Headers you really cannot lose out. However depending on what's done to the car is how much you may benefit from them. I am sure you can find 3.4 headers but finding a set to fit in the fiero could be a bit challenging. The stockers ported out are not all that terrible. If I was going to change anything it would be the ypipe that connects the headers together.
[This message has been edited by Spadesluck (edited 11-30-2018).]
Since the sprints were discontinued the only headers available are for the S10 . They won't fit a Fiero install without serious modifications. I have a new S10 set but they won't fit without serious modifications.
------------------ " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "
[This message has been edited by Dennis LaGrua (edited 12-17-2018).]
Porting the stock manifolds and Y-pipe helps, but headers are even better. It's been proven that porting the stock exhaust yields 8-10 HP. And my dyno test of the Trueleo headers yielded 7 HP more than a ported exhaust. So a good set of headers should yield about 15 HP over stock.
As for sources... good luck. Headers used to be available from the Fiero Store and Trueleo, but not any more. I'm hesitant to recommend WCF, because of their shaky reputation.
Edit to add: Keep in mind the Camaro / S10 headers are for a longitudinal setup. And the Fiero has a transverse setup. The S10 headers will probably interfere with the Fiero transaxle, plus the exit will be in the driver side wheel well. In other words, you'll need to make some serious modifications to make those headers work.
[This message has been edited by Blacktree (edited 12-16-2018).]