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The Story of Turbo'ing a Hybrid Prius, Part 2

Will it work?

by Julian Edgar

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At a glance...

  • Oil and water connections
  • Exhaust and intake
  • First test results
  • Part 2 of 5
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This article was first published in 2005.

Last week we introduced the madman’s project – turbocharging a hybrid petrol/electric Prius. We covered the turbo selection (an IHI RHF4), modifying the turbo to make it fit, and making the exhaust manifold. This week we plumb oil and coolant lines, equip the car with a new exhaust and intake, and finally get out onto the road.

Oil and Water

The biggest problem to overcome in the installation of the turbo water and oil lines was the oil drain hose back to the sump. Many turbo kit instructions suggest that it is fine to attach this fitting with the sump in situ. That is, a hole is drilled in the sump and the wall tapped. The oil is then drained, hopefully taking all the metal particles with it. A fitting is screwed-in and a hose attached.

But I thought that a bloody awful way of doing it – how to remove all the metal particles, how to seal the inside of the fitting, and what’s this about tapping the wall of a sump that might be only 2mm thick?

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The Prius sump comprises two parts. Most of the sump is an alloy casting, however, on the very bottom, there’s a small cover plate made from pressed steel. To remove the alloy part of the sump not only meant undoing lots of bolts, but the front cam timing belt cover plate would need to be removed, as would the valve cover. And to remove the timing belt cover, the engine mount would also have to come off. In short, while possible with the engine still in the car, taking off the full sump looked like a helluva job. But the bottom metal cover plate, well, that would be easy.

So would it be possible to remove the cover plate and then reach up inside the alloy part of the sump to install the fitting? The answer to that depended on two things: how high on the wall of the sump the fitting had to go, and what the clearances inside were inside – could a hand be fitted inside? With the bottom of the sump off it could be seen that it was just possible to insert a nut and washer on the inside of the fitting. The space for a hand was tight – too tight for my big hands – but with some assistance we managed it. Getting this fitting in place was a huge relief.


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As part of a series of stories I was doing on putting together a cheap exhaust, I decided to source pretty well all the exhaust components secondhand. The standard Prius exhaust is very small and so I settled on (much bigger!) 2-inch components. A wrecker revealed a long thin cat (the shape required to fit under the car), a stainless steel flex joint and a long resonator. All the parts were factory – ie none aftermarket. The muffler was sourced from a muffler shop – it was a near brand-newie from a late model Corolla and is slightly smaller than 2 inch.

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However, before any of these bits could be put into place, the first section of exhaust (off the turbo) needed to be made. As this part of the exhaust was accessible only with the right-hand driveshaft removed, this couldn’t be done at the muffler shop. I sourced three mandrel bends (1 x 180 and 2 x 45 degrees) at what I later realised was an exorbitant price and had Peter-the-welder arc them together. I was able to use the exhaust flange that came with the turbo and used my lathe to turn-down a 19mm nut for the front oxy sensor mount. While we were at it, we also attached the cat and resonator – both going on after a removable flange that followed the flex joint. These latter bits were added to make the car (hopefully!) quiet enough to drive to the exhaust shop, some 20 kilometres away.


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When I’d first fitted the supercharger, I’d used an airbox sourced from a Daihatsu Sirion. Despite using a filter with the same area as the standard Prius filter, the pleats are much deeper, resulting in more filtration area and so (presumably) lower restriction. However, the box-like shape of the pictured Sirion design had allowed a massive amount of intake noise to be emitted by the supercharger, so I had reverted back to the standard Prius airbox, mounted in the front guard.

I’d expected to go back to the Sirion box with the turbo, but in the mean time had spotted the incredibly compact Falcon XR8 airbox design, which I figured would also fit into the front guard. Designed for a flow commensurate with 260kW, I thought the pressure drop (ie flow restriction) at 50-odd kilowatts wasn’t going to be too great... Plus, the filter should need changing only very infrequently – a good thing as the bumper cover would need to come off to gain access to it.

