Which type/style of high-flow Cats to use?
 

I'd like to replace the aged and restrictive catalytic converters on my 1990 560SEC. There are several types of cats with varying oxegen sensor placements in addition to several different manufacturers available from Summit. Does anyone have the choice dialed in yet?

Ideally, I'd like to get some that fit near the stock location and require less head tube modicication etc. I've already replaced the kidney shaped resonators with a pair of straight through 18 inch glasspacks and changed the muffler to a 3-chamber flowmaster. It sound sweet now, but I know the original cats are the final restriction in the system that need to be updated.

You should get the kind that places the O2 sensor as close to the original location as possible. Sensor placement in relationship to the cat is very important to how the computer determines mixture control both up and downstream of the cat. I've seen driveabilty problems in vehicles occur in non-MB vehicles that had the O2 sensor mounted 1-2" away from "stock".

If the style/price of cat doesn't have the sensor hole where you need it, you can always purchase and weld in an O2 bung in the desired location.

It's also important to note that some amount of backpressure (restriction) in an exhaust system is desireable.

Have you actually checked your cats to see if they're clogged/restricted? I know it's common lore that big pipes and no restrictions means extra HP, however there are many more factors at play than most people realize when it comes to optimizing the power output of an engine. Granted, 20 year old cats are not likely to be in great shape (especially if your engine has ever run rich), and universal ones are generally cheaper, but you should know what you're paying for.

If you really want to "make more power" you should get a little dyno time and see for yourself what modifications/improvements actually work. One of the instructors whose lectures I pay out of pocket to attend has a friend who owns a custom muffler shop and says that about half the time the new $3k system they install actually puts out the same or less HP than the stock one - but the sound is exactly what the customer is looking (and paying) for.

Besides, the biggest enemy in an exhaust system is turbulence - you can do what you like downstream, but until you ditch the exhaust manifold and switch to some properly designed headers, you're really just changing the way your exhaust sounds, not how much HP you're making.

I usally ram rebar thru my cats to improve flow.Cheaper better fix.

I have run megaflow cats for a number of years with glasspak resonators, and magnaflow exhausts...truth be told as far as power...did not notice much of a difference until I added a K&N filter to the front end...then the difference was noticeable, but whether it was the combination or just the better flowing intake I do not know...I have four set ups but in 190E.s with this set up...one of them is on a 3.2 with a nominal 215 hp at the flywheel. I seem to get about 50,000 - 80,000 miles before replacement is needed.

Your profile states "west coast", which I'll interpret as the Republik of CA. In CA, you can put any cat on your car as long as that specific cat's application sheet lists YOUR car. The Emissions Twits can look up the model number and check to see if it has you car listed as one of its applications.

The rules absolutely vary by state, so before you purchase, call the Emissions Twits to see what they require.

Here in Nevada, we're going to cut the exhaust pipe just below the "Y", weld on a flange, then down to a high-flow cat, weld on an O2 sensor bung at the cat outlet, then a single pipe out to a muffler in the back. If I can find some Euro manifolds, then it will be duals out the back.