I've heard before that the LS1 needs backpressure, and that's why it came from the factory with the cat-back setup that it did.

When I installed my Rumbler I felt a small loss of low-end grunt and my tach. can redline off the line. I personally think that a free-flowing exhaust is the best because it allows the engine to breathe.

Here's my question:

Can the addition of deeper gears like 3.42s or 3.73s make up for the loss of low-end grunt that comes with free-flowing exhaust?

 

it comes with the "cat back" setup it does for clearance, and to pass federal emissions laws. Backpressure is a side effect.

 

There in lies the myth ... I measured no loss in low end on several dyno pulls over several different mods, between a cut-out and no cut out

What I saw was less gain at the low end relative to the gain on the high end ... just adding the cut-out was only worth 1 rwhp at the low end, compared to 12 rwhp on the high end ... over a 10 fold difference between the high over the low.

It's that difference that most people interpet as "feeling a loss" on the low end.

 

backpressure is a lie. LS1s have that crappy exhaust because of ease of mass production and elimination of noise level.

You shouldn't loose low-end grunt with any exhaust, you probably arn't hooking up because you have more power. bigger gear ratios give you more torque down low, but say good bye to hooking up with stock tires.

I say get some better rubber.

 

Hooking up isn't a problem. I know how to launch at the tree, I'm cutting 2.1s and 2.0s on street tires.

I'm saying that after I installed the Rumbler, the car takes about a split second longer to accelerate down low than when I had the stock cat-back in.

Even my GM Master Tech. (who races) recommended that I leave in my stock cat-back and install longtube headers just so the engine has enough backpressure. Maybe he doesn't know what he's talking about.

It's not a big issue, more power = better, regardless of the backpressure nonsense I'm thinking of, I just wanted to see what others thought of this.

 

my car was funky like that when i had my cutout open. so i started opening the cutout after a launch. could have been my imagination too but who knows. have the same problem now too but thats because of my cam doesnt really kick in til 2k rpm

 

that is odd. If you're hooking up fine, your problem could be with a sensor. In any case, don't believe backpressure.

I recommend you get headers and a tune, that'll fix up your acceleration problem, but a tune is pretty useless and a waste of money without headers and/or a cam.

 

Higher exhaust velocity = more torque at lower RPM.
Smaller pipes = more velocity
Smaller pipes = low RPM more torque

It has nothing to do with backpressure.
The stock system is 2.75" and Rumbler is 3", there isn't that much difference. My guess would be that you may have lost a little throttle response because you're A/F ratio is probably a little off now from less restriction.
The flat spot of the stock Y has the same crossectional area as an aftermarket. The only gains you'll see from an aftermarket Y is !cats.



Get a looser converter for your A4 and don't look back.