When lower boost doesn’t slow a car…

Posted on December 11th, 2005 in Opinion by Julian Edgar

As regular readers will know, my NHW10 hybrid Toyota Prius has been turbo’d and intercooled. To fit in the available space, the turbo – one of the puffers from a twin turbo Subaru Liberty – required that its wastegate mounting system be modified. A spacer ring was used to allow the wastegate actuator to be placed in a different orientation to standard. This ring put a small preload on the wastegate rod, resulting in a minimum on-load boost level of 7 psi. That’s 7 psi, even with the wastegate hose connected directly to the turbo compressor outlet – ie no bleed or aftermarket boost control fitted.

Now that’s generally well and good, but sometimes at high loads, 7 psi can cause a problem. Intermittently – and for only a very short duration – the hybrid control electronics closes the electronic throttle. I assume that this occurs because I have exceeded a preset internal safety trip-point for the engine or electric motors. That implies that if boost can be dropped a little at the top end, the throttle shut-downs are likely to stop. (And in previous short-term testing with lower boost levels, the problem did in fact disappear.)

Since the minimum boost level the wastegate can be set to is 7 psi, dropping boost below that requires bleeding air from the manifold. One easy way of achieving this is to allow the blow-off valve to leak, something which can be achieved by pulse-width modulating the boost/vacuum feed to the valve. Working with the airflow meter signal, the Simple Voltage Switch kit allows this boost drop to be triggered at a preset load. This boost leak doesn’t cause any fuelling problems, because a recirculating blow-off valve is used and the air is returned to the intake after the airflow meter.

Using this approach, I initially dropped peak boost back to 5 psi at loads over about 80 per cent of max, with little discernible difference in performance. Since dropping from 7 psi to 5 psi apparently made little variation in the available top-end power, I then decided as an experiment to allow the blow-off valve to leak all the time. (The aftermarket GFB valve runs a variable preload on its internal spring, making this easy to achieve with some spring adjustment and pulling off the vacuum/boost feed hose.) This resulted in a slow rise in boost to a max of only 4 psi.

Of course, slow rising boost is an anathema in a turbo car – you always want boost to come up as fast as possible. In a normal car, the difference between this and the previous boost-as-fast-as-possible-to-7-psi-and-then-hold-it-at-that-level would be like chalk and cheese. The configuration with the slow-rise-to-4-psi would feel half-dead and power would be clearly way down.

For Godsake, for some testing forget the bloody dyno – get out on the road!

Posted on December 4th, 2005 in Opinion by Julian Edgar

Why is it that people put so much faith in dyno testing? I have written about this topic before (see Driving Emotion – August 2004) but it needs to be continually shouted from the rooftops. Dynos are bloody useless in so many areas of car modification testing that I don’t know even where to start. But I’ll try.

As I wrote in that previous column, they’re pretty well useless for testing turbo boost controls. Why? Well they:

  • Don’t take into account the acceleration rate of each gear – vital because boost overshoot on transients is hugely affected by the rate of engine rpm increase.

  • They don’t allow the testing of boost behaviour of full-throttle gearchanges (very few people do full throttle gearshifts on the dyno). Again, it’s in just these conditions that you look for boost overshoots and/or slow increases back to peak boost after each gearchange.

  • No one ever does a full-bore launch from a standstill on a dyno. And the speed with which boost can be brought up in these conditions – ie controlling wastegate creep – is a major aspect of good boost control.

The benefits of road testing modifications

Posted on November 27th, 2005 in Opinion by Julian Edgar

Yesterday I did some serious testing. Well, ‘serious’ as in I set out to specifically test and fine tune the air/fuel ratios in my project car.

Despite frequently writing that you should always have an assistant in the car to read gauges and hold hand controllers and make tuning decisions, I must admit that circumstances often force me to do it all myself, as I am driving. (I am cognizant of the dangers involved in doing this; once many years ago while on-road tuning, I ran straight into the back of a car that had unexpectedly stopped.) So yesterday, for the fine tuning, I did in fact sit in the passenger seat. My lady drove the car and our little boy was in the back.

Despite needing to use full throttle as part of the tuning process, the performance of the car (a turbo hybrid Prius) is so slow that it can all be done on open public roads without breaking the law or posing danger to anyone.

While setting-up this car I have at times run numerous in-car instruments, but on this occasion they were limited to a  MoTeC air/fuel ratio meter running from a tail-pipe mounted probe, an LCD intake air temperature display, a LED indication of factory oxy sensor output, and an LCD hand controller connected to a Digital Fuel Adjuster kit. I also brought along a screwdriver and spanner to allow me to adjust the high and low fuel pressure regulators (this car uses a system that switches out the closed-loop oxy sensors and simultaneously switches in higher fuel pressure). A smaller screwdriver was also carried that could be used to adjust the switch-over point between fuel pressures, a change which is triggered by a Simple Voltage Switch kit monitoring airflow as measured by the airflow meter.

