Project S2000: Part 20 – Stock Air Box Modifications And Not Going Faster

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So, hacking the stock air box appears to have done very little besides increase the intake noise slightly. A click or two up on the volume of the radio takes care of that in street driving. It was time to test out the other theory of removing the air box lid completely. I went back out to my secret test location, did two pulls with the lid on, stopped, took the lid off with the car still running, and did two more pulls all within a few minute span. Taking the lid off with the car still running made it easily apparent the car was going to be quite a bit louder. At WOT, it sounded like S2000s equipped with K&N intakes, but a bit louder.

Comparing the two runs with the lid on versus the two runs with the lid off, you can see that removing the lid did increase the intake manifold air pressure about 0.025-0.050 bar. Did that translate into more power?

Quite the opposite, removing the lid cost a ton of power. In just doing 3^rd gear pulls going from about 3700rpm to 7800rpm, the time required increased from ~6.3 seconds to ~7.6 seconds. You can see this was repeatable too as two runs were done for each.

What was the culprit for the power reduction? The hot air coming off the radiator caused drastically higher intake manifold air temperatures. The points I have tagged are at 7800rpm at the end of the first pull. Lid on versus off, the IAT was 131F vs 144F. Ignition timing was 25deg vs 24deg. Check out the ignition timing curves however and you can see with the lid on, the timing was running much higher through the entire pull. Looking through the complete plots and the second pulls, you can see the trends are consistent.

Using my handy dandy dyno spreadsheet, removing the lid is costing ~5-10 lb-ft of torque at the wheels across the engine speed range tested. That’s about 15whp at the upper end of engine speed range.

So, one lesson learned is Honda did a damn good job on optimizing the stock air box design and there’s nothing to be gained in power by hacking it up. The other lesson is colder is better (as should be obvious). Removing the air box lid will make your car much louder making everyone look at you. Removing the lid will also reduce your power significantly giving everyone more time to look at you. If you crave attention, I guess remove the lid. If you like more power and not drawing attention leave the lid on.