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changing compressio rate
http://www.international2cvfriends.com/forum/viewtopic.php?f=7&t=4502		 Page 1 of 3
Author:  azuiper [ January 23rd, 2013, 1:11 am ]
Post subject:  changing compressio rate
Hi guys,

at the moment I am dismanteling a 2cv6 engine type A06/664 S6 from a German 2cv6 which I bought some time ago in a very bad condition - chassis already broken and 1/3 missing - carrosserie/coach, hood, fenders etc,etc totally rotten. So as I expected the only interesting parts remaining are engine, gearbox, driveshafts and some other small stuff.

taking the engine apart I was surprised that under a lot leaves, dirt en oil after 10 years of standing outside in all weatherconditions we have here in Holland the inside was in a remarkable good condition. According the dashboard the car had run some 90.000 km. Taking the cilinderheads and barrels off I removed the pistons. Pistons and barrels are fit for a second live as the honig-marks (my English in not that good - sorry) or whatever you call the diagonal scratches on the inside of the barrel where still present.

It's the pistons and barrels I have a question about : on top of the pistons you can read 7.9 - after some reading this matches with the last 2cv6 sold to Germany amongst others with the lower compression rate of 7.9 for environmental reason I far understand. After complete dismanteling this engine I am now measuring all the relevant parts as far as I can so I know what is still good and what has to be replaced.

when I'm up to putting the engine together I want to change the compression-rate from 7.9 to approximately 8.5 by machining some material from the top of the barrel. Reason : I want to use this overhauled engine in my Dyane with at the moment a fatique engine.

HOW MUCH SHOULD I REMOVE for this change in compression ratio ?? Does any of you goeroes have any information on that ?

Also I would like to know the differences in Dyane6 engine and a 2CV6 engine - I know a Dyane (1983) has a little higher compression rate 9.0, but is the camshaft the same ?? is the carburettor the same ??

I hope to find some usefull answers - thank ou in advance for your replies.
Author:  ken [ January 23rd, 2013, 2:33 am ]
Post subject:  Re: changing compressio rate
azuiper,
I had something similar recently with an Ami engine, for which someone had the bright idea to fit a set of those 7.9:1 pistons.
I haven't got round to doing anything with it yet, apart from the calculations, so here you go.

Combustion chamber volume with 7.9:1 pistons = 43.63cc, compared with 40.13cc for 8.5:1 pistons, which is 3.5cc difference.

Each millimetre removed from the barrel changes the combustion chamber volume by 4.301cc, so in theory, you'd need to shorten the barrels by about 0.8mm to increase the CR to 8.5:1.
However, I've got a feeling that this would result in the pistons coming into contact with the cylinder heads, so unless you do some work with candle wax moulding to check the clearance, it might be best to err on the side of safety and settle on 0.5mm reduction of the barrel height.

That would give 'around' 8.25:1 CR, so you can set the maximum ignition advance to 34*, which should be fine on 95 octane fuel.
Also, change the primary petrol jet from a 102.5 to 110, since Citroen used the same jetting for 'blown' induction engines as they did on 'normal' induction, whereas it should have been richer.
The camshaft on Dyane engines is the same as on 2CV engines, so nothing to worry about there... ;)

ken


azuiper wrote:
It's the pistons and barrels I have a question about : on top of the pistons you can read 7.9 - after some reading this matches with the last 2cv6 sold to Germany amongst others with the lower compression rate of 7.9 for environmental reason I far understand. After complete dismanteling this engine I am now measuring all the relevant parts as far as I can so I know what is still good and what has to be replaced.

when I'm up to putting the engine together I want to change the compression-rate from 7.9 to approximately 8.5 by machining some material from the top of the barrel. Reason : I want to use this overhauled engine in my Dyane with at the moment a fatique engine.

HOW MUCH SHOULD I REMOVE for this change in compression ratio ?? Does any of you goeroes have any information on that ?

Also I would like to know the differences in Dyane6 engine and a 2CV6 engine - I know a Dyane (1983) has a little higher compression rate 9.0, but is the camshaft the same ?? is the carburettor the same ??

I hope to find some usefull answers - thank ou in advance for your replies.
Author:  Harley [ January 23rd, 2013, 3:01 am ]
Post subject:  Re: changing compressio rate
Hi Azuiper

I see Ken got in before me, but here goes!

