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1 1/2 headers would be better cause they match the ports on your stock heads and the smaller tubes will maintain your torque while increasing HP.

1 5/8 headers are larger than the ports on your heads wich reduces the velocity of the exhaust gases leaving out reducing your low end torque. Stock heads are not capable of making use of the extra air flow capacity that 1 5/8 headers provide and would probably lose HP over the stock headers.

That being said my 1 5/8 equal length headers that I have on my car are overkill but I will be able to take advantage of there flowing capacity when I get my nitrous.

Later,