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.

• On the dyno people never trial all the different combinations of throttle position, load and engine rpm that you’ll find in a few days of road driving. (I originally said in 10 minutes on the road, but let’s be scrupulously fair.)

None of this is hard to understand: making sure that boost doesn’t exceed a certain level in relatively slow-changing engine conditions is vastly easier than doing the same on transient – but major – changes in engine load that might be completed over just a few seconds.

And finally, I made the point that an intercooler cannot be effectively tested on a dyno.

Recently I have been testing intercooler water sprays. As you would expect from this prelude, the testing has been done on the road. We have written about it in the past: all other things (like drop size) being equal, the effectiveness of a water spray is highly dependent on the mass-flow of ambient air through the ‘cooler. But let’s take it further. In fact, it’s the global airflow over, under and around the car which will determine how well a spray works. Why? Because it’s this airflow that determines how much air passes through the intercooler.

If there is little air passing through the core, the water droplets will evaporate on the surface of the core, cooling only a tiny proportion of it. On the other hand, if there is plenty of air passing through the core (and plenty of evaporating water droplets to go with it!) the majority of the core thickness will be cooled.

Irrespective of the size of the fan stuck in front of the radiator, a dyno simply does not replicate this global airflow. It doesn’t even come close. And that applies to some dyno fans that I have seen that are massive – housed in a cube-shaped frame standing taller than I am. To get airflow that is characteristic of the road – and those characteristics include speed, degree of turbulence, temperature and relative humidity – a full climate-controlled wind tunnel is needed. And those don’t use fans as tall as I am – instead they use a fans many metres in diameter and driven by enormously powerful electric motors. Most wind tunnels are also of the recirculating design and have high-speed moving floors.

So, recently seen a modified car workshop with a climate-controlled, moving-floor wind tunnel with a dyno in it? No, neither have I.

Very, very clearly, writing something like: “I experimented with a water spray when my car was being dyno’d recently and it made stuff-all difference – there was no noticeable improvement on the dyno with my front-mounted 600x300x75 cooler” is rather like saying: “When I moved my car from the driveway into the shed, I could detect no improvement in the handling of the new tyres”. If you are not exactly replicating the real-world conditions in which you are trying to gain an improvement, what worth is the testing?

In fact it is worse than useless – it is potentially misleading.

It’s not as if on-road testing is difficult or expensive. Just put a fast response temp probe in the intake air stream and watch it as you drive around. [And if the car is of such performance this cannot be done on the road, (1) you wonder what use it is as a road car, and (2) you can always hire a track.] Have a switch that manually turns the spray on and off, and remember what the numbers show in different situations.

Without spending a cent, you have a moving road, the correct global airflow including such subtleties as real world turbulence, accurate engine loads for the available airflow, and so on and so on.

It’s in this way that you can find that in some turbo intercooled cars, peak intake temps on load occur immediately after being stopped in traffic. Or in other intercooled turbo cars, a slow climb up a long hill when stuck behind a truck can cause intake air temps to go higher than when on a full-boost, through-the-gears 0 – 150 km/h run. Or in other intercooled cars, 15 minutes of consistently hard driving will blow intake air temps out of the water.

You’ll also be able to see in what conditions of road speed, ambient temperature, and load history an intercooler spray is effective. In experience of my own turbocharged intercooled road cars (13 different intercooling systems on nine turbo cars over nearly 20 years), an intercooler spray that is triggered by a dumb boost pressure switch spends most of its time wasting water. The spray control system must have the intelligence to at least monitor intercooler core temp (or, less preferably, intake air temp) and engine load.

To think that a quick dyno run at full throttle (or even 30 ramped dyno pulls, one after the other) is going to tell you anything much about on-road intake air temps, or the efficiency of an intercooler water spray, is completely fallacious.

And the great thing is, it’s so easy and cheap to get absolutely cast-iron validity in your results. Just do the testing where you drive the bloody car…