[JulianEdgar]

I have been testing my car modifications on the road since the mid 1980s. That included the basics like stopwatch testing - but basics most people seldom then seemed to do.

I remember, in about 1988, testing the performance 'gain' of the expensive new exhaust I'd just had fitted to my turbo 3-litre - only to find that against the stopwatch, there was no measurable performance improvement at all. That was about the time I decided following the prevailing wisdom ("Fit a two-and-a-half-inch exhaust to the Commodore and there'll be a real performance gain") was best ignored.

My next car, a Subaru Liberty RS (Legacy in most parts of the world - the predecessor to the WRX) I decided to modify very carefully. An incentive was that I was now also writing freelance articles for modified car magazines, and so if I could show people some good techniques, the articles were more likely to be bought. (I ended up selling 1000+ articles in Australia, the UK and the US.)

I decided to modify the intake system of the Subaru. I was talking to an experienced race engine builder (he had his own engine dyno, etc) and he made a throwaway line that fascinated me. He said, "Just put a vacuum gauge on the intake before the throttle and that will show you the intake restriction".

I don't think he'd ever actually done it, because I found that a normal vacuum gauge had no-where near the sensitivity. But I think he'd watched intake ducting being pulled closed at full load on his engine dyno, and made the deduction. So I investigated what instruments that could be used to measure very small pressures, and discovered manometers. I then built a simple vertical tube water manometer and found that it was brilliant.

By connecting the manometer to the intake at different positions, and then driving the car at full load on the road, you could directly measure the flow restrictions of each part of the intake system. Even better, once you'd found a specific restriction, you could then modify just that part of the intake and then measure the difference - all at full engine airflow. (So, vastly better than testing the intake on a flowbench.)

For example, say the outlet of the airbox was measured as being restrictive. You could then increase the outlet pipe size, or add a better bellmouth, and directly measure the improvement - eg a pressure drop here of 4 inches of water dropping to 2 inches of water. By finessing the intake, typically at almost no cost and using just basic hand tools, I found it wasn't hard to drop the flow restriction of the Subaru by over 60 percent. The acceleration improvement was immediately measurable against the stopwatch on the road. (Incidentally, I have also always recorded a fuel economy improvement with better flowing intakes.)

But the water manometer was a bit messy, and I found out about Magnehelic gauges. By using a Magnehelic gauge, I could measure intake restriction without needing a passenger to hold and observe a water manometer. And since the Magnehelic gauge had two ports (ie pressure and vacuum), I realised I should technically be able to measure positive pressures in the intake - but only if I sited it in an area of high aerodynamic pressure. To my astonishment, the readings worked - with the intake moved to the right place, positive pressure was measurable in the intake prior to the filter box.

I've since used the Magnehelic measuring technique to modify the intake systems on many of my cars.

To me, current DIY aerodynamics modification has many parallels with my pressure measuring technique for modifying intakes. To decide which modifications will work, you can guess, using generic rules of thumb or look at what others do. But it's all hit and miss, and because so many of the modifications don't do anything positive, costs a lot in terms of the outcomes that are actually achieved.

To be honest, it is also an approach that I just can't understand. Why would people often rather waste money, time and effort than do some simple testing to find out what works?

But after all, decades after I first wrote about the pressure drop testing of intake systems - articles that were and remain very popular - most people still use rules of thumb for improving intake systems. Unfortunately, my testing suggests that a significant number of those rules of thumb are wrong.