catalytic converter

Ederd · Ederd's Garage

While coming home from work, my catalytic converter had an issue with being attached to the car, well it just fell off. I assume after 27 years, it just gave up. I was driving about 75 ish and passing a 18 wheeler going up hill bc I was slowing down traffic. Upon inspection on the side of I65 today it appears the CC had been gutted at some point. There is not a single thing inside.

Options
1. Re-weld old one back in place
2. Get a straight pipe installed
3. Get a new CC
(which isn't going to happen due to $ restrictions at the moment)

So, besides just being old and thin rusted metal, what would have caused the old welds to give way?

Any advice on the cheapest and easiest route to take to get the muffler back on so i don't sound and look like a moron going down the road.

carsonmx114 · 09-26-2011, 06:37 PM

The metal actually looks to be in good shape. Any muffler shop should be able to weld the cat back on without much trouble at all.

Arizona_928 · 11:04 PM

if you have emission laws get a new one put in. If not weld in straight pipe. You might like the sound.

fatigue from thousands of heat cycles, rust, cracks, then you hit something in the road and it gives way...

those cats have pellets inside. you could sell it online for $$

or you can get rid from the cat back, and go race car loud... LOL

whitesmile · 09-26-2011, 11:18 PM

Quote:

Originally Posted by AZ_porschekid

if you have emission laws get a new one put in. If not weld in straight pipe. You might like the sound.

fatigue from thousands of heat cycles, rust, cracks, then you hit something in the road and it gives way...

those cats have pellets inside. you could sell it online for $$

or you can get rid from the cat back, and go race car loud... LOL

Yup, you could sell it !!

Ederd · Ederd's Garage

well after having to have ear plugs for a day, i finally got the exhaust fixed. i went with a straight pipe to the existing Dansk catback. i've already burned up a 1/2 tank of gas just driving around listening in glee!! local shop welded it on and charged $75. Happy as kid in a candy shop!

Ederd · Ederd's Garage

im sure the sound will get annoying after a while but for now i'm quite alright with it.

iplagolf · iplagolf's Garage

Ederd...Would love to hear what it sounds like! Took a ride with my wife today and she said the 944 sounded like a sewing machine and not a sports car. I've been following a thread about removing the CC and installing a cherry bomb type muffler instead. They have tried various types and combinations. I don't have the equipment or the lift to do so. I do have a few questions about removing items and adding items to an exhaust. Several years ago when Indian Motorcycle was in business in the garlic capitol of the world in California (Gilroy) I was doing some work for them. One of the biggest questions I got from new buyers or prospective buyers was about exhaust modifications. They wanted loud pipes. Since the bikes were made in California which has a very strict emissions law(s) the exhaust system made them sound like a Vespa. I got with S&S Motors, the brand Indian was using, and put the question to them. The worst thing you could do is a drag or straight pipe. They said you loose power and performance. The reason is harmonics. A sound wave travels down the pipe and then back up the pipe into the motor and into the carb to do God only knows what, (my eyes were rolling back in my head, too much tech for me) anyway you need something in the pipe for back pressure was the answer. So I guess the question is, if you remove the CC whats going to happen, if anything. Someone with way more knowledge designed the system with the components to achieve an end. Does the CC put any back pressure on the system or is the muffler enough? Any ideas anyone? I just want a throatier sound. Thanks, Fritz

Aufgeladen944 · 10-02-2011, 01:21 PM

Chopped out my cat last month...and wow, what an enormous difference. Mine was clogged completely and the car runs much better. The exhaust sounds awesome.

I do have some postfiring when decelerating, but I rather like the sound of that, too.

ERAU944 · ERAU944's Garage

the sound without cat is way different on a 944 vs a 951. the 944 sounds great with the cat in and a flowmaster IMO.

the turbo takes a lot of the rasp out of the exhaust - if you delete the cat on the 944, you will get a ton of rasp even through the muffler.

the 951 exhaust sounds excellent, even with the cat in. I think I will try installing a delete pipe to see if I can't get me some of that sweet, sweet postfiring...

9FF · 05:56 PM

The most restrictive part of the 944na system is the cat. If your state allows the best thing you can do is remove it. I fitted a Hushpower in place of my cat, its virtually unrestricted but has some silencing properties, it made a huge difference with a decent sound you could live with and also a bump in performance.

Straight pipes get old after a while and actually decreases performance on the na, you need some backpressure in there. The Hushpower I bought was cheap and stainless steel from Summit. It was also oval in shape and a direct replacement for the cat. Here's a link: Hushpower 12518400 - Flowmaster Hushpower II Mufflers - Summit Racing Equipment

v2rocket_aka944 · 10-02-2011, 08:57 PM

for the 800 thousandth time, backpressure is always the enemy...what you want is exhaust velocity in the pipes...velocity promotes the scavenging effect.

Aufgeladen944 · 10-03-2011, 03:16 AM

That is true about the Turbos. The turbo itself acts as a muffler.

9FF · 10-03-2011, 04:24 AM

Quote:

for the 800 thousandth time, backpressure is always the enemy...what you want is exhaust velocity in the pipes...velocity promotes the scavenging effect.

