So I've got an '05 MCS with a few parts from the JCW kit and a few from elsewhere. I run the car semi-regularly in autocross, where I'm stuck in SM because my SC pulley is aftermarket - and since I'm on street tires, I'm pretty much non-competitive. I was considering DOWNGRADING my pulley from 16% to the JCW pulley (~12%) so as to compete in STX where I'm much, much more competitive, but I can't help but be apprehensive about intentionally reducing my power output. So let me ask a few questions to clear my mind:

- How much of a power difference will it make? Will I actually feel it? Could I conceivably gain it back elsewhere? I'm seeing a claimed 8-10hp/tq gain from an Alta header / hi-flo cat, would that be enough?

- I have an RMW tune on the car that was done with the 16%. Presumably it would be recommended (or required?) to re-map the ECU post-downgrade?

Engine mod list (for reference): '05 MCS, RMW tune, JCW injectors, JCW CAI, M7 16%, Milltek (cat-back only). 195.6whp, 186.4wtq.

 

I seem to remember about 5hp as the max difference between a 15% and a JCW. Not sure that would make that much of a difference in autocross. As to the Alta header, my previous mechanic told me that every single Alta header he sold came back with CEL problems.