L31MaxExpress
I'm Awesome
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Install complete , went well no leaks ....quality pieces IMO . Bolted right up to the factory Y pipe. Surprisingly it isnt any louder than the factory manifolds and you feel a slight power increase right off idle then especially in the mid range and upper rpm where it just feels like its revving freer if that makes any sense. Number 6 and 8 could use a shorty plug probably but I put in a pair of DEI insulated boots as a precaution. The Accel shorty plug p/n is Accel 0574S-4 . The flange is .38 thick too .
Exactly my experience with a quality shorty as well. More power everywhere than manifolds but not as much average torque gain as long tubes. Tri-Ys build the most low-midrange torque but the only option was the $$$ Thorleys and I am not sure they make them any longer for a GMT400.