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I managed to pick up the airbox for nothing and after cutting off the lower prong mount and making a heavy duty aluminium bracket, had it in place.

The next step was to connect the airbox to the airflow meter and then to the intake plumbing leading to the turbo. I made a bellmouth adaptor from an old cake dish (see the end of Driving Emotion for more on this technique) which nicely dropped down the huge Falcon airbox exit from 4 inches to the 2 inches I needed. Plastic 2-inch plumbing fittings were then used to lead the air to the airflow meter, which I’d cut from the lid of the factory airbox.

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The car had already been equipped with a front-mount intercooler – it was installed for the supercharger and retained for the turbo. It consists of a diesel Pajero ‘cooler which is mounted ahead of the three existing radiators – for engine coolant, power converter coolant, and the air con condenser. To allow the intercooler to be squeezed in, the power converter rad was moved back a little and the front anti-intrusion bar modified.

The Smoke Test

With all that lot in place, three weeks had passed. (In the same period I also needed to do other work!) With the radiator refilled with coolant, oil back in the sump and the ‘gearbox’ filled with its special trans fluid - and the wastegate disconnected - I could finally hit the road, boost gauge in hand.

Would the turbo be too big for the application?

Would the exhaust manifold perform badly?

Would the high pressure oil line or the water hoses leak at their modified fittings?

Filled with equal amounts of dread and expectancy, I turned the key. Nothing happened: the 12V battery was flat.

I charged it but the car still wouldn’t start – it appeared that the high voltage battery had dropped too low for the ECUs to give permission for the starting procedure to occur. How I developed a charger to change that situation – and how I accessed the high voltage battery pack’s innards – is another story (see When One Mistake Can Kill) – but a day later, the car started. With half the exhaust still missing, it was pretty loud, but not impossibly so. There were no apparent fluid leaks, so I backed the car down off the ramps and drove onto the road.

My street heads uphill and I hadn’t gone even a few hundred metres before seeing positive boost on the gauge and hearing the faintest of whistles from the turbo.

The system worked!

More prelim testing revealed that up to 15 psi was available – much more than I wanted – and with the wastegate connected, a very satisfactory 6 psi was achieved early in the rev range. (How early? It’s hard to tell as the car doesn’t run a tacho and the actual exhaust gasflow depends on the electronically selected throttle position and the revs that the ‘CVT’ allows the engine to run at.) But certainly, whenever the engine was audibly under moderate load, the boost gauge was showing a wastegated 6 psi.

Finishing the Exhaust

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But any further testing would need to wait for the rest of the exhaust to be fitted – I didn’t want to be stopped by a policeman suspicious of the exhaust noise – and so I headed for the exhaust shop. From the resonator back, I was happy to have the exhaust press-bent and so it didn’t take long before the ex-Corolla muffler was in place and the rest of the exhaust properly suspended from rubber mounts. (I’d just temporarily wired it in place!)

As expected, the exhaust was dead-quiet – it had a deeper than standard note but the system was completely free of drone, burbles, resonances and all those other things I didn’t want. (Remember, the petrol engine in this car automatically starts and stops a lot. Having a loud exhaust boom every time the engine fired into life was not wanted!)

In fact, the quietness of the car was astonishing. Instead of the awful supercharger wail, now all that could be heard was the needle in my undamped temporary boost gauge jumping with the pressure pulses in the intake manifold. Crimp off that hose and the very faintest of turbo whistles could be heard – almost lineball in loudness with a typical factory turbo car. Even the engine noise seemed less than I remembered it – before the supercharger was fitted, I could recall the engine noise rising with the ‘CVT’-directed changes to a clearly audible state. Now the engine seemed muted – refinement was better than standard (and soooo much better than with the blower fitted, it’s not even funny) and yet now the high voltage battery level stayed much higher, especially up hills. That in turn resulted in full electric power being available pretty well anytime I put my foot down.

Was all this good or what?! Well it was until I started measuring air/fuel ratios....oh hell!

Next week: sorting out the fuelling

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