Every once in a while when the car shifts at a high rpm from 1-2 there is a large bang (my car shifts very hard) that comes from under the dash and it shakes, it doesn't happen all the time but ever since I got headers/catback it will do this randomly. Any thoughts on what it could be?

Broken transmission mount.

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1988 Formula 5L/5sp 3.45 gears, SLP cold air kit, MAS gutted, TPIS air foil, TPIS AFPR, MAC headers, Mac under drive pulleys, MAC cat back, Relocated MAT.
Best ET street tires 13.85
Best 60 FT street tires 1.930
Best MPH street tires 99.29

I'll second what bjankuski said. It's probably just a coincidence that the problem started after you put on your exhaust.

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91 Z28
Come see The Vicious...

Check your torque arm mount on your tranny also.Mine did the same thng.

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'91 RS 5.0 TBI
New paint-solid black
ROH Snyper wheels-17x9 wrapped in 275/40/17
ASCD SS hood, High-rise Z-28 spoiler,shaved handles/antenna,
Hooker super comp headers,dual 3" exhaust(yeah,too much for my set-up,but it was free!),two 1-chamber Flomasters,Eibach Sportline springs,KYB struts/shocks,rebuilt steering linkage,Iroc sway bars,IROC steering box,wonderbar,140 Iroc gauge cluster,Auto meter gauges,dual guage A-piller pods,open air filter,AFPR,smog delete
TBI mods,180* thermostat,
383 stroker in the works

President-Austin 3rd Gen F-body c.c.
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If I don't get it fixed right away will it cause problems, it doesn't occur often only occasionally, I will check it out though, I just want to know if I'm going to do any serious damage if i drive it. I also have a dyno scheduled in 2 days but since it doesn't require hard shifting I should be ok, right? Thanks

Drive it like grandma and you'll be ok. Make sure to get a poly mount to replace it like energy suspention, so you never have to do this again.

uhm, while on this topic...my car thumps under the seat when gunned at the beginning of a gear...feels like i lose performance...do i have the same problem? how much are mounts and arms?

yeagh its probably the mount. mine started to do it after I replaced my clutch. I didnt realize how bad my clutch was slipping, until it wasn't any more. Any time I gunned it, the tranny would shoot up and hit the floor plan and make4 a loud bang. The mount isnt that much. I think poly ones are like $20.