An interesting article in this month's Porsche Post, as to the pro's and con's of aftermarket
air filters V the standard set up.
The performance figures are suprising!

The personal philosophy I apply to modification is not too radical- it should be unseen or relatively easy to reverse - that way I keep my 'on the fence' feelings appeased. With that in mind one of the first modifications I undertook was to change the air filter. The air filter is often branded one of the greatest restrictions to your car's performance - replacement filter manufacturers often state 10+ bhp improvements. What makes modifications like this one attractive is the ease of access. It is fairly easy to perform, regardless ofability as a mechanic, and it's relatively cheap compared to other options.
"I went for a K&N replacement panel filter in this instance - it is a known quantity in terms of
quality, and is readily available from most places including internet stores. Expect to pay between £40 and £100 approx. for a direct replacement filter, depending on the manufacturer. It is a direct replacement for the standard filter and is simple to change. Just undo four clips and swap the filters. There is a clip on each corner of the air box cover. The two at the back can be a bit tricky to manipulate. On reassembly, the filter needs to be placed in the box before refitting the coverdue to the filter depth.
"It is always nice to see actual results of a modification, so a trip to the rolling road and we
get to see 'what's what'. "First off, it is a run with original filter with car up to temperature, to set a base line and ensure all is well with the motor. Theoretically this should be the run with least power. "Next a run without any filter or air box cover. I've noticed covers are available with most of the material removed, so wondered if the cover and snorkel is a restriction in itself. This run should give the best power as there will be no restriction to impede air flow into the air box inlet. "Just as an experiment, I did a run with no air box - that is a naked airflow meter. Call it curiosity. "The last run is with the replacement K&N filter, air box and cover, all back together. Interesting to see from the table below that this falls between run 1 and 2. Dyno run Modification bhp Torque on test.

1] Standard. BHP 225 TORQUE 190
2] No filter or cover. BHP 227 TORQUE 190.4
3] No air box. BHP 216.9 TORQUE 188.5
4] K&N filter. BHP 226.3 TORQUE 189.4

"It can be seen that we are not talking about massive power figures here - Porsche had a
pretty good handle on their intake design. The air box has an integral snorkel or trumpet. which
does a good job of smoothing air flow into the air flow meter . "Without the air box in place there is a significant drop in power. A lot of people junk air boxes, but I'm not sure about this one. I've read similar reports, with Clio Sports having aftermarket induction kits that do not better the factory effort.
"It takes me back to the days of just passing my driving test. A pal took his mum's 'hand-medown' to Halfords, junked the exhaust. and fitted the loudest they had - I had pity for his
neighbours. But it wasn't long before he drew the unwelcome attention of the law. He told me of
one dressing down, where the officer asked him what on earth led him to believe he was more
qualified than the car manufacturer, with their million-pound R&D department. It not only
stuck with me for being so funny, but also for being so true.
"In the end, the replacement filter gave me a 1.3bhp gain; and only O.7bhp down on no filter at all. "The bhp figures are down on Porsche's claimed 231, but I guess a 6bhp drop over 22 years is not bad for a non-rebuilt motor. More importantly, it picks up cleanly, doesn't burn oil, or hesitate, or have any flat spots. It may just be the rolling road I use, and it is important to use the same one regardless of how unflattering the figures may be
- it's the best way to gauge improvement (or not).

That's really interesting Hu.

There's so many people claiming massive increases with aftermarket filters or cones.  This goes to show two things.  (1) Porsche knew what they were doing in the first place, and (2)  leave it alone 

The only air change that makes a real difference is throttle bodies, but they cost a fortune....

That's brilliant.  I agree with Pete.  So many people believe the hype and buy cone filters believing they are getting more power. 

The only problem with throttle bodies is mapping the air fuel ration at low revs/idle in the cold.  Other than that they are fine.

After reading this I think If I change anything? It will be the stock filter for a K & N stock shape one and leave well alone!

The stock system is best