04 am ] |
| Post subject: | |
THAT's the answer I'm looking for, thanks.....means with a standard chamber shape I should still be able to easily get it up over 10. |
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| Author: | drmini in aust [ Wed Aug 29, 2007 8:01 am ] |
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When doing the 12G1316 head (same chambers as a 12G940) for da Moke, I did some volume testing with the standard chambers, and found that 1mm off head = 2.5cc approx.
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| Author: | minicranks [ Mon Sep 03, 2007 5:02 pm ] |
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sounds to me like its not being worked out right either. [mod edit - rules 1 & 4] |
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| Author: | smac [ Tue Sep 04, 2007 6:16 am ] |
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ummm...what? |
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| Author: | mason [ Tue Sep 04, 2007 6:32 pm ] |
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Thats not a very nice thing to say jai |
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| Author: | smac [ Wed Sep 05, 2007 11:06 am ] |
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Seems to have crawled back under his rock.... |
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| Author: | minicranks [ Wed Sep 05, 2007 4:25 pm ] |
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thats ok im used to beeing bled dry or questions and then give the work away to someone else. na no rocks down here just motors being built flat out busy. |
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