Twincharging your engine
Volkswagen were one of the first mainstream car companies to pioneer and introduce twin charging, although Lancia pioneered a Twincharger set up back in the 80's on the Delta S4 rally car.
What is twincharging and why is it beneficial? Are there any downsides?
Twincharging is where both a supercharger and a turbo are fitted to an engine.
The merits of superchargers lie in the fact that they produce linear power gains in relation to the RPM of the engine, mimicking a large capacity engine. Superchargers can run out of steam at higher RPM levels which is where a turbo charger comes into its own.
A turbocharger takes a while to get going as it needs a good flow of exhaust gases but when it does it can make large power gains from mid range right up to the high end of the RPM range.
When you have both fitted you quite literally have the best of both worlds, lots of low down power and plenty of extra boost in high RPM levels. The reason for doing this is that in applications where you need loads of boost and not a lot of air flow like in diesel engines (due to lack of revs) and small petrol engines you need forced induction.
No turbo will flow 30 lbs/min at 40 psi, so you use 2 to make it up and keep both of the turbos or supercharger and turbo operating efficiently, you have the smaller of the 2 making loads of boost pressure and the bigger being able to flow the total volume of air not just half, it needs to flow all of it. So put the larger/better flowing unit nearest the engine and leave the smaller compressor near the air intake.
Thanks to compounding, the effect of two stages of compression will actually mean the air compression is much higher than that just adding the two compressors boost amounts together. For this reason you can get a substantial boost increase from relatively low cost components. (10psi Turbo + 10psi supercharger = just under 30psi of boost!)
Most Twincharging applications will follow a few common routes.
Asynchronous or Series, where both work together simultaneously. This is probably one of the easiest set ups to build.
Then we have the inline or Parallel setups, as used by the VAG group where the supercharger provides boost at low RPM and the turbo takes over at a set point into the higher end of the RPM range. A diverter or electronic relay is used to cut between the supercharger and turbocharger.
Cheap superchargers and turbos can be found in your local breakers yard. The trick is getting them plumbed into your engine and working effectively. If engine bay space is limited this may restrict your options, although relocating the battery to the rear can free up some vital room.
So how do you create a Twincharging application for your car? Our members have played around with various set ups and Claymores Turbo volvo is a good example of what can be achieved quite easily. His set up takes the output of compressed air from the turbo and then uses the supercharger to compress this further.
Obviously there is quite a large heat build up issue so a larger intercooler is a pre-requisite. Some set ups use a twin intercooler at each stage, others stick with a larger capacity intercooler and these can always be sprayed with water or nitrous to further reduce the intake temperatures.
Roots superchargers work best in this set up according to our members so it is worth sourcing an old Roots charger from your breakers yards and finding a mounting point for it in the engine. A screw supercharger would work as you just need a positive displacement but there will be issues present that you don't get on a Roots setup.
You could argue that having 2 compressors in the intake will impede flow rate but the key is that the compressed air from one is further compressed in the second more than making up for any restrictions introduced. Inline setups use separate paths for each charger so they are not flowing through one another and this avoids the "restriction" debate altogether.
The trick after you have plumbed in the second compressor is getting the mapping and air fuel ratios right. Failure to get this right can lead to over fueling and/or detonation in the engine. A twin charged engine is best set up on a rolling road with proper diagnostic equipment and most cars will need an aftermarket ECU to cope with the extra mapping requirements.
A small word of caution on Twincharging relating to throttle body placement: It needs to go between the Turbo and supercharger if used in compound charging or the back pressure when the throttle is closed at high revs can destroy the supercharger. You will also need to lower the compression ratio of the engine if you want to run high boost and the overall principles of adding a turbo to a non turbo engine apply to situations where a second charger is added.
There are an increasing number of Twincharging kits coming on to the market for popular turbo engines. A kit will usually provide all the mechanical components you need to get a Twincharger set up and working correctly but you will still have to get the engine mapping sorted out for best results. For more tips on Twincharging a car we suggest you join our forum and discuss this aspect of tuning with our experienced members.
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