Major Back Pressure Problem off New Exhaust System

Ok I just had my new cat back exhaust system put on my SL in place of stock back in february. My problem is after i put the exhaust on i noticed a horrible low end loss of torque. The loss is very evident on the car. It takes off horribly slow. Now I am trying to find out the problem of the system. The system includes 2.5 inch piping from the pre cat all the way back with a resonator in place of the cat to quiet things down. After the resonator it runs into a 2.5 single in dual out FLOWMASTER 50 series muffler. Out the outlets it runs into a dual setup. Now is my back pressure problem probably:

1. 2.5 is too big, should get new piping (2 1/4)

2. the new resonator i had put on to quiet it down, very baffled.

3.the FLOWMASTER 50 series muffler itself.

4.runnign dual exhaust

Sounds to me like you are just experiencing what happens when you upgrade your exhaust. Your powerband shifts to a higher RPM giving you more HP up top.

2.5 inch is not too big. In fact for a NA its about optimal. Is your car ATX?? If so then thats the culprit.;):p

^ I've heard that some people notice a loss in difference, and that others don't when they switch to larger exhaust systems, I know Musashi had the same problem :(

Yeah i'm with talskinyguy, increasing the size of your exhaust decreases low end torque and puts it up top, so you probably have a higher top speed, but your acceleration is worse.

well guys is really doesnt matter anymore, out of fucking nowhere my automatic tranny went out on me...... i have to get a new tranny now, im thinking of manual conversion

well guys is really doesnt matter anymore, out of fucking nowhere my automatic tranny went out on me...... i have to get a new tranny now, im thinking of manual conversion
BINGO, thats the answer. It had nothing to do with the exhaust size.

Yeah i'm with talskinyguy, increasing the size of your exhaust decreases low end torque and puts it up top, so you probably have a higher top speed, but your acceleration is worse.

you missunderstood what I said. Acceleration will not be slower. What I said was that you will have less torque in the lower RPM's, and more power in the upper RPM's.

Is there anything that we could do to fix this problem? Top end power is great but if it reduces your torque at lower RPMs then that sucks....especially for us NAs. Would it be a bad idea to just stay stock? I know stock is restrictive but just how restrictive are we talking about?

my vote: stay stock, add testpipe only, or go aftermarket SINGLE exhaust, dont loose torque low just so you can accelerate faster at 100 mph, just not worth the loss

A custom 2.5 inch exhaust will not make you lose power. I know someone who has a ATX 92 base 3k with a custom 2.5 Magnaflow exhaust. He said power was gained in all ranges and he lost no low end. You just wont lose low end because most of the stock NA exhaust is near 2.5" anyway.

And its kinda funny, soon after you get a custom exhaust your tranny goes out. Well a week after I added my TT catback my clutch went out. Maybe it was all that extra power.;) :p

What's the difference between a "custom" 2.5 setup versus a 2.5 piping from say Borla? I thought 2.5 is 2.5 and will all be the same... What's the stock setup? 2"?

00blackzx3, testpipes....are those CARB certified? Because I live in CA and have to worry about emissions. DOH!!!

Testpipes are anti-emissions. I don't think you would pass emissions in CA with a testpipe... you would with a cat though. Just a tad more performance to your ride, but just a tad more destructive to the environment. :icon16:

my vote: stay stock, add testpipe only, or go aftermarket SINGLE exhaust, dont loose torque low just so you can accelerate faster at 100 mph, just not worth the loss


Does no one listen to what I say? Higher RPM not higher speed.

Does no one listen to what I say? Higher RPM not higher speed.

Maybe I don't get it....care to explain? :uhoh:

RPM is gear dependant, not speed dependant

when racing you stay in the upper RPM band as you shift gears. so you now have more power because your engine can breath better in the upper rpms but you lose a little down low due to less backpresure.

What's the difference between a "custom" 2.5 setup versus a 2.5 piping from say Borla? I thought 2.5 is 2.5 and will all be the same... What's the stock setup? 2"?

00blackzx3, testpipes....are those CARB certified? Because I live in CA and have to worry about emissions. DOH!!!

The stock setup is right around 2.5" for the most part. This is why I said your new exhaust wasnt the cause of your slow acceleration. I have the TT setup on my car and I lost hardly anything down low. You are not going to lose much, if any, at all with 2.5". And the reason his was custom because it was built, and didnt come prefabbed. It was a pipe coming off the cat that went to a single in dual out magnaflow muffler, then exited out both sides in the stock locations.

Something was probably about to let go in the tranny and thats why it felt sluggish. I gurantee once you get running again you will realize ho w much better off it is.

Slade, no testpipe will ever be CARB certified, because it takes hte place of your main cat.

Something was probably about to let go in the tranny and thats why it felt sluggish. I gurantee once you get running again you will realize ho w much better off it is.

How would you explain Musashi's case then? As Rocker pointed out, he (Musashi) had the same problem!?! I'm just asking because in the future, I wanted to get a new exhaust system as well (with the stock look of course) but just didn't know what the best approach was!?!

I have changed exhaust out to 2.5 inches and you do notice a slight decrease in torque down low... but it is far from "horrible"... It rev's a lot freer when you get the engine spinning past 4000.
If it's enough to call "horrible" it most likely something else... The most this could vary is 10lbs-ft or so...