compression ratios

[GT6 M]

This just aint as straight forads as it seems

every time I doo it, keep getting diff results

Butt,most likely I aint got the inputs bunged in
as cant find owt ont interweb that,ll help me.

Wot im trying to fathom is this

Chamber vol for a  46 cc
butt, take 40 thou off,  { 1 mm } fair enough, easy peasy

Butt, the chamber walls slope, so as ye go deeper into chamber, then the CCs will alter  faster{ go down } cos of  the slopes , so chamber is getting less,for same depth,
{ think i got it reet,!!!}



So, any Boffins on here tell me how much the chamber vol will alter
10 thou is roughly 1 cc on a streight sided thingy
so 40 thou would =4 cc,s ish

butt what would it be, tekn into consideration the slopes.

420  cyl share 46 chamber  = 9.14
wot would it be with 1 mm off, tek,n into account sloping  chamber walls.

OR, put another way, would a 40 thou skim, only need to be a 25 thou skim,

Answer here please.....

M

[TedTaylor]

It is very rare that you skim a head where the sides are sloping - they are usually still parallel at 40 thou.  In fact if you skim so much off that you are into the area where the chamber sides are sloping you would probably be taking too much off .......

MUT

[piman]

Hello Marcus,

because there is only a small section with a slope, it is quite a mathematical task to give a definitive formula that X skimmed equals y compression ratio. The practical way is to talk to someone who skims a lot of 2000 series heads, but I doubt that there are many (any?) doing that these days? It's fine if you can skim your own heads but quite a pain and expensive to keep taking it back to a machine shop until you get it right. An alternative, but it is not entirely accurate due to the meniscuc is to fill the chamber with the correct volume you want for your desired compression ratio and use a depth micrometer (or less accurate vernier calipers) to measure the depth to the liquid surface.

Ted, I think you've forgotten the Triumph head slopes to the face on the spark plug area?

Alec

[TedTaylor]

Yes Alec you are dead right and I had overlooked that - been thinking about other heads.

However as you say the section with the slope is small, and there are other factors in the head design.  I'll dig out a spare head when I have finished the roof of the garage (so that I can get on with AFH) and dig out a head and do some measuring.

I was thinking of what you suggested re filing the chamber with the volume you want.  While there is an issue with the miniscus I believe it is quite small with low RD liguids (petrol and similar) and I think that adding a tiny quantity of liguid soap to water it will case the miniscus to collapse as it will affect the surface tension of the water.

MUT

[GT6 M]

Ello Ted, Elec,
Ted says
you are dead right and I had overlooked that - been thinking about other heads.

Or, yer heed is full of rubbish like mine, so cant think at times, !!!
join the club.

I recon  it,ll be aboot 1.5 ccs
as spark side and squish side both slope.

1.5 ccs will make a fair bit of diff really

420 /46.0 = 9.14
420 /44.5 = 9.44

on top of what would be took off
so will err ont safe side, then check

M

[Hogie]

Hi Marcus,
              that is a challenge.  On my TR4A head the squish area tapers in bit the undercutting around the valves tapers out - ooh eck.

Roger

[Nick Jones]

I used graph paper with 1mm squares to measure the starting x-sectional area and then estimate the reducing area then final checking by cc'ing.  Valve heights will have more effect.

It's more significant on the shallower 2L heads as the squish ramp comes right to the head face as well on the spark plug side.

Nick

[GT6 M]

well  i did it t,day, and had to actually take out 37 thou,
as it had a bit of a dip in the middle of all the small bits between cyls
the most important bit for sealing

what Rodger was saying, i have found too
as where the valve wall was  opened to unshroud  valves

and gasket is now v v close to chamber sides

will cc it the morrow,
hoping for a 9.7 ish cr

M