It’s easy to think that vehicle aerodynamics is an esoteric topic not really relevant to typical modified cars. After all, goes the common refrain, if you’re not driving at 150 km/h, who cares? That viewpoint can be supported by some recent Australian cars which seem to be developed more with styling in mind rather than slipping cleanly through the air – the latest Mitsubishi Magna didn’t see the inside of a wind tunnel and HSV products are now developed without tunnel testing.

However, to take that view would be short-sighted – as car manufacturers chase increasingly better fuel economy, you can be sure that low-drag aero will start making a comeback. Of course, for many manufacturers, aero attention never went away…

But what’s that got to do with modified cars? Well, just today I chased and chased a problem that turned out to be completely aerodynamic in origin. As some of you will have read, my 1988 Maxima V6 Turbo has been fitted with an underbonnet intercooler (see “DIY Budget Intercooler Fitment”). It uses a manually-operated water spray (also covered in AutoSpeed) and air is forced through the intercooler via a front-facing small scoop. More recently I modified the intake system so that it draws in air at the airbox, which is placed directly rearwards of the intercooler.

The testing I was doing revolved around different arrangements of foam rubber sealing around the intercooler and air intake. The first approach removed the rear strip of foam rubber from above the intercooler so that some air could pass over the intercooler core and flow back to the airbox. The next approach sealed both the airbox and intercooler to the bonnet scoop. (This test session followed the insulating of the intercooler > throttle body duct to prevent the intake air getting heated on the way back to the engine.) The intake air temp was all the time being monitored by a fast response LCD probe, placed just before the throttle body.

With all this testing I was getting a real feel for the intake air temp. For example, in ambient conditions of about 20 degrees C, the car in cruise would have an intake temp of about 30 degrees C. It used to be closer to 40 degrees, but the lagging of the return duct had made a distinct difference. Or had it? The day before yesterday it was 30 degrees in cruise – but today it appeared to be back up to 40 degrees!

Same sort of day temp; different intake air temp.

Then I noticed that the on-boost temps were much higher than I had seen previously. Two full-throttle runs to about 130 km/h would normally bring up intake air temp to about 55-60 degrees C (higher than I have quoted in previous stories but the boost has been raised from 0.45 to 0.6 Bar). Now the two runs were taking the intake air temp to a huge 75 degrees C! Running the water spray knocked about 5 degrees off this, but the temps were still unacceptable.

What was going on? I started doing the full throttle runs then quickly pulling over to the side of the road and popping the bonnet, measuring by hand what was hot and cold. The inlet tank of the ‘cooler was bloody hot; no surprises there. The outlet tank was warmish – warmer than I expected but then the boost is higher than when I previously felt temps. And the exposed air filter? Hot – too hot. This was when I had the scoop sealed to both the intercooler and the airfilter, and there was obviously too much air flowing over the hot ‘cooler before it reached the airfilter.

I reorganised the foam strips, sealing the scoop to just the intercooler. Sure, the airfilter would be breathing even hotter underbonnet air, but I wanted to make certain that at least the intercooler was working efficiently. And it seemed it was – the outlet tank was cooler after the full-throttle runs. But now the airfilter was getting really hot.

I went back home and cut a hole in the bonnet above the airfilter. A drastic move? Not really – I knew that some time in the future I’d either need to get a larger scoop that would cover both the intercooler and airfilter feed holes, or leave the intercooler scoop in place and install a grille over the hole above the filter.

So what happened now? Back on the road the airfilter stayed cool to touch and the intercooler kept working as it had last run.

But the measured intake air temp was still high!

But how could that be? The air reaching the filter was definitely colder than before, while the bonnet scoop was sealed to the intercooler and all the air being pushed in through the scoop was being forced through the core.

Something had changed though – definitely. The intake air temp in cruise was still 10 degrees higher than it had been, and the intake air temp rose dramatically quickly when boosted. Like, two 0-130 km/h runs would give an intake air temp of around 65 – 70, when I was expecting 55 – 60. Sure, I’d brought the temp down from today’s starting point of 75, but that 65 – 70 was way higher than I was used to seeing in these conditions.

What had changed? I started developing weird theories. Perhaps the turbo was on the way out and somehow its outlet temps were being elevated? Maybe I’d dropped a rag over the intercooler core – an impossibility when every time I popped the bonnet I could quite clearly see that nothing was obstructing it.

Well then, exactly when had I last been testing like this? It wasn’t yesterday, I thought to myself, because yesterday I had the wheel alignment done and bought those two new tyres.

Wheel alignment…. reduced intercooler efficiency – surely there could be no connection? If the alignment guys had knocked off an intercooler clamp or something like that then boost would be noticeably down – and why would a knocked clamp increase intake air temp anyway? But then I had a thought. Perhaps they’d altered the fixing of the undertray and the air that was previously flowing out past the engine was being blocked?

I parked by the side of the road and dived under the front. And there was a change, clear to see. Normally the short front undertray of the Maxima hangs down at the back. In fact, it hangs so low that at times I can hear it scraping when negotiating steep driveways and the like. I was never sure if it was meant to hang down so low, but after working on the car I always left it as it was. But the blokes at the wheel alignment place weren’t going to have bits of plastic hanging down low, were they? They did a neat and workmanlike job by using a cable-tie to lift the tray up into place.

And by doing so they blocked the air exit of the intercooler, located high above….

I grabbed the cutters and nipped the cable tie, letting the rear of the undertary fall down as always. And yes, now the intake air temps in cruise were back to around 30 degrees C and the peak on-boost temp that I saw was just over 60 degrees.

As I write this I am fascinated – what happens if I take off the undertray entirely? What happens if I remove the undertray and then place a little aluminium spoiler across the underside of the car? Or what about making a new undertray that has a broad opening all the way across the rear?

Tomorrow I think I’ll try some quick and easy variations in front undertray design – y’see, in some cases, aerodynamics matters a great deal…