The first step was to check intercooler efficiency. While I had previously measured intake air temps, I’d only done so with the front bumper and number plate off the car. I’d physically felt the temperature of the plumbing after driving the car hard, but hadn’t quantified the numbers. And I must admit, watching the LCD temperature display, I was appalled. Initially, on the long trip down from the hills on which we live, the temp had stayed low – about 10 degrees C above the 20 degree ambient. But then, whenever boost was called upon, the temperature rocketed. Like, it would rise to 20 degrees C above ambient after just a few seconds of boost, and 40 degrees C above ambient after perhaps 10 seconds of boost!

When getting good fuel economy doesn’t mean driving like a tosser

Posted on November 13th, 2005 in Opinion by Julian Edgar

The stories we ran a few months ago on getting best fuel economy included one on driving techniques (see Savings on Fuel – Part Three). None of the techniques mentioned were startling or new; if you read pretty well any of the books on driving published over the last hundred-odd years, you’ll find mention of being smooth, ‘reading’ the traffic flow, rolling up to halts rather than braking at the last minute, and so on.

And while they might not be new ideas, they’re still certainly quite valid when it comes to getting best fuel economy.

Another characteristic of those habits is that they all fall into the category of ‘good driving’. It doesn’t really matter what vehicle you’re pedalling – whether it’s a huge prime mover or a tiny economy car or a turbocharged rocketship – all will respond favourably to these driving habits… habits which will cause little or no concern to other drivers. In fact, you’re much less likely to have accidents if you drive in these ways.

I kinda took that symbiotic relationship for granted – good driving, low accident rate and better economy – until I was rudely awakened by discussion of an ‘economy’ driving technique that seems custom-designed to infuriate other road users… and simply could never be described as good driving.

Thankfully, it’s currently not a widespread practice, is limited to just one type of car, and is pursued by those who can only be – quite charitably – referred to as tossers.

Economical engines

Posted on October 30th, 2005 in Opinion by Julian Edgar

Start talking the fuel economy of different petrol engine types and designs and things get complex, fast. Amongst other factors, fuel economy is affected by internal friction, pumping losses, combustion inefficiencies and the air/fuel ratio that is used.

The biggie with internal friction is, literally, how big the engine is. A larger engine has longer internal bits rubbing other bits, and so a 5.7 litre V8 is always going to have poorer fuel consumption that a 2-litre four cylinder. That statement applies when both engines are producing the same low power required for cruising, but may not be the case when the power demand is high – climbing a hill while pulling a trailer, for example. In the latter situation, the smaller engine will have to rev very hard to develop adequate power, and the higher the speed of the engine, the greater the power loss through friction. Because of the high loads to which it is being subjected, the small engine might also move out of closed loop (ie ~14.7:1 air/fuel ratio) to a much richer mixture. So as the power demand increases, the practical on-road fuel consumption may not so clearly favour the smaller engine over the larger engine.

Another way of seeing this is to look at the fuel economy gained from a small engine car that always has to have the ring driven out of it to keep up with traffic. In this situation, the fuel economy is often poorer than the larger engine car that is always just loafing along.

Pumping losses refer to the drag caused on the movement of the pistons on their intake and exhaust strokes. Any restriction on the intake – including, critically, the partly closed throttle – will lower the pressure of the air that fills the cylinder on the intake stroke. Rather like drawing down a syringe that has the needle opening blocked, power is needed to overcome this partial vacuum. On the exhaust stroke, anything that restricts flow out of the cylinder – from a poorly flowing muffler to bad port design – will again require power that’s subtracted from what is available at the flywheel.

Some BMW engines dispense with the throttle – and instead change intake flows by varying valve lift and timing – however the intake pumping losses remain. (Diesel engines, of course have no throttle and so much smaller pumping losses.) A better approach to reducing pumping losses is to adopt the Atkinson or Miller cycles, where the closing of the intake valves is much delayed at lower engine rpm. This poorer intake flow requires the driver to more widely open the throttle for a given power output, so reducing pumping losses. However, the engine also develops less power because its volumetric efficiency is much lower than an engine with conventional valve timing. Atkinson/Miller cycles are therefore used only when there is forced induction at low revs (eg a supercharger as in the Eunos 800M) or power is available from an electric motor (Toyoya Prius and most other current hybrids). 

Oldies but goodies

Posted on October 23rd, 2005 in Opinion by Julian Edgar

I love reading; in fact it puzzles me slightly that anyone who has any interest in anything wouldn’t love reading. I also love buying secondhand goods; put those two together and lots of old books come my way. Via eBay, from garage sales and secondhand book shops, at auctions and by tender.

Over the last year or two I have been buying lots of old car books, especially those that deal with car technology. Given that my major car modification interests are electronic systems, turbocharging, aerodynamics and hybrid cars, you might wonder why I’d bother buying old car engineering books. After all, aren’t they all way outdated?

Well, yes and no.

Sure, you won’t find mention of the latest in Bosch electronic stability controls, or active aero, or ball-bearing turbos. But equally, there’s been almost zero change in car fundamentals. Engines still have pistons and cams and crankshafts, the concept of valve timing hasn’t changed much in 100 years, and Ackerman steering geometries have as much validity then as now. Even more importantly, the physics of power and torque and engine revs; sprung and unsprung weight; engine balance – and a host of other topics – hasn’t changed one whit.

