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The Story of Turbo'ing a Hybrid Prius, Part 5

The final result

by Julian Edgar

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At a glance...

  • Massively improved hill-climbing performance
  • Improved fuel economy
  • Reduced noise
  • A transformed car
  • Part 5 of 5
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This article was first published in 2005.
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So it’s been lots of work and there have been a helluva lot of problems to overcome along the way – but now, finally, what’s the turbo Prius like on the road?

In a word – fantastic.

The turbo NHW10 Prius now has better than standard fuel economy. And performance? Well, the key aim has been realised – country road hill-climbing performance has been completely transformed.

Fuel Economy

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On an open road cruise at 100 km/h, the turbo Prius will turn in a best economy of about 4.6 litres/100km [21.7 km/litre, 61.4 miles per imperial gallon, 51.1 miles per US gallon]. In the same conditions, the dead standard car used to get about 6.3 litres/100 km [17 km/litre, 5.9 litres/100km, 48 miles per imperial gallon, 40 miles per US gallon], so the modified car has improved the fuel economy in these conditions by up to 28 per cent!

However, that’s a very best case scenario.

In a mixture of some urban, a fair amount of open road cruising at 110 km/h, and some steep country road hill-climbing (in other words, my most frequent driving mix), the economy now averages 5.5 litres/100 km [18.2 litres/100km, 51.4 miles per imperial gallon, 42.8 US miles per gallon] . In the same driving conditions it used to be 5.7 litres/100 km [17.5 km/litre, 49.6 miles per imperial gallon, 41 miles per US gallon], showing that there’s been an improvement of 3.6 per cent.

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It’s hard to make a comparison in heavy traffic urban driving, because economy varies so much with the conditions. However, a best economy of about 4 litres/100 km [25 km/litre, 71 miles per imperial gallon, 59 miles per US gallon] can now be gained in these conditions. I don’t remember previously ever seeing economy that good.

And it needs to be kept in mind that’s all achieved in a car with comfortable seating for four adults (it’s a very roomy cabin for the size of the car), and having dual airbags, climate control, excellent NVH, etc. In other words, it’s not a small, poorly riding economy box with a screaming engine.

Why the hell are the fuel economy figures being quoted is so many units?! There’s a good reason: in Japanese domestic NHW10 Prius models, the display is in kilometres per litre. In Australian models, it’s in litres/100km. In the US, in miles per US gallon. And well, after all of those, we thought we’d better include miles per Imperial gallon as well!

Performance

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As mentioned earlier in this series, the Achilles Heel of this NHW10 domestic Japanese model is the lack of open-road performance when the high voltage battery is short-term exhausted – as happens after full-throttle has been used a lot. Without the additional electric motor power then being available, the performance is miserable. In fact, up a very steep open-road test hill, the full-throttle performance at the top could drop as low as 47 km/h, and was typically only about 50 km/h. But with the turbocharger, the Prius can consistently top the same hill at 86 km/h – an absolutely incredible 70 per cent improvement in real world open-road performance.

But what about the car’s quarter mile and 0-100 km/h times? Well, while improved, the gain is nothing like as great as the lift in hill-climbing performance. What happens is that the hybrid control system acts in such a way that the peak combined power of the electric motor and the petrol engine is not substantially increased. In other words, total peak power is not much changed over the best it could previously ever be.

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Part of the difficulty in describing the performance change is because the Prius Power Split Device varies engine revs according to speed and throttle position. So to reach peak engine power (as measured by the airflow meter signal), the car needs to be travelling at over 100 km/h at full throttle. At anything less than about 100 km/h, full throttle makes use of the typically much improved mid-range power. (It’s improved because there’s always plenty of electric power available.)

That’s why dropping the boost from 7 psi to 5 psi at high loads makes little difference to normal on-road performance – in fact, it’s hard to even tell the change. But drop 3 or 4 psi of boost in the mid-range - and then climb a steep hill - and the engine can immediately be felt revving harder to generate enough power. And after only a short time of this (eg 30 seconds), Myrtle the Turtle will come on indicating that the high voltage battery is down in level... .and then performance is just woeful. With the turbo boosting the mid-range by 7 psi, Myrtle is completely banished.

But there’s no getting away from it – even with the turbo, the petrol engine is still a low powered one. On the freeway at 110 km/h, the level of turbo boost varies from 0-2 psi – the engine is working much harder than you would expect in a conventional car. On extremely steep grades (eg marked at 18 per cent!) the Prius still struggles...although then, so do lots of other low-powered cars.

NVH

Amazingly, noise, vibration, harshness are now improved over standard. This is primarily because with the greater mid-range torque produced by the petrol engine, the ‘gearbox’ keeps engine revs lower for a given power output. So instead of engine revs flaring loudly at each small hill, the car now just torques its way up with engine revs and noise both much lower.

When the front undertray was off the car, some intake noise from the large airbox could be heard, but with the tray back in place, this is inaudible. There may – may – be a slightly deeper exhaust note on the overrun, where the injectors are switched off and the engine is freewheeling, but from inside the car, the new exhaust is otherwise dead quiet.

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Outside, the exhaust has a deeper note and the turbo can be very faintly heard whistling-up – those noises overlay the (also faint) whistling/whine of the electric motor and power converter and the normal sounds of the combustion engine. From inside the car, about the only time you can hear any performance is at full throttle at higher speeds, where the engine makes a muted growl.

