Measured Backpressure on the Vette

[Vettezuki]

Long saga, but finally thanks to ThrottleCrazy and 94cobra69ss296 got to measure the back pressure on my Vette. It has some "resistance to flow". It's about 1 psi on idle and 3psi at full throttle. The cats are the main issue, but unfortunately with recent CA law those have to be EO numbered. I could put on a set of LT headers and hi-flow cats and keep the stock stuff for "contingencies" and probably pick up a fair bit of power. I'm at 391WHP now. The motor with headers and hi flow cats (but an LS1 and not LS6 intake) and a few other differences, made about 430 WHP. It's a guess, but I'm think LT headers, hi flow cats, (maybe a new set of injectors as at least one of my is dodgy sometimes) should put conservatively at 410+ I would think :huh:

[enkeivette]

Will the LS6 intake not fit? You only have 2 1/4" pipes too right? Need to step it up to the 3" pipes and some long tubes.

[Vettezuki]

Will the LS6 intake not fit? You only have 2 1/4" pipes too right? Need to step it up to the 3" pipes and some long tubes.

I have the LS6 intake. The Vette the motor was on had the LS1. I mean to say in this case I breate even better than it did. I have 3" pipes (I think, definitely bigger than 2 1/4") after the cats. I have stock manifolds and cats and that's where the bottleneck is.

[enkeivette]

After the cats? :toetap:

[Vettezuki]

After the cats? :toetap:

The stock manifold are not pipes. I thought you were referring to after the cats. Are we gonna have another semantics smack down.

[BADDASSC6]

Good do all that then drop a big cam in it!

[94cobra69ss396]

If I remember correctly the tubing up near the cat was pretty small. I would guess that it was only 2.25. I would also recommend larger exhaust but I don't think you would need larger than 2.5. You would definately make more power with a free flowing exhaust system.

[BRIAN]

Hey Ben do you have a dyno chart you can share here? How did you measure "back Pressure?" I may want to try it on my exhaust.

[Vettezuki]

Hey Ben do you have a dyno chart you can share here? How did you measure "back Pressure?" I may want to try it on my exhaust.

There is a pressure gauage that inserts into the O2 bung ahead of the cat.

Here's my last dyno sheet, with the setup as it is. 391/369.

http://www.motorgen.com/pic/data/515/BirdRunDyno1.jpg

[Vettezuki]

Good do all that then drop a big cam in it!

Would love to, but then I wouldn't pass the sniffer anymore. I'm have a 114LSA 224/224. My emissions are still safe, but pushing the boundary.

[BRIAN]

Awesome, now I just have to wait for you to uprade your exhaust and dyno again :)

[94cobra69ss396]

I'd like to see that too. I've had some discussions about the negetive effects of backpressure and I would love to remeasure the backpressure on your Vette after the upgrades and then see what the difference is on the dyno.

[Vettezuki]

I'd like to see that too. I've had some discussions about the negetive effects of backpressure and I would love to remeasure the backpressure on your Vette after the upgrades and then see what the difference is on the dyno.

I want to do it for sure, but new headers, cats, replumbed exhaust and new tune is going to be well over a grand and probably closer to two. I'll be putting that into the project car first. But I will get to this. Not quite sure how best to do it to isolate the effects of back pressure. Just inserting some hi-flow cats in place of the stock cats, measuring back pressure, and if essentially gone, dynoing might be the best way. Changing to LT headers, new injectors and tune, etc., there's just too many contributing factors to increased power.

[94cobra69ss396]

I want to do it for sure, but new headers, cats, replumbed exhaust and new tune is going to be well over a grand and probably closer to two. I'll be putting that into the project car first. But I will get to this. Not quite sure how best to do it to isolate the effects of back pressure. Just inserting some hi-flow cats in place of the stock cats, measuring back pressure, and if essentially gone, dynoing might be the best way. Changing to LT headers, new injectors and tune, etc., there's just too many contributing factors to increased power.

I agree. Just because there is no longer back pressure doesn't mean that the flow is optimum. Are your cats bolted in between the exhaust manifold and the aftercat or is it welded? If it is just bolted we could probably make a test pipe to replace the cat and then measure the back pressure again.

