Dynos and air filter testing

This has always been a hot topic with any marque of car, not least of all the MX5 market.

The other day we did a panel air filter test. By this I mean we swapped out just the filter element itself inside the original airbox. I’m going to call the original style of filter ‘Paper’ I’m not sure they are made of paper these days looking at them, but we’ll stick with that.

A little bit about chassis dynos

A chassis dyno is basically a tuning tool, it’s for tuning or remapping cars. You cannot take a car from one dyno to another and expect the same figure, end of story. Some dynos don’t even give the same power/curve shape as others, nothing like on some! You’ll see in the pics below that the same car run over and over again gave a different figure everytime. That’s because things are constantly changing within the car. The first run is always the lowest, the second will make more power after that it can be up and down and all over. The reason for this is that many items are heating up and cooling down. Friction (and also intake air if it’s a turbo car) therefore changes throughout. Changes which alter the power output (due to temperature) are tyres, diff (oil), gearbox (oil), driveshaft CV joints (grease), intercooler on a turbo car and of course engine temps – the temperature of the actual parts, the oil and the water and the intake air perhaps. Some things get more efficient as they get warmer, some not, all have a peak operating temperature which you can not reach, or exceed.

Did you know (all things being equal) that an engine with a plastic intake manifold will make more power than one with an aluminium one? It will and the reason is plastic doesn’t conduct heat very well so it doesn’t get hot from being bolted to the engine, therefore it doesn’t transfer unwanted heat to the intake air.

If the Dyno has ‘calculated’ power at the flywheel for the graph then things get even worse! Dynos guesstimate (sorry for our overseas readers there) the power at the flywheel using the figure taken from the wheels. They all use a different method to do this so whilst the Wheel HorsePower figure will be a little different from one dyno to another, the Flywheel Horse Power will be even less accurate, often some 5 to 10%. The Americans are better at this as they usually quote WHP not FWHP. The only issue with that is an engine will show a different figure at the wheels depending on whether it is FWD, RWD or 4WD, but then as we learned from above, you wouldn’t sensibly compare them anyhow.

The reason for that ^ little diversion is that you have to be very careful of what you read when people post up dyno results. If they’re from different dynos, or even the same dyno, but with some time lapsed between runs the figures will be less than accurate. Cheap bolt-ons like air filters show very small results anyhow so these results can be mixed up with, or even overshadowed by other changes which are happening all the time .

I have never tested an MX5 with one of those snorkel type replacement air intakes, you know the type. The airbox is completely removed and a length of pipe put in place leading to the front area of the car with a filter on the end. I am extremely suspicious of the figures bandied around as they seem to rewrite the laws of physics. Now if you were to do a test on such a type you would need to do what we did and do multiple runs, all one straight after the other. Therein lies a potential issue with testing these type, you would have to have pre-rehearsed how to change from one to another so you could do it really quickly. If a lot of time had gone by whilst you undid everything then some accuracy will be lost. I know that actually fitting them requires taking the front off the car to get it in.

You will have seen these K&N panel filter copies for sale here and there, mainly on FB. The vendors claim a power increase. Someone also incorrectly independently tested one. They took out a USED paper filter and replaced it with a NEW ‘uprated’ panel filter. They did one run and it showed a gain. Well no surprise, taking a dirty filter out and replacing it with a clean one gave a better result.

So anyway, onto our little test. We aquired one of these things and a brand new paper one too.

The car was a 2006 with a BBR 200 in it, power output as you can see was hovering around 200bhp.

6 runs were done overall, the first three were with the aftermarket filter in (Filter 1). The car was then stopped for no longer than a few short minutes whilst the paper one (Filter 2) was put in as quickly as possible. 3 more runs were done.

An average was then taken of the 3 runs to give wheel horsepower figures:

Paper averaged out at 149 WHP

Aftermarket 143.8 WHP

So an average loss of 5.2 WHP torque similarly lost 5.2 Nm

With a paper filter the car more or less matched BBRs figures.

Previous to that test we did another on a different dyno, it showed a loss too, It was done the other way around, paper in first then aftermarket after that. I honestly didn’t believe it hence us doing it again on a different dyno.

Don’t believe any of this tripe about the engine learning how to cope with this new found airflow, if there is more air available it’ll show as a power increase immediately. We don’t remap cars and wait a week for it to catch up and show a power increase.

I would guess that (or I would hope!) the aftermarket may outflow the paper after both have done 10 thousand miles or so and got some dirt in them. That would be a difficult test to do accurately. Mind you, with the size of the holes in an aftermarket filter it’ll let a lot of dirt through in the meantime.

Looking through a genuine K&N into the sun.
Looking through a paper filter into the sun.

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