Little background first: I loaned my truck out to a good buddy. He had it for a couple months so I don't really know what led up to the failure. He called and said that it "lost all power and made a loud hissing noise along with some clanking from underneath." I lurked around here long enough to deduce it was a plugged cat, Thanks to this forum and all of you guys that are 100 times better than a Haynes manual

The cat was definitely coming apart, so i cut it out and am on my way to a Summit Racing retail store, lucky i know, to pick up a direct fit. Here is the question, I drove the truck a bit with the pipes cut just in front of the cat, it was loud as a train, but still felt a little flat and uninspired. I expected it to run wild unrestricted and it just felt weak.

Is this the "needs back pressure" issue or is there something else that I should be looking for? Good oil pressure and not using water so i think the head gaskets are good, seems to idle OK, just sucking wind when you crack the throttle under load.

Any help or thoughts are much appreciated.

 

Any time you reduceback pressure you loose low end torque and usually gain high RPM horsepower. It will most likely fell much better with a new cat on it and you will beable to hear yourself think.

Will

 

Also don't confuse backpressure with exhaust velocity. More exhaust backpressure is not good, more exhaust velocity is. Reducing the diameter of the exhaust pipes generally will increase the velocity, but is seen as backpressure when running higher rps such as running at WOT, especially with a modified engine. With a stock engine, don't open up the exhaust or velocities will drop and so will lower end torque. I agree with priest1981, when you install the new CAT, it will run better than with the open exhaust pipes and sound much better. You have pretty much doubled the pipe volume by cutting them in fromt of the CAT where they normally are combined into a single outlet. You have lost the accumulative velocity by separating them.

I am by far no expert, but I do understand, to a limited degee, how these things work.There have been more discussions than I can remember on this topic. There are many theories and science to support them. Fine tuning your exhaust system isn't necessarily rocket science, but it sure does seem like it sometimes...

 

I have a question, I see where they off a single exhaust that is 2 1/2" or 3' from flowmaster, gibson that says to use when towing. My original exhaust is 2". Am I going to loose torque by installing the larger pipe?

 

these motors need some sort of backpressure to function correctly in the right power range. too much will cause misfires too little will cause lack of low end tq.

to find out easily if everything else is up to par, hook a vaccum gauge up to the intake manifold and check the reading. should be between 17-22 in HG.

 

Just wanted to say thanks real quick. Bolted on a directfit magnaflow cat from Summit and she ran much better. Lord knows how long it has been partially clogged.

Ron333, What you said about doubling the exits makes sense. I try to take each repair and learn from it. Can you explain how a true dual exhaust system doesn't have the same velocity problem based on replacing a 2 into 1 cat. Layman's terms for a lame man would be great.

Ryan

 

I won't be able to explain it very well. As I mentioned I only have a limited understanding on this subject. If you search this forum, and the internet, you'll be getting much better answers than what I will be able to give.

I'll do my best, and await the criticism from those who know that will no doubt follow...

If there isn't enough exhaust velocity, then the exiting exhaust gases don't create enough momentum to pull allong (syphon) the rest of the exhaust gases behind it. Engines use an overlap between closing the exhaust and opening the intake valves to benefit from the vacuum of the exhaust gases exiting the combustion chamber. When the exhaust gas velocity is down, there isn't enough vacuum created in the combustion chamber to adequately pull out all of the exhaust gases and benefit from the overlap between the intake and exhaust vales pulling fresh fuel/air mixture for the next compression cycle. When there is enough velocity / vacuum in the expelling exhaust gases, this will help pull all of the spent exhaust gas and pull a little more fresh fuel/air into the cylinder before the exhaust valve closes. This gives more of a bang in the combustion cycle, thus more power. Now, if you're running higher rpms with an open exhaust, this will increase the exhaust gas velocity and begin creating the vacuum during valve overlap as I mentioned. The more the exhaust system is open, the harder it is to create this vacuum, so the engine has to increase its rpms to make power. If you plan on using power at HIGH rpms only and built you engine accordingly, then this would be a good thing. Since your engine isn't built to run at the high enough rpms to benefit from open exahust pipe, it will feel sluggish because it is not getting that desired pull on the exhaust gases exiting the combustion chamber.

