I just heard a claim that fitting a new 9:1 set on an old 8,5:1 engine is bad for the crankshaft? How much can the slightly increased power affect the life of the crank?

Technical and theoretical maybe but realistically no.
The increased compression 8.5:1 to 9.0:1 will only slightly change the max compression without more pressure on the piston crown. If the claim is correct then the opposite should be the case on 435 and 425 engines with lower compression. The real issue is lack of service, running with ‘’old’’ engine oil and at the same time running the engine flat out.
Should mention that the barrels are the same. The difference is the piston crowns depth which is determine the compression ratio

Is there a difference between "Dyane/Ami" and "2cv" cranks from new? I've only ever seen one part available.

Also, do originally 9:1 engines tend to die of bottom-end failure more rapidly than 8.5:1? I've never heard of 2cv bottom ends going except for lubrication abuse.

Most of the people who'll know will be back on Monday...

No all the same




No big end failure are the same for both ratios




Interesting...!! My explaination is no good

My thoughts too...
My AMI 6 with the AM2 engine (602 with 9:1) had a busted big end bearing, but I have heard it's because this engine doesn't have a proper oil filter.
I think I will buy 9:1 instead of the 8,5:1 for my 2CV, when I need to change.

Everyones thoughts are welcome, and appreciated

Do the 9:1 cars work safe with 95 octane fuel?

I've got 9/1's in my '88 spesh engine.
Been in for 4000miles with no probs.
Engine had originally done 80,000m when it suffered a cracked head,scored barrel & piston and broken rings.
It seems to run OK on 95 octane,but I alternate each tankful of "95" with a tankful of "super" .....[some peeps have told me this aint neccessary ....but what the hell,it certainly wont do it any harm]
Theres more on the topic here ....
viewtopic.php?f=7&t=1747

hth
T.

Put the paranoia away. "Most"...

Pinging is more likely to happen with 9.0:1 than 8.5:1 on low octane fuel but correct service (new advanced weigths) should minimize that. Our Ami is running 9.0:1 (new set fitted 12 month ago) and running on octane 95 without any problems. Our 2cv (original engine 1990 8.5:1) are having pinging problems under load on 95 octane. I boil it down to weak weights. In any case should pinging be avoided. The piston gives up long before the crank shaft

EirikJ,
Citroen's owners' handbook for vehicles with both 9.0:1 and 8.5:1 engines recommended 4 star fuel of 97-99 octane, if I'm not mistaken.
Because of manufacturing tolerances in the ignition advance mechanism, you _may_ be OK running them on 95 octane fuel, but on the other hand, problems with pinking can still occur.

Peak cylinder pressure during normal combustion are around 5 times that at cranking speed, which translates to 875psi for a 9.0:1 engine compared to 750psi for an 8.5:1 engine.
Put simply, this is how the engine with a higher compression ratio produces more torque and power than its lower rated equivalent and if used to its full capacity, it also puts more load on other engine and drive train components...

However, if pinking occurs, 'spikes' of cylinder pressure can be around 15% to 20% higher than the above figures, with accompanying abnormally high engine (and oil) temperatures.

< http://www.google.co.uk/imgres?imgurl=h ... 29,r:0,s:0 >

So, fitting 8.5:1 pistons gives a larger safety margin against engine damage if you intend to run your engine on 95 octane unleaded fuel.

If you do fit 9.0:1 pistons, extra care must be taken to check maximum ignition advance, which can only be carried out reliably using a strobe.

ken

I thought 2cvs (with 8.5:1) were OK on 2* - 92RON - back in the day?