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From the Editor

9 November 1999

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Our job at AutoSpeed is to write stories about cars. To find unusual and interesting modified cars, to do tech stories on DIY modifications, and to cover all exciting things automotive. As you'd imagine, we are always looking for new stories. So when I was in a workshop the other day, I asked - as I always do - what work they had on. Amongst the upcoming jobs they had was the fitting to a BMW 316i of a European aftermarket cat-back exhaust and a chip. While the 316 is no ball-tearer, it's often the less powerful cars that are treated to this type of upgrade - the owner deciding that they'd like just a little more grunt.

So I was interested - could we get the car onto a dyno for 'before' and 'after' tests, and preferably drive the car as well? The workshop was happy, but who was to pay for the dyno runs? Mmm, perhaps the importer of the exhaust and chip? I rang that company and they stated that, sure, they'd pay for the dyno runs. I discussed the situation with the workshop - accompanied by one of their guys, could I drive the car the across-city jaunt to the dyno and back again - both before and after the mods? That way, I'd drive the car a total of about forty kilometres, twenty without the mods and twenty kays with them. And the workshop had no problem with that as well.

Fine, I thought - a short self-contained feature story. You'd expect a little more peak power from the exhaust (my guess'd be 5-6 per cent) and from the chip a little more throttle response and mebbe a smidgin of power. Once I wouldn't have said the 'throttle response' bit, but dialling-in heaps of off-load ignition advance with the A'PEXi Power FC that I'd fitted to my previous car had shown what a difference that can make.

The only fly in the ointment was that by the time the work was done and the dyno testing carried out, it looked like taking two full days. But, anyway, it was all not to be. A few hours before the testing was to begin, I talked again by phone with the importer. He asked: how was I going to do the testing, exactly? I reiterated what I had said the day before, adding only the details of the driving.

"No, that's not a fair test," said the man. "You need to drive the car for a week before you can effectively test it in its modified form. Even a set of floormats you couldn't test by just a quick drive."

Er, right. So the dyno testing wasn't relevant, then?

"No, I think that you'd be putting too much emphasis on full-load testing."

I pointed out that advertising literature for the chip has quite specific claimed power improvements - was that not full-load dyno testing? And why on earth did I have to drive the car for a week? Any other performance improvement - acceleration, throttle response, smoothness, tractability - you'd be able to feel during a drive of 20 kilometres...... wouldn't you? Well, no, not according to the man.

The test was off.

No forgive me for being cynical. But the only reason that I can see that it would take a week of driving to find any improvement is that by the end of the week, you'd have forgotten how the car drove previously. Or, perhaps, the improvements are so subtle that they aren't really there.....

As much as I try to educate myself otherwise, it keeps getting confirmed - an aftermarket chip in a near-standard naturally aspirated car apparently offers an awfully borderline improvement. After all, what you think of paying over A$600 for a chip modification that you can't feel in 20 kilometres of driving?

It goes without saying that smoothing the path of the intake and exhaust flows benefits power. Porting the head, fitting an exhaust that uses only gentle, open-angle bends, making sure that the intake system is as free-flowing as possible - all give power improvements. But it's an all-too-common belief that these basics don't apply to forced induction engines. After all, it's easy to wind up the turbo boost a bit or drive that supercharger just a little bit harder, isn't it? Pressure drops - what do they matter? No-one worries too much about the restriction caused by an intercooler, do they?

But every flow decrease in the intake and exhaust plumbing of a forced induction engine will also harm power. Sure, you can turn up the boost to compensate - but that means exhaust back-pressure (turbo) or parasitic drive power losses (blower) will also escalate. When I was a kid, my father - a physicist by profession - used to constantly reiterate "You don't get something for nothing". And you don't lose something without it having an affect somewhere else, too! Dad wasn't talking about dollars and cents, but about energy and power. The turbulence and flow restriction of poor intake plumbing will drop engine efficiency - and that means a decrease in flywheel power output.

I well remember - a decade ago, now - doing the intercooler plumbing on my Commodore VL Turbo. On that car, the pipe exiting the turbo has to have a very sharp bend in it to clear the inner guard - and that bend caused me considerable heartache. But there was no alternative - a sharp right-angle had to happen. Then I found that I needed to use a further two 90 degree rubber bends off the 'cooler itself - each was a personal pain to be endured. To try to compensate, the inside radii of the fabricated steel plumbing was filed and filed, every internal imperfection smoothed.

Sure, a forced induction engine isn't as sensitive to flow restrictions as a naturally aspirated engine. But to say about an overly small throttle body, or bad intake plumbing, or a restrictive intercooler, "Doesn't matter, it's a turbo (or supercharged) car" is just plain silly. About the only place where a pressure drop is a worthwhile trade-off is in the intercooler - and that's because internal air turbulence will improve the heat exchange efficiency.

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