Turbo Rallye project – the plan

PlanSpeccing a turbo conversion for a naturally aspirated motor is not an easy task. Every man and his dog has an opinion, and everybody knows best. My approach is to find a reliable manufacturer that produces a turbo which is well matched to the high revving 1600cc engine. I’ll be doing this by analysing turbo compressor maps to find an efficient turbo that is also available at a low price. I’ll also be using empirical data gathered by the 106 community – first hand experiences can be invaluable and save a lot of time and effort. The aim will be a fast road car that is equally at home on trackdays.
It helps to be clear what you want to achieve with these types of builds or they tend to spend years half completed languishing in a garage surrounded by expensive parts that were bought on a whim.
So with that in mind the ethos will be:
  1. Quality – use proven parts, measure twice, cut once
  2. Low cost – good use of second hand and essential parts only
  3. Reliability – no chasing figures at the expense of reliability
The approach for this project will be to turbocharge the 16V 1600cc TU5J4 engine from the 106 GTi. I will not be lowering the compression, or using forged steel components to keep the cost down. The TUJ4 is a relatively high compression engine at 10.8:1 so I will be looking at running low boost – probably 7.5psi or half a bar of boost depending on how much air the turbo can flow. Any more than this and cylinder pressures rise and the risk of detonation and a melted engine goes up.
Benefits of a low boost standard engine approach
  • Cost – Tu engines are plentiful and cheap. Should the engine let go, replacing it will be easy and cheap. Small turbos are cheaper.
  • Less heat – Having to run relatively low boost should mean less heat management issues to contend with (generally speaking)
  • Responsiveness – high static compression means good drivability off boost and should give good turbo spool
  • Kinder to the transmission – Less boost, less power, less torque through the drivetrain.
Drawbacks
  • Risk of pre-ignition – A bad batch of fuel, or hot intake temperatures could pre-ignite the fuel and melt the engine
  • Limited boost means limited power before knock occurs.
  • Increased wear – Running over 50% more power than the engine was designed for, will increase stress and fatigue
  • High cylinder pressures – risk of head lift and head gasket blowing
So given our clear objectives, next time we will delve into the black art of choosing a turbo.
Remember to put your nerd specs on and bring a calculator.

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