3" pipes, etc.

[hoser]

I have been poking thrrough the existing threads on the subject, and I still have some unanswered questions. Don't care much about sound, just minimal restriction without being ridiculous.

On the e34 540,

1. Can we fit 3" pipes from the cats back, before the cats, or anywhere at all? My mechanic said no way after he looked it over, but he could be wrong (i hope).

2. Can we use 2 of these bullet mufflers side-by-side instead of a dual-inlet universal muffler?: http://www.prostreetonline.com/buy/d...ler/dyn-24215/
I would have to cut some plastic since the existing opening is only 7.25" or whatever, but I'm not too concerned about that. I realize this might be ridiculously loud

3. Of course I would never consider removing working cats since it's illegal and all, but hypothetically, would new high-flow cats be a big improvement over the originals? Assuming the originals aren't rusted out of course.

thanks guys - sorry if some of this has been covered, if so I blame my poor researching skills.

[repenttokyo]

personally, I wouldn't go for 3 inch pipes unless you were making about 450 horsepower.

[hoser]

personally, I wouldn't go for 3 inch pipes unless you were making about 450 horsepower.

you mean i'm not?!? seriously though, what's the scientific explanation? And is 2.5" better than 2.25" for my 282hp or whatever (i'm chipped, if that matters). thanks.

[Anton CH.]

You can fit 3" pipes but why on earth would you? Mufflers and exhaust piping provides restriction which is bad. But when you do have exhaust the idea is to keep the gas velocity high which means keep things hot.

If the pipe is too small then the engine is being forced to move gases.
If the pipe is too big then your exhaust gas velocity drops too low.

So by changing the diameter all you are doing is changing where peak HP is at. Changing old cats to newer high-flowing ones is a good idea though.

[repenttokyo]

you mean i'm not?!? seriously though, what's the scientific explanation? And is 2.5" better than 2.25" for my 282hp or whatever (i'm chipped, if that matters). thanks.

personally, 2.5 is all I would go for on a 280 hp application. I was running 2.5 on my last v8 with a double x pipe with good results. But anything above that, and you starting running into the velocity problems the previous poster explained.

[Gayle]

I would like to hear more about the newer high flow cats. Can anyone inform me?

[hoser]

Right, because higher velocity => lower pressure! I should've known that one...thanks for the answers. Still waiting to see what everyone's opinion of those bullet mufflers is. I'm guessing they're way too loud...

[hoser]

Double x pipe eh...I was planning on putting one before the cats. What was the placement of the crossovers in your setup?

[repenttokyo]

the crossovers were just behind my cats, about 8 inches down. This wasn't on a bmw, btw. The double X really reduced drone on the highway.

[hoser]

After doing some more research, i found out that bigger is definitely not always better (contrary to what my wife says). I also found a couple of figures: 2.2cfm/hp is sort of the gold standard for a perfect flow rate, and straight pipe gives about 115cfm per square inch cross-sectional area.

So 2.25" straight pipe would give about 457.2cfm, or 914.5cfm for my 2 2.25" pipes. Assuming I'm making 290hp (estimated 282hp new + 18hp chip - 194k miles, very scientific), I would need 638cfm to be near optimal flow rate.

So that leaves 914.5-638 = 276.5cfm that I can lose with cats and muffler, and still be near the optimal flow rate. Not sure how to calculate the drop in flow rate caused by those components, but it seems like even going to 2.5" might not be worth the loss in gas velocity/scavenging effect. Some have reported gains from 2.5" over 2.25". Does anyone have real numbers on those gains?