guttted cat vs. down pipe

[Lance_S]

Just thought I would share my experience. I had a downpipe (mandrel bent with heavy flanges) that needed to be installed. The main cat was already out and there is a 2.5 pipe going into a highflow muffler at the back. The only thing left to change was the downpipe. I swapped it out a couple days ago. I thought the precat was original, come to find out it was gutted. I was a little dissapointed because I thought it wouldn't make much of a difference. Gutted precat vs. downpipe. But it did.

 

A couple things changed. First, I pull about a lb. more of boost. Which is always nice, free power. It's not dramatic. But it is noticable. Second, the power is more linear. When I leaned on the gas slightly, before the swap, the turbo would, rather abrubtly, go from negative boost, right up to around 7 lbs. The car would have what felt like turbo lag because it went from nothing to ON, like right now. With the new downpipe installed, the turbo now comes up slower. In fact, I have to push the excellerator down farther to get into the boost. At WOT, the boost is more, but at less throttle, the boost is less. This is actually better because I now have a more usable throttle. Before, just slight throttle would give me a jump into the 5 to 7 lb range, now, slight throttle slowly comes on and hangs around 3 lbs. To get more, I need to give it more throttle. So the throttle is more usable if that makes sense.

 

And finally, when I let off the gas, before the change, the car would pull down hard, like a jake brake. When I let off the gas it was like there was a suction pulling on the turbo and forcing the engine to pull down the revs. Now when I get off the gas the car floats down. It doesn't pull down hard on decell with my foot off the gas, it still decelerates obviously, just not as hard.

 

So, that's my 2 cents. For those that have gutted their precat and think it's as good as a downpipe, think again. A good, mandrel bend downpipe is definately a improvement for your SQ.

 

L

[BrazilBoy]

That's why full exhaust is the very first mod that should done on these cars.. Second should be intake..get the air in and out smoothly..obviously after she is running perfectly..

[yellow_turbo88]

when you talk about a full exhaust do you mean gutting the cats and then have a new exhaust put on

[Lance_S]

In my situation, the main cat was already gone and the precat (original downpipe) was gutted. That's why I posted. I was pretty amazing that even with the original cat gone and the precat gutted, putting a downpipe on the car made a significant difference. I can't imagine what it was like with the full cat, small piping and original precat on it. The back pressure must have been immense.

 

On a side note, the car is too loud for my taste so I will be adding a mid-pipe resonator to quiet things down. That will induce back pressure back into the system but not nearly as much as a cat will.

 

L

[Shelby]

the problem with useing the pre cat with no moding other then guting is the out let it's still small , think of it as placeing a slug in side a 5/8 water hose and seeing what happens to the water flow out the end of the hose

a chain is only as strong as it's weakest link,,a pipe will only flow what it's smallest opening will flow

[Starfighterpilot]

the problem with useing the pre cat with no moding other then guting is the out let it's still small , think of it as placeing a slug in side a 5/8 water hose and seeing what happens to the water flow out the end of the hose

a chain is only as strong as it's weakest link,,a pipe will only flow what it's smallest opening will flow

 

In addition to what Shelby said above, altho there is turbulent flow coming out of the waste gate induced by the turbo, as the exhaust gas goes through a gutted precat it has eddies and increased turbulent flow, induced by the expansion and contraction of the exhaust gas as it passes through the "UNSTREAMLINED" precat inner shell. These eddies and turbulent flow characteristics increase resistance to the exhaust flow. But nowhere near as much as running with the pre cat internals still installed. That's why even gutting a clean precat noticeably increases performance.

 

BTW - I hope that you have a heated O2 sensor installed in your new down pipe so it will warm up enough to allow it to send the correct feed back signal to the ECI ECU to go into closed loop. Many SQC'ers have noted that with the new down pipe installed the unheated OEM O2 sensor does not get hot enough, cuz of lower exhaust temps caused by increased exhaust flow, to start sending the feed back signals.

 

For What It's Worth

 

KEN

[Lance_S]

In addition to what Shelby said above, altho there is turbulent flow coming out of the waste gate induced by the turbo, as the exhaust gas goes through a gutted precat it has eddies and increased turbulent flow, induced by the expansion and contraction of the exhaust gas as it passes through the "UNSTREAMLINED" precat inner shell. These eddies and turbulent flow characteristics increase resistance to the exhaust flow. But nowhere near as much as running with the pre cat internals still installed. That's why even gutting a clean precat noticeably increases performance.

 

BTW - I hope that you have a heated O2 sensor installed in your new down pipe so it will warm up enough to allow it to send the correct feed back signal to the ECI ECU to go into closed loop. Many SQC'ers have noted that with the new down pipe installed the unheated OEM O2 sensor does not get hot enough, cuz of lower exhaust temps caused by increased exhaust flow, to start sending the feed back signals.

 

For What It's Worth

 

KEN

 

 

 

Ken,

 

Thanks for the reply. I actually can't answer that question because I don't know what I am looking for. What does a heat vs. unheated O2 sensor look like? The bung is in approximately the same place as the old one, right off the back of the turbo so it gets really hot. I would be surprised if it didn't get up to temp but I have been surprised before! I don't have any codes and the car is running great but I guess that doesn't mean it's not doing what it's suppose to.

 

Thanks,

 

Lance