Calculating Compression ratio is better left to rocket scientists and witch doctors...

This article explains in-depth how and what it is: Article
And here is a calculator to accompany it: Calculator

Correct ratio pistons are more effective at their given ratio compared to shortened barrels, as this alters the combustion chambers 'squish area', a small area or ring around the edge of the combustion chamber. The performance engineers say this area must not be messed with, treat it like your in-laws, respect it but don't f@ck with it

I think the best would be to purchase a set of pistons with the required CR.

Harley
Author:  Sean [ January 23rd, 2013, 9:47 am ]
Post subject:  Re: changing compressio rate
just get a new set of barrels and pistons for the dyane (as far as a recon engine goes its a relativley cheap fix)

sell the low comp pistons to someone who thinks they want to fit a turbo to a 2cv :P
Author:  2CViking [ January 23rd, 2013, 11:45 am ]
Post subject:  Re: changing compressio rate
I have some used 9.0:1 pistons.

No farting around :mrgreen:
Author:  ken [ January 24th, 2013, 1:45 am ]
Post subject:  Re: changing compressio rate
Sheesh,
I guess Harley's account has been hacked, as this doesn't sound like the same person who twin-plugged Visa cylinder heads and had a set of special pistons machined for the same engine, not to mention such odd diversions as channeling a 2CV or converting the track rod ends to rose joints... :roll:
http://snail.s4.bizhat.com/viewtopic.ph ... orum=snail

As regards calculating the combustion chamber volume of a 2CV engine, the equation is so simple it's probably within the capability of a primary school child, so there's no need for either rocket science or witchcraft.
Can it be any easier than this?
Solve (301 + x) / x = 8.5

Lastly, probably worth mentioning that a four stroke engine with hemispherical heads rarely has squish bands.
A 2CV engine certainly doesn't, so it's going to quite tricky to f@ck with something which doesn’t exist. ;)

ken

http://books.google.co.uk/books?id=-kBj ... nd&f=false


http://www.ducatitech.com/2v/maint/rebu ... dwork.html


Harley wrote:
Hi Azuiper

I see Ken got in before me, but here goes!

Calculating Compression ratio is better left to rocket scientists and witch doctors...

This article explains in-depth how and what it is: Article
And here is a calculator to accompany it: Calculator

Correct ratio pistons are more effective at their given ratio compared to shortened barrels, as this alters the combustion chambers 'squish area', a small area or ring around the edge of the combustion chamber. The performance engineers say this area must not be messed with, treat it like your in-laws, respect it but don't f@ck with it

I think the best would be to purchase a set of pistons with the required CR.

Harley
Author:  PatrickMason [ January 24th, 2013, 3:09 am ]
Post subject:  Re: changing compressio rate
What happened to your 2cv Harley?!
Author:  Harley [ January 24th, 2013, 4:09 am ]
Post subject:  Re: changing compressio rate
PatrickMason wrote:
What happened to your 2cv Harley?!

:lol: Ken knows me better than I know myself sometimes :lol:

ken wrote:
Lastly, probably worth mentioning that a four stroke engine with hemispherical heads rarely has squish bands.
A 2CV engine certainly doesn't, so it's going to quite tricky to f@ck with something which doesn’t exist. ;)

Well love me tender and call me Elvis, I just had a closer look at the Visa pistons and head, there is no clear squish band, BUT! due to the volcano type shape of the piston crown I do believe it exists, see this link for a bit of a similar pic and explanation: http://www.nrhsperformance.com/tech_squish.shtml

I beleive in voodoo and witchcraft, it's how i've been building my 2cv :twisted:

BTW Kens' linked articles are better than mine! nice find.

Harley
Author:  Sean [ January 24th, 2013, 9:34 am ]
Post subject:  Re: changing compressio rate
looking at how the original pistons and heads mate there is a degree of "squish" around the slope of Harleys volcano.

its a bit "angels on a pin" for the OP who just needs to get a set of original pistons fitted ;)
Author:  PatrickMason [ January 24th, 2013, 10:44 am ]
Post subject:  Re: changing compressio rate
PatrickMason wrote:
What happened to your 2cv Harley?!
To clarify - I read through your Kermit thread on the Snail forum and saw your first post in the new year...?
Harley wrote:
:lol: Ken knows me better than I know myself sometimes :lol:
I think he knows most things...!
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