Backpressure ALWAYS decreases power

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OS

Lifer
Oct 11, 1999
15,581
1
76
Originally posted by: Howard
The very definition of backpressure means is that it's a pumping loss.

By properly sizing the exhaust, you REDUCE backpressure and free up lost power. Who disagrees this time?

You get power from correctly sizing the exhaust because exhaust gas velocity is affected by the size of the piping. By the bernoulli equation, too large a pipe reduces gas velocity.
 

Vic

Elite Member
Jun 12, 2001
50,415
14,303
136
Originally posted by: Howard
Originally posted by: Vic
It's not a true statement. Power is a function of torque and rpm (work over time). In other words, all things being equal, power is the ability to produce a similar amount of work (torque) but at faster speeds (shorter durations of time). An increase in backpressure (to a reasonable extent of course) will actually increase torque
I disagree. If this were true, choking an oversized exhaust at any point (with a physical bottleneck of sorts) would increase power.

In fact, what we are really after is increased exhaust gas velocity, which increases volumetric efficiency (so to speak) because more of the cylinder can be filled with fresh air and fuel. The scavenging effect, as it were. By decreasing the exhaust pipe diameter, velocity must increase because mass flow remains constant.
Your initial question was vague, general, and open-ended. Thus, I based my rebuttal on the assumption of starting from an ideal sized exhaust, or "reasonable" as I said it.
 

Howard

Lifer
Oct 14, 1999
47,989
10
81
Originally posted by: GalvanizedYankee
A high performance 4 stroke gasoline engine w/o variable valve timming can suffer from
a huge flat spot in it's mid-range when run w/o any backpressure. This hole in the torque
curve is caused by over-scavaging the cylinder, not only causing an increase in pollutants,
but a real reduction of cylinder filling. It can approch 15% or more.
Definitely news to me.

Ah, I see why now. In most engines, the exhaust valves close after the intake valves opens because the cylinder has not completely rid itself of the exhaust. As the piston goes down for the intake stroke, both the intake and exhaust valves are open after TDC for a period of time, effectively reducing displacement. In fact, it's physically impossible to get rid of all the combustion products without having both intake/exhaust valves open after TDC at the beginning of the intake stroke...

Thanks for helping me realize this. :eek:

However, should the engine be fitted with an "ideal" exhaust (zero backpressure at any RPM), reducing the overlap would bring the effective displacement back up.
 

CVSiN

Diamond Member
Jul 19, 2004
9,301
0
0
Originally posted by: OS
Originally posted by: Howard
The very definition of backpressure means is that it's a pumping loss.

By properly sizing the exhaust, you REDUCE backpressure and free up lost power. Who disagrees this time?

You get power from correctly sizing the exhaust because exhaust gas velocity is affected by the size of the piping. By the bernoulli equation, too large a pipe reduces gas velocity.

is that why dumps on a street car is actually bad? i never understood how all this worked was just always told at least in the 5.0 stang world that dumps are not good over a well tuned 2.5 inch full exhaust
 

Howard

Lifer
Oct 14, 1999
47,989
10
81
Originally posted by: CVSiN
Originally posted by: OS
Originally posted by: Howard
The very definition of backpressure means is that it's a pumping loss.

By properly sizing the exhaust, you REDUCE backpressure and free up lost power. Who disagrees this time?

You get power from correctly sizing the exhaust because exhaust gas velocity is affected by the size of the piping. By the bernoulli equation, too large a pipe reduces gas velocity.

is that why dumps on a street car is actually bad? i never understood how all this worked was just always told at least in the 5.0 stang world that dumps are not good over a well tuned 2.5 inch full exhaust
By dumps do you mean open headers?
 

OrganizedChaos

Diamond Member
Apr 21, 2002
4,525
0
0
i agree. back pressure is bad.
velocity and scavenging is good.

unfortunately most of armchair experts around can't understand the difference.
 

CVSiN

Diamond Member
Jul 19, 2004
9,301
0
0
Originally posted by: Howard
Originally posted by: CVSiN
Originally posted by: OS
Originally posted by: Howard
The very definition of backpressure means is that it's a pumping loss.

