My sudden interest in this subject happened on “The little run” when Putt Putt’s engine was detonating severely going up the Brookton Highway hilly bit. I knew she was subject to this problem and had previously retarded the ignition and fitted cold spark plugs. Alas, once she was working hard in hot weather, this retardation caused combustion chamber overheating, leading to detonation. We stopped in the National Park and adjusted the ignition back to normal, significantly reducing, but not eliminating the problem. Returning home, I started investigating the fuel needs of Citroëns, as the CX2500 and BX16TRS both ping severely when asked to really put out in hot weather.

The CX Glovebox Handbook specifies 97 to 98 octane fuel. The 1983 2CV Factory Manual specifies “Super” fuel for a high compression engine and “Standard” fuel for the other engines. This “Super” fuel is English 4 star 97 plus octane and the “Standard” fuel is 95 plus.

Fuels available from your local petrol station are:

1. Unleaded (ULP) 91 octane (RON method).
2. Lead Replacement (LRP) 96,
3. Premium Unleaded (PULP) 95.

At specialised outlets you can obtain Super Premium Unleaded (SPULP) rated at 97 plus octane.

Are your using the right fuel in your CX, BX or 2CV?

If you have a CX, BX, or high compression 2CV, you are most probably not, unless your into the habit of searching out more expensive special SPULP fuel like Shell Optimax or BP Ultimate.

Australia downgraded its common pump petrol octane ratings many years ago when leaded petrol disappeared, and its octane ratings above, are well below that needed for our beloved later model Citroën’s.

SO WHAT CAN YOU DO?

CX2500 IE & EFI BX’s

If you have these cars, ideally you should be using SPULP. There is nothing you can do when using normal PULP or LRP, except to back off the accelerator as soon as you hear detonation and to not use full power for an extended period. The whole shooting box which works these engines is done by pre programmed computers.

CX2200, CX2400, GS, Carburettor BX16 &19

Again, ideally you should be using SPULP. You could have the engine adjusted by retarding the static ignition timing, at the peril of piston overheating in prolonged high power use and/or in hot weather. This could be offset to a degree by putting in larger carburettor jets. A simple solution is to back off on the throttle when you hear detonation and to not use full power for an extended period.

2CV

The biggest challenge is with this little monster which needs every ounce of power it can develop and in which detonation is almost impossible to hear at “speed”, particularly on indifferent road surfaces.

If you have a 9:1 compression ratio engine and the luxury of a lube oil temperature gauge like I have, you will soon see the gauge moving from its normal 95C max to 130 C plus when using ULP for more than 15 minutes in 30C plus weather. Even with LRP and PULP, my 9:1 engine detonates in hot weather, retarding the ignition aggravates the situation once the engine has warmed up. One can only guess the increased and undesirable heat loading on the poor little pistons and their critical top ring lands.

It may be possible with ignition retardation and re-jetting of the carburettor to reduce this ULP piston overheating tendency if you have the know how and equipment far beyond my means needed to verify this. I am hearing stories that a 107.5 primary main jet instead of the normal 102 helps.

There are at least three sets of engine compression ratios available in the 2CV engine:

1) 9:1 Needing 97 octane SPULP
2) 8.5:1 Needing 96 Octane LRP or 95 Octane PULP
3) 7.9:1 which will run on common 91 Octane ULP.

How can I tell what my 2CV engine compression ratio is?

For an engine in good condition a compression test will show:

170 psig for a 9:1 engine.
150 psig. for an 8.5:1 engine
135 psig. I would hazard to guess about for a 7.9:1 engine

Alas, ascertaining compression pressure is an engine condition indicator, so you must be confident the engine being tested is in good condition first. If in doubt you will need to find the compression ratio which is stamped on the pistons inside the engine.

So there we have it folks, check your car manual for your fuel specification, get the highest octane available. “Take it Easy” on the gas pedal if you can when the engine detonates. Don’t be tempted to retard the ignition, it will bring overheating problems in hot weather.

Whatever you do (unless you have a 7.9;1 2CV) try not to use standard unleaded petrol in a late Michelin/early PSA era Citroën. If you are forced to use this ULP in these Citroëns, be very careful not to push the car to anywhere near its full potential for an extended time, especially in hot weather.

Regards - Chris