Yup,Jason(88Workcar) is right.I know because when I went to buy my paxton blower kit,there was a speed density kit AND a mass-air kit also.I'm not sure what Vortech offers.
I do know a guy who(used to be a member on this very site even) had a Paxton blower on a 87GT who ran a best ET of 12.166 @ 112mph
with a speed density kit installed on his car.
There is nothing wrong with speed density,you just have to understand it and know how to tune it correctly.
Kinda like Paxton ball-drive blowers,I get all kinds of people saying they never hold up,you can't make them run good ET's,etc,etc,whatever.Well guess what,there will always be us underdogs that prove otherwise
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Paxton Blown 87GT