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ok the info i got from stillen on the exhaust is for the 3.5 not the 2.5 so there will be a diffference there. but HS is claiming like 10 hp, AEM says about 7 to 10 and the majority of aftermarket exhaust systems are good for about 7 to 12 hp +or-
look in the june 2002 issue of Sport compact car they get 40hp with just boltons in a WRX. granted its a turbo but these kinds of gains are becomming more common with the new engine designs and learning computers. the older ecus went off hard set codes and didnt stray from them. the new generation of computers actually learn drivers habits and make adjustments for all kinds of variables like more airflow less back-pressure and such to make the power that make. thats why its going two ways either no ecu upgrades or huge gains in some cases depending on the application. the days of slapping a monster cam and high comression pistons with a gas chugging holley double pumper are over. but the time tested tricks still work, sometimes rather well.
most dont know how these bolt on parts can create more hp, its simple. the internal combustion engine is basically an air pump. the more you can get in it the more power you can make. basically.
cai's work off drawing cooler air through a smooth bore tube increasing its velocity, more air in more power. headers and exhaust systems , same principle smoother less resistant path for the air to travel in, and removal of the primary cats in most cases (qr25)helps a great deal. also headers as well as exhaust systems work on a simple point they make use of the exhaust pulse of the engine to create a vacuume in the pipes. so on each exhaust stroke it literally sucks the spent gasses from the cylinder thus making more room for the intake charge. as to allow more clean air fuel mix into each cylinder ie: more power...
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Smeeeeeeeeeeg Heeeeeeead
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