Thoughts on this Cat bypass

[khamul02]

Originally Posted by rnln

khamul02,
you mean the same one the original poster posted?
Is it a cat by pass or is it a premuffler (has resonate part inside)?
How is the fitting? Do you have to modify the O2 bung or anything else?
Thanks.

It is empty inside. It has an O2 "hole" I made no mods. The stainless has held up well. There is some discoloring on the end pieces with it connects to the rest of the exhaust after two years. But over all it looks great, sounds great, and has worked without issue. I believe I bought mine for 149 2 years ago for the exact same seller with those exact same images.

[khamul02]

My mistake... this is the one I picked up for 149:

75-89 Porsche 911 SC 2.7 3.0 3.2 Cat-Bypass Pipe New : eBay Motors (item 360245069145 end time Apr-18-10 09:50:22 PDT)

Sorry the images on the second or thrid post confused me as to which one was being discussed.

[rnln]

So it is not a premuffler then. It is a cat by pass, no different then a straight pipe but they made it bulge for the look only. Still good price for the polish ss. I think I like it.
Thanks Khamul02.

Originally Posted by khamul02

It is empty inside. It has an O2 "hole" I made no mods. The stainless has held up well. There is some discoloring on the end pieces with it connects to the rest of the exhaust after two years. But over all it looks great, sounds great, and has worked without issue. I believe I bought mine for 149 2 years ago for the exact same seller with those exact same images.

[Steve W]

Economics and politics aside, the item referred to in the OP's post is a cat bypass pipe. This is as opposed to a Euro style premuffler which has a large diameter muffling chamber around the straight section - typically 4 to 5" diameter just like the original premufflers that come on the ROW/Euro 911s. The difference is that a premuffler will control and attenuate the exhaust note better than a bypass, especially in conjunction with a sport muffler where a bypass would end up being too loud and raspy for a lot of people. The OBX pipe looks almost like it's made with a outer muffling chamber, but it so little volume that on a scale of 1 to 10, 10 being a real premuffler, this would be a 2.

Performance wise, I'd think both probably perform about the same, although long ago Bruce Anderson told me that when he did dyno testing between a bypass and premuffler, the bypass for some reason didn't seem to do much, while a premuffler gained about 6-8 hp, and combined with a good sport muffler made almost the same hp gain as SSIs with a good dual out sport muffler.

The M&K premuffler was a unit I designed in conjunction with Ben to address issues that existed amongst such products at that time. The only premuffer at the time I would have recommended was the Dansk as it was made exactly like the factory unit, but the problem was it was basically very difficult to non-existent for a long time. Another popular unit for some reason has the O2 sensor bung too far upstream, reading only half the engine. So the M&K, besides having sufficient chamber volume for sound attenuation, features a mitered merge collector that those that understand header design, pay big bucks for because it widens the powerband of the engine while maximizing top-end horsepower by controlling the tuning waves present in the exhaust. It takes much more time and labor to fit and weld a mitered collector as opposed to the swaged canister type collector as you see on the OBX unit. Take a look at the Burns Stainless site to get an idea of what such things cost.

Burns Stainless LLC - Merge Collectors

So just like tools, you get what you pay for. You can buy Craftsman, Snap-On, or MAC, or go to Harbor Freight and find something that is supposed to do the same thing. How well it does it and how long it lasts is another issue. Not to say the OBX unit will fail, but their headers crack after about a year and a half.

[Targa Me]

Just in case anyone is interested; this is what it looks like inside the Maxspeed cat bypass.
Its not just a straight tube, there's some kind of baffling of sorts.
I wonder, does this qualify as a premuffler?

[911st]

Steve,


Thank you much for you input. Your opinion is valued and appreciated.

Agreed, the M&K is a beautiful piece and a value considering what one gets.

However, can you say that your numerous tests have shown that the M&K pre-muffler, or the factory pre-muffler for that mater, out performs a Bursch Cat Bypass in terms of power production?

If so, what are you seeing?

I thought we used the Pre-Muffler chip with the Bursch on my car at first.

The best.


Good info on the O2 sensor as can be seen below.

[Steve W]

No, I never claimed a bypass made any different power than a premuffler - I think it's all the same, but I'm just relaying what B.A. found was different. There's so much different stuff out there, no one, or myself is going to spend the time to buy everything and bolt all this stuff in and out of a car on the dyno to test every possible combination of mods in a scientific method just for the sake shi tz and giggles. Parts, labor, and dyno time cost lots of money, and has to serve a purpose to the guy forking over the money and sweating under a hot dirty exhaust changing this stuff out.

With the stock muffler, a bypass pipe or premuffler, the exhaust is not all that much louder than a cat car. But change to a sport muffler, and the bypass pipe setup will get significantly louder and raspier, vs keeping the cat or using a real premuffler which keeps that under control.

[Steve W]

If you look carefully, you can see that the diameter of the pipe steps down to a smaller diameter with an abrupt transition, reducing the cross sectional area to 50% of the primary pipe - a very poor and restrictive design. Again you get what you pay for.

Originally Posted by Targa Me

Just in case anyone is interested; this is what it looks like inside the Maxspeed cat bypass.
Its not just a straight tube, there's some kind of baffling of sorts.
I wonder, does this qualify as a premuffler?

