Hi guys, I have a 01 procharged cobra, that I recently swapped a built 03 short block on with 04 heads that were just refreshed to stock specs. I reused my manifold. As soon as I start the car it'll start to backfire pretty constantly at idle. Im pretty sure there is no exhaust leaks, or vacuum leaks. I'm seeing 20lbs vacuum at idle. My fuel pressure does seem a little higher than I remember. I think it used to be 30 at idle but now it seems to be at 37 but it's been almost 5 months since I last had it running so my memory is foggy on that. Plugs are new tr6s gapped at .30. The car was tuned by lasota and it was pretty good before the swap. The only other difference other than the new block is the new longtubes. I'm getting a retune next week, but am just trying to see if you guys think this is a tuning or mechanical issue? Would longtubes be causing a backfire without a tune for them? Thanks for any input guys.