By properly sizing the exhaust, you REDUCE backpressure and free up lost power. Who disagrees this time?

You get power from correctly sizing the exhaust because exhaust gas velocity is affected by the size of the piping. By the bernoulli equation, too large a pipe reduces gas velocity.

is that why dumps on a street car is actually bad? i never understood how all this worked was just always told at least in the 5.0 stang world that dumps are not good over a well tuned 2.5 inch full exhaust
By dumps do you mean open headers? Yep, less scavenging, but I'm not well enough versed in the physics of fluid mechanics to fully understand (or explain) why.

Ah okies thanks for the answer... yes from the header even long tubes.. but also even after the muffler was always told even the turndowns behind a muffler would adversly affect a streetcar as well.
 

GalvanizedYankee

Diamond Member
Oct 27, 2003
6,986
0
0
I should have said under-scavaging. With little back pressure cause by an over sized
low velocity exhaust system, the exhaust wave can reverse the flow in the intake port
at TDC while valves are at overlap. Introducing exhaust into the fresh charge reduces torque
and increases pollutants.
At it's worst an intake manifold will start filling with carbon.
Two plane intake manifolds and dual tract (for injection) are used to maintain intake velocity at lower RPM.
 

Howard

Lifer
Oct 14, 1999
47,989
10
81
Originally posted by: GalvanizedYankee
I should have said under-scavaging. With little back pressure cause by an over sized
low velocity exhaust system
That's the thing. An oversized exhaust increases backpressure, while a correctly-sized exhaust has a minimum of backpressure (higher scavenging).
 

kumanchu

Golden Member
Feb 15, 2000
1,471
4
81
i think that responses should keep in mind the topic is about backpressure and not actually about exhaust sizing. Exhaust sizing affects back pressure, however this does not answer the call of the question.

are there cases where increased backpressure increases hp?
 

Toastedlightly

Diamond Member
Aug 7, 2004
7,212
5
81
Originally posted by: Howard
The very definition of backpressure means is that it's a pumping loss.

By properly sizing the exhaust, you REDUCE backpressure and free up lost power. Who disagrees this time?

Well, one has to take into account the exaust velocities, cam profiles, firing order, ect. Can't generalize like this.
 

Howard

Lifer
Oct 14, 1999
47,989
10
81
Originally posted by: Toastedlightly
Originally posted by: Howard
The very definition of backpressure means is that it's a pumping loss.

By properly sizing the exhaust, you REDUCE backpressure and free up lost power. Who disagrees this time?

Well, one has to take into account the exaust velocities, cam profiles, firing order, ect. Can't generalize like this.
The only relevant factor you listed IMO is cam profile (which I addressed in an earlier reply to GY). All else is related to the exhaust geometry.
 

Toastedlightly

Diamond Member
Aug 7, 2004
7,212
5
81
Originally posted by: CVSiN
Originally posted by: OS
Originally posted by: Howard
The very definition of backpressure means is that it's a pumping loss.

By properly sizing the exhaust, you REDUCE backpressure and free up lost power. Who disagrees this time?

You get power from correctly sizing the exhaust because exhaust gas velocity is affected by the size of the piping. By the bernoulli equation, too large a pipe reduces gas velocity.

is that why dumps on a street car is actually bad? i never understood how all this worked was just always told at least in the 5.0 stang world that dumps are not good over a well tuned 2.5 inch full exhaust

Well, if you have a turbo-car, then you get all sorts of goodies from a cutout on the downpipe.
 

Eli

Super Moderator | Elite Member
Oct 9, 1999
50,422
8
81
Originally posted by: Vic
It's not a true statement. Power is a function of torque and rpm (work over time). In other words, all things being equal, power is the ability to produce a similar amount of work (torque) but at faster speeds (shorter durations of time). An increase in backpressure (to a reasonable extent of course) will actually increase torque and thus power at lower rpms but will hamper the ability of the engine to rev efficiently at higher rpm, thus reducing torque and consequently power at higher speeds. Reducing backpressure reduces torque (and thus power) at lower rpm, but allows the engine to rev more efficiently at higher rpms and thus retain more torque at those higher speeds, hence more power.
This is the correct answer.

