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Citroën A-Type Tuning

Citroën 2 Cylinder Engines

© Copyright: J.Cats.

Disclaimer: This page only provides information based on the personal opinion of the author. The author can't be held responsible for damage in any way that could result from modifications mentioned here.

Contents of this page:

Tuning the Citroën 602cc 2 cylinder engine

The big advantage of the 602 engine is that it fits in the car without any modifications.

Starting point for most people will be the A06/635 engine. These have the double choke carburetor as standard. An even better starting point is the AM2 engine as found in the Dyane and Ami. These have different pistons to give 9.0:1 compression ratio but also a higher lifting camshaft.

Airfilter:
On this a low restriction air filter is fitted (K&N, Jamex etc) with proper sump breather.
Ignition:
The standard ignition system is far from perfect so it's good to fit an electronic system. These are available in 2 types. One is the full electronic version such as the Luminition or 123 on which the points are completely removed. The other is the version on which the points are left but now only act as a switch. Both versions give an improved spark but neither will give any performance increase over standard ignition. The big advantage is that once the ignition timing is ok, you hardly need to look after it, (or not at all on the full electronic type). Optimum maximum total advance is a little over 33degrees.
Exhaust:
The exhaust is not as restrictive as many people think. A good indication is that Citroën used nearly the same system on the 53bhp Ami Super. Engines that still have their original camshaft don't need an uprated exhaust. Now depending on who you ask there are some options: For racing 2 into 1 systems are very popular. Here both pipes coming from the manifold and fed into a Y piece. Keep the sections between engine and Y piece equal length and quite long to give good low and mid range torque. Look at the french/belgian 24h racers for interesting exhaust design. Another option is a system where both sides are fully seperated. In most cases this will only give power losses, mostly in the lower rpm range and especially if no balance tube is fitted between the 2 sides. There is also a very short 2 into 1 section available to replace the crossbox. The same as above applies to this.
Flywheel:
The flywheel can be lightened and for a nice engine you are advised to do so. This will give improved acceleration but tractability on hills and in city traffic is reduced. Too much lightening will give you rough idling and less smooth running. Don't take off more then 1/3rd of the original weight to start with.
Compression:
Without any internal modifications around 32bhp should be possible on the 8.7:1 compression engine. Offcourse the 9.0:1 pistons as fitted to Dyane and Ami give a nice power and torque gain. The compression can be increased further by reducing the length of the barrels (machining a small amount of metall off the outer end of the barrel). 10.5:1 is a practical maximum and this will already need altered ignition timing to keep it from pinking. With the increased compression ratio and after removing all the casting marks from the heads you should have around 39bhp.
Camshaft:
The original camshaft has very little overlap so some improvements can be found in this area.

For the 602 engine the following displacement increases are possible:

  • Capacity: 650cc, Bore: 77mm
    There are a number of ways of builting this engine:
    1. Use the barrels, heads and pistons from the 652cc Visa/LNA engine and fitting these to the 602cc block. When using the 652 heads, these have to be modified to fit on the 602cc block/sump. The oil return pipes with the pushrods going through them have different connections on the block.
    2. Fitting the whole 652cc Citroën engine. See the next section for more info on this.
    3. Get a 650cc package available from 2cv tuners / specialists which can be bolted straight on.
    4. Make the package yourself.
      The 652cc engine has Nickasil coated alloy barrels. The 602cc engine has cast steel barrels. Because of this the original 652 piston rings and 602 barrels don't match, destroying the barrels very fast. So you get the Citroën 652cc pistons, either new or used. Take your 652cc pistons and a set of 602cc barrels to an engine rebuilter. They can bore the barrels to fit the pistons. They can easily make up new pistons rings suited for the barrels or adapt your pistons to take off the shelf rings. Yes, it's that easy.
  • Capacity: more then 650cc
    More then 650cc is also possible. 670cc with 78mm pistons has been done before but here the barrels get very thing and reliability is greatly reduced. Over 78mm special bigger barrels are required which need modification of the cylinder heads and crankcase to fit.

Note:
2cv barrels are not hardened as many people think. They are the same hardness through and through. Therefore overboring the barrels can be done without risk as long as the walls remain thick enough and the right piston rings are used.


Converting to a Citroën 652cc 2 cylinder engine

Citroën developed their own flat twin 652cc engine from the 602cc engine. This was fitted to the cheaper Visa and LNA models. Both cars had either the flat twin, fitted front to rear, or a four in line, fitted left to right. The flat twin from these cars can be quite easily fitted to any A-type. For more info on the 652cc conversion check out 652cc conversion.


