DODGE RAM FORUM - Ram Forums and Owners Club! - Dodge Truck Forum

Go Back   DODGE RAM FORUM - Ram Forums and Owners Club! - Dodge Truck Forum > Dodge Ram Forum - Mod Zone - Custom Dodge Ram Mods - Dodge Ram Performance > Custom Dodge Ram Performance Mods - Engine - 5.7 HEMI V8 > Exhaust


Notices

Exhaust 5.7 HEMI Exhaust Discussion


Latest Vendor News
AutoAnything
Addictive Desert Designs!
United Radio
Bayou Goat Mounts

Reply
 
Thread Tools
  #1  
Old 05-12-2014, 04:23 AM
Planert360 Planert360 is offline
Dodge Ram Forum Member
 
Join Date: Sep 2013
Location: Wisconsin
Age: 24
Posts: 62
Gender: Male
Vehicle: 2013 Ram
Trim Level: Express
Color: Cherry Red
Rep Power: 0
Rep:54
Planert360 will become famous soon enough
Default Back pressure VS Velocity (Exhaust Speed)

So In searching the exhaust section these last few days, Iv noticed a few things, and Im curious on some other peoples input on the topic of 'Back pressure' and 'Velocity' when it comes to exhaust (I know technically I should be saying speed, but lets not be picky here, the direction is in general 'out'). Im reading how more and more people are concerned with having enough 'back pressure' but from my understanding, and Ill explain more, back pressure is absolutely bad, isn't it? Few years ago in my SR year of high-school, I recall getting our Physics teacher (Who was a gear head) and our shop teacher together and asked about exhaust, this is what Iv learned, and basically always believed, so Ill explain, and would love to get some input.


Back-pressure is always bad, an ideal exhaust system has 0 back pressure, and the most velocity possible, so the delicate balance is the fastest velocity, while maintaining the lowest back pressure. Think of Exhaust as a brick on a table. Pushing it uses (X) amount of force, this is the basic amount of force needed to push the exhaust out of the motor, so X=(F)orce needed. If you add piping, and create back pressure, your essentially having someone hold the brick while you try to push it (Y Is the amount of pressure being applied to hold the brick, or Y=Back-pressure), thus the equation is now X+Y=F as the motor is basically having to work harder to not only move the vehicle, but also working to push the exhaust out of the motor, and now through the pipe

Making sense so far? So then in all Back-pressure is bad, why not add huge pipes. Okay, I added big pipes, but now I actually LOST power, why? Velocity (Speed, shut up)!

Velocity is the rate at which exhaust "flows" through the piping. Think of a 2 water hoses, one is a 1/2 inch hose, the other is a 1/4 inch hose. Both hoses are hooked up to a 10 gal. per minuet water source, so both hoses are pumping out the exact same amount of water, (Now lets compare this quickly to your motor, at 2500 RPM your motor will put out the same amount of exhaust, no matter the size piping) However the water has to flow faster in the smaller pipe, to match the output of the larger pipe, correct? so the smaller pipe has a higher Velocity.

Back-pressure is the enemy to Velocity, as back-pressure is basically slowing the velocity down (The more bends you have, the higher back-pressure you have=slower velocity vs a straight flowing faster system)

So then why is velocity good? If the gasses are getting out, and there's no back-pressure, what role does velocity play? When the exhaust gasses get flowing quick enough, they actually help SUCK the exhaust out of the motor by creating a negative pressure behind it (Think of the air bubble you get in the front of the bed, while traveling down the highway. Now throw an empty soda can towards it,it will get sucked in, and most likly the can will move around, but it stays in that bubble, why? Because the bubble is acting like a vacuum, and has a lower pressure then the air around it.) In the exhaust system, that bubble is right outside the exhaust valve. Exhaust gets sucked out of the cylinder, and into that bubble. when the valve closes the bubble goes away and the exhaust gasses that were caught in it now become fast moving fumes like those around it. Going back to that brick, velocity is as if someone tied a string around that brick, and is actually pulling it the same direction your pushing it. The amount there pulling it can be (P) for example, so now our equation is X-P= F

Lets just throw numbers in here so some more can understand. Lets say it takes 10 units of power to simply push the exhaust gasses out of the motor (This is similar to running open headers), so:

(X)=10

And lets say there is 1 unit of back pressure, so:

(Y)=1

And to make it easy, lets say there is also 1 unit of velocity (Or 1 unit of suction, helping pull the exhaust out of the motor) so:

(P)=1


With backpressure the equation is:

(10)+(1)=11;

So 11 units of power to push the exhaust out, whereas the equation for velocity is:

(10)-(1)=9; so it only takes 9 units of power to push the exhaust out.

