Bischoff 2 in 1 out muffler for cars with EGR filters

[75 911s]

I've searched the forum and I've been unable to find out anything regarding the stock sausage Bischoff two in, one out muffler. My part number is BISCHOFF 911.025.02/974 and this one was the stock muffler on my CA destination 75 that had Thermal Reactors/EGR



Here is what I know:

They were installed on thermal reactor cars from 75-77 and had an EGR filter bung.

They are mild steel and weigh approximately 30lbs

They have two inlets and one exhaust outlet


Here is what another forum member said about them:

Originally Posted by alwaysflat6s

Hi Duane,

I referred to it as a "Factory Sport Muffler" because it was designed to be used with those terrible Factory Thermal Reactors, which were very restrictive. They didn't need nearly as much baffling to have a suitable noise level and Porsche wanted whatever HP was available. I first encounted this it was with a California smog car using thermals. As soon as we swapped in a set of early heat exchangers [the engine had a pulled stud from all the heat and we went through the engine while we were at it[ the noise level was much louder, but still nice.. and sweet! With the simple 2.7, stock cams, and CIS there wasn't all that much power but it sounded like one of the better early 911s.

BTW, at one point I bought any of these that I found and redid the outlet for good early 911 cars. I have run these on 2 different '73 RSs and have one on a '74 Carrera with a built 3.0 engine. All sound neat and "sporty". Actual HP different? Never dynoed one but suspect "a few" HP increase.

I hope that answered your question. To summarize: I am positive these have less baffling and are good for a few HP.. and since they are "Factory" I like them. [I must have a half dozen aftermarket exhausts sitting around but these are pretty much my favorites, but now getting rare.

What I would like to know:

Any additional anecdotal information you may have on this particular muffler

Any additional real world dyno / testing information

Any pictures of one cut open and or baffle configuration - is it possible to tell where the internal baffles are via the weld marks on the outside?

Anyone who has converted this to a 2 out or gt3 style muffler.

Would love to know more as I'll probably put it back on my car with the SSI HEs I just installed. And why spend $$ on a dansk sport when this could effectively be a very similar beast?

Thank you in advance

[911 SLANT]

I have the same muffler. It came on my 1976 911S. The bung is cut off and welded. I alway thought it was stainless, but wasn't sure. It does have a nice performance sound to it. I have it paired with early SSI style headers. I think I will have it polished, now that I know it's stainless. I do have a polished stainless Dansk on the way. Thanks for info.