Exhaust mod questions
JK Junkie
Joined: Jul 2008
Posts: 3,017
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From: Lakewood, OH
JK Enthusiast
Joined: Apr 2009
Posts: 140
Likes: 0
From: Fayetteville
Check this out...Re-read my post CAREFULLY. I never said "back pressure was a good thing."
http://www.uucmotorwerks.com/html_pr...torquemyth.htm
JK Enthusiast
Joined: Apr 2009
Posts: 140
Likes: 0
From: Fayetteville
Jpop, We are on the same side...Too much Back pressure at high RPM's is bad. This is caused when you are trying to achieve the greatest velocity by narrowing the pipes. (smaller pipes than stock.) Greater velocity equals greater torque. Achieving this results in high back pressure.
3rd grade explanation..
3rd grade explanation..
JK Junkie
Joined: Jul 2008
Posts: 3,017
Likes: 0
From: Lakewood, OH
Well, I don't know how well I can explain it either as I'm 20+ years removed from taking fluid dynamics in college. There is a definite sweet spot in pipe diameter for moving exhaust gases, depending on volume. The primary benefit of the proper flow of exhaust gases is the scavenging of spent fuel from the cylinders. Too much back pressure or a constricting pipe will limit the volume of gases that can be moved and also limit the amount of spent fuel that can be pulled or scavenged from the cylinder.
Conversely, too large a pipe allows plenty of volume but reduces velocity. A lack of velocity will limit the amount of scavenging as well. Furthering the issues with too large of a pipe is that it allows more time for exhaust gases to cool and contract further slowing velocity.
My personal experience was with a drag car that was right beyond the hp of dual 2.5" pipes. I moved to a 3" exhaust, picked up horsepower on the dyno, but lost ET while gaining speed at the top end. Ended up being a lot of dollars spent to get things right and one of the cheaper components was where the problems stemmed from.
Today it's pretty easy as there are some online calculators for header lengths and exhaust diameter. The whole back pressure thing seemed to come about with the tuner crowd who realized their 4" exhausts weren't working and realized a need for velocity which they referred to as "back pressure". That 4" pipe needed 600hp to fill it up and the majority of them didn't have it under the hood.
Conversely, too large a pipe allows plenty of volume but reduces velocity. A lack of velocity will limit the amount of scavenging as well. Furthering the issues with too large of a pipe is that it allows more time for exhaust gases to cool and contract further slowing velocity.
My personal experience was with a drag car that was right beyond the hp of dual 2.5" pipes. I moved to a 3" exhaust, picked up horsepower on the dyno, but lost ET while gaining speed at the top end. Ended up being a lot of dollars spent to get things right and one of the cheaper components was where the problems stemmed from.
Today it's pretty easy as there are some online calculators for header lengths and exhaust diameter. The whole back pressure thing seemed to come about with the tuner crowd who realized their 4" exhausts weren't working and realized a need for velocity which they referred to as "back pressure". That 4" pipe needed 600hp to fill it up and the majority of them didn't have it under the hood.
JK Enthusiast
Joined: Apr 2009
Posts: 140
Likes: 0
From: Fayetteville
Well, I don't know how well I can explain it either as I'm 20+ years removed from taking fluid dynamics in college. There is a definite sweet spot in pipe diameter for moving exhaust gases, depending on volume. The primary benefit of the proper flow of exhaust gases is the scavenging of spent fuel from the cylinders. Too much back pressure or a constricting pipe will limit the volume of gases that can be moved and also limit the amount of spent fuel that can be pulled or scavenged from the cylinder.
Conversely, too large a pipe allows plenty of volume but reduces velocity. A lack of velocity will limit the amount of scavenging as well. Furthering the issues with too large of a pipe is that it allows more time for exhaust gases to cool and contract further slowing velocity.
My personal experience was with a drag car that was right beyond the hp of dual 2.5" pipes. I moved to a 3" exhaust, picked up horsepower on the dyno, but lost ET while gaining speed at the top end. Ended up being a lot of dollars spent to get things right and one of the cheaper components was where the problems stemmed from.
Today it's pretty easy as there are some online calculators for header lengths and exhaust diameter. The whole back pressure thing seemed to come about with the tuner crowd who realized their 4" exhausts weren't working and realized a need for velocity which they referred to as "back pressure". That 4" pipe needed 600hp to fill it up and the majority of them didn't have it under the hood.
Conversely, too large a pipe allows plenty of volume but reduces velocity. A lack of velocity will limit the amount of scavenging as well. Furthering the issues with too large of a pipe is that it allows more time for exhaust gases to cool and contract further slowing velocity.
My personal experience was with a drag car that was right beyond the hp of dual 2.5" pipes. I moved to a 3" exhaust, picked up horsepower on the dyno, but lost ET while gaining speed at the top end. Ended up being a lot of dollars spent to get things right and one of the cheaper components was where the problems stemmed from.
Today it's pretty easy as there are some online calculators for header lengths and exhaust diameter. The whole back pressure thing seemed to come about with the tuner crowd who realized their 4" exhausts weren't working and realized a need for velocity which they referred to as "back pressure". That 4" pipe needed 600hp to fill it up and the majority of them didn't have it under the hood.
That is what I was trying to tell him. You did a much better job explaining seeing that we started shouting!
JK Junkie
Joined: Jul 2008
Posts: 3,017
Likes: 0
From: Lakewood, OH
By the way, here's a real quick little calculator that takes all the guess work out of it.
http://www.mk5cortinaestate.co.uk/calculator5.php
JK Newbie
Joined: Jul 2009
Posts: 3
Likes: 0
From: Lone Tree, CO
A great alternative that will save lots of money is to go muffler-less. It actually sounded pretty good after I routed the exhaust to the back of the Jeep. Without the muffler the JK still doesn't sound loud. The only difference I've noticed is cruising in 6th gear on the highway is a lot easier, a lot less downshifting when power is needed.