Who likes maths.
Im planing on puting together my 1426 at some stage in the future.
bores are 74.7mm (triumph 2.5), stroke is stock and the block is decked so the come up flush, assuming a stock 12g940 head, how high is my compression?
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| Compression ratio - maths test https://www.ausmini.com/forums/viewtopic.php?f=5&t=41358 |
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| Author: | jbeenz [ Wed Apr 23, 2008 4:46 pm ] |
| Post subject: | Compression ratio - maths test |
Who likes maths. Im planing on puting together my 1426 at some stage in the future. bores are 74.7mm (triumph 2.5), stroke is stock and the block is decked so the come up flush, assuming a stock 12g940 head, how high is my compression?
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| Author: | 1018cc [ Wed Apr 23, 2008 4:49 pm ] |
| Post subject: | Re: Compression ratio - maths test |
jbeenz wrote: Who likes maths.
Im planing on puting together my 1426 at some stage in the future. bores are 74.7mm (triumph 2.5), stroke is stock and the block is decked so the come up flush, assuming a stock 12g940 head, how high is my compression? You will also need to provide at least combustion chamber volumes of the head. Are the pistons flat tops? |
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| Author: | jbeenz [ Wed Apr 23, 2008 4:52 pm ] |
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hehe thanks 1018, yes, pistons are flat tops, stock 12g940 is 21.5cc. Just thought it would be an interesting excercise, i have done the maths, just want to make sure im not nuts! |
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| Author: | 1018cc [ Wed Apr 23, 2008 4:56 pm ] |
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I don't know what you got but when I just did a really quick calculation (not taking into account the volume created by the H/G etc) I actually got 18.9:1 
Obviously this will be a little higher than it will be (as I said before I did this REALLY quickly) but I didn't expect it to be that high.....I'll have another look at it 
<edit> I think I f**ked it up...hang on a bit longer |
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| Author: | 1018cc [ Wed Apr 23, 2008 5:09 pm ] |
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Ok, I have no idea then. I keep working it out and getting really high compression ratio's each time. e.g. Stroke: 81.28mm Bore: 74.7mm Chamber Volume: 21.5cc Gasket: 2.5cc (guess) Putting all that in gives me a C/R of 15.84:1 - which is pretty high still. Maybe someone else can see what I'm doing
Without taking into consideration my gasket guess I still get 17.57:1 
Sorry jbeenz I must be doing something wrong (unless your numbers are around the same). <edit> Working off stock 1275cc sizes (70.6mm bore, 81.23mm stroke & 21.5cc chamber volume) in order to get 8.8:1 compression (stock) there is 19.23cc taken up elsewhere (not part of the stroke / chamber volume etc). Working off this number for your engine I get 9.73:1 compression. I don't think that this is enough - if your pistons are coming to the top of the block and there is less than 30cc's in the chamber volume + gaskets etc (again 30cc is a guess - I wouldn't think there would be that much per cylinder) then your C/R should be more up around 11.57:1 I'm so confused 
Hopefully someone else can clear this up |
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| Author: | jbeenz [ Wed Apr 23, 2008 5:26 pm ] |
| Post subject: | |
1018cc wrote: Ok, I have no idea then. I keep working it out and getting really high compression ratio's each time.
e.g. Stroke: 81.28mm Bore: 74.7mm Chamber Volume: 21.5cc Gasket: 2.5cc (guess) Putting all that in gives me a C/R of 15.84:1 - which is pretty high still. Maybe someone else can see what I'm doing Without taking into consideration my gasket guess I still get 17.57:1 Sorry jbeenz I must be doing something wrong (unless your numbers are around the same). Thats about what i got, i allowed 3cc for gasket, so mine was a little less....perhaps i could use diesel 
thanks! |
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| Author: | Matt68 [ Wed Apr 23, 2008 5:29 pm ] |
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Area of cylinder = 2 Pie r2 x Height Edits to include MadMatt's terms for clarity: = 356.2168 cc's per cylinder. Plus 21.5cc head chamber and 3.5cc head gasket; Total volume of 381cc, (CV + SV) Compressing to 25cc (CV). So total volume/CV = 15.24:1 compression ratio, static. Nick, we must be missing something in working out static compression ratios? |
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| Author: | 1018cc [ Wed Apr 23, 2008 5:39 pm ] |
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I did an edit just before ^^. At least someone else is getting high compression ratio's too! I hear that methanol is a cheap racing fuel Jed, just a pity because your car will use more methanol to produce the power
Decompression plate anyone? |
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| Author: | jbeenz [ Wed Apr 23, 2008 5:46 pm ] |
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i think im going to need a bathtubed head! id like to get down to 11-12 to 1. Plus a real lumpy cam i guess!
Block pic! 
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| Author: | 1018cc [ Wed Apr 23, 2008 5:53 pm ] |
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Jed, I have just done the maths. Still assuming 2.5cc gasket size you will need a combustion chamber volume of about 29.9cc's for 12:1, 31.4cc's for 11.5:1 & 33.1cc's for 11:1
I wish you luck with your bath tub head 
There isn't too much meat between those bores now......But it will be so much fun - 1426cc's........ |
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| Author: | spraycanmansam [ Wed Apr 23, 2008 6:32 pm ] |
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Don't worry, my usual calculator worked out a crazy high C/R too 
Big bathtubs for you
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| Author: | gafmo [ Wed Apr 23, 2008 6:52 pm ] |
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Where's Dr Mini when you need him |
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| Author: | jbeenz [ Wed Apr 23, 2008 6:56 pm ] |
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out with the angle grinder
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| Author: | DOZ [ Wed Apr 23, 2008 7:23 pm ] |
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You need to put a dish in the piston for a start, you only need squish at the edges of the piston. I would say 6-8cc should be attainable. Add to this your gasket and bit above the ring land before you start grinding the combustion chamber. Daniel |
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| Author: | TheMiniMan [ Wed Apr 23, 2008 9:50 pm ] |
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Allow 3.5cc for the gasket please The formula is combusted volume(CV) plus the swept volume (SV), then divided by the Combusted Volume (CV) so,,, seeing that CV divided by CV = One Now all we have to do is divide the SV by the CV ,,, & then add one (for the CV divided by CV bit) get it???? heaps of people muck it up & forget to add the one NOw!!!!!!!!!!!!!!!!!! In saying that you will definately want to CC (Volumetriclly measure) the space above the piston at TOP DEAD CENTRE,,, If there`s a tiny bit there then good ,,, add that into the chamber volume with the 3.5ccs for the head gasket Now,,, You will NEED to CC (Volumetrically measure) the cyl head chamber too, if there`s been "ANY" previous chamber griding &/or facing then you are all only guessing Do it right,,, buy a burrett off e-bay, the 50mm plactic ones are cheap as chips ($23) i paid for my last one get one of those, & cut a piece of flat perspex out to suit a little bit bigger than the bore, drill a hole in the guts of it for a filler hole,,, a smear of grease over the block & head faces & fill the dam things up & measure them both then & only then will you get any accuracy i do this crap probably more than anyone (maybe except GR) but still,,, this is how it`s done NO GUESSING !!!!!!!!!!!!! Class dissmissed 
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