There are several ways to increase the boost on a biturbo, all of which need to be done carefully. Before I go into any more detail about boost here are a few things to consider:

1. Is the MABC (Maserati Automatic Boost Control) working correctly to begin with. Are the vacuum chambers of your distributor advance intact - along with the ball joint on their shaft? Is your ignition static timing correctly set? Are your valve clearances correct and set so that there is no rattle from the cam-boxes? Is the engine generally healthy?

2. Is the car running a correct fuel mix? If the mixture is running lean then increasing the boost is asking for trouble. A lean mixture combusting under a higher boost results in vastly increased EGT (Exhaust Gas Temperatures) and can wreck your engine.

3. Have you thought about the effects of increasing boost with respect to the exhaust gas temperature? If you have more fuel and air in the chamber you will have a hotter combustion and if the EGT gets too high you can wreck your engine.

4. Have you thought about the effects of increasing boost with respect to the intake manifold temperature? If the intake manifold temperature is too high it can lead to pre-ignition of the fuel (knock) and wreck your engine.

Points 3 and 4 can both be addressed by fitting a good pair of intercoolers and/or a direct water injection system (see www.aquamist.co.uk and find system 1S). The intercoolers fit in between the turbo compressor and the plenum chamber and they cool the air before it reaches the plenum. I fitted a pair of modified Spearco intercoolers to my 425 and fitted ducting from the radiator grill to the fronts of the intercoolers to force air through them. At 5500rpm in 3rd gear the temperature of the turbo compressed air was 100°C, the ambient (outside air) temperature was 25°C and the intercoolers dropped the turbocompressed air to 28°C. Mine are very efficient. By intercooling the air you also increase its density, i.e. there will be more atoms of oxygen in a given volume of air. By having a denser air intake you can 'squash' more oxygen into the engine which, in turn, leads to a better combustion. You must remember that air-to-air intercoolers MUST have a good flow of air through them to work. To get a good flow the car must be travelling at some speed (over 40 mph). Below this speed the intercoolers offer very little cooling effect.

Water injection works by injecting an atomised mist of water into the airflow, either before the carb or after. The tiny droplets of water absorb heat in the intake manifold and cool the intake charge. They also absorb enormous quantities of heat inside the combustion chamber and turn into superheated steam. This, without going into technical detail, is down to the high "Latent heat of evapororation" of water. Simply put - it takes a lot of heat energy to turn water from a liquid to a gas. The water mist thus removes heat from the combustion and carries it out of the exhaust resulting in cooler EGT. Another firm advantage, besides reduced EGT, is that water injection STOPS PINKING! And another advantage is that as the water is absorbing the heat and turning into steam it 'flattens' the combustion, i.e. it prolongs the burn. This means that instead of the exploding air/fuel mix thumping the top of the piston it firmly pushes it down the barrel. The superheated steam also keeps your plugs clean! I cannot sing the praises of water injection high enough.

A combination of intercoolers and water injection means that the intake is cooled at all speeds. From 0 to 40 mph the water injection take care of it and from 40 mph+ the intercoolers become efficient. The water injection in my car is set to inject at boost pressures in excess of 10 psi (standard pressure). It is not dependant on speed, it injects when the boost pressure rises over the factory set pressure.

As for point 1. You need to check your mixture. If anything, set it slightly rich. If you have fitted intercoolers you may want to consider re-jetting the carb to account for the denser air intake. Running a rich mix will result in a cooler EGT as the unburned fuel will evaporate in a similar way to the water droplets in my explanation above. A richer fuel is a way of preventing knock although it is wasteful.

I appear to have gone on a lot about intercooling and water injection but not even touched on boost yet! This is because it is SO important to prepare your engine for what you are about to put it through. Increasing the boost with a correctly set up, intercooled engine is no great trouble - but increasing the boost without thinking what you are doing can make it "POP"!!

So, how do you increase boost?

You can either fiddle the amount of air flow controlling the wastegates, "fudge" the MABC so it doesn't know whats going on or you can modify the MABC so it controls the boost at a higher pressure.

Fiddling the air flow to the wastegates is simple but crude. It is the way that most boy-racers tweak their Renault 5 Turbos and so on. Check out any car accesory catalogue (www.demon-tweaks.co.uk for example) and you'll find bleed valves that fit in the line between the AMAL valve and the pressurised feed from the turbo charger. You could even make your own (I've been there and done that). These valves effectively reduce the amount of controlling air being fed to the AMAL valve and, in turn, make the air to the wastegates less. This increases your boost. A variation on this is to play around with the restrictors in the amal tubing. Make the C restricor smaller / make the R restrictor bigger = less air to the wastegates = more boost. As an example of how it works; pull off the W pipe on the AMAL valve and take your car for a drive... you will experience the maximum boost possible from your turbochargers! DONT BLOW UP YOUR ENGINE / BE CAREFUL !!

