Giving up….

Posted on July 24th, 2005 in Opinion by Julian Edgar

I made the decision at 4 am. Or perhaps a little after, in fact. Lying awake in bed I realised – with the startling clarity than only predawn ratiocination can bring – the project on which I had been spending every free hour for more than four weeks was a disaster. Well, not a disaster, but it didn’t meet the criteria that I had (retrospectively!) laid out for it.

So rather than going on, it was better to stop.

It was ironic. Every single aspect that I had expected to cause problems was working superbly. The belt tensioner, the blower mount (which had also become the new right-hand engine mount), the intercooling and the engine management. Even the hybrid control system had coped with the increased engine output like it was, well, made for it. There was no detonation and the standard injectors had enough capacity to flow the required extra fuel.

In short, the positive displacement supercharger that I’d fitted to my ’99 Toyota Prius – making it the world’s first supercharged, intercooled, petrol/electric car – worked brilliantly.

Except for one aspect.


I’d vaguely known that Roots-type supercharger could be noisy. I’d even known that Eaton had changed their straight Roots rotors for a twisted type in order to reduce noise. I’ve driven a few aftermarket blown cars where the blower had wailed and whined – but then again, I’ve also driven a number of supercharged factory cars where the blower could only just be heard.

In short, during the planning stage, noise wasn’t a major concern to me.

The first inkling that there could be a problem occurred when I ran the blower without it pumping into the engine. It was a test of the belt drive, nothing else. But the noise of the blower – completely unmuffled with both inlet and outlet connected straight to the atmosphere – was incredible. By the time I got the blower drive finished, it was relatively late at night – and the noise was enough to wake the neighbourhood.

Well OK, I thought. But surely the supercharger will be vastly quieter when its intake is connected to the airbox, and its outlet to the intercooler?

Let’s start at the airbox side. I’d selected a new airbox from another car – it had a larger filter and was also more easily fitted inside the front guard, which is where the airbox was now required to go. But with the box connected to the blower, intake noise – even at idle – was LOUD. I then went back to the standard Prius airbox – a smaller design with much better muffling capabilities. Mounted inside the guard, it was going to be hard to change airfilters, but oh well. That cut down on intake noise considerably, although there was still plenty at the entrance to the airbox. So then I tried different airbox intakes (including tuned Helmholtz resonators, a long absorption muffler made from poly pipe and quilt wadding, and resonant chambers) finally deciding on a short snorkel with multiple bends as the most effective compromise between reduced intake noise and best flow.

But the blower was still plenty noisy at idle.

I then measured the pressure build-up between the closed throttle and the supercharger. With a blower bypass valve installed, this pressure should have been near zero – but instead it was 2 psi. Changes to the spring tension within the bypass valve (a turbo blow-off valve) fixed this problem. At last, after about a week of work, the supercharged car was relatively quiet at idle. As a guide, the supercharger noise was about the same as the noise generated by the electric radiator fans when they were running.

Click for larger image But on the road the blower was still very noisy. Part of the problem is that the Prius runs what is effectively a CVT – a continuously variable transmission. As a result, when you boot it, the engine revs can immediately rise to high rpm – there isn’t the gradual wind-up in revs within each gear. Any car with a CVT tends to be subjectively noisier than one with a conventional trans, and in the case of the supercharged Prius, it wasn’t just subjectivity – it was noisy.

The supercharger in this situation could be best characteristised as having the sound of an air-raid siren. And it wasn’t just the sheer decibels – it was also the odd note emanating from a car. People on kerbsides would turn and look from 20 metres away – and that wasn’t even at full throttle.

The original plumbing for the supercharger – an ex-Subaru Vivio design – incorporates a small resonant chamber in the blower outlet. So I added just such a design into the Prius plumbing. The difference to the SPLs? Minor.

I listened all over the plumbing (using a piece of tube as a stethoscope and blocking the bypass valve to simulate high load boost pressure) and found that nearly all the noise was coming from the plumbing between the blower and the front-mount intercooler. On the other side of the intercooler the plumbing was quiet – the ‘cooler was acting as a big damping chamber and the pressure pulses causing the sound weren’t getting past it. So what about acoustically insulating the intake plumbing ahead of the intercooler?

Sounds easy, but the trouble was I had very little clearance around the pipe. The tight space ruled out traditional acoustic foam and the like so I pulled out the big gun – and wrapped the plumbing in 3mm lead sheet. If anything was going to stop the noise, the lead sheet would be it. I also placed under the bonnet a big pad of noise-absorbing foam.

And the results? Well, the subjective loudness had perhaps been halved. Now, instead of hearing the supercharger at all throttle positions, you could only hear it above about half-throttle. Trouble is, in a hybrid you are quite often using more than 50 per cent throttle – especially in the hilly country in which I live. Whenever you needed the supercharger, you could sure as hell hear it. So how loud was it by this time? At its loudest, it can probably be described as being like a police siren heard inside a car with all the windows up – and with the police car one or two car-lengths behind you!

And that was just too loud. Maybe in a conventional car, where high revs wouldn’t have been being used so often, it would’ve been fine – with the smaller air-filter, the 3mm lead sheet lagging and the underbonnet sound insulator all in place, that is. But in this case, the detrimental effect on driving quality was marked… it almost completely offset the increased performance. You could have a loud whine, or no performance. Take your pick.

In the middle of the night, lying awake and thinking through the possibilities of better sound insulation, more resonant chambers, more sound-proofing, I suddenly made a decision. Forget the supercharger…

Yes, I’ve learned a lot about fabricating supercharger brackets, about belt-drives and about making plumbing that can fit into very tight spaces. I’ve also written many articles about these topics (and after all, that’s what pays the bills) but I’ve spent a massive amount of time and energy and effort.

To end up with something I’m not happy with.

After my decision, I went back to sleep, more comfortable in my mind. When I woke up, I went browsing eBay for little turbos…..

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