Low idle after cat eliminating downpipe?

[grantman10]

I just got my car back from an exhaust shop. I had them fix an exhaust leak between the head and exhaust manifold, and put on a megan 3" cat eliminating downpipe.

Pulling out of the shop i noticed that the car ran a lot better now that the exhaust leak was taken care of, but shortly after driving it it stalled when i was in neutral. Weird, so i started it back up and went on my way. It happened again, so i pulled over to look for some boost leaks. Nothing visible.

The idle wasn't jumpy like a boost leak, it was just low. So I went to my friends house (he has 99 gsx) and he said that maybe the lack of back pressure caused the engine to run like this. So we adjusted the biss screw and it idles perfectly.

So i went to drive it and the turbo was spooling slower, and felt like less power. I was REALLY hoping to lower my spool.

I plan on making a boost leak tester tomorrow but i was just wondering if anyone has had to adjust the biss because of loosing the cat?

also i have a 2.5" catback is that changes anything, i figured the 3" dp should flow better than stock, and i am planning on upgrading the catback later this year.

Let me know what you think.
Thanks

 

[grantman10]

nothing?

 

[kenamond]

If the mechanic disconnected the battery (as he should have), the ECU got reset. Sometimes, you won't notice anything. Other times, it has to recalibrate itself w.r.t. idle. You can speed it up with the BISS adjustment, and you did that. So that might be what that was all about...not necessarily related to the other problem.

As far as the slow spoolup, go ahead and do the BLT and see if anything shows up there. Did you boost higher after the DP installation? Usually, your boost will go up when you free up the exhaust restrictions, and you have to readjust your MBC. Have you logged any pulls since this started happening?

 

[grantman10]

Yes the mechanic disconnected the battery, because my cd player settings were reset. As for boost, my mbc was still at the same settings. I think that maybe the fact that i have a 3" to 2.5" exhaust that its not that big of an improvement.

As for logging, i have not. I don't have a logger. Trust me, thats on my list!

Also something weird happened tonight when i was getting on her in 3rd. i was about ~5500 rpms and i heard a loud BANG. It sounded like when my inter cooler pipe would pop off. By instinct i immediately pushed the clutch in and was expecting the car to stall, but it didnt. Wincing, i slowly let the clutch out hoping my transmission was fine. Everything seemed fine. Later that night it ran fine, just scared me!

My two assumptions:

1. Fuel cut (i have never hit it, but it wasnt a brick wall. Maybe i pushed in the clutch too soon?).

2. maybe timing belt jumped, it was really wet out. But i think there would have been some damage.

I was under wot so i don't think it was a backfire.

Let me know what you think!

 

[kenamond]

Hmmm. That bang is strange. You didn't hit something? It could have been a backfire. Have you had any misfires/stuttering? You don't notice *anything* different after the bang? I'd do a visual inspection of the intake and exhaust. If the car is acting differently, I'd be cautious and keep my eyes/ears open.

Exhaust backpressure is a function of size and length within a section of pipe. If you replace a long section of pipe with a larger diameter pipe, it will decrease backpressure regardless of what's before or after that section. Also, you removed the cat which will help even more. A 3" catless DP should definitely help with exhaust flow over a stock DP with a cat. Any improvement in flow after the turbo helps the overall volumetric efficiency of the engine (the engine flows more pounds of air at the same boost). Also, the turbine is like a pressure multiplier. The manifold pressure and O2 housing pressure ratio is pretty constant while boosting; the downstream pressure is smaller than the upstream pressure by some constant ratio, so if you drop the O2 housing pressure (better DP), you drop the manifold pressure by even more psi. That means less pressure in the cylinders pushing down on the piston crown during the exhaust stroke, and that means less negative torque (more torque) - keep in mind that the piston has to push that exhaust out through the turbine. This gets more air into the cylinders during the intake stroke (there is less exhaust left over due to the lower exhaust pressure, so there's more room for intake charge). And that means you've got more air/fuel mix to burn.

So a lot can be affected by improving the exhaust. Because you flow more air at the same boost, you might be knocking now. Or you might be misfiring occasionally where you weren't before. A logger would certainly help with answering some of these questions.