compression ratio

[raivz]

Can someone recalculate how many mm below the deck my piston needs to be in TDC to accomplish 8.5:1 compression ratio. Or near that. my calculations said that 1.7 is good. But can someone whos better at math can confirm my results.

[bmwman91]

Did you measure the combustion chamber volume / squish volume with the piston all the way up? Obviously that's not the easiest thing to do. If so, what did you get as your volume measurement?

[raivz]

I didnt mesure the volume.I calculated it. i'do know its not the  most accurate way but i can't take my engine apart just yet.
 i did this: stroke 81mm. bore 84mm. 1 cyl volume is 499cc. 

499/(10-1)= 55.4444444444   
this is chamber volume with HG.

I found the equation in turboclub forum.

Maybe i just need to measure

[bmwman91]

Haha yeah, ultimately if you are going for a forced induction build (which I assume you are given the low compression target), you should just measure things directly once the engine is apart.

[MLM]

Your calcs are pretty good. I have a spread sheet I use from real measurements. OEM is 9.94:1. Decking the piston 1.65mm gives 8.56:1.

So your 1.7mm was pretty much on the money.

Note: My calc (and yours) assumes you maintain the piston dish cc which isn't true if you just skim the piston.
The dish is 3.5mm at its deepest leaving you with some dish after skimming but not the full 7.4cc and therefore a higher CR when taking into account the changing volume sizes. For your calc to work you must maintain the original dish volume.

[raivz]

I don't know if skimming my pistons are so good idea. Orig piston compression height is 31.65.  Maybe it makes pistons  to weak. Or does someone done it before?

[varg]

Your calcs are pretty good. I have a spread sheet I use from real measurements. OEM is 9.94:1. Decking the piston 1.65mm gives 8.56:1.

So your 1.7mm was pretty much on the money.

Note: My calc (and yours) assumes you maintain the piston dish cc which isn't true if you just skim the piston.
The dish is 3.5mm at its deepest leaving you with some dish after skimming but not the full 7.4cc and therefore a higher CR when taking into account the changing volume sizes. For your calc to work you must maintain the original dish volume.

Would you mind sharing that spreadsheet? I'm still wondering exactly what my CR is with the .120" head gasket.

[varg]

I have been able to get results that make sense and fit what I've read about the stock stuff with this calculator: http://www.csgnetwork.com/compcalc.html

Using the following figures, some of which I found searching this forum.
Bore: 84
Stroke: 81
HG Bore: 85
Compressed HG: 1.73mm
Combustion chamber volume: 33.5cc
Piston dish volume(negative is dish): -7.4cc
Piston above deck height (negative is proud): -0.15mm

These figures yield a result of 9.99:1

Adding my .120" (3.05mm) MLS head gasket I get a not-exactly-what-I-wanted figure of 8.8:1. Based on this I'm considering buying and installing a 0.098" (2.49mm) MLS to bring my compression ratio up to 9.3:1, hoping for a significant increase in responsiveness and low end/spool.

If someone can chime in with verified figures, especially piston height and CC measurements, it would be much appreciated. I do not have access to a spare M42 to perform this on. The head gasket firgures are taken directly from my removed stock head gasket, however, I am measuring with harbor freight calipers, getting different measurements depending on where I measure and am not sure how much the head gasket grows when it is un-clamped.

(edit)
for future reference, I found someone's measurement of the combustion chamber at 27cc
still no consensus