[rnln]

that is the resonator, premuffler in european term I think. Cat by pass should be blank on the inside. It should be just a pipe, no muffle stuff.
As SW pointed out, can you measure the most inner diameter of that maxspeed?
Sounds like there are different versions on this china made ebay thing only. Khamul got the bypass, Targa Me got the remuffler type. Anyone know the inside diameter on the orginal premuffler?

Originally Posted by Targa Me

Just in case anyone is interested; this is what it looks like inside the Maxspeed cat bypass.
Its not just a straight tube, there's some kind of baffling of sorts.
I wonder, does this qualify as a premuffler?

[Targa Me]

Steve, the step you're seeing is at the tail end of the bypass, not where the exhaust enters, so i wonder if what you say about it being restrictive still is true?

[rnln]

Targa Me,
What he is saying is the diameter of the inner part (which has honeycomb holes) can be too small. If you can measure the diameter of that, and if someone can help measure the original premuffler, or MB permuffler for comparison. Then we can tell if it is a problem or not. This is all assumming the original premuffler, and MB premuffler is perfect.

This pic is from the $99 ebay one, I download and lighten it up. It looks different form the Maxspeed Targa Me posted above. It does look like a straight pipe.



Your pic, the red line is the restricted area, the difference between the inner core and the tail pipe diameter.

[Targa Me]

I'm at work now, when i get home this evening I'll measure the I.D. and post it.

[911st]

Originally Posted by Steve W

No, I never claimed a bypass made any different power than a premuffler - I think it's all the same, but I'm just relaying what B.A. found was different. There's so much different stuff out there, no one, or myself is going to spend the time to buy everything and bolt all this stuff in and out of a car on the dyno to test every possible combination of mods in a scientific method just for the sake shi tz and giggles. Parts, labor, and dyno time cost lots of money, and has to serve a purpose to the guy forking over the money and sweating under a hot dirty exhaust changing this stuff out.

With the stock muffler, a bypass pipe or premuffler, the exhaust is not all that much louder than a cat car. But change to a sport muffler, and the bypass pipe setup will get significantly louder and raspier, vs keeping the cat or using a real premuffler which keeps that under control.

Thanks Steve,

Good point about the effect of a pre-muffler with a stock v sport muffler. I did not notice a change in my car's sound with a bypass and stock muffler. With a to loud sport muffler it might be a different case.

I heard of the BA comment before in reference to the 'factory' pre-muffler but it never seemed to make it to his book or writings that I have seen.

I know it is not possible for you to test every mod combo. However, it was inferred that you had seen enough systems and that you might have observed a noticeable difference with the M&K.

I am not a big fan of the OBX and would not touch that Maxspeed if it is dose not maintain a full diameter.

I suspect there is opportunity to correct or improve the 3.2 over a cat and even the 'factory' pre-muffler but it might require a redo the cross over area to see a significant gain. The 993 sure picked up a bunch by getting ride of the unbalanced, hard elbow turn, and restrictive cross over. Maybe even not merging the secondaries with them dumping into an expansion area separately or some effort to equalize there length.

Not sure about the benefit of the O2 connection being merged or not. At WOT or with aggressive accel it is not going to have any effect.

If just one cylinder goes out of spec with the merged system the O2 correction might just mask it or adjust around it. With the O2 on one it may not correct around it or bring on more correction.

Maybe running with the O2 disconnected is better yet?

Again, thank you and keep up the good work.


ps (Hope your MAF system is coming!)

[911st]

How about a 'race-cat' in M&K quality?

If it made mostly the same HP as a bypass or Porsche pre-muffler, it should be another winner! That I would have bought in a heart beat living in CA if under $500.

Some times it might not sound like it as I prob for details & solid info while I am learning but I am a big fan of the Brian designed, Ben built turbo header system. It is is a 'best of class' system.

[Dr J]

You may want to listen to one before selecting. As some have mentioned, there is a difference in sound. I had a gutted cat which is shaped like the premuffler and I liked the sound, nice but not so loud. I went with a cat bypass and its too loud for my taste. I can even set off other people's car alarm!

[rnln]

update,
My burch (is it how it spelled?) bypass inner core is 2.375" diameter.

[911st]

Dr. J.

Do you have a stock muffler. When I removed my cat I did not notice much change on my 85 we stock muffler.

[Dr J]

Not stock muffler. Don't know what kind it is. It was in the car when I purchased it. It looks like a muffler for non-porsche vehicles (highly oval) chromed. Unfortunately, I don't have a picture handy that I can upload, but if I get a chance, I'll load one.

[Silver Surfer]

So, lets say I have a 1984 3.2 with a catalytic converter, and I want it taken off, so I can install a cat bypass, a premuffler, or backdate the whole exhaust system. Lets also say I can't do the work myself. Question: will I have trouble finding a good shop that will do this for me?

[911st]

Not sure about Washington but in California I believe it is supposed to be a big deal for a shop to mess with disabling smog equipment. A good relationship, a cash transaction, and/or a lot of begging helps.

Still, I have seen many Porsche shops change out exhaust systems.