986 still down on power. Thoughts?

[Morph_986]

I would like some advice about my 986 (2001 Boxster S, manual, 66k miles) that still seems down on power after completing most of the normal checks and maintenance.

First things first. Power (at wheels) is being measured by a Vbox Sport on the same piece of road with the same driver in pretty much identical conditions. Second I live at 7500ft and where the road is results in almost exactly a 20% reduction in power for a normal aspirated engine. I am therefore always going to have a weak car by normal standards. However, I am 10-15 HP down at the wheels on where I should be accounting for 20% losses in the drivetrain. Some recommended a figure of 15% for a manual and that makes things worse.

I have the Durametric software. It shows only one issue I will come to later. No pending or actual P codes. I changed the air filter, cleaned the MAF sensor and throttle body, changed the spark plugs, power cycled the engine computer, recalibrated the throttle and cleaned the air plenum. Vacuum level is appropriate for the elevation suggesting no great vacuum leaks. The old plugs did not show signs of running lean. I also did a concentrated sea foam gas treatment to clean up the injectors.

Net result, no change in HP at all, except for maybe 2-3 HP after the sea foam, but no more.

The only known issue is with the bank 2 variocam where the o rings are failing. I see green chunks in the oil filter. The Duratech software reports that both banks 1 and 2 variocam systems are working as the engine revs change as they should. The 23 degree indicated on bank 2 is steady. If I pulse the variocam in the software at low rpm, bank 1 holds steady at 24 degrees while bank 2 goes to 23 degrees for a second and then creeps back to 0 over 2-3 seconds. So that actuator is clearly going bad but is not totally failed.

I did a secondary cat delete. Absolutely no change in HP after.

This makes me think that I have a limiting fault somewhere else that is preventing maximum power. Thoughts on what this could be? No signs of worn piston bores or rings but I have not done the tedious business of a compression check. Changing the plugs was tedious enough.

The only two items I can think of is a blocked fuel filter limiting the availability of fuel under load, or garbage in the air box before the air filter. Can the Duratech software tell me if I am starved of fuel under load or do I need to squirt the fuel line into a container for 30 seconds as noted in the Bently manual? I can recheck the air flow. I noted nothing when I swapped the air filter, but I was not looking hard.

Many thanks for your time.

Philip