CatBack...school me

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Been reading some stuff over at ford-truck.com Reading a lot into a "catback" exhaust system. What the hey is that? An exhaust system that bypasses the cats? Speaking of bypassing the cats, could I do that, totally remove it if I could find someone to do it? Would that give me some more oomph in a 01 5.4 4x4 truck? I know I'll have to get some MIL Eliminators to fool the O2 sensors that run in the back of the cats, but would it be a help, hinderance or a pain in the @ss?
 
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Originally posted by Schmoe: Reading a lot into a "catback" exhaust system. What the hey is that?
"Catback" means "from the cats on back". The exhaust bolts just downstram from the stock cats, which are left installed. Catback systems usually don't improve power much in normally aspirated engines. Two other options that might give you actual gains are: "Shorties": short headers that go from the engine block to the stock collectors (upstream from the cats). Shorties are usually a bolt-on replacement for the stock (usually cast iron) headers. Shorties usually provide small power gains and the stock cats remain in place so the car passes smog. "Long tubes": like shorties, only longer, bypassing the stock collectors and usually bypassing the stock cats too. They come in two flavors: "street legal" which include their own high flow cats, or "offroad" which do not have any cats at all. Long tubes provide bigger power gains than shorties but even the street legal ones won't pass smog in some states.
 

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OK, so what if passing smog test was not in the question, like a farm truck or in a state that doesn't have smog checks? Would removing the cats be a good idea?
 
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quote:
Originally posted by Schmoe: OK, so what if passing smog test was not in the question, like a farm truck or in a state that doesn't have smog checks? Would removing the cats be a good idea?
Not necessarily. Ignoring the obvious pollution issues, the exhaust is a *system* that works in conjunction with the intake and the carbs or MAF program to provide the correct A/F ratio for the current driving conditions. Generally, if you open up the exhaust or intake to let it flow more, the engine will run leaner. This may be good or bad, it could cause power gains or losses, depending on the situation. Furthermore, there is the geometry of the exhaust which times pulses reflected back to the engine. This pulse timing determines the RPM at which the exhaust "resonates" to increase the efficiency of the engine. What it boils down to is that DIY shade tree mods to the exhaust system are generally not a good idea, as they could cause power losses or A/F ratio problems. If you want to make more power, best bet is to go with a well engineered off the shelf exhaust system with dyno charts to demonstrate the performance impact. Here is a link you might enjoy: http://auto.howstuffworks.com/question172.htm
 
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NA engines do not benefit as much from larger exhausts/ less restriction as much as do turbo charged motors. NA engine needs back pressure to work efficiently for normal driving. A well tuned exhaust is the critical part. This may include headers/ hi flow cats etc. BTW: In some places removal of the cat may incur significant fines. However on Turbos the bigger and less back pressure the better. Thats why you will see 3" and larger exhausts commonly on turbo charged cars. I did see a report somewhere on the internet on a NA motor and tested different cats and well as complete removal. There was very little difference in power output even with no cat. Many standard cats are fairly efficient these days. [ December 16, 2003, 07:01 PM: Message edited by: theguru ]
 
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Cat back is a little more appropriate to modern cars that have a "pre-muffler" resonator between the catalytic converter and the mufflers. Cat-back simply means you replaced everything after the emissions control system, not just the mufflers but also what's in-between.
 

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I can understand that, but if you go over to f150online and ford-truck.com's, a lot, I mean a lot of owners have said that they have noticed significant increases in hp and fuel economy when they installed shorty's and catbacks. Somethings got to be working. Also, like changing your pulleys can up your hp. That still baffles me. The cats, yeah, I can see that they are a lot more efficient than the old pellet designs, but I was surprized that it really doesn't over a difference in power whether they are installed or not.
 
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I can understand an HP increase with a cat-back system, especially on a truck engine. There may be some loss in low end torque though. As ekrampitzjr said, these items are designed the way that they are for a reason, usually to optimize specific characteristics. I don't know. Putting underdrive pulleys on will reduce the parasitic power loss from your accessories.
 
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Most ford cats have an O2 sensor before and after the cat they use teperature of the 2 cats to tune your fuel. If you put any other cats on you may get a check engine light which one is annoying and two your mileage will suffer. The header issue is Ford uses a robo-torquer so the header from the factory must allow easy installation of all fasteners at once. If you ever remove your factory headers you will see how small your primary tubes are. Underdrive pulleys will spin your items at different RPM's than intended which could cause your ALT to be below RPM for charge at idle and you water pump to rotate too slow on those warm summer days in traffic. My 2 cents thanks for your time.
 

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BUT they basically reduce your serp. belts rpm down by 25%. What I've learned from that is during idle or low speeds, the AC doesn't get as cold and the alternator doesn't crank out the amps that it should to run all those systems. Boy, the more I read into all this (as far as milking some extra horses and better mpg) the more I'm confused (which doesn't take much) and/or scared. Like the Ford 5.4L piston slap or blowing spark plug problem. Scared to even drive my Dad's truck. Thinking about liquidating and buying a brand new one, but I couldn't do that to dear ol' Dad.
 
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The factory stuff usually works the best as it is a complete balanced system "usually" . It would need a dino test to prove a difference "Your Own" .Louder always seems more powerful. Headers usually make more power. I'm cant belive an after cat would make a whole lot of difference the worst that can happen is money spent. Leave it stock and enjoy the truck. Do something wild change all the lubricants to soon. Don't worry about piston slap and if a plug blows out there are kits ,heli coil that can repair the threads without removing the heads. Fords are the best of the big 3. [ December 17, 2003, 04:06 PM: Message edited by: Steve S ]
 
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Lately, the factory systems have been getting much better, but the system on my car was designed primarily to make it ghostly quiet. Simply putting an H-pipe in gave it more midrange power, and actually made it quieter. Slipping the $15 Summit Turbo mufflers in helped out a bit with the top end. Pulling the cats off an LT1 actually results in torque loss, though it does give you an extra couple of HP.
 
