I have a 1970 nova with a stroked 350 motor, everything with it origonaly seemed to be running well. I took the car to a shop for some new wheels and tires, and ended up lowering it as well. in doing this my custom dual exhaust that ran over the axles became a rubbing issue, so I chopped them off right before the axle. Now for some reason i get this weird pulsating noise at high rpm right before a shift change... sounds almost like floating valves or bouncing off a rev limiter. Did i loose back pressure or something? It seems that the car runs perfectly fine while in park, no matter how i rev it. This would be the easiest fix (i'm guessing just throw on a couple of crimped turndowns?)but their are also other possibilities... While my car was in the shop the 'shop retard' tried to pull it over a floor jack that ripped a gaping hole in the oil pan. This required them to pull the entire motor in order to fix it... but i haven't been able to notice anything that might be adjusted wrong, the tranny vacuum line and kickdown cables seem to be operating correctly, and not crimped or pinned. Anyone got any ideas??? The amount of pipe that was cut off was about 3.5-4 feet of pipe from both tailpipes  

Hard to say what a "weird pulsating noise" is for sure. But it could be that you're now able to better hear a problem that was there all along.

Thinking logically about the the change in pipe length, and the other symptoms you described, perhaps the car is now too lean on fuel at high r.p.m. under load.  Note that just reving the car to maximum r.p.m. while standing still doesn't really test the maximum capacity of the air/fuel delivery system.  In fact a car with a seriously occulded fuel pick-up or underjetted carb/undersized carb will pass a curb-side, no-load "rev" test with no problem.

There is no need for exhaust backpressure in any high performance engine. (Except for perhaps before the turbine in a turbocharged engine wherein some backpressure is unavoidable and even helpful to drive the turbine)  Backpressure is an indication of a restriction that reduces the air flow potential through the engine.

However, if the engine is jetted for a lower "fresh air consumption" level because of backpressure and the backpressure is removed (increasing the ability of the engine "system" to flow --and thereby consume-- fresh air at a given r.p.m./load level), the engine will generally run lean.  That's one of the reasons why you usually have to increase the jet size when switching from iron log manifolds to exhaust headers (excluding of course the "active" effects of tuned headers, which magnify the lean-out).  The "need for backpressure" wives-tale comes from addressing the lean-out problem by reducing air flow across the engine instead of enrichening the fuel to match the increased air flow capacity.

On the other hand, unless the tail pipes are very restrictive (undersized, too many crush bends, etc.), they have a lesser effect on backpressure because the exhaust gasses have dissipated a great deal of their sonic, heat and kinetic energy by the time they have traveled through the headpipes and mufflers.  

the noise is also joined with a complete loss of power until the trans changes gears... guess i should have mentioned that as well  

Sounds like the valves are floating to me what cam is in the engine maybe the valves were set to tight.  

not sure on the cam anymore, but it's pretty mild, and i don't believe it's floating valves either... why would it do that instead of just changing gears?  

so i went and bought a piece of pipe, cut it in half, crimped it on the ends and attached it to each pipe so that there's about 1/4 of the opening and the problem seems to be gone... course it sounds weird as hell now, lol  

Merely cutting off the exhaust pipes and replacing the oil pan wouldn't make the valves float. So if it's valve float, it was either a preexisting condition that you couldn't hear/feel with the tailpipes on (unlikely) or you're reving the engine harder now that it sounds "badder."

Valve float usually will also show up in a curb-side, no-load rev test.  

Fuel starvation can cause a loss of power and even fluctuations as the float bowls rapidly "cycle" (drain and refill).  A minor lean-out because of incorrect jetting shouldn't cause this, though.

Could there be a fuel line smashed? The tank filter clogged?  

Country_Punk:
so i went and bought a piece of pipe, cut it in half, crimped it on the ends and attached it to each pipe so that there's about 1/4 of the opening and the problem seems to be gone... course it sounds weird as hell now, lol

In essence you've just "throttled" the exhausts, which would increase dilution in the combustion chambers (the result of more backpressure), reduce fresh air flow through the system, and richen the mixture.

In other words, you've addressed symptoms and not the underlying problem.  

well i also tried rejetting and stepped it all the way up from 85 to 100. After taking it apart 15 times i decided that wasn't going to fix it, and according to what you've said letting in more gas should compensate for the extra airflow right? unless i should be jetting way over 100... which seems a bit nuts to me but i'm no expert either?  

What carburetor do you have?

What is the coloration on the spark plugs?

What is the fuel pressure?

What is the color of the inside of the exhaust pipes?

If you can eliminate fuel supply as a variable, go next to ignition and make sure that it isn't breaking down under higher cylinder pressures (load + r.p.m.).  

carb, holley 750 elec choke
airgap intake
plugs are normal looking, maybe showing signs of being to hot on one side?? pics are here
http://i259.photobucket.com/albums/hh301/Country_Punk_bucket/spark%20plugs
fuel pressure is 7.25
heddman headers
2.5 inch exhaust
x pipe
tail pipes are black on inside  

okay so i removed my tialpipe restrictors and wokred at it all day long. I found that the secondary bowl level was really low, and ended up working my way down to 70's for jets in the front. I still have a little bit of hesitation on take off which i think will be cured after stepping down a few more jet sizes, but the other problem i had been describing has completely gone away. Thank you for all the help!