Sorry V2, us old guys still use that term even though we know it's the technically wrong term.

Guest765 · 10-03-2011, 05:27 AM

Technically you do want backpressure in the form of a harmonic wave as has been posted above... I don't know why so many people think this is wrong?

9FF · 10-03-2011, 06:11 AM

Manufacturers design exhaust systems as a "whole" system. They know from the start that the system will effectively change diameter in at least two places, the cat/pre-muffler and the rear muffler. These areas will reduce the velocity of exhaust gases overall.

To overcome this they calculate the ideal pipe diameter for the system without any cat or mufflers and then reduce the diameter to make up for the increase in diameter at the cat and muffler areas.

So if the system ideal is 2.5in they might reduce that to 2.25in to make up for the increase in diameter at the cat/pre-muffler and rear muffler.

That is generally why you will lose power by replacing the cat with a straight pipe. You need to replace it with a pre-muffler of about the same diameter as the cat. This will keep the velocity of the whole system as designed to work best.

At least that is what I was taught about na engines a long while ago, maybe things have changed.

Turbos, now that's a completely different animal and they go against all the na rules.

iplagolf · iplagolf's Garage

Great stuff here. Over the weekend I spent way too much time looking through Porsche forums. I think it was here on the 951 thread, there was a set of great pictures of what the muffler inside looks like. (Tried to find it again and I can't) As I remember it has two chambers with holes in the pipe. The pipe is staggered in length inside Also I believe it said that the CC has no restriction on the exhaust flow. So, is there any or enough back pressure from the muffler on the NA if there is no restriction from the CC? It seems to me if the muffler has an expansion chamber it acts as a venturi and will put back pressure on the system. Was the Flowmaster installed for back pressure or to quiet the system down? I'm just trying to understand the posts to decide what I want to do. Thanks, Fritz

9FF · 10-03-2011, 08:33 AM

I bought the Hushpower to reduce the noise and maintain the power. I couldnt stand the constant drone and loudness of the straight pipe. It is almost The same size as the cat and about the same weight so it's not a light piece. You can actually see straight through the Hushpower so there are no restrictions, but the baffles either side increase the effective diameter in that area. As I said I like the sound and I noticed a "seat of the pants" performance gain after I replaced the straight pipe, best described as more low end torque.

iplagolf · iplagolf's Garage

9FF, Good stuff in your post No. 25 in this thread. Ok, so the CC does restrict the exhaust flow from the engine. What I read was incorrect about no restriction. It makes sense since the exhaust gases flow into the CC and mix with the pellets contained therein. There is a huge amount of surface area of the pellets. The exhaust gas expands into the CC and are then forced back into the exhaust pipe to travel to the muffler which is another expansion chamber and then out the tail pipe. When the exhaust gas is allowed to expand and then forced into a smaller pipe the velocity of the flow is increased and thus back pressure is produced by the venturi effect. There is a sticker on the side passenger window the says the vehicle conforms to California emissions...The 944 is being choked to death. As with my Indian, the same thing. Now here comes the problems and more questions...When I removed the "guts" from the Indian exhaust and replaced it with an insert from Hacker Labs (They were making prototypes for aftermarket exhaust systems for the Indians, S&S motors) I ran into problems with the carburetor and the factory set up. I had to change jet sizes to make the motor preform as it should. After I did this and we put it on the dyno there was a 10 hp increase. So now the Porsche. If you install a straight pipe or a Master Flow or a expanded diameter insert pipe the same size of the CC without the pellets, how will is change the fuel/air mixture of the factory set up? In this thread someone posted about a "rack off" of the exhaust with the straight pipe installed. This sound like a "rich" condition of the fuel/air mixture. Does anyone agree with that? Disagree? I know some readers may be thinking, "Geez, give it a rest." I don't want to screw this car up. It runs great, looks great, I just want it to sound great. Thanks, Fritz

Guest765 · 10-03-2011, 11:35 AM

It was put there by Porsche for more than just emissions and is a calculated part of the exhaust system. End of story. Without it being there or without a suitable same sized replacement you're changing the entire dynamics of the exhaust system. The prosche isn't being "choked to death".

flash968 · 10-03-2011, 03:10 PM

all too often people refer to "back pressure". obviously you do not want back pressure. what they are really referring to is restriction or low gas velocity.

that being said, you also do not necessarily want high gas velocity. while it can help in upper end horsepower, it can hurt in lower end torque.

exhaust systems and designing them is a lot more complicated than just sticking on a pipe and opening it up. bends, reducers, mufflers, resonators, and yes, catalytic converters, are all items that are calculated and tuned in a properly designed system

in the case of the 968, for example, removing items, and opening up the flow, while it can add 2-3hp way up top, can cost as much as 20 ft/lbs of torque at 3400rpm. porsche really went a long way on designing that system. i spent $11k fiddling around trying to come up with a cat-back that did not lose torque.

every model of car is different, and they all respond differently to exhaust changes. be careful when approaching this kind of modification, and definitely test things to verify results, and eliminate the "adrenaline factor" that can make you "think" it's faster, when in reality, it isn't