[Vettezuki]

I agree. Just because there is no longer back pressure doesn't mean that the flow is optimum. Are your cats bolted in between the exhaust manifold and the aftercat or is it welded? If it is just bolted we could probably make a test pipe to replace the cat and then measure the back pressure again.

They're bolted. Pretty easy to remove actually. Good idea. :pot_stir:

[94cobra69ss396]

They're bolted. Pretty easy to remove actually. Good idea. :pot_stir:

Is it a straight shot? If so, it would be really easy to make them. Just go to Autozone and pick up some tube.

[BRIAN]

If I may hypothesize here, the torque curve will simply shift to the right on the dyno graph with only an exhaust size upgrade with no tune or cam replacement in mind.

edit: wait wait....if I have this right in my head, increasing the exhaust size too much would cause the curve to move to the right (RPM range). Why? Well I figure too large of an exhaust would take a higher amount of exhaust pressure to create velocity, in other words higher RPM's needed. Too small an exhaust we have a restriction of the velocity trying to release.

[94cobra69ss396]

If I may hypothesize here, the torque curve will simply shift to the right on the dyno graph with only an exhaust size upgrade with no tune or cam replacement in mind.

I don't think so. If anything he'll see an increase across the entire range that the dyno sheet shows but mostly in the upper rpms. Remember that he already has a restriction that even at idle is creating 1 psi of pressure in the system and 3 psi with just a rev. I had a plugged cat on my Suburban and it didn't even have back pressure at an idle. It did however have 3 psi on a rev but only on one side.

[BRIAN]

Hmmmm I have my head rapped around this now. Need to find something else to occupy my time haha. We'll leave it up to the dyno :)

edit: YA! Without using any formulas and just winging it, anywhere between 2.5" and 2.75" should yield a better power curve. ^^^^This guys is right!! As I said though I think too large and the gains will only show on the top end. I don't know why I'm bringing all this up, it just snapped into my head this whole exhaust theory ordeal I heard about.

[enkeivette]

You do realize that your car is already really fast don't you?

[enkeivette]

Btw the torque curve can never just shift. If it changes, it will still cross over at 5250rpm.

[BRIAN]

I understand, the whole unit conversion causing the curve to cross that exact point every time. You get what I meant though no? lol Just more torque higher up in the range.

[Vettezuki]

You do realize that your car is already really fast don't you?

Yes. But Sean needs to be put back in his place.
http://www.motorgen.com/garage/timeslip.php?SlipID=243&SlipID2=136

[94cobra69ss396]

Btw the torque curve can never just shift. If it changes, it will still cross over at 5250rpm.

That is incorrect. A change to a larger cam will shift the torque curve up. Horsepower and torque will still cross at 5250 but the peak torque will be at a higher RPM.

[Throttle Crazy]

My guess considering what your current set up and readings are is about 7.5% increase at the rear wheels across the board with headers and straight pipe. 420rwhp/396rwtq. Ron how much did you gain with just the x pipe instead of the high flow cats? I know the 1/4 mile time went from 12.1 down to 11.6.

[94cobra69ss396]

My guess considering what your current set up and readings are is about 7.5% increase at the rear wheels across the board with headers and straight pipe. 420rwhp/396rwtq. Ron how much did you gain with just the x pipe instead of the high flow cats? I know the 1/4 mile time went from 12.1 down to 11.6.

I'm not sure because I never dyno'd it but I picked up 4 MPH in the 1/4.

[Throttle Crazy]

That is incorrect. A change to a larger cam will shift the torque curve up. Horsepower and torque will still cross at 5250 but the peak torque will be at a higher RPM.

That is correct. Both numbers will just cross at a higher point in the 5250 rpm range. The curve has still shifted.

[94cobra69ss396]

That is correct. Both numbers will just cross at a higher point in the 5250 rpm range. The curve has still shifted.

What?

[BRIAN]

He's agreeing with you.

[94cobra69ss396]

I was questioning his "Both numbers will just cross at a higher point in the 5250 rpm range". I'm just not sure how they can cross at a high point in the 5250 range. Maybe he meant at like 5251 or 5252.:rolling:

By the way Throttle Crazy is my brother so I have to give him a hard time.

