Gut the catback or build new exhaust?

So I pulled the exhaust from my challenge project this weekend (this is the bottom side of it):

That sucker is one big piece and is 80 lbs! It has a few small rust holes in one of the pipe sections, but otherwise looks ok. It does do a good job of quieting the engine. Tubes are 2.5 inches I believe.

Here's my question at this point. I want to get some sort of exhaust back on the car for neutral budget. It's loud without this catback right now, which isn't all that bad, but the exhaust does exit right under the middle of the car so it is loudest inside the car. Not to mention the badness of all those fumes seeping up into the passenger compartment. I'd like more noise than stock, but probably not as much as I have now. I'd also like more power, but am realistic. Really, I'd just like to not lose power from stock. Losing a little weight would be nice too.

Should I:

1. Gut the two mufflers on this one and put it back in?
2. Build a new exhaust and try to sell this one (or scrap it) to recoup? Full length or side exit? (Not a ton of room for side exit though).
3. Leave it off and live with it? Run only with the windows down?
4. Something else? Maybe replace that final muffler with two straight pipes?

I do have some 2.5 inch tube and u-bends laying around from a prior project, enough I think to remake this exhaust.

I vote remake, since it will get you less weight and more noise than stock. You can add some noise reducers if it is too loud once fitted. even better if you have most of what you need to build it.

Ya I would agree. 80 lbs is like carrying half a passenger for the race. Make up a new one. Can you bring them together in to one pipe and run a single pipe out the back?

Or run two separate pipes out the side of the car. I like side pipes for race cars as any rear end hit does not kill your exhaust.

BMW?

Anyway- what's wrong with a side exhaust? Maybe one dual in/out silencer, and then dumped out right in front of the rear tire.

Back when I was more an autocrosser- I kept reading of SP cars that dumped the exhaust on the ground just before the axle- as the rule was just to be behind the driver. And that was as far back as one could get and not strain to get over the axle and/or subframe.

The under car exit is lot of the noise, a side exit a few inches past the pinch weld will make it a lot quieter in the cabin, a turn down will make it quieter outside.

I've done the turndown-at-an-angle thing before and the sound was fine, but it was a truck so I can't comment about exhaust smell for the rear seat passenger.

The Accord got an exhaust with a turndown at the rear bumper, but it doesn't have to go up and over the axle and I haven't driven it for long enough to know how the noise it.

Anyway, I'd try and fab one up from scratch if you can weld and don't mind a few iterations.

Yes, its a BMW, a 1990 750iL to be precise.

Side pipes and one single pipe are hard (but not impossible of course) because of ground clearance. The single pipe would need to be bigger (3 or 3.5 inch I would guess) as I don't want to make a restriction. Side pipes are what I immediately thought, but the exhaust tunnel molded into the unibody floors is a neat and tidy place to keep the pipes, and it doesn't go to the side of the car exactly. I realize this is me complaining.

I do really like the side pipes for less complexity/fab time/weight sake.

What do you guys budget for exhaust tubing that is already laying around in your garage? I could possibly dig up the receipts but I'm sure I bought this stuff 3-4 years ago.

i don't know how much power you are making, but a single 3" pipe is good for about 400hp... a single 2.5" pipe is good for about 300hp..

In reply to novaderrik:

And those are probably at the collector measurements, the farther down the pipe you go, the smaller the pipe needs to be, as the gasses cool and condense. That's why a lot of older econobox type cars had tiny tailpipes.

I vote dual NASCAR boom tubes.

Nova and Kenny,

Interesting. I have always heard stuff like this, so I am surprised the BMW factory spec'd 2 2.5 inch pipes. Its a v12 yes, but only about 300hp. So they are double what they need to be?

Furious_E wrote: I vote dual NASCAR boom tubes.

Boom tubes (one or dual) would solve the side exit clearance issue, easy enough to fabricate, and you could make up a set of baffles for them if they're too loud. Just use heat shields (simple metal sheet should be fine) to keep the heat off the floor.

3in side exit exhaust never scraped on my lowered Volvo, doubt it would be any issue on a stock height older bmw.

Side pipes ftw

Ok, wanted to bring resolution for now incase those who offered input did not seem my build thread. Here's what I did.

Sound is much more manageable now, sounds pretty good. Spent about 7 feet of pipe ($23 to the budget I think). Also, I lost about 35 lbs in the process (and all that weight is from behind the axle - good), and it was super easy and took about 2 hours including setup and cleanup. Welds aren't great but they will hold, and I ran out of wire.

I also pulled off that heat shield and quickly regretted it. Here is what is underneath. Oh well.

Thanks to everyone for your suggestions!