Fudging the MABC is another way. The MABC uses various inputs from around the engine to judge how much to boost the car. If you fudge one of the inputs it doesn't quite know what is going on and will either shut the engine down or give you more boost. The pressure transducer is a very important controlling input. It is a pressure sensitive variable resistor and its resistance varies with pressure. The MABC is set for maximum (over-boost) when the car is being driven slowly or is at idle - this is the MABCs "Default" duty cycle. As the pressure increases the MABC allows it to peak for a short time and then reduces it to a steady controlled value. If the pressure transducer is fudged so that the MABC 'thinks' that the boost hasn't reached the permitted maximum it will control the wastegates to keep the boost rising. Short circuiting the pressure transducer with a length of wire takes it out of the equation all together and the MABC then allows permanent over-boost and stays in its default duty cycle - i.e. the turbos boost like crazy / like pulling the W pipe off the AMAL valve - BE CAREFUL!!

Finally the best way to increase the boost is to alter the MABC. The MABC works by sensing various inputs from the engine and then sending out a signal to the AMAL valve. This signal is a series of pulses and the way the control works is that the width of each pulse is changed. If there are 12 pulses per second, for example, and the width of the pulse is 1/12th sec then the pulse is almost one big one. If the width of each pulse is 1/24th sec then you could say that the pulses are only there half of the time. The AMAL valve operates on each pulse so if the width of the pulses is fat the valve will be "ON" more than if the width of the pulses are thin.

The AMAL valve has a pressurised air feed from the turbocharger on the C port. As it switches on and off it passes air down the W port to the wastegates and, alternatively, down the R port away from the wastegates. The more air that reaches the wastegate actuators the less boost the turbochargers will deliver.

The MABC is very similar to the APC used on 1980s SAAB turbo cars and some volvos. There has been some stuff published on the internet about tweaking saab APCs and I have learned a lot from these articles. The MABC has three potentiomaters on its circuit board, P, F, and K.

P= Percent (Solenoid "Default" duty cycle

F= Full Boost (Max boost vs RPM map)

K= Knock (Knock sensor sensitivity)

(http://web.inter.nl.net/users/turbo-team-europe/apc.htm is where I found all this information)

In my own words:

The MABC runs in its "default duty cycle" when you are not accelerating hard. Under this condition the MABC allows an 'over-boost'. In other words the controlling air from the AMAL valve to the wastegates is at a minimum and maximum possible boost is allowed.

As the pressure builds up to a certain point and the RPM reaches a certain level the computer switches to its "active control mode". The MABC then gets the AMAL valve to pass more controlling air down to the wastegates to reduce the amount of boost from the turbos.

It is reckoned that the P potentiometer controls the rate at which the boost rises in the default duty cycle and the F potentiometer controls the maximum level of boost in the active control duty cycle.

So effectively, P sets the peak boost and F sets the maximum constant boost level that the MABC settles at. To increase the boost by tweaking the MABC is a balancing act between the P and F potentiometers - If you are going to do this make sure you mark their initial positions with a scriber or small blob of paint so you can put them back to standard if you need to!!

The article I referred to earlier goes into some more detail and talks of the Saab "red-box" APC that was available to buy to increase the boost of the car. A bit like 'chipping' a modern computer. The author has played around with the red-box and tracked the changes to the orignal APC circuit design. From this he has then created a modification to the APC circuitry to change the way it works. Essentially his modification removes the F circuitry from the APC and alters the P circuitry so that the only control is the P. In other words, a potentiometer is placed on the dashboard of the car that controls the maximum boost.

I have done this mod on my car and it works. So I have the MABC in control but I set the boost. Its good this way because the MABC is still in control so if it senses that something is wrong (engine overspeed or knock for example) it can reduce the boost or cut the ignition as it normally would... but when nothing is wrong it permits as much boost as I want to 'dial in' with the knob on my dashboard!!

The circuit modifications are simple and comprise of:

a. Replacing 1 resistor with a different value

b. Soldering in a wire bridge between two points on the circuit board

c. Removing the P potentiometer and replacing it with a different value potentiometer mounted on the car's dashboard.

CLICK ON THIS IMAGE FOR A MORE DETAILED VIEW!