I am looking at getting a 2 3/4" catted downpipe (Randon Technology DP), and I plan on keeping the stock exhaust. As of now the car is 100% stock until I can get time to have the dealer flash to the GM S2 calibration so I can install my injectors and pulley. GM stage 2 is probably I will do for power mods other than cooling mods.

Since I am all stock for now, will using a 2 3/4" catted downpipe make a noticeable decrease in low end torque and power? Will I notice much of a gain anywhere with it, if any at all?

I know that the DP is one of the more restrictive areas of the stock exhaust, so I am hoping there is something to be gained by adding one. I know that there are many LNFs out there that have seen a benefit to adding a DP on the stock exhaust, does this really work the same for LSJs?

 

I recommend going with the 2.5" catted downpipe instead of the 3" on the stock cat-back exhaust. Your car will be noticably louder with a high-flow catalytic converter compared with the stock catalytic converter.

 

I don't think the LSJ will see as much of a gain as the LNF does, but it certainly can't hurt. Obviously the more power you throw at it, the more you'll gain from going catless. I do not believe you'll have any performances losses to speak of, and the exhaust should keep the same note, just louder under throttle.

 

I thought I read that the stock diameter DP is 2.75" on the LSJs, but I could be wrong on that. If that is true, a 2 3/4" DP all the way to the outlet should smooth out the exhaust flow where the stock DP has bend restrictions in it. Or will I just be creating a big bottleneck by having a 2 3/4" all the way back DP going into the stock 2.5" to 2.25" over the axle stock exhaust?

By going with a 2 3/4" full lenght catted DP on the stock system, will I increase the potential for knock and higher EGTs?

 

these people need to stop this crap.

Pick up the 3' dp catted or not dont matter. Once you mod a little more get a retune and enjoy the gains.

 

Ok, I'm an idiot. For some reason I though that the Random Tech DP was 3", but turns out it is 2 3/4" from header inlet to exhaust outlet. Isn't the Hahn DP and Random Tech the same exhact DPs?

Anyways I have updated all posts with this, and am still wondering if it will be a good idea or not to use it

 

I have a question not to jack the thread but I am planning on doing an entire exhaust, header dp and cat back on a basically stock engine, will I need a tune after installing the exhaust?

 

You don't NEED a tune, but a tune will help improve and get the most power gains from it.

Ok, end thread jacking contine in a new thread. Back to the original thread

 

my buddies IRL sounded pretty good with the BADMAB shorty header and a 3" downpipe on stock exhaust... it was loud but not obnoxious unless he was high rpm.

 

I am not considering a DP for sound purposes only folks

 

If you look at the official dyno thread, there is a clear advantage to having a high-flow catalytic converter. It appears that they help improve performance by about 8 to 10 horsepower to the wheels.

My reasoning for picking the 2.5" downpipe over the 3" was that it didn't make any sense to have a 3" pipe in the middle of an exhaust system that was smaller on both ends of the downpipe.

 

the stock cat is a high flow......but all the restrictions in the pipe is where u will see the improvements. i.e. where the o2 sensor is.

 

Aftermarket DPs are a good mod on LSJs.

 

I have photos on my Streetfire.net account of the differences between the stock catalytic converter and the 2.5 inch ZZP downpipe with the Mangaflow high-flow catalytic converter. The differences are dramatic. To emphasize it, I shined light through the downpipes and took a photo. You can clearly see that much more light is shining through the aftermarket catalytic converter. I also took many images of the stock downpipe next to the ZZP downpipe (for comparison purposes).

 

i installed a zzp 3" dp on my friends stock lsj and from what he says it runs much better and i noticed a nice more agressive sound outta the exhaust. i personaly like it. Just giving out my opinion on this

 

So no one here has the Random Tech downpipe at all? I have heard that it is a 2.75" and I have heard that it is a 3" downpipe. Can anyone who has it confirm the correct diameter of it?

 

just get the zzp 3' downpipe with a cat and call it a day they is a reason zzp is the most trusted company for our car parts.

 

I had it!

It is a 3" downpipe from the inlet flange to the front of the converter, and then goes down to 2.75" from the rear of the converter to the exit flange.

They come with a shitty flex section with inner braids that ended up warping & choking off my exhaust. Random installed a new interlocking flex section (as per my request) under warranty. Mine was bolted to a 2.5" Corsa catback and worked just fine.

I would just pick up a ZZP downpipe though. The Random tech downpipe is rather expensive.

Here's what it looked like installed on my car:

 

I like my ZZP Long Tube Header/ 3" Cattted Downpipe to stock exhaust setup
Gave me roughly 10 whp.

 

Dyno?

 

So do you think that the RT DP will bolt to a stock catback?




I'd go with the ZZP for the price factor, but I have my reasons for wanting to go with the Random Tech.
1 = I'd prefer a catted downpipe, and ZZP uses the craptacular Magnaflow cats on their DPs. I've known a few people who had thier cats and they didn't last too long at all.
2 = The RT DPs are known for not setting the p0420 or any other DTCs. I don't have HPT or know anyone remotely close that can disable it permanently. Constantly resetting the code will get old quick.

If ZZP used a spun metal matrix cat or offered the option, I'd have had one already

 

I have wondered why they don't. It wouldn't hurt to ask.

 

They will install a spun metal cat if you ask them to.

They just made me a downpipe with a spun metal cat

 

It will not bolt up to the stock catback.

The exit flange is much larger than the stock catbacks 2-bolt flange.

Also, the stock catbacks entance to the resonator is only 2 1/16", so even if it did bolt up you would potentially have an exhaust leak due to the much larger diameter of the random tech downpipe.

ZZP will weld on a spun metal cat if you ask them to. They just did that for me Plus ZZP uses an interlocking flex section. Random Tech uses a craptastic braided wire mesh flex which ends up warping do to the heat from the exhaust gases and chokes the flow off. Ask me how I know I'll try to find some pics of what it did on my car.

 

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