- Citroën 602cc engine tuning
- Citroën 652cc engine conversion
- Citroën 652cc engine tuning
- Gearbox
- Clutch
## Tuning the Citroën 602cc 2 cylinder engineThe 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.
For the 602 engine the following displacement increases are possible: - Capacity: 650cc, Bore: 77mm
There are a number of ways of builting this engine:- 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.
- Fitting the whole 652cc Citroën engine. See the next section for more info on this.
- Get a 650cc package available from 2cv tuners / specialists which can be bolted straight on.
- 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: ## Converting to a Citroën 652cc 2 cylinder engineCitroën 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
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: ## Selecting the gearbox
So how does gearbox tuning work? - 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:
Now a special example of a tuned gearbox on a modified car: 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.
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 clutchThe 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. |