Obviously you would rather only waste 9 units of power vs 11 when pushing exhaust out.


So, to recap; Back-pressure bad, velocity good. Now the problem is, your exhaust cant be changed while your running down the drag strip (Well it can, and some fancy cars actually do modify the exhaust on the fly, but we wont get into it) So it can only be "setup" for one ideal RPM. usually, this is aimed right in the middle, so that's why when you take a stock exhaust and rip out the muffler to create a nice free flowing setup, you'll possibly see a loss in low end, and a gain in the high end, because low end you'll have less velocity, and high end you'll have more, while decreasing the back-pressure that used to be there. Also this is why some say "This muffler didn't hurt my performance like that one did" Chances are the "better" muffler just lined up your Back-pressure vs Velocity better then the "Worse" one.

Ideally you'd have smaller pipes, or less possible flow at lower RPM's, and bigger pipes or more possible flow in the higher RPM. My suggestion is on performance cars, find a way to set a trigger depending on RPM. My old MSD box had one (and I sure others sell them), some racers use them to trigger NOS at a specified RPM range, but if you hook that trigger up to a small exhaust cut out, for example, you could in fact alter your Back pressure and Velocity, automatically, on the fly.

Input?

And don't say that's too much to read, lol. I'm by no stretch of the imagination claiming to be an expert on the topic, but I do feel I am a higher educated individual, and fairly smart, also good when it comes to critical thinking, so I'm hoping there's other like minded individuals on here (Or possibly an expert).

Also to the Nazi's, sorry for any and all missed grammar mistakes, its 2:30 am here, and I've been up for about a day in a half, so suck it.
Reply With Quote
Sponsored links
  #2  
Old 05-12-2014, 09:33 PM
Planert360 Planert360 is offline
Dodge Ram Forum Member
 
Join Date: Sep 2013
Location: Wisconsin
Age: 24
Posts: 62
Gender: Male
Vehicle: 2013 Ram
Trim Level: Express
Color: Cherry Red
Rep Power: 0
Rep:54
Planert360 will become famous soon enough
Default

Nobody eh, No input at all?

Hmm, okay.
Reply With Quote
  #3  
Old 05-12-2014, 09:47 PM
Bob Chapman Bob Chapman is offline
Dodge Ram Forum Member

 
Join Date: Jan 2014
Location: Marquette Michigan
Posts: 98
Gender: Male
Vehicle: 2014 Ram Sport
Rep Power: 0
Rep:10
Bob Chapman is on a distinguished road
Default

You need to ask that old physics teacher to critique this - I am afraid that you are way over most of our heads.
Reply With Quote
  #4  
Old 05-12-2014, 09:49 PM
Planert360 Planert360 is offline
Dodge Ram Forum Member
 
Join Date: Sep 2013
Location: Wisconsin
Age: 24
Posts: 62
Gender: Male
Vehicle: 2013 Ram
Trim Level: Express
Color: Cherry Red
Rep Power: 0
Rep:54
Planert360 will become famous soon enough
Default

Haha, Fair enough. I wasn't sure if there were any guru's here who may be able to add some input. Thanks anyhow.
Reply With Quote
  #5  
Old 05-12-2014, 09:50 PM
cvieke's Avatar
cvieke cvieke is offline
Dodge Ram Forum Member
 
Join Date: Oct 2009
Location: IN
Age: 48
Posts: 65
Gender: Male
Vehicle: 2008 Ram
Trim Level: TRX
Color: Black
Engine: 2003-2008 345ci (5.7L) Hemi V8 345hp 375lb/ft
Rep Power: 0
Rep:10
cvieke is on a distinguished road
Default

Very well said. Makes since. I am a registered Vascular Technologist. A lot of physics involved, but I deal with the same issues with velocity and pressure with-in the arteries in the human body.
__________________
Volant CAI, Magnaflow muffler, 2" leveling, 3" body lift, Bushwacker fender flares, 4.56 gears
Reply With Quote
  #6  
Old 05-12-2014, 10:49 PM
Planert360 Planert360 is offline
Dodge Ram Forum Member
 
Join Date: Sep 2013
Location: Wisconsin
Age: 24
Posts: 62
Gender: Male
Vehicle: 2013 Ram
Trim Level: Express
Color: Cherry Red
Rep Power: 0
Rep:54
Planert360 will become famous soon enough
Default

Thanks, and I commend your job!
Reply With Quote
  #7  
Old 05-13-2014, 12:55 AM
Gecko's Avatar
Gecko Gecko is offline
Dodge Ram Forum Senior Member!
 
