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:
- August 1972 onwards the 1015 is modified to overcome oil surge problems. Avoid the early engines.
- July 1981 onwards the rubber hoses on the right hand (2 piece) intake tubes are no longer fitted on both G11 and G13.
- July 1981 onwards all 1300cc engines are fitted with electronic ignition (Hall effect sensor in distributor).
Before you consider the flat 4 conversion, there are a few things you should know:
- Fitting a flat four involves a lot of modifications, no matter how you do it.
The 2 cylinder engines fit in the car without any modifications.
- The 2 cylinder engine is very light.
Since the engine sits in front of the front wheels, any increase in engine weight will deteriorate the handling.
In 1973 Citroën introduced their own flat four conversion, called the Ami Super.
This is an Ami 8 with modified chassis to take the modified GS 1015 engine and gearbox.
If you still want to go for the 4 cylinder engine, you should start by getting a Citroën flat four (GS, GSA or Axel) engine.
But fitting such an engine in a 2cv means you have a lot more to change than just the engine.
Depending on the configuration you choose, the chassis and / or body will need modifications.
The inner wings will always need modifications because of the larger width of the 4 cylinder engine.
The width of a GS(A) engine can be reduced by fitting Axel manifolds.
Using the 2cv chassis
There is one big problem with this.
The chassis isn't strong enough. An Ami Super chassis is made from 1.2mm steel.
The 2cv chassis is made from 0.8mm steel.
This problem can be overcome by fitting one of the tubular replacement chassis available (such as the Wheels/SLC one).
These are already stronger then the original, on request they can be made even stronger.
Another problem is that the chassis will need an additional cross member.
On the 2 cylinder engines, the engine mounts are on the front of the engine.
On a GS(A) engine the engine mounts are on the back of the engine.
Try and get the engine and gearbox as far back as possible.
This will improve handling and reduce the amount of modifications needed to the bonnet and wings.
But make sure the steering rods clear the gearbox in all positions.
With the 2cv chassis you have the choice of 2 types of gearboxes:
Using an Ami Super chassis and Ami Super/GS(A)/Axel 4speed gearbox
- 2cv or similar gearbox.
This isn't really an option because this gearbox has difficulties withstanding the torque of a slightly tuned 652cc engine.
It will not withstand the torque of a flat four engine.
That's one of the main reasons why Citroën gave the Ami Super a GS gearbox and didn't use the normal Ami 8 one.
When using this gearbox, apart from the inner wings, no or few other modifications are needed.
The modifications depend on type of car and how far back the engine is.
- GS(A)/Ami Super/Axel gearbox.
Ami Super, GS(A) and Axel 4 speed gearboxes are completely interchangeable.
The engine will move forward because of the increased gearbox length when compared to the 2cv gearbox.
The gearbox support has to be looked at.
One option is to use the original 602 support but this will give problems with the bigger weight and torque of the engine.
It's better to make a support on the rear cross member using the original mount on the gearbox.
This is what Citroën did on the Ami Super chassis.
Depending on the type of car more modifications then just the inner wings have to be made:
2cv: the bonnet and wings will have to be lengthened by 10cm or 4 inches.
Dyane: everything will fit under the bonnet when the frame in front of the engine is removed.
Mehari: everything will fit under the bonnet.
Ami: everything will fit under the bonnet.
This chassis is originally equipped with the modified GS 4 speed gearbox and the modified GS 1015cc engine.
All Ami chassis have longer and bigger spring retainers when compared to the 2cv chassis.
The spring retainers stick out above (and below) the chassis and are longer then those fitted to the 2cv, Dyane and Mehari.
The floors on these cars need humps in the floor and the reinforcements underneath the floors have to be modified to clear the spring retainers.
The AK, AKS and Acadiane vans have a chassis with similar spring retainers.
The 1015 engine can be replaced by any Citroën GS(A)/Axel flat four engine.
The Ami Super, GS(A) and Axel 4 speed gearboxes are completely interchangeable.
The Ami Super was only built between 1973 and 1976 so they are now quite rare and rust can be a big problem.
These chassis (even equipped to take normal 2cv springs pots) are available from some replacement chassis suppliers.
