does anybody know how much extra hp i would get with a set of twin 670 pipes from fast? im prolly gonna bye a set from a friend for $150 also where should i start with the jetting? clutching? any ideas or good setups?

I ran a set of Fast twin eliminators on a 97 MXZ 670. Fast claims around a 10 hp gain if you run with K&n filters and a longer duration RV.
I ran mine with K&N's, .8 base gasket, .065 squish, 150-80 RV. This makes it a 91 octane minimum sled. I ran 410 mains on both sides with the K&n's at low altitude.

I ran 145 ramps, 17.5g(Ithink) 200-320 or 230-320 depending on track you are using and a 50 deg helix @ c6 with a beige spring. They liked to spin around 8400 on my tach.

I could not get them to work correctly with a gutted box. At that time FAST did not recommend running the stock airbox. Either gutted or K&N's. Remember to gear down. I ran 24-44 with an 1 1/4 and I weigh 170.

Sled was a rocket in the right conditions, a dog in the wrong ones.

My opinion now after looking back on it would be to leave the stock pipe on.

Run short boots, 501 RV, .065 squish, y pipe mod, and a good clutch setup and you will be miles ahead and money in the bank.

Many people have commented on TT670's setup over the years.

Check with him but I think it is PX ramps, 200-350, 18g, 48-44 BI @b6.

Good Luck