And the best thing about some of these books is that the way they explain these concepts is simply second to none. Perhaps in times past it was much more common for someone to get interested in a topic, buy some books and then set out to teach themself the whole thing from scratch. (These days, the same person just subscribes to a web discussion group and gets a mix of advice that is typically 80:20 in quality… and no, that’s not 80 per cent good stuff!)

The weight changes caused by turbocharging and supercharging

Posted on October 9th, 2005 in Opinion by Julian Edgar

Many performance mods add weight to a car, so reducing their real world effectiveness by at least a little. A big exhaust, a supercharger or turbo – all make the car heavier than it was standard. (Of course there are a couple of mods that make the car a tiny bit lighter – eg porting a cylinder head or lightening a flywheel – and there are other modifications that make no difference at all to car weight – eg increasing turbo boost.) But in general, even changes like bigger wheels add mass.

Recently I had the opportunity of comparing the weight gains made to a naturally aspirated car that was firstly supercharged, and then the supercharger removed and the engine instead turbocharged. How much weight was gained by each approach? The car was my first series Toyota Prius.

The blower installation involved the fitment of:

Driving the XR8

Posted on September 11th, 2005 in Opinion by Julian Edgar

Drove a car the other day that’s a good example of a fundamentally excellent design ruined by some strange decisions. The car? A Mark II BA Falcon XR8 – that’s the one with the 6-speed trans.

So what is wrong with it? Well firstly – and probably most critically – the gearing is simply way too tall. The very sweet 260kW 5.4 litre DOHC-per-bank engine has peak torque (a massive 500 Nm!) up at 4250 rpm. That’s not a problem in itself, because the engine is also superbly mapped, being tractable and progressive at any rpm. However, with an engine like this, you can’t run ultra-tall gearing and expect a strong performer. Not unless you drive around always two or three gears lower than ideal.

Added costs of turbo/blower

Posted on August 28th, 2005 in Opinion by Julian Edgar

When Japanese import engines and gearboxes first started flooding into wreckers, I can remember writing cautionary tales spelling-out the fact that while the engines were incredibly cheap, by the time you got one installed in a car and had provided engine management, engine mounts, radiator modifications and sometimes a new tailshaft, the price may well had gone up three or even four times. These days, with many engines available with uncut looms and the factory ECU, you could probably reasonably budget on a doubling the in-car price.

And a similar price multiplication also occurs for individual engine parts.

Over the last few months I have fitted firstly a Japanese-import supercharger, and then latterly a Japanese-import turbo. Both were installed on a car that is normally naturally aspirated. And rather like the old days of buying cheap engines, I’ve found that the main cost of both the supercharger and turbo conversions hasn’t been the initial cost of the blower or turbo, but instead all that is required to accompany it.

The little supercharger cost me $250 from a wrecker, while the turbo was even cheaper. (All dollars are Australian.) Hell, that’s good value! In both cases, the devices were in excellent condition and compared with buying new, represented savings of hundreds and hundreds of dollars. In fact, you could get very excited walking out of the wrecker with one of these in hand….

But you really need to budget at least another $1000 to get either a supercharger or turbo into a car and working. And that’s doing as much work as possible in your own garage at home…

Unique turbo matching…

Posted on August 14th, 2005 in Opinion by Julian Edgar

The matching of a turbo to a particular application is something about which the ignorant knowledgably proclaim – and about which the experts are very cautious and tentative indeed.  In short, matching the compressor to the required airflow is difficult (what with the variations in air density caused by temperature changes and boost, and with the variation in engine air consumption caused by throttle position, variable valve timing and different engine speeds), and sizing a turbine to suit both the compressor and available exhaust gas flows is something that can send you around in over-decreasing circles of frustrated indecision.

So when confronting a unique turbo situation, one of the best ways is to take the lead from OE manufacturers. In short, they’ve done the hundreds of hours on the engine dyno and road (chassis dynos are rarely used in new car R&D labs, except for emissions testing) that result in a turbo that has minimal lag, flows enough air, has low exhaust backpressure, and is durable in the application. If you’re dealing with modified road car engines developing sane power levels, the role models are the single and twin turbo production engines of the world. Sure, you can run a different turbo arrangement (for example, one huge turbo instead of two smaller ones), but usually that will involve a drawback that a car manufacturer wasn’t prepared to embrace. (Terrible lag from the single turbo versus the twin – especially sequential – turbos, for example.)

All these thoughts have been running through my mind. You see, today Michael Knowling and I were prowling the wreckers of Adelaide looking for a turbo to suit my Toyota Prius. Adelaide is the cheapest city in Australia that I’ve found for bits, and furthermore, the available range is second to none. And the Prius? As regular readers will know, the small supercharger that I had fitted to the hybrid petrol/electric car worked superbly in every respect – except for noise. If you wanted a small police siren hard at work under the bonnet, it was good. But if you wanted a quiet, effortless power – well, it wasn’t. So despite the massive amount of work that fitting the supercharger had involved, it was time to move to the other forced aspiration option. A turbo.