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Exhaust systems are designed the way they are for a reason. Usually the reason is cost, and to be completely silent, like 99% of buyers want them to be. Cat-back systems can often give more power, depending on backpressure created by the stock exhaust. For example, I shaved 2/10th's in the quarter when I installed my cat-back system on my truck, but there is a big difference between 1 2 1/2 inch, crimped and flattened pipe, and 2 2 1/2 inch mandrel bent pipes and a free flowing muffler.
 
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Contrary to popular belief, most catalysts add very little resistance to an exhaust system. The old GM "pellet" designs were a different story, but I think everyone's gone to the far better "honeycomb" type. The overall exhaust system is engineered at great expense by the manufacturer; most of today's designs are excellent and highly efficient. Most resistance in an exahust system is before the catalyst, especially in front-drive cars. The exhaust manifolds and pipes before the cat(s) often have tricky bends in them that add back pressure. Often there is little you can do about that: headers and other aftermarket gizmos still must fit in the same tight spots as the original manifolds, etc. Cat-back systems are of limited use, and many are made of inferior materials to the stock systems, which are now usually made of stainless steel and can last the life of the car. (My '88 Festiva still had its original exhaust system after over 350,000 miles. My '97 Escort still has its after 235,000.) Don't mess with the cat(s). And I personally wouldn't bother with a cat-back system unless you really like the sound the exhaust will have.
 
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Cat backs are popular on trucks for two reasons: First, people usually replace the muffler with somethign much louder and go with dual tips out the back. If you like listening to your engine rumble (which in a truck is generally that nice V8 burble), then by all mean, consider a change. Second, they do work well at freeing up some HP on the upper end of the powerband. If you drive your truck like a racecar, this might be nice. Otherwise, you are giving up some of the low end torque, which is considered a bad thing in a truck application. I'd rather have all the low end torque I can get. Underdrive pullies free up some HP as they take less "oomph" to drive, as you are slowing the belt speed down. It means the alternator, water pump, ac compressor, etc... are all runiing slower and this may be especially noticable if there is any extended idling. Don't get confused. The reality is there is no such thing as a free lunch. Catbacks increase top end hp at the expense of low end torque and generally the quietness of the stock system. Underdrives free up minimal hp at the expense of more strain on the electrical, cooling, and AC systems. Decide whats best for you...
 
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If it were me I wouldn't do it. I've owned a 95 F150 w/302 and a 94 F150 w/351. I put a 3inch catback on the 302 and killed the bottom end. Trucks are built for bottom end and it's the bottom end that gets you from one light to the next quicker than the guy beside you. Also, it's what pulls a load. I put a 2 1/2 inch catback on the 351 with a magnaflow straight thru pipe and it was ok but I won't waste the money again. The bang for the buck isn't there. At most, I might swap a straight thru muffler for the stock one and leave the factory pipe. But, you won't gain enough to be worth it. If Ford could gain 10hp with a better pipe/muffler then they would have done it because this would give them the marketing numbers to compete with Chevy & Dodge numbers even though Ford's lowend TQ is where it's at for truck use.
 
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quote:
Catbacks increase top end hp at the expense of low end torque and generally the quietness of the stock system.
All depends on the catback. If you stay stock size and switch to a good mandrel bent exhaust, with long-case free-flow mufflers and resonators, you will not lose low end because you are still keeping the velocities up. The long-case mufflers and resonators will keep the noise down, though still slightly louder than stock, while the mandrel bends will decrease flow restriction at higher RPMs. Adding an H-Pipe, for V type engines, will also increase mid-range, while decreasing noise. Most people fall for the "bigger and louder" is better, and then do see a torque loss. Exhaust systems must be tailored to the engine and amount of power being developed. My 300HP V8 does best with a 2.25" system, though a 2.5" system does not hurt bottom end too much. Going to a 3" would kill my bottom end though. Factory exhaust systems, for the most part, are designed with low cost and low noise in mind with little thought for performance. A well tailored exhaust IS a free lunch, except for the actual money you have to lay out for it. It can have mufflers that are as quiet as stock(with some sacrifice in all out performance over free-flows), while still giving a HP and TQ boost, with no sacrifice in fuel economy. It is pretty hard to find an aftermarket system that meets those specs though, as most people wanting cat-backs want the noise and the bragging rights of the big pipes. There is always the custom route though.
 

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I appreciate your replys. Now, let me sit back and decipher exactly what I need/want to do. Steve S.....everything I've read about the blowing plug syndrom on 5.4, they say that when the plug blows, it ruins your heads, and then your screwed. You got any ideas on how to prevent this? I know Ford covers themselves by placing in the owners manual to check torque on the spark plugs every 3K, but that is totally unreasonable. I've read somethings that you will hear a popping sound coming from the heads that is indicative to the plugs going. You know anything? [ December 18, 2003, 05:06 PM: Message edited by: Schmoe ]
 
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quote:
Originally posted by Schmoe: I appreciate your replys. Now, let me sit back and decipher exactly what I need/want to do. Steve S.....everything I've read about the blowing plug syndrom on 5.4, they say that when the plug blows, it ruins your heads, and then your screwed. You got any ideas on how to prevent this? I know Ford covers themselves by placing in the owners manual to check torque on the spark plugs every 3K, but that is totally unreasonable. I've read somethings that you will hear a popping sound coming from the heads that is indicative to the plugs going. You know anything?
The heads can be repaired with heli-coils, here is a company that makes an application specific kit. http://www.timesert.com/sprksert.html
 
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