Tuning the Citroën 652cc 2 cylinder engine

There are 3 basic 652 engine types:

  • V06/630, fitted to older LNA and Visa, low 9:1 compression but very good camshafts
  • V06/644, most common 652, 9.5:1 compression but "ECO" short overlap camshaft
  • V06/66?, last model Visa, suitable for 95 octane fuel, low 9:1 compression, ECO camshaft
A very good starting point is the combination of the 630 camshafts and 644 pistons. On some 652 engines the sparkplugs don't sit deep enough in the head to give good combustion. To improve this, longer shaft sparkplugs can be used or the seats modified to let the standard sparkplug sit deeper. Last point of easy improvement is porting the cylinderheads, especially on later engines the standard porting can be quite a bit off.

From here on the tuning possibilities are about the same as on the 602cc.

For the 652 engine the following displacement increases are available:

  • Capacity: 710cc, Bore: 80.5mm
    This is the maximum displacement increase that can be achieved by enlarging without altering the crankcase and cylinder heads.
  • Capacity: 750cc, Bore: 82.5mm
    With this displacement increase the barrels are replaced by larger ones. For this the cylinder heads and crankcase need to be altered.

Note:
Citroën 652cc barrels have a special Nikasil coating. Overboring will remove this coating and the alloy barrels will quickly disintegrate. Overboring these barrels (and recoating) is a specialist job as is honing these barrels. The Nikasil coating also requires special piston rings.


Selecting the gearbox

So how does gearbox tuning work?
There are 2 things for which a gearbox can optimized:

  • Top speed
  • Acceleration
Top speed

Tuning a gearbox for top speed means that in a given gear, the theoretical top speed is reached when the engine is running at maximum power rpm. The maximum power in your engine gives the car a theoretical top speed which is limited by the total drag of the car at that speed. In order to reach this top speed, the maximum power needs to coincide with this speed. The maximum power is only available at a certain rpm, so your overall gearing needs to match the maximum power rpm with the theoretical top speed. Anything else and your car will not reach it's theoretical top speed. So the common idea of just getting a longer ratio gearbox isn't always the right one. Also, the gear in which the car does top speed at maximum power rpm doesn't have to be top gear.

Some practical data on a normal 2cv6 1981-1990:
In 4th gear it does 20.2 km/h @ 1000 rpm. Maximum power is at 5750rpm which equals 116 km/h or 73 mph in 4th gear.
115km/h is the theoretical top speed for a 29bhp 2cv. So the power and gear ratios are very well matched. Citroën has clearly done a good job on this because either longer or shorter ratios decrease the top speed.

Now a special example of a tuned gearbox on a modified car:
We take an 2cv, with modded visa 650cc engine, ami super 4 speed gearbox and 135 tyres. The engine gives about 40bhp and gives the 2cv a theoretical top speed of 130 km/h. The 40bhp is delivered at 5500 rpm. At 5500 rpm in 4th gear I will be running 130 km/h, the car's theoretical top speed at the maximum power rpm. So this gearbox is a good match for this car and engine combination.

Usually, increasing power (tuning an engine) means that the top power is reached at a higher rpm. So you have to look at the power curve of your engine to see what gearbox ratios you need.

Acceleration

For optimal acceleration the following gearbox features are advantageous.

  • First gear is low enough for a proper launch, not too much wheel spin but no bogging down of the engine. Just between 3000-5000rpm and then side step the clutch.
  • All gears are equally spaced without too much overlap.
    No big holes between gears.
    No gears that are very much alike.
With these things in mind look at the speed through the rpm range of every gear on a certain gearbox and see if it fits your car.

Lets take a look at the different gearboxes in more detail:

  • 2cv gearbox.
    For sprinting the 2cv4 / Dyane 4 gearbox is the best suited one with very low ratios. For cruising, racing and the 652 conversion, high gear ratios are better suited. The most common gearbox to be used for this is the Dyane 6 / Ami 8 / Acadiane gearbox.
  • Weber 5 speed gearbox.
    Eugene Weber in Switzerland has built 5 speed gearboxes using 2cv casings. Gear ratios are to customer specification. His sons are/were continuing his work.
  • Visa / LNA / GS(A) gearbox.
    Especially with high torque tuned 652 engines, these gearboxes can be a good alternative. The various 4 speed gearboxes have slightly different ratios. For more info on using these gearboxes check out G series gearbox conversion.
  • General gearbox caution.
    Apart from the previous points, there is another point you have to consider. Will the gearbox you'd like to fit withstand your engine? The normal A-type gearbox will just about cope with a 652 engine. A tuned 652 engine is asking for troubles, the gearbox and clutch simply aren't strong enough. As soon as you start driving hard the gearbox will break.

Selecting the clutch

The 652 and 602 engines have always shared the clutch (plate and pressure plate) so for most 602 applications the standard clutch will be strong enough. If needed a sintered plate available from 2cv specialists. On the original plate the center of the plate can break away from the rest of the plate when driving hard (drag racing). The original pressure plate is strong enough, the diaphragm type is a little stronger then the 3 finger type.


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