Buying the wrong turbo
When buying a replacement turbo for a Volkswagen group car there are many options.
1. Buy a brand new OEM Turbo
You can buy a brand new OEM turbo (Volkswagen don’t make their own turbos). In the case of my 2002 Audi TT it’s a Borg Warner produced K04 turbo. However, they will cost the guts of €1000.
2. Aftermarket Turbo
You could also buy a non-OEM aftermarket turbo. They vary in quality. At the higher end, Frankenturbo make a hybrid turbo upgrade for about the same price as an OEM turbo. At the lower end, you could buy a cheap chinese turbo on Ebay for less than €200. I would highly recommend that you do not buy one of these turbos. Particularly if you’re fitting it to an Audi TT or other transversely mounted 1.8T engine because when it inevitably fails it is extremely difficult to get it out without removing the engine from the car.
3. Reconditioned OEM Turbo
I opted to buy a reconditioned OEM turbo from a UK company who recondition turbos and then give you £100 for your old turbo to recondition. With a reconditioned turbo you typically get at least a 1 year warranty and the peace of mind that this turbo was used on your car previously.
However, as it turns out, the Audi TT 225ps shipped with two different versions of the K04. The earlier BAM engine codes shipped with a K04-023 and the later APX engine codes shipped with K04-022. The difference? A single port on the exhaust intake on the turbo for an Exhaust Gas Temperature (EGT) sensor. Of course I purchased a K04-022 without the port so I’m going to be sending mine back.
My Final Turbo Choice
Unfortunely, the company I’m buying from do not have a K04-023 so I’m going to opt for an Irish company to collect the original turbo, rebuild it and send it back to me. Apparently this can all be done within a week which is very impressive!
What is the Exhaust Gas Temperature Sensor for?
It turns out that there’s a lot of debate online around Audi TT engine codes, mostly about which engine code is the best to have. There seems to be some particular hype around the BAM engine code. I would recommend taking a look at this thread for a detailed breakdown of the different 1.8T engine codes.
To give a TLDR of that thread, there were two Audi TT 225ps engine codes, APX which was used in cars from 1999-2001. In 2002 it was replaced by the BAM engine. They are very similar with only a couple of additions with the BAM engine:
Variable Valve Timing (VVT): this was introduced purely for emissions reasons.
Two Lambda Sensors: they are used to get the oxygen levels in the exhaust gases. APX has only one lambda sensor post Catalytic Converter to ensure that it is operating correctly. The BAM has an additional pre-cat lambda sensor that, from what I can tell, is used to adjust the valve timing to produce less emissions if needed.
Exhaust Gas Temperature Sensor: this is used to protect components such as the turbo from high temperature exhaust gases that can cause damage to the turbo and Catalytic Converter. The EGT sensor relays the temperature information to the ECU and the ECU can in turn increase the amount of fuel being injected or decrease the turbo boost pressure if the temperature is very hot.
So from what I can tell, people who are in to tuning their cars (can you tell I am not :) like the BAM engine because they can monitor the EGT so that they don’t destroy their engine. I for one prefer a reliable car and see and EGT as another thing to break so I’m not a fan.
What is Variable Valve Timing (VVT)?
The ability to change the timing of the opening and closing of the intake or exhaust valves. This is typically done by shifting the camshaft back slightly. This is done using a solenoid to slide the camshaft backwards and forwards.
The Audi TT BAM engine in my TT has VVT on the intake camshaft.