It's something entirely different for two strokes.

You do need some backpressure, even in a two stroke. On engines with a tuned exhaust, the energy pulses travel better through a more dense medium - in this case, exhaust gasses.

On two-stroke engines with no tuned exhaust, removing the muffler or putting a straight pipe will reduce low end torque considerably.
 

Toastedlightly

Diamond Member
Aug 7, 2004
7,212
5
81
Originally posted by: Howard
Originally posted by: Toastedlightly
Originally posted by: Howard
The very definition of backpressure means is that it's a pumping loss.

By properly sizing the exhaust, you REDUCE backpressure and free up lost power. Who disagrees this time?

Well, one has to take into account the exaust velocities, cam profiles, firing order, ect. Can't generalize like this.
The only relevant factor you listed IMO is cam profile (which I addressed in an earlier reply to GY). All else is related to the exhaust geometry.

Here I thought it was about exaust :p And I shoudl read the thread before posting :(
 

funboy6942

Lifer
Nov 13, 2001
15,290
389
126
Yup y you never see a drag car with large pipes straight out of the heads, or huge exhaust ports and valves on them either. No need to want to get that exhaust out of there as fast as you can :p
 

Howard

Lifer
Oct 14, 1999
47,989
10
81
Originally posted by: Eli
Originally posted by: Vic
It's not a true statement. Power is a function of torque and rpm (work over time). In other words, all things being equal, power is the ability to produce a similar amount of work (torque) but at faster speeds (shorter durations of time). An increase in backpressure (to a reasonable extent of course) will actually increase torque and thus power at lower rpms but will hamper the ability of the engine to rev efficiently at higher rpm, thus reducing torque and consequently power at higher speeds. Reducing backpressure reduces torque (and thus power) at lower rpm, but allows the engine to rev more efficiently at higher rpms and thus retain more torque at those higher speeds, hence more power.
This is the correct answer.

It's something entirely different for two strokes.

You do need some backpressure, even in a two stroke. On engines with a tuned exhaust, the energy pulses travel better through a more dense medium - in this case, exhaust gasses.

On two-stroke engines with no tuned exhaust, removing the muffler or putting a straight pipe will reduce low end torque considerably.
4-strokes, we're talking about 4-strokes.

Even if energy pulses (what kind of energy, btw?) do travel better through a denser medium, are we not concerned about the flow of the actual matter of the exhaust?
 

GalvanizedYankee

Diamond Member
Oct 27, 2003
6,986
0
0
Howard, there are so many variables that impact engine performance that it's mind numbing.

Google and buy this program...Desktop Dyno. It's supposed to be pretty good.


...Galvanized
 

Eli

Super Moderator | Elite Member
Oct 9, 1999
50,422
8
81
Originally posted by: Howard
Originally posted by: Eli
Originally posted by: Vic
It's not a true statement. Power is a function of torque and rpm (work over time). In other words, all things being equal, power is the ability to produce a similar amount of work (torque) but at faster speeds (shorter durations of time). An increase in backpressure (to a reasonable extent of course) will actually increase torque and thus power at lower rpms but will hamper the ability of the engine to rev efficiently at higher rpm, thus reducing torque and consequently power at higher speeds. Reducing backpressure reduces torque (and thus power) at lower rpm, but allows the engine to rev more efficiently at higher rpms and thus retain more torque at those higher speeds, hence more power.
This is the correct answer.

It's something entirely different for two strokes.

You do need some backpressure, even in a two stroke. On engines with a tuned exhaust, the energy pulses travel better through a more dense medium - in this case, exhaust gasses.

On two-stroke engines with no tuned exhaust, removing the muffler or putting a straight pipe will reduce low end torque considerably.
4-strokes, we're talking about 4-strokes.

Even if energy pulses (what kind of energy, btw?) do travel better through a denser medium, are we not concerned about the flow of the actual matter of the exhaust?
Sound energy.