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Author: Subject: Working out compression ratio
mcerd1

posted on 15/8/16 at 01:45 PM Reply With Quote
Working out compression ratio

I've managed to find another 5min in the garage !

so I figured I'd best check my compression ratio properly - so I got some thick clear acrylic sheet, big syringes an some food colouring



and I make that about 42 cc in the combustion chamber (going to double checked /

the pistons are 93mm with a standard 76.95mm stroke = 522.7 cc

and I'm planning to use a 1mm thick gasket with a 94.5mm bore (so 7 cc)

so that makes the static CR ~ 11.67:1 - a bit more than I'd planned for RON97
but with a 5.06" rod length and ~63 inlet valve closing - so a dynamic CR of ~9.4:1


any thoughts ?





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dave_424

posted on 15/8/16 at 02:02 PM Reply With Quote
Do you have the CC of the piston dish/valve cutouts? also do you know if your pistons come up flush with the deck, or is there a deck height you haven't included?

http://www.csgnetwork.com/compcalc.html

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mcerd1

posted on 15/8/16 at 02:21 PM Reply With Quote
^^ oops

should have said its a pinto so the pistons are flat and its been decked to make them flush to the top of the block - so zero cc for both of them

[Edited on 15/8/2016 by mcerd1]





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britishtrident

posted on 15/8/16 at 02:30 PM Reply With Quote
Headgasket is normally just a tiny bit bigger than the bore size.





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mcerd1

posted on 15/8/16 at 05:08 PM Reply With Quote
quote:
Originally posted by britishtrident
Headgasket is normally just a tiny bit bigger than the bore size.

yes, but this isn't a standard bore size - its 93mm bore rather than 90.7mm so gasket choice is a bit more limited

so it'll be 2090cc rather than 1993cc

[Edited on 15/8/2016 by mcerd1]





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snapper

posted on 15/8/16 at 07:24 PM Reply With Quote
Done this several time, got several T shirts
You'll need a thicker gasket
If cast pistons then over 10.5 to 1 or 10.7 if your brave
To be safe retard ignition but that compromises power
Your choice is to open out the chambers a touch best unshroud the valves near chamber edge
And look for a thicker gasket which will cost over 100
The Adjusa gasket which I have used is ok but won't like 11 to 1
You can add 1cc of volume for the top ring depth and if using a 94.5 gasket that'll give another couple of cc over the standard 92.5 bore gaskets
I have made an excel spread sheet that takes in to account all the elements if you know bore, deck height etc





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mcerd1

posted on 15/8/16 at 08:56 PM Reply With Quote
the pistons are forged, so no worries there





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ashg

posted on 16/8/16 at 12:14 AM Reply With Quote
just stroke it :-) that will solve the problem :-) little bit of a pricey option lol.

if there is enough material get some valve recesses cut in the pistons that will drop the compression and let you run a bit more lift so a win win.





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snapper

posted on 16/8/16 at 05:30 AM Reply With Quote
Athena or Cometic gasket of 1.3 to 1.5mm will do it and is the most simple way to achieve the drop





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snapper

posted on 16/8/16 at 05:35 AM Reply With Quote
What can are you running?





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AntonUK

posted on 16/8/16 at 07:33 AM Reply With Quote
i ran 11.5:1 on an a-series lump no problem. always premium (97-98) fuel.

It was a daily drive too

[Edited on 16/8/16 by AntonUK]





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mcerd1

posted on 16/8/16 at 08:48 AM Reply With Quote
quote:
Originally posted by snapper
What can are you running?

I've got a piper 285 for now - on paper the inlet valve closes at 63

its also got bronze guides and I'll be running it on megajolt, so I should have plenty control over my ignition timing

I'm thinking that 11.67:1 is just a little high, but can't decide if a 1.3mm [0.051"] gasket would be safe enough as it would still give me 11.24:1

0.056" might be better at 11.06:1 and 0.060" would be getting pretty safe at 10.92:1 and safer yet at 0.066" and 10.73:1 - but at 82+ a go I don't really want to buy multiple gaskets to try.....


de-shrouding the valves may still be an option, but its a little outside my comfort zone - besides I may not keep this head forever (I have a spare in need of a recon and dreams of NA cossie heads too)
the 42cc chambers are probably quite a good size for bolting on to an otherwise standard engine


valve pockets aren't really an option as my pistons have already had the tops machined down to fit in the block with the longer cossie rods (back when I got them the 'long rod' versions of these pistons wasn't available)



[Edited on 16/8/2016 by mcerd1]





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mcerd1

posted on 18/8/16 at 04:00 PM Reply With Quote
I've just about convinced myself that a thicker gasket would be the sensible / easy option

Off the shelf the best I can find is 1.3mm [0.051"] which would be 11.24:1

I can get cometic ones made thicker, only problem is Burtons are looking for 130 for a custom order and a 6 week lead time


Still I'm half thinking that a 0.070" gasket might be a good bet at 10.6:1 - with a bit of luck that might even just be enough for RON95 on a good day......

Any other thoughts before break out the credit card ?





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snapper

posted on 18/8/16 at 04:58 PM Reply With Quote
You've engineered yourself into a corner
Gasket is your only option & 11.whatever to 1 is still high for a 285 cam RL31 better
I always keep a bottle of octaine booster in the boot





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mcerd1

posted on 18/8/16 at 05:39 PM Reply With Quote
quote:
Originally posted by snapper
You've engineered yourself into a corner

I know it
one little error in my spec when I was getting all the machining done....

what CR and cam are you using just now ?





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mcerd1

posted on 23/8/16 at 11:53 AM Reply With Quote
well the bullet has been bitten - a 0.075" thick 93.5mm bore gasket is on its way from the other side of the pond





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