AXT Turbo Pty Ltd

If you are thinking of purchasing a turbo to suit a 3B , we suggest you read the following :

The 3B engine design lends itself to turbocharging , but it must be treated appropriately to gain the most longevity, reliability and of course power output.

The 3B engine has been built to survive some of the harshest climates on the face of the planet.

In Australia, we see the extremes of hot weather, as well as the cool sub zero's in our winter months.

In testament to the original engine design, excellent examples are readily found some 30 years later.

When considering turbocharging the 3B engine, it is important to recognise the overall design, to plan the appropriate type of system to suit this unique engine.

The basic engine design of the 3B is a crossplane head, fitted to a high compression block.

The pistons are normal diesel, heavy duty cast alloy units.

The crank is a typical steel unit .

As this engine was originally designed in the 70's, the design does not include any additional combustion chamber cooling that is common-place on later model engines.

The reason for this is in standard trim, the combustion temperatures are quite low, with very little backpressure to retain heat. The common way to remove heat from the combustion chamber at the time of designing this engine, was to run a very large heavy piston skirt, to act as a "heat-sink".

This limits the engine's ability to run at high RPM and also increases the frictional loses as well as increasing the overall inertia.

The problem with making more power, is that for an increase in power to occur, the engine must either make more heat ( through a larger combustion burn ), rev harder and have more friction.

In the 3B, the engine design leads to a significant overhead, creating the potential to have a large rise in potential power, as long as certain aspects are kept within limits.

The main limiting factor of the 3B engine is the pistons.

The standard cast pistons are more than strong enough to cater for increasing the combustion pressures, however the ability to expel excess heat is limited.

The large piston skirts do a great job, however, they are not like later model designs.

Later model designs have small oil squirters underneath the piston, which spray a small amount of oil on the back of the skirts and piston crown. This dramatically reduces the temperature of the piston, meaning hotter
combustion can occur.

Unless we are prepared to dis-assemble the engine and re-design this weakness, then we need to work out other ways of reducing the combustion temperature, yet still gain power.

The first way to do this, is to literally extract the hot combustion gases from the chamber very quickly, so the heat never has a chance to build up. The next step in this process is to ensure the gas velocity does not slow down at any point, and produce interference.

The problem with a turbo, is that not only is it close to the exhaust ports, it also requires exhaust pressure to drive it.

AXT Turbo has spent copious amounts in both money and time, developing a suitable flow, that does not add significant combustion temperatures, or more importantly, combustion backpressure.

The AXT Turbo system manifold is quite a complex design, with very little flow reversion, so once exhaust gas exits the port, the backpressures can never force it to flow in the wrong direction.

Once the gases hit the turbo, they tend to want to flow through the turbocharger, as it is the path of least resistance. Using a unique turbocharger exhaust housing and wheel combination, the turbocharger unit is able to spool very rapidly, to provide an end sensation of simple torque amplification.
( Not the typical jap car feeling of nothing, nothing, wohaa there we go ).

Part of the turbo design process is ensuring the largest diameter exhaust wheel is used where possible.

The exhaust wheel of the turbocharger, is basically like a lever turning a shaft. The longer and bigger the lever, the less effort is needed to turn the shaft. Using a large wheel combined with a housing that accelerates the gas flow, leads to a smooth flowing turbocharger, that actually aids the combustion processes, without introducing any extra heat or back-pressure to the overall combustion process.

The next part of the process is quite simple. Use a liquid cooled centre cartridge...

The reduction of heat being transferred from the exhaust end to the compressor end of the turbo, immediately reduces the compressor outlet temperatures quite considerably. The net effect of this, is that with cooler intake temperatures, cooler combustion occurs.

The next advantage of a liquid cooled centre cartridge, is that the oil lubricant to the bearings never gets hot enough to start burning or "coking" up on the bearing surfaces. Oil starts to burn above 250DegC, so as long as it stays below this point , it can never burn. Typical diesel exhaust gas temperatures can be well over 300 degC , so without any water-cooling, you can eventually expect the turbocharger centre to become overwhelmed with carbon, generally causing a failure in the long term....

The next part of the design process, is to ensure the compressor stage of the turbocharger actually matches the flow requirements of the engine it is to be feeding. Incorrect compressor selection leads to both turbo lag and increases in combustion temperatures, through increased intake heat. The compressor design is often overlooked, with many people believing that the compressor side of the turbo can really only be too big or too small for the application. Much more importantly, is that the compressor housing sizing matches the peak flows required, but also the wheel design allows accurate flow to match.

Far too often AXT see's customers who have decided to try generic "off the shelf" turbo's. Inevitably we see the end result of these, in the form of catastrophic engine failure caused by;

Too much retained heat in the combustion chamber...

Further to this, they show us the Pyrometer fitted to the vehicle , that showed the exhaust gas temperatures were never getting very hot. This is basically because, just like any furnace or a kiln, the temperatures inside the combustion chamber are dramatically hotter than on the exhaust. The heat is insulated and retained by the rest of the engine components.

Although we appreciate that there may be some initial cost savings by trying to make your own kit, we highly advise that , through our knowledge gained in the turbo design process, this is one engine that must be finely matched, to gain optimum power AND reliability...

Finally, if any of your mates are suggesting bolting a GT28 ( Generic Garrett Turbocharger, originally designed for the SR20DET 2.0L Petrol Engine ) they should also be offering to pay for your engine rebuild
WHEN it becomes needed...

We hope this article has given you some insight into AXT Turbo's Turbocharger System design process...