Well I have the 2.7 so my base hp is slightly higher then the 2.5 was, but as far as modifications I beleive the both the 2.5 and 2.7 benefit from many of the same type of modifications.
As far as tweeks to make the car faster:
1) Fuel chip tuned on a Dyno can add about 25-30 hp. This process requires a chip to be removed from your Cars ECU and then flashed based on the stats that come from a Dyno test of your specific car. This must be done by someone who truly knows what they are doing.
2)AirBox. You can gain about 7-9 hp. There are a few kinds of airboxs available for the Boxster. I ended up ordering mine through TTP in Germany which makes many modifcations which I have done as well from them. This item give a nice sound!
3) Iridium Spark Plugs. THis is a simply tweak some guys and I came up with to replace the Bosche Plugs that are standard in the vehicle. If you decide to do this option I would suggest doing it prior to having your Fuel chip tuned. You could possibly end up with Air/Fuel mixture rations going bonkers.
4) Custom Headerby TTP. This mod completely removes the two front Cat Converters due to the new heads being extra long. This is a great mod! Probably 5-10 hp depending on who you buy yur pipes from. Craftmenship means alot in this area. FYI removing teh front to Cats is how the Euro version of the Boxster is.
5) Replace the two rear stock cats with a High Flow Cat. This will only provide maybe 2-3 more hp but allows your engine to breath a lot better. A must if you do both the headers and muffler.
6) replace he stock muffler. you wont get a huge gain here, maybe 5-7 hp, but can truly change the sound of the too quiet Boxster and make your 986 sound more like a 996.
7)And lastly something I have been working on with some local engineers with TTP is putting a twin turbos on the car. Through some feedback from engineers at TTP if done properly can add about 100 hp to your vehicle. I hope to do this mod maybe this fall. Its an 80 hour job.
Hope some of this info helps
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