But how big should the turbo be? Or, since buying a brand new turbo was never really a viable financial option, what should the donor turbo be taken from? This model Prius runs a 43kW 1.5-litre engine. The small peak power output is primarily because the engine revs to only 4000 rpm; however, it’s also an Atkinson cycle design, a valve timing approach which affects the mass of air that gets breathed each intake stroke. (And so consequently the exhaust output at low revs is likely to be lower than a conventional engine.) The supercharger had been from a Japanese Kei class car – initially I thought it was from a Subaru Vivio, but with the proliferation of small Aisin superchargers that I’ve since seen at wreckers, I can no longer be certain of the model. Anyway, the blower was from a small – probably 660 or 550cc – car and in terms of airflow, it worked brilliantly in the Prius application. In fact, using just the standard Prius crank pulley and the standard supercharger pulley, boost was around 6 psi… all the way from idle upwards.

There might have been boost everywhere, but the peak power of the engine was probably little changed. Huh? What happened was that the mid-range (and bottom-end!) torque was increased, but the hybrid control system tended to limit the max power. Which was fine: having more midrange gave the desired results.

So, what were the criteria a turbo would need to match? A peak power of around 43kW, but from a 1.5 that revs to 4000 rpm. Based on the power number – and the experience with the supercharger – a Kei class turbo sounded the go. So, from a 660cc Subaru, Daihatsu, Suzuki or Mitsubishi. (I’ve never seen a turbo off the Mazda Kei class cars.)  A long time ago I played with a Daihatsu Mira 660cc turbo, so I know what these turbos look like. And how durable they can (not!) be… But when at the first wrecker I saw an ex-Suzuki Cappuccino turbo, I was surprised. These turbos are tiny – way smaller than the smallest current Garrett, the GT12.

That particular wrecker turbo was completely stuffed, with huge shaft play. And the next couple of K-class turbos that we saw weren’t a lot better. They had seized shafts – and I wasn’t all that rapt with the wrecker’s offer to just soak them overnight in diesel fuel so that they’d “come good”. The condition of these little turbos brought back to me the way that the RHB31 turbo on my Mira had worn out so quickly – although admittedly I was boosting it pretty hard.

But one thing’s for sure – in the Prius application I don’t want to be changing turbos very often. Not only will it be a bastard to fit, but once fitted, it will be a bastard to change. I want this to be a strictly fit-and-forget affair…

Michael also made the point that if I used a Kei class turbo, and it failed in service, sourcing a replacement would be pretty hard. Hmmmmm.

So what about using one of the turbos off a twin turbo engine? Like one from a Toyota 1JZ-GTE, or one from a Nissan VG30DETT, or one from a twin turbo Subaru EJ-20? It was the EJ20 turbo that we found first. So, one of the two turbos from a 2-litre four-cylinder engine developing 190kW. It’s a bit simplistic, but in this sequential turbo engine, we’ll assume that each turbo flows about 95kW. Since – at least in the Subaru engines we inspected – the turbos were very similar in size, and since the second turbo is brought on-stream at about 4000 rpm (see Subaru Liberty B4 Twin-Turbo), you can think of one of the turbos as being off a 2-litre, 95kW, 4000 rpm redline engine. Ah, but how much boost was that turbo capable of developing? In the case of the sequential twin turbo EJ20, about 12 psi.

That’s more boost than I’ll need in the Prius application (I was quite happy with the results gained from about 6 psi), so the question becomes: will a turbo that produces 12 psi boost (and is fast to boost) on a 2-litre, 95kW, 4000 rpm redline engine be suitable for a 1.5-litre engine with 43kW and a 4000 rpm redline? The immediate answer is ‘no’, that the much smaller turbo from the Kei class 660s will be better – those engines have a peak power of only 47kW. But in the Prius application, I need only half the peak boost pressure of the Subaru.

The single turbo from the EJ20 twin turbo is also very close in size to the Garrett GT12 – a turbo that on paper looks fine for the Prius. And with the Prius’ transmission – which acts very much like a CVT – the engine revs can flare quite high even at low throttle angles. That would help the turbo spool up, but then again there’s likely to be quite a lot less exhaust gas available to do the work.

But other specific Prius aspects also come into it. The supercharger had worked well because it boosted mid-range and bottom-end torque – mid-range especially. That in turn had confused the hybrid system into putting more charge into the high voltage battery, which therefore made more electricity available more often for the 30kW electric motor. So in other words, the supercharger’s main advantage was that it increased the effectiveness of the electric motor. A slightly over-large turbo that produces (say) an unwastegated 3-4 psi in the mid-range and a wastegated 6 psi at the redline would possibly achieve much the same outcome – but with less exhaust backpressure than a smaller turbo. So that would benefit fuel economy, a very important part of the requirements for the finished system.

To be devastatingly honest, I simply don’t know if one half of the turbo system from a twin turbo Subaru EJ20 is suitable for a first generation Toyota Prius. But today I plonked down the cash for one of the two EJ20 turbos – so I guess we’ll all find out. A major reason for my choice (I was initially dead-set on a tiny Kei class turbo) are concerns about durability. It’s pretty rare to go along to a wrecker and find a turbo that’s no good – and to find all three Kei class turbos completely stuffed is certainly not a good sign. At least the Subaru turbo is large enough and – in this country – commonly enough available that if I need to replace it, that won’t be difficult.

But as to its suitability for this application, I am not sure I have much idea…..