With the Ami Super chassis and Ami Super/GS(A)/Axel 4speed gearbox, you will need to use the Ami Super driveshafts.
Ami Super driveshafts are unique.
However, they can be replaced by shortened GSA ones.
There are 2 different type GS(A) driveshafts.
One has the same number of splines on the wheel side as the Ami Super / 2cv driveshaft.
The other has a different number of splines.
With these last driveshafts, the GS(A) driveshaft/wheel retainer in which the driveshaft goes at the wheel end has to be replaced by a GS(A) one as well.
Combining GS(A)/Axel engine and Ami Super/GS(A)/Axel 4speed gearbox
In between the engine and the gearbox sits one final component, the bell housing.
The bell housing fits over the flywheel and clutch and carries the clutch mechanism.
As the bell housing gets longer, the engine moves further forward (at the rear there is no room because of the front axle).
The more the engine moves forward, the handling of the car gets worse and more modifications to the nose are needed.
There are 3 bell housing options:
Using the 5 speed GSA or Axel gearbox
- Ami Super bell housing.
This is the shortest one.
When using the Ami Super bell housing you'll also need the following parts:
If one of these components isn't available, move to the next option.
- Ami Super input shaft (sticks out of the gearbox/bell housing).
- Ami Super flywheel.
- Ami Super or Axel engine mounts.
- Axel bell housing.
This is 10mm longer then the Ami Super one to fit over the thicker flywheel.
When using the Axel bell housing you'll also need the following parts:
If one of these components isn't available, move to the next option.
- Axel input shaft (sticks out of the gearbox/bell housing).
- Axel or GS(A) flywheel.
- Ami Super or Axel engine mounts.
- GS(A) bell housing.
This is several cm longer then the Axel bell housing.
When using the GS(A) bell housing you'll also need the following parts:
Note: The VERY early GS bell housing (pre 1973 I think) is identical to the Ami Super bell housing, and then so are input shaft, flywheel and engine mounts.
- GS(A) input shaft (sticks out of the gearbox/bell housing).
- GS(A) or Axel flywheel.
- GS(A) engine mounts (Axel/Ami Super fit as well).
The GS(A) / Axel 4 speed gearbox can be replaced by the GS(A) / Axel 5 speed gearbox.
This gearbox is about 10mm longer then the 4 speed gearbox measured from the rear mount to the end of the gearbox / start of the bell housing.
However, the 5th gear is in the rear lid.
So in order for the steering rods to clear the rear gearbox lid, the gearbox (and engine) has to be moved forward about 50mm.
With the Ami Super bell housing the following modifications are needed:
5 speed gearbox caution
2cv: 15cm or 6 inches lengthening of the bonnet and wings.
Dyane: 5cm / 2 inches lengthening of the bonnet and wings.
Mehari: 5cm / 2 inches lengthening of the bonnet and wings.
Ami: 5cm / 2 inches lengthening of the bonnet and wings.
With the Axel or GSA bell housing fitted, even more lengthening will be needed.
There is of course a very easy solution to the gearbox problem.
Lifting the gearbox onto the front axle instead of trying to fit it in front of the front axle.
This can be and has been done but there are some point which must be considered:
- Both the gearbox and engine can get oil surge problems when going through long corners.
Oil surge means the engine and gearbox will not be lubricated.
The engine oil pump won't have any oil because this has moved away from the pickup.
Over filling the engine oil will put oil in the (front) barrels.
The increased sump pressure will blow all seals out of the engine.
The gearbox will have no lubrication on the back cogs because all the oil is in the front of the gearbox.
Over filling the gearbox oil also gives seal problems, especially on the front one, giving an oily clutch plate.
- The increased angles on the driveshafts can reduce driveshaft life.
This is already a problem with these cars.
- Some problems can be solved by lifting the engine at the front as well.
However, putting all that weight higher up on the chassis will greatly deteriorate the handling.
- When lifting the engine onto the axle a hole has to be cut in the footwell.
This is originally the strongest section fo the front of the body.
So a hole cannot be cut without additonal strengthening to the body.
