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back pressure,needed or not

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fordpower View Drop Down
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    Posted: 19-July-2017 at 5:31AM
Well thinking of replacing my cats.
tried to do some research on back pressure.I found it very confusing to say the least. does anyone have opinion . I like sound with cats and no mufflers.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GranTorinoSport Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 7:12AM
Back pressure is needed for lower end power and driveability.

Lack of back pressure is desired for high RPM and race applications, however those vehicles don't have to worry about driving on the street.

The absence of your catalytic converters with the muffler(s) remaining will likely reduce some back pressure but not to an exceptional degree. I'm not sure how much back pressure the original factory cats provided, but like anything I would imagine the early ones were probably far worse than a more modern one.

If you want more sound, I would not remove the mufflers, rather choose a new muffler. There are several good offerings that will reduce back pressure and increase sound. I know many members here will have some strong opinions on good and bad mufflers. I think I used a Dynomax on mine (I'd have to look up which one). I got a higher flowing model that still was touted as "quiet", coupled with dual 2.5 inch exhaust (and an H-pipe) for my 521. I'm happy with it, but you can tell between the 600cfm carb and the 2.5 exhaust that it hits a wall when you are WOT. The 600 cfm carb is me just being cheap (it was laying around) until I get fuel injection, but the 2.5 inch was intentional because I've heard that quieting down 3 inch exhaust is nearly impossible.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nuggets Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 7:56AM
Its not back pressure! I hate this term......you are SO WRONG when you say back pressure.....or lack of etc etc

You want as close to ZERO back pressure as possible.

What you want in an exhaust is HIGH VELOCITY, this allows gas to escape quickly. This is not back pressure, has never been and will never be. It is a case of keeping exhaust gasses as hot as possible 

Its why if you run a full 3" system on a 289 it'll run worse and feel more gutless than a 289 on a twin 1.5" system. What you're doing is lowering gas velocity, in turn this can create back pressure as the gas slows down so much that you have a backup right at the end.

Run whatever the hell you want with regards to cats, no cats, mufflers.......just keep the pipe size in line with the engines requirements. 

If you create back pressure you harm performance. End of story. 



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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72GTS351CJ Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 9:06AM
Take a breath and calm down. It a friendly forum.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nuggets Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 9:32AM
As far as I'm concerned there was nothing unfriendly about my post.....purely informative and not sugar coated.

When we design exhausts in an engineering environment we design mufflers, cats, bends and the entirety of the system to have as close to zero back pressure as possible whilst in keeping with the given constraints both dimensionally and budget wise. 
Barnaby

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Post Options Post Options   Thanks (0) Thanks(0)   Quote fordpower Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 9:35AM
Could be a difference in meaning is all. I run only my cats. I like the sound so have a lot of others. I just wondered if by not running mufflers I create the velocity problem
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nuggets Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 9:54AM
Back pressure in an exhaust sense is essentially a restriction, thats its only definition. Not what you want on any car.

Yes you potentially create a velocity problem, will you notice it? Probably not. 

Its hardly a performance car you're toying with, if you enjoy the noise just do it
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Big Bird Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 10:27AM
You need to find a balance. Exhaust that's overly restrictive hurts performance, but enormous pipes cause the exhaust gasses to move too slowly, which reduces scavenging and forces the engine to "push" to get the exhaust gasses out. If the engine is a stock engine with a factory tune, EGR will not function correctly with no back pressure, for example. On a non-stock setup, keeping the exhaust moving at proper speed will help scavenging and increase intake charge flow.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nuggets Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 10:44AM
You have to compromise. However.....given all design restrictions in a full under car system you will never eliminate all back pressure anyway so the EGR should always function. 

I cant remember the figures on the GT3-R exhaust or stock W12 system on the Continental but even with the optimum routing you still get it and thats with super high flowing cats, mufflers and pipework routed in such a way to minimise kinks as well as being of optimum size. 

The only way is to essentially run it like a drag car with zoomie headers, but given this is a full system car having the positive or negative pressure valve in the system it should not make a difference to its operation. 

They want deleting anyway. Horrid things.


Edited by Nuggets - 19-July-2017 at 10:45AM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lynchster Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 1:42PM
[QUOTE=Nuggets]

Since you seem to be an expert on the subject............

I'm guessing that having my manifolds with four or five feet of pipe is not optimal.

Edited by lynchster - 19-July-2017 at 1:45PM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72FordGTS Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 3:07PM
The modern hi-flow cats don't really seem to have much restriction.  We've come a long way from the old restrictive 1970's cats.   My brother bought a set from a speed shop for one of his cars.  It did muffle the sound slightly, but the exhaust also stinks way less than a car without cats.  I'd stick with mufflers, just stick to small ones for more sound (if that's what you want).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nuggets Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2017 at 10:55PM
Originally posted by lynchster lynchster wrote:



Since you seem to be an expert on the subject............

I'm guessing that having my manifolds with four or five feet of pipe is not optimal.



Probably not. But that's how I run too. I like the sound and look of true duals....could I gain more with an X or H set up. Yes.
Do I want to. No?


You have to weigh up, cost of making your exhaust, sound quality which is always subjective and performance. Could you gain something with the tuning of the system through pipe sizing and bend radius etc. Yes.

Would it be worth it. Doubt it.