Join Date: Aug 2011
Location: New Jersey
Age: 24
Posts: 959
Gender: Male
Vehicle: 2004 Quad Cab
Trim Level: SLT 4x4
Color: Black
Engine: 2003-2008 345ci (5.7L) Hemi V8 345hp 375lb/ft
Rep Power: 4
Rep:214
Gecko has a spectacular aura aboutGecko has a spectacular aura aboutGecko has a spectacular aura about
Default

What you said makes sense and I know some guys have tried opening their cutouts on the way down the track to see if it allows them to run better or not. I don't remember off the top of my head what their results were though.

I'm curious to see what others have to say about this, such as cdnoilram and chefred for example.
__________________
Supercharged, cammed, other things too!
Reply With Quote
  #8  
Old 05-13-2014, 01:01 AM
Planert360 Planert360 is offline
Dodge Ram Forum Member
 
Join Date: Sep 2013
Location: Wisconsin
Age: 24
Posts: 62
Gender: Male
Vehicle: 2013 Ram
Trim Level: Express
Color: Cherry Red
Rep Power: 0
Rep:54
Planert360 will become famous soon enough
Default

Yes, I haven't been around long enough to know whos who, but if they could chime in, that would be great!

Also the thing I would think with opening the cutout going down the track is, it would have to be done each and every time the trans shifts. Lower rpm=Closed, hit say 3000 RPM and open it up, then as you shift, close it again. Could be hard on the trigger finger, so it would idealy be automated.

I know that would be possible with RPM triggers and a relay, as I know some racers who do that for NOS so they dont have to worry about pushing the button during the correct RPM range.
Reply With Quote
  #9  
Old 05-13-2014, 01:08 AM
Gecko's Avatar
Gecko Gecko is offline
Dodge Ram Forum Senior Member!
 
Join Date: Aug 2011
Location: New Jersey
Age: 24
Posts: 959
Gender: Male
Vehicle: 2004 Quad Cab
Trim Level: SLT 4x4
Color: Black
Engine: 2003-2008 345ci (5.7L) Hemi V8 345hp 375lb/ft
Rep Power: 4
Rep:214
Gecko has a spectacular aura aboutGecko has a spectacular aura aboutGecko has a spectacular aura about
Default

Are those triggers instant open? I know with my cutouts, and I'm assuming with all cutouts, that it takes a few seconds to open so unless it was an instant open and close I could see that being difficult to make happen.
__________________
Supercharged, cammed, other things too!
Reply With Quote
  #10  
Old 05-13-2014, 01:14 AM
Planert360 Planert360 is offline
Dodge Ram Forum Member
 
Join Date: Sep 2013
Location: Wisconsin
Age: 24
Posts: 62
Gender: Male
Vehicle: 2013 Ram
Trim Level: Express
Color: Cherry Red
Rep Power: 0
Rep:54
Planert360 will become famous soon enough
Default

Honestly I'm not sure, as I've never dealt with a cut out. The trigger I was referring to is just the RPM trigger, ie when the RPM is above a certain limit, it would switch on power to the relay, the relay would then have to put the cut out. However if cutouts take a second or two to open/close, I feel this would not work.

I do recall reading some higher end car manufacturers dabbling in exhaust modification depending on RPM, I may have to giv into and see how they achieved it.
Reply With Quote
Reply

Bookmarks

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


Advertisement






All times are GMT -4. The time now is 03:50 PM.


= Copyright RamForumZ.com a Gigathreads.com Network Site =Ad Management by RedTyger
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
vB.Sponsors


=Gigathreads Network=
CLUBS MOPAR NATION DSM GPMM MAMM JAM
DIESEL Diesel Forum      
CHEVY Chevy Camaro Forum Chevy Volt Forum    
CHRYSLER Chrysler 300C Forum      
DODGE Dodge Avenger Forum Dodge Caliber Forum Dodge Challenger Forum Dodge Charger Forum
  Dodge Circuit Forum Dodge Hornet Forum Dodge Magnum Forum Dodge Nitro Forum
  Dodge Ram Forum Dodge Dakota Forum Dodge Durango Forum Dodge Dart Forum
FORD Ford Raptor Forum Ford F150 Forum Ford F100 Forum  
JEEP Jeep Forum      
PONTIAC Pontiac G8 Forum      
SRT SRT Viper Forum SRT Cuda Forum