Running the GS(A) engine without hydraulic pump
When using a GS(A) engine there is one small problem.
A hydraulic pump is fitted and this has to be removed.
To do this:
- Unbolt the pump. It slides right off.
- Take off the small piston by pushing out the little pin.
- Remove the oil pump. The bolts can be reached through the pulley.
The pump never wants to come out easily. Make sure all bolts have been removed.
- Take out the little hydraulic pump con-rod.
- There is a small hole on the oil pump shaft to lubricate the con-rod.
- Just leaving out the con-rod gives oil pressure loss.
Fit an Axel oil pump. This doesn't have the lubrication hole.
Have a ring made that press-fits over the shaft to tightly cover the hole.
Cut the rod bit off the con-rod to leave a ring and put that ring back in the pump, covering the hole as per original.
- Cut the flange from the hydraulic pump.
- Weld up the hole in the middle. Make sure it properly seals.
- Fit the flange on the engine.
Normally when using the Ami Super chassis, Ami Super driveshafts are used.
However, these have always been problematic even on the original Ami Super.
When using these in a more demanding enviroment it soon becomes clear that they are not up to the job.
An option is to use normal 2cv driveshafts.
These can even be fitted to the Ami Super front axle without modifications.
Adaptors can be made up but are also commercially available to fit the 2cv driveshafts to the GS(A)/Ami Super/Axel gearbox and brakes.
The different engines
The 1015 has 1 advantage, it's less wide then the other 3 engine and is the only one that will fit the ami super chassis without modifications.
Apart from that there are only disadvantages, low torque, high revving but breaks easily.
Because of it's small capacity this engine is often skipped but it should be given a lot of thought.
It's the strongest engine of all, can rev over 7000rpm without falling apart, has big port/valves heads and produces plenty of torque and power.
No known problems, bulletproof even when abused.
Very flexible engine, well suited to 4 speed gearboxes which speeds up reaction times a lot and makes the car feel even quicker then it already is.
Not much wrong with it.
Doesn't have the high revving capability and the flexibility of the 1130.
Misses the all out power of the 1300.
Quite highly tuned by Citroën (X2) so not much more power easily available.
Evidently the most power of all, but more important loads of torque.
There are 3 different types 1300 but there is very little between them.
Unfortunatly not high revving so 5 speed gearbox is needed.
Lots of tuning potential, especially in the heads.
The myth of the GSA X3 engine
Everybody seems to be looking for a GSA X3 engine, or has a X3 stashed away for conversion/tuning purposes.
Newsflash: There is no X3 engine!
All GSA X3 were fitted with the normal GSA engine of the time, either the G13/625 or G13/646 (ECO!) engine.
So what is so special about the X3?
It has a short ratio 5 speed gearbox.
This gives the car some improved acceleration if you are willing to keep changing gears all the time.
It is unsuitable for high speeds as it over-revs the engine.
In standard form the 1130 is the nicest engine of them all.
However if more power is needed, bigger capacity is the way to go.
By just using the standard Citroën components from the engines, quite a performance increase can be achieved.
This package / combination of parts is also the optimal starting point for further modifications.
Crankshafts for 1130, 1220 and 1300 are identical but the 1130 uses main bearings from different material, more suitable for high rpm.
Bigger capacity is best so go for 1300 barrels and pistons.
Without modifications, these only fit on a 1300 crankcase.
- Cylinder heads.
The 1130 and 1220 heads have a much bigger inlet port then the 1300.
To use 1130/1220 heads on the 1300 cylinders, the edge where the cylinder sits in the head has to be enlarged.
Some metal has to be removed from the outer edge of the combustion chamber as well for the 1300 piston to clear the 1130/1220 head.
These modifications are easiest on the 1220 heads.
The 1300 has 38mm intake and 35.7mm exhaust valves.
The 1015, 1130 and 1220 have 39mm intake and 34mm exhaust valves.
So if you have the 1130 or 1220 head, optimal valve sizes are 39mm intake and 35.7mm exhaust.
Your local engine rebuilter will be able to put in even bigger ones (40mm inlet, 36mm exhaust).
The best camshafts are the camshafts from the G12/619 GS X2 engine.
