to the 13's

[86GT]

The intake is restrictive compared to the 87+ ones. They were off of the Crown Vics and had a 58mm throttle body instead of the 60mm. The easy way to check is look under the cover plate. If it has HO cast into the intake, then you've got the 87+ one, if not, get it off of there. I did that swap about 5 years ago, and it made a noticeable improvement.

Change the heads to something better too, TFS TW will fit the flat top pistons that you have, as long as you don't put a huge cam in there.

Hope that helps.