you got true dual from headers to the bumper? or Cat-back dual? Could have something to do with the whole highly debated Back-Pressure thingey... Or, if you did true dual, you might have moved the O2 sensors out of the optimal flow of exhaust. OR maybe it always has done this, you just haven't been looking as close before.
no mufflers? hmmm...youve gotta have at least a LITTLE backpressure to keep the car runnin...my 56 mercedes, after i re-installed another motor from a running parts car, there was no exhaust system (still isnt a total one yet, but a rusty exhaust manifold with a glasspack clamped right behind it)...JUST to keep it from stalling, i had to put tension on the throttle linkage to the carbs, had it idling at 1500k...every now and then, there'd be a little flame comin from the exhaust ports (where the headers SHOULD have been) it runs like crap so far, even though the motor had a fairly recent restoration, just swapped out the orig. carbs for dual 2-barrel weber carbs before i headed off to school...we'll see how it runs come springtime!! anyway, bout that setup, that suckers gotta be annoyingly loud. that benz, even though its a 4cyl, sounded like a shrieking frieght-train when it was fed some revs...lotsa lifter click-clacking...
These engines use a measured amount of back pressure in order to create low end power. Going straight from the cats-out, there's very little pressure available. Top end must scream, but your going to lose the off the line "oomph" these have. If it bothers you that much you could clamp in a pair of "turbo style" cheapo mufflers. They'll quiet the car down abit but also help to bring the low end back up to snuff.
You really opened up the exhaust didn't ya? LOL. I can't imagine the sound response you got out of it. Only thing is you lost so much low end power, intrepids are the worst at lowest speeds, especially with the 3.3. If you happy with it you know, dont fool with it, waste of money, but if it were me, I would attach some sort of muffler, maybe like a lil 20 dollar thrust muffler on each side from Advance. OR move up to the Dynomax Ultra Flows or Flowmaster. Just a couple of ideas. You race it much? How does it do??