However, these camshafts are very hard to find.
The next best thing is to use the camshafts that come with 1130 or 1220 heads.
ECO camshafts have rockers with a trapezium head.
Other camshafts have square head rockers.
Don't mix camshaft and rockers.
- Intake manifold.
From July 1981 onwards there are no more rubber hoses on the right hand side pipes.
Manifolds without rubber hoses should be used.
There is no performance difference between Solex and Weber carburetors.
Any carburetor properly adjusted / jetted for the engine will give optimal performance.
An engine built from the parts described above, with rejetted carburetor, large bore low back pressure exhaust system and low resistance air filter should make over 75bhp with a very wide torque spread.
For the GS(A) (1300) engine the following displacement increases are possible:
- Capacity: 1400cc, Bore: 82-83mm
This uses VW aircooled 1500cc (oversize) pistons (not quite sure of this).
The original Citroën barrels are bored to take these pistons so no modifications on crankcase and heads are needed.
However, the thin walls of the barrels will reduce reliability.
The outside barrel diameter (1300) is 86mm, so with a 83mm bore the wall is only 1.5mm thick!
- Capacity: 1600cc, Bore: 88mm
This uses a popular VW aircooled upgrading kit with special 88mm (forged) pistons and barrels (?).
To fit this kit both crankcase and heads have to be machined making room for the bigger barrels.
The 1300 crankcase has 86mm holes for the cylinders.
There isn't much room between the cylinder to go much wider, and you'll run into the head studs pretty soon anyway.
88mm is therefore also the maximum possible.
The VW 88mm kit exists in 2 types.
One is the "slip-in" kit with smaller outside diameter of the cylinder, this has a thin cylinder wall but reduces the amount of metal which needs to be removed from crankcase and heads.
The other is the thicker wall cylinder kit, where much more metal needs to be removed from crankcase and heads.
GS(A) / Axel 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 when the walls remain thick enough and the right piston rings are used.
So how does gearbox tuning work?
There are 2 things for which a gearbox can optimized:
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 Ami Super, with Axel 1300cc engine, GSA long ratio 5 speed gearbox and 145 tyres.
The 61bhp of the Axel engine gives the Ami a theoretical top speed of 150 km/h.
The 61bhp is delivered at 5500 rpm.
At 5500 rpm in 5th gear it would be running 185 km/h, but the car will never reach 185 km/h with only 61bhp.
At 5500 rpm in 4th gear I will be running 149 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.
The car can cruise in 5th but will not reach 150km/h.
To reach 150 km/h (or when power is needed to go uphill for example) a change back to 4th is needed.
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.
For optimal acceleration the following gearbox features are advantageous.
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.
- 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.
This gearbox is not suited for the 4 cylinder engine, there is way too much torque and this will break the gearbox.
- GS(A) gearbox.
When fitting a flat four engine, most people will start with the Ami Super or GS(A) 4 speed gearbox.
The various 4 speed gearboxes have slightly different ratios.
The GSA 5 speed gearbox is basically a GSA 4 speed with a 5th gear put on top.
This 5th gear is located in the gearbox back cover.
When fitting this gearbox, the engine and gearbox will have to be moved forward for the gearbox (cover) to clear the steering rods.
Apart from that, the GSA 5 speed gearbox is also 1cm longer then a GS(A) 4 speed gearbox, moving the engine further forward even more.
From my own experience I can say the 5 speed box (with 145 tyres) provides pleasant cruising at low rpm (low noise) levels.
But as soon as power or acceleration is asked for, a change to 4th or 3rd is called for.
- 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 Visa engine.
A tuned Visa 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.
An A-type gearbox with a 4 cylinder engine is an absolute no-go.
After having experienced the torque a 4 cylinder GS(A)/Axel engine can produce, you'll know an A-type gearbox just won't cope.
Nevetheless, quite a number of people have tried, none that I know of managed to keep the gearbox in one piece.
The original GS(A)/Axel clutch will stand up to 90bhp (or to be precies, the torque produced at those power levels).
More power/torque will inevitably introduce clutch slip.
I haven't found an uprated clutch / supplier yet.
A-type Tuning Index
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