Exhaust tubing and optimisation is only generally worth it on high performance motors or where you're being stifled by emissions to start with so the tuning of the system post cat becomes even more critical in order to keep power high. It's how a Bentley still makes 500+ hp with pre and post cats, giant mufflers and emissions regs.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Don V. Quote  Post ReplyReply Direct Link To This Post Posted: 25-July-2017 at 6:50AM
When it comes to IC engines an interesting debate is what term is more misused and misunderstood, back pressure or torque?
 
 
Unless the laws of physics have been recently rewritten backpressure is absolutely necessary for an engine to run. I suppose it's possible to get an engine to idle poorly for a while without backpressure because the 3 requirements for ignition are present but a load would stall it.
 
The basic analogy might be a syphon hose. What happens when a syphon hose loses back pressure? Flow stops. An IC engine is defined as an air pump and this is basic fluid dynamics. BTW, fluid is not the same as liquid in fluid dynamics.


Edited by Don V. - 25-July-2017 at 6:50AM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aquartlow Quote  Post ReplyReply Direct Link To This Post Posted: 25-July-2017 at 9:08AM
Originally posted by Don V. Don V. wrote:

When it comes to IC engines an interesting debate is what term is more misused and misunderstood, back pressure or torque?
 
 
Unless the laws of physics have been recently rewritten backpressure is absolutely necessary for an engine to run. I suppose it's possible to get an engine to idle poorly for a while without backpressure because the 3 requirements for ignition are present but a load would stall it.
 
The basic analogy might be a syphon hose. What happens when a syphon hose loses back pressure? Flow stops. An IC engine is defined as an air pump and this is basic fluid dynamics. BTW, fluid is not the same as liquid in fluid dynamics.
 
Not trying to be an ass here, but how does a top fuel dragster/funny car make 10K horsepower with just zoomie headers if back pressure is absolutely necessary for an IC engine to run well. In another IC example I have also seen numerous turbo diesel engines with little more than a 18" piece of 4" pipe for an exhaust ran up through the hood and those ran "real" well. Please explain.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote californiajohnny Quote  Post ReplyReply Direct Link To This Post Posted: 25-July-2017 at 12:18PM
slight amount of back pressure works better on most street engines.

todd, to answer your question about the top fuel engines... they breath by help of the big belt driven blower!Big smileThumbs Up
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aquartlow Quote  Post ReplyReply Direct Link To This Post Posted: 25-July-2017 at 12:51PM
Originally posted by californiajohnny californiajohnny wrote:

slight amount of back pressure works better on most street engines.

todd, to answer your question about the top fuel engines... they breath by help of the big belt driven blower!Big smileThumbs Up
 
Definitely agree about street engines liking a bit of back pressure and the TF cars with blowers, but I also see pulling trucks with zoomies through the hood and they run tunnel rams with 2-4's(and they run pretty "fair" Wink).
   It seems on a street car engine, the exhaust system doesn't necessarily need to introduce back pressure to help the engine run better, but rather be tuned to some extent to help keep exhaust pulse velocities up so the exhaust flow itself will help or aid in scavenging or pulling the exhaust volume before it cools enough to slow down the exhaust pulse(which would equal back pressure). I believe exhaust back pressure, unto itself, is generally thought of as a bad thing for performance and/or efficiency.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote californiajohnny Quote  Post ReplyReply Direct Link To This Post Posted: 25-July-2017 at 1:12PM
very true!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aquartlow Quote  Post ReplyReply Direct Link To This Post Posted: 25-July-2017 at 1:26PM
Originally posted by californiajohnny californiajohnny wrote:

very true!
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 25-July-2017 at 1:35PM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Don V. Quote  Post ReplyReply Direct Link To This Post Posted: 25-July-2017 at 3:46PM
Originally posted by aquartlow aquartlow wrote:

 
Not trying to be an ass here, but how does a top fuel dragster/funny car make 10K horsepower with just zoomie headers if back pressure is absolutely necessary for an IC engine to run well. In another IC example I have also seen numerous turbo diesel engines with little more than a 18" piece of 4" pipe for an exhaust ran up through the hood and those ran "real" well. Please explain.
 
I don't think you're being an ass at all. It's a great question. I don't have the time right now to answer your question but I will quickly address Zoomies.
 
First and foremost, zoomies are cool.
 
A top fueler engine is designed for zoomies. I've seen more people use zoomies and destroy their heads than I've seen people successfully use them. Zoomies aren't something the average garage mechanic should be screwing with.
 
Back pressure is an important engine design parameter that involves the entire engine. It's often seen as an exhaust only parameter by a lot of people so they don't consider the effects inside of the engine from whatever they are doing with an exhaust. A lot of engines have been detrimentally effected because of this. For example, CATS require specific exhaust temperatures to perform their designed emission function. People remove CATS and alter their exhaust system but that doesn't change the engines design parameters like exhaust temperature. There is a measure of adaptability designed in to an engine but when the optimum performance requirements of an exhaust system exceed the engines adaptability to deliver the demanded performance requirement either performance is reduced, more often then not because of insufficient exhaust velocity, or money has been wasted on something with an engine that can't deliver the necessary performance to meet the exhaust systems potential. This example has more to do with modern engines than the engines that came in the cars that are the topic of this website.
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