Thats not allowed Ken! i've been sitting here for an hour now trying to figure this out on my fingers...

This is how I rationalise it...
2cv6 piston = 74mm dia = 7.4cm
Combustion chamber volume with 7.9:1 pistons = 43.63cc
So 43.63 / 7.4 = 5.9cm combustion chamber height
43.63 / 5.9 = 7.39cc per vertical mm (although as the chamber is hemispherical this is completely accurate)
How do you get 1mm = 4.31cc?

Oh btw, it took me fark'n ages to solve your equation:
(301+x)/x=8.5
which is x=602/15

Go easy on me, I have taken a few to the head over the years...


Sorry I should have said craters, from extinct volcanoes, thats where all, or lack there of, power is coming from
Here is my crater, I excavated it


reason being is that the piston maker pointed out the uneven burn pattern left behind from carbon build up and said this will create a more even flame front.
I also remember asking him how much I would have to add to the base of the barrel gasket, Visa engine, to decrease the CR to 8.5:1 from 9:1 I think he calculated 22thou = .56mm but again due to the aforementioned my memory is hazy...

I think I need to go lie down now...

Harley

Hi guys,

thank you all for you answers/replies - interesting to read your stories. I think I will stick to Ken's advice to mill 0.5 mm from te top of the barell. After mounting pistons and barrel I will take some lead wire which I will tape at the center of the piston running in 4 directions to the outside of the piston - after mounting the heads properly I will rotate the crankshaft carefully to TDC and a little beyond. After dismantling the heads I will be able to see if and howmuch clearance between piston and head. Perhaps I can also see if this gap is a little parallel.

What do you recon would be an acceptable size for the distance between piston and head ?? (I am reluctant to call this space the squishband as you notice)

in case Ken turns out to be wrong in his calculations (relax Ken !! this is only hypothetical off course ...) I will cut some rings to put on the bottom side of the barrel to adjust. I have a laser cutting machine and specialize in this precision metal, so I can take this from the shelf.

If this excercise will turn to one big mess I can always buy a new set of pistons & barrels and keep on enjoying driving my A-types ....

Cilinderhead : what can be done in terms of optimalisation with the cilinderheads ?? Yes, cleaning, grinding valves, new seals, but has any of you ever done something with the shape of the internal or with flowing inlet/outlet-side ??

Harley,
the problem seems to be that you've got a bit of confusion going on with your 'centimetres', 'square centimetres' and 'cubic centimetres'.

Q. How do you get 1mm = 4.31cc?

A. Cross sectional area of barrel = ( 3.142 x 7.4 x 7.4) / 4 = 43.01 sg. cm.
For each mm of barrel height, or piston movement, volume is 43.01 x 0.1 cubic cm, which is 4.301 cc. (Fat fingers with calculator on earlier posting? )

Q. Oh btw, it took me fark'n ages to solve your equation:
(301+x)/x=8.5, which is x=602/15

A. (301 + x ) / x = 8.5
Cross multiply, which gives (301 + x) = 8.5 x , then 301 = 7.5x, last step is x = 40.13


Anyway, here are some old school graphical short cuts, luckily it was common practice to work in cubic centimetres, not cubic inches, when this book was written by Phil Irving back in 1948...


Compression ratios, ignition timing & detonation. by slcchassis, on Flickr


p32 TfS by slcchassis, on Flickr



You're not alone, went to the pub late yesterday evening to celebrate completing the tax return.
So few folk in that the landlord decided to do a line clean, which meant beer at less than half price, so finally got home at gone 2 am, after a snowball fight between the drunks from the pub and a mob of youngsters who'd walked back from town because all of the taxis had stopped running.
Proper vicious when it comes to a snowball fight, some of these young lasses nowadays...

azuiper,
I doubt that you will have any problem with removing only 0.5mm from the barrel top spigots, as I've just remembered this advice from ECAS a few years back.

http://www.ecas.co.uk/2cv_tuning.htm#A10

Your idea for using lead wire (soft solder?) to check clearances is rather neat, mind you, much less messy than the wax castings I made in early days of building 2CV race engines...

ken

i thought plasticene was used to check clearances just put some little balls on the piston crown and turn it over TDC easier to compress than tin/lead/antimony.

Hi Sean,

For many years I used an MZ ET250 motorcycle as a daily driver to work. This is a 2-stroke engine. To check the squishband (which is an important issue for 2-stroke engines) I also used solder tin wire - you only had to remove the sparkplug, put the wire in, turn the crankshaft carefully till over TDC, take the wire out and measure the deformation of the wire. you didn't have to take the cilinderhead of for this measurement - makes the job as easy as can be.

I have no doubt plasticene (which is new to me) will work the same but you have to remove the cilinderhead twice for this ....

If you look at the shape of the cilinderhead of a 2cv with the sparkplug hole more or less in the center I think that it must be possible to do this measurement - I will give it a try and let you now how it turns out.