Working out compression ratio
mcerd1 - 15/8/16 at 01:45 PM
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 ?
dave_424 - 15/8/16 at 02:02 PM
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
mcerd1 - 15/8/16 at 02:21 PM
^^ 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]
britishtrident - 15/8/16 at 02:30 PM
Headgasket is normally just a tiny bit bigger than the bore size.
mcerd1 - 15/8/16 at 05:08 PM
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]
snapper - 15/8/16 at 07:24 PM
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
mcerd1 - 15/8/16 at 08:56 PM
the pistons are forged, so no worries there
ashg - 16/8/16 at 12:14 AM
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.
snapper - 16/8/16 at 05:30 AM
Athena or Cometic gasket of 1.3 to 1.5mm will do it and is the most simple way to achieve the drop
snapper - 16/8/16 at 05:35 AM
What can are you running?
AntonUK - 16/8/16 at 07:33 AM
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]
mcerd1 - 16/8/16 at 08:48 AM
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]
mcerd1 - 18/8/16 at 04:00 PM
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 ?
snapper - 18/8/16 at 04:58 PM
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
mcerd1 - 18/8/16 at 05:39 PM
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 ?
mcerd1 - 23/8/16 at 11:53 AM
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
