Project 718 Cayman T: Part 8 – BMC Air Filter

Here is everything put back together. If you look really closely, you can see the cut line in the carpet. The next time I have to get under the engine cover, I just have to remove the panels on the front-side by the seats and the chassis brace. Fewer steps, less time, less chance for breaking stuff, and less chance of losing those mini black rubber traffic cones.

Is there a measurable difference in power going to the BMC air filter compared to stock? I would add my stock air filter was only very slightly dirty, so I’d expect essentially no performance drop. Looking at this first run using Virtual Dyno to estimate the power, it looked like the BMC freed up a big chunk of power up top. That was suspicious to me…

So, I ran the test again and this result passes the sniff test. Which is to say, small gains were expected. I bet with the first run, I just happened to get a gust of tailwind at the top end of the pull. All Virtual Dyno is looking at is the rate of change of engine speed to estimate torque and power.

Using the data logging capability of the COBB Accessport, we can dig into the results more. Above is the plot of the Torque Actual value. You can see the BMC air filter runs had a lot more drops in the torque value which was due to more knock and ignition timing correction. I would add this is the same tank of gas and same tune on all of the runs. My hypothesis is uneven distribution of the Race Gas Concentrate in the tank. I’m working on being able to run straight 91 octane only while making more power so I don’t have to use any more octane booster.

To make more power, you need more air. Fortunately, we can data log the calculated Mass Air flow value. It’s clear that on both runs with the BMC air filter, the engine was getting more mass of air compared to the stock air filter. The ambient air temperatures were within 2degF of each other (62F-64F) and the Intake Air Temps within 4degF (75F-79F) as data logged with the COBB Accessport for all three runs; on top of that, the BMC runs where the higher temperatures which would have a negative impact on mass flow. This comparison is as good as you’re going to get short of doing an engine dyno in an environmentally controlled lab.

The data shows the BMC air filter allows the engine to suck in more air compared to the factory air filter with everything else being equal. There was another immediate indicator that the BMC air filter flowed more than stock; when the car was completely stock everything, I noticed compressor surge during spool-up when the ambient temperature was below 70F and driving near sea level. Maybe it occurred when the air temperature was warmer too, but it for sure was happening with the cooler ambient air. I was surprised as the car was completely stock, but it had the very distinct choo-choo-choo noise of compressor surge. How can you prevent compressor surge? You reduce intake flow restriction which means the compressor can operate at a lower pressure ratio for the same flow and intake manifold boost pressure. With the BMC air filter installed, there were no more choo-choo noises.

Reducing flow restriction before the compressor inlet has compounding benefits to making power. You can build your own calculator and play with numbers, but the concepts are simple. The first benefit to reducing pressure drop in front of the compressor inlet is you can run a lower compressor pressure ratio. This immediately drops intake air temperatures because you don’t have to compress the air as much. Because you’re now operating at a lower compressor pressure ratio, that means the compressor needs less power resulting in less turbine power required. Lower turbine power means lower turbine pressure ratio which reduces the exhaust back pressure on the engine. Lower intake air temperatures and lower exhaust back pressure are both good things for making more power.