Cat Surgery (+ Air Dam Electronics)

#1

Two questions:

-I just did surgery on my cat. I uncrimped the shroud that holds the insulation to the cat, and then used a ferrous metal blade on my circular saw to cut a huge hatch out of the top. Now, it’s TOTALLY clean–none of that ram rod crap. Question: any compelling reason to replace this insulation shroud, or can I leave it off?

-A collision with my driveway apron just forced my hand to finally remove the entire front air dam off my '87 325is. I was surprised to find a wire going to a hole in the air dam on the driver’s side corner. What the hell is this?

Thanks,
Sasha

#2

it’s for outside air temp
Igor

#3

Sasha,

Some guys over hear told me yesterday (I was talking to them about how to get rid of the cat for racing and still keep it for street legality). They said gutting the cat and leaving it in place can actually hurt the power/torque because the air flow doesn’t like the completely open chamber left behind. They recommended to me (If I was going to gut the cat) to put straight pipes through the middle of it before closing it back up.

I can’t really state this as fact, but this comes from a shop here in Atlanta.

Later,

  • Kelly
#4

I had a street 87 iS and that airdam took a serious beating over the years…

#5

kchildre wrote:

[quote]Sasha,

Some guys over hear told me yesterday (I was talking to them about how to get rid of the cat for racing and still keep it for street legality). They said gutting the cat and leaving it in place can actually hurt the power/torque because the air flow doesn’t like the completely open chamber left behind. They recommended to me (If I was going to gut the cat) to put straight pipes through the middle of it before closing it back up.

I can’t really state this as fact, but this comes from a shop here in Atlanta.

Later,

  • Kelly[/quote]

I thought about this very issue, but the guy at the exhaust shop I’m going said it wouldn’t be a problem when I asked him about it. I could argue both sides and convince myself either was correct. When I saw all the layers of that honeycomb stuff inside with teeny tiny holes, though, it seems like that cannot help the flow. I suppose that perhaps the overall area of holes may have equalled the area of the pipes though, so conservation of mass would apply and the flow would continue through without problem. All the holes could help align it and keep it laminar, too. It would still basically have had to expand, pass through the ceramic, and then compress again, though, just like it will have to do now with the honeycomb removed. The only difference is that more swirling could occur, which could be bad. I’m going to reinstall it and see what happens. I’ve already got the top welded back on and putting straight pipes through the cat would have taken quite some doing. Just getting the honeycomb out was trouble enough!

Sasha

#6

The surgery is complete, and here’s what my butt dyno says. Low end torque is a little compromised and I believe there’s increased back pressure at low revs. This statement derives from the fact that with the cat intact but two 2" glass packs installed, when I pushed the clutch in to come to a stop at idle, the RPMs dropped to about 500 and then recovered, like the engine management system wasn’t set properly for the lower back pressure. Now it doesn’t do this, but just settles in like it did with the completely stock exhaust system.

The higher RPM range (like 4500 to redline) seems to be improved, however. I bumped off the rev limiter for the first time ever today because it was still pulling hard in 2nd. I also chirped the tires shifting at redline from 2nd to third. Plus, the sound is a real ripping noise that was absent before.

The bottom line: until a real dyno other than my butt says otherwise, I think this set-up is an improvement for the track environment.

Sasha