If you've ever noticed when looking at camshaft profiles, they always talk about lift, and duration, and valve overlap, etc. These things are built into the lobe shapes to make power in specific rpm ranges, and they are based to a degree on this overlapping of the valves.

 

I had a similar issue with my 1500 INDYRAM installed headers that didn't match up with y-pipe so ran open headers for 3 days until I was able to get it to the shop and have it custom welded (the guy was a friend of my grandfather and did it for practically nothing). In those three days I drove maybe 45 miles went through half a tank of gas and had no low end but once I hit about 50-55 mph it turned into a beast. Backpreasure helps with low end torque witch is why you truck moved like a geriatric snail off the line.

 

Some backpressure is needed to keep you from burning valves, but, as stated, velocity is more important. Velocity is the whole reason for installing headers. If all of the exhaust ports on a head are shooting into the same box (a manifold), the velocity from any one port is lost right there. A properly designed exhaust system will have primaries based on compression and valve size that go into a large collector. Form there the pipe should decrease in size to accomodate the cooling of the exhaust gases. As the exhaust cools it contracts, so to maintain velocity the pipe should get smaller. That being said, if you are running a cat it doesn't matter what you do after that where exhaust velocity is concerned. Not only is it larger to varying degrees than the pipe, it is also restrictive in varying degrees.From a backpressure standpoint it does matter. More backpressure requires less fuel to maintain a given air fuel ratio. Less backpressure requires more fuel.The pcm will need to compensate for any major changeto your exhaust system, and, while the pcm will eventually adjust for a major change at cruise, a flash may be required to get wot a/f ratio correct.

People are correct when they say that they lose low end torque when they install a bigger exhaust system because, in most cases, that's all they did. You can't go and drastically reduce backpressure without tuning the pcm for it.

More reading:

http://www.cargather.com/engineering...cal-00311.html
http://forum.camarov6.com/showthread.php?t=77275
http://www.uucmotorwerks.com/html_pr...torquemyth.htm

 

I won't be able to explain it very well. As I mentioned I only have a limited understanding on this subject. If you search this forum, and the internet, you'll be getting much better answers than what I will be able to give.

I'll do my best, and await the criticism from those who know that will no doubt follow...

If there isn't enough exhaust velocity, then the exiting exhaust gases don't create enough momentum to pull allong (syphon) the rest of the exhaust gases behind it. Engines use an overlap between closing the exhaust and opening the intake valves to benefit from the vacuum of the exhaust gases exiting the combustion chamber. When the exhaust gas velocity is down, there isn't enough vacuum created in the combustion chamber to adequately pull out all of the exhaust gases and benefit from the overlap between the intake and exhaust vales pulling fresh fuel/air mixture for the next compression cycle. When there is enough velocity / vacuum in the expelling exhaust gases, this will help pull all of the spent exhaust gas and pull a little more fresh fuel/air into the cylinder before the exhaust valve closes. This gives more of a bang in the combustion cycle, thus more power. Now, if you're running higher rpms with an open exhaust, this will increase the exhaust gas velocity and begin creating the vacuum during valve overlap as I mentioned. The more the exhaust system is open, the harder it is to create this vacuum, so the engine has to increase its rpms to make power. If you plan on using power at HIGH rpms only and built you engine accordingly, then this would be a good thing. Since your engine isn't built to run at the high enough rpms to benefit from open exahust pipe, it will feel sluggish because it is not getting that desired pull on the exhaust gases exiting the combustion chamber.

If you've ever noticed when looking at camshaft profiles, they always talk about lift, and duration, and valve overlap, etc. These things are built into the lobe shapes to make power in specific rpm ranges, and they are based to a degree on this overlapping of the valves.



For not being an expert as you said,you damn well explained it like one.

The only thing I can do on your post is to bump it!!

Great explanation and understanding of this.