I have bought a zenden extractor from kent, to fit my 660, it is 3-1 extractor. About the performance, low rpm torque is totally gone, but the strange thing is, the car is go more faster.
I test it with my friend, first my friend say that he can't feel any pick up from my engine, but look at the speedo meter, the speed is increasing more faster then before.
About zenden extractor, I can say that quality is poor, the extractor gasket can't fit perfectly, then I send it to my friend workshop to do minor modify. Ofcourse the zenden extractor look nice... if you think you really want to buy a performance extractor for your kancil, try my friend one, good finishing and more light weight.
By the way, my exhaust pipe is stock, only the extractor is zenden.