Conclusion

To achieve modification results on such a complex car that include superior fuel consumption and vastly improved open-road performance are very satisfying results.

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The upshot is that the Prius drives exactly like a turbo factory NHW10-model Prius would. There’s no added induction noise, no blow-off valve noise, no exhaust noise. There’s no rush of boost as the turbo spools-up, no change in the sensitivity of the electronic throttle, no increase in vibration or harshness. Instead, the car is punchier in urban cut and thrust, and much more powerful when being driven hard along a sinuous and hilly country road. Drive it everywhere at full throttle and fuel consumption is far poorer than standard, but drive it normally and the fuel consumption is a little better than standard – the consumption depends entirely on the mood of the driver. Having said that, it's rare not to get 750 kilometres out of a single 45-litre tank...

In short, all the criteria for improved open road driving performance without a severe overall fuel economy penalty or poorer NVH have been met.

The raison d’etre of the Prius can be summarised in these words: fuel economy and emissions. The designers aimed at making it the most fuel-sipping, cleanest car in the world.

And despite the addition of the turbo, maintaining these characteristics was near the top of the priority list.

As discussed in the main text, the turbo Prius is often more economical than standard. Its emissions in nearly all driving conditions are the same as standard (using a new cat converter and staying at stoichiometric air/fuel ratios) but it must be admitted that at high loads, the richer than standard air/fuel ratio that is used results in higher hydrocarbon and CO outputs (although lower NOx).

Summary of Turbo Modifications:
  • Turbo - ex-Subaru Liberty twin turbo IHI RHF4
  • Fabricated exhaust manifold
  • GFB recirculating blow-off valve
  • Fabricated intake and intercooler plumbing – 2 inch and 1.75 inch mandrel bent
  • Turbo oil and water cooling lines
  • Intercooler - ex-diesel Pajero air/air
  • 2-inch exhaust – 2 inch cat, 2-inch resonator, ex-Corolla rear muffler
  • Airflow meter – original sensing element in larger body
  • Airflow meter electronic interceptor – Digital Fuel Adjuster kit
  • Intake airbox – Falcon XR8
  • Fuel supply system – in-tank pump, external pump, two pressure regulators, solenoid, one-way valve, two control relays
  • Decrease in high load boost using a solenoid-controlled bleed on BOV pressure line via Simple Voltage Switch working off airflow meter signal
  • Engine 'on' signal provided for 30 seconds after boost event via a modified Simple Voltage Switch working on hybrid ECU 'full air con request' input
  • One heat range colder Iridium spark plugs
  • Electronic switching of oxygen sensors and fuel pressure via the Simple Voltage Switch kit working off airflow meter signal
So How Does it Compare?

So what are the different Prius models - and how does the turbo’d one compare?

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The NHW10 model was released in Japan in late 1997.

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The first model Prius sold outside of Japan was the NHW11 model, which looks much the same as the NHW10 but has some significant underskin changes.

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The current model, the NHW20, has both a new body and even more significant driveline changes, although the fundamentals remain the same.

So is the turbo NHW10 now a match for the current model NHW20? The short and blunt answer is: it’s not even close.

The NHW20 gets to 100 km/h in about 10 seconds, helped hugely by its near-doubled (at low speeds) electric motor power. The turbo NHW10 takes 14 seconds (see, told you it was still slow in standing start times!).

The NHW20 also shows superior fuel economy to the turbocharged NHW10, although this is a much closer contest. In fact, on a cross-country haul of the sort I did in an NHW20 (see Toyota Prius: Across a Continent) the economy sat for – at times – hundreds of kilometres at 5.3 litres/100km, a figure I am sure the turbocharged NHW10 could equal. (In fact, I would be hopeful of high Fours.)

But, as impartially as I can, the following table shows a comparison of the vitals.

Model

NHW10

NHW11

NHW20

NHW10 turbo

Engine Power (kW)

43

53

57

55?

Electric Power

(kW)

30

33

50

30

0-100 km/h(seconds)

16

13

10

14

Japanese 10-15 test fuel economy*

km/l

28.0

29.0

35.5

-

litres/100km

3.6

3.4

2.8

-

mpg (Imp)

79

82

100

-

Mpg (US)

65

68

83

-

Typical on-road fuel economy

km/l

17.5

19.2

21.2

18.2

litres/100km

5.7

5.2

4.7

5.5

mpg (Imp)

49.6

54.3

60

51.4

Mpg (US)

41

45.2

50

42.8

(* apparently there was an NHW11 mid-model update that resulted in the 10-15 economy test figure improving by 6.9 per cent. Note: the Japanese 10-15 mode fuel economy test is notoriously optimistic compared with real world fuel economy)

So hell, if in performance the turbo’d NHW10 is still slower than the standard NHW11 and NHW20 models, and in economy it’s often no better, why didn’t I just go buy a current model – or even an Australian-delivered NHW11 – and save myself the trouble?

Well, one really good reason is cost. The JDM Prius cost me only AUD$12,500. Add the turbo, fuel pumps, etc (which is hard to cost because fitting all this stuff also earns me money!) and you might add another $3000. I am still way ahead of a locally-delivered NHW11 (at about AUD$22,000) or a current model NHW20 (AUD$35,000 secondhand).

However, clearly I’d like to apply the turbo technology to a current model NHW20. Perhaps one day I will...

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