[Throttle Crazy]

What?

The intersection of hp and torque can still meet at 5250 rpm but be higher numbers.

[94cobra69ss396]

The intersection of hp and torque can still meet at 5250 rpm but be higher numbers.

I knew what you meant. I just wanted to give you a hard time.:D

[Throttle Crazy]

I knew what you meant. I just wanted to give you a hard time.:D

Why, I would neeeever do that to you.:rolleyes:

[Throttle Crazy]

That is incorrect. A change to a larger cam will shift the torque curve up. Horsepower and torque will still cross at 5250 but the peak torque will be at a higher RPM.

Are you talking about the torque curve moving and hp numbers staying the same? Are you saying that by changing cam profile you can move the torque curve without effecting hp?

[94cobra69ss396]

Are you talking about the torque curve moving and hp numbers staying the same? Are you saying that by changing cam profile you can move the torque curve without effecting hp?

No to both.

[BRIAN]

LOL at these guys:lmfao:

[Throttle Crazy]

I was questioning his "Both numbers will just cross at a higher point in the 5250 rpm range". I'm just not sure how they can cross at a high point in the 5250 range. Maybe he meant at like 5251 or 5252.:rolling:

By the way Throttle Crazy is my brother so I have to give him a hard time.
you were trying to make a joke, but the fact is that torque and horsepower intersect at 5252 rpm, horse power was invented by James Watt's after observing that a horse could lift at a rate of about 550 foot-pounds per second for about an 8-hour shift. horsepower = torque * rpm / 5252

[enkeivette]

That is incorrect. A change to a larger cam will shift the torque curve up. Horsepower and torque will still cross at 5250 but the peak torque will be at a higher RPM.

That's not a shift, that's an expansion or a change. A shift denotes moving the exact curve either up or down the rpm axis. :judge:

[enkeivette]

The intersection of hp and torque can still meet at 5250 rpm but be higher numbers.

Another example of a change, being an expansion, not a shift. :)

[enkeivette]

And before you say it it shifting up the axis, stop. Because since you can't make hp at zero rpm, the angle of the torque line will be different to originate at 0 and 0.

[enkeivette]

You let Sean beat you Ben?! Don't feel bad, he does have a blower. So Sean did manage to pull off a 12.5? Why are you guys still giving him shit?

12 second car, 12 second driver. Thumbs up for Daddy.

[Vettezuki]

The intersection of hp and torque can still meet at 5250 rpm but be higher numbers.

Will ALWAYS intersect at 5250. It's a "magic" number. Go to How Stuff Works if you want to know the full derivation for it.

[BADDASSC6]

That's not a shift, that's an expansion or a change. A shift denotes moving the exact curve either up or down the rpm axis. :judge:

Sounds like that first semester of LAW SCHOOL again.

[enkeivette]

Sounds like that first semester of LAW SCHOOL again.

Why does everyone think that I'm in my first year? First year was sooo last year ago!!!

[enkeivette]

Will ALWAYS intersect at 5250. It's a "magic" number. Go to How Stuff Works if you want to know the full derivation for it.

Motorgen.com is a better source for info, don't go anywhere.

HP is a mathematical calculation from measured torque, which is actually what a dyno measures. It always crosses at about 5250 because of the math equation.

[Vettezuki]

Motorgen.com is a better source for info, don't go anywhere.

HP is a mathematical calculation from measured torque, which is actually what a dyno measures. It always crosses at about 5250 because of the math equation.

Uhh, 5250 is used in the equation, not because of the math equation. And to be extra annoying, it's actually 5252, derived thusly.

The constant 5252 comes from (33,000 ft·lbf/min)/(2π rad./rev.).

NOTE: The 33,000 ft.lbf/min. is the definition of one HP. Why it's divided by 2π rad./rev. I don't recall. But that's the constant you use.

HP = T[ft.lbf.] x RPM/5252

[Throttle Crazy]

Uhh, 5250 is used in the equation, not because of the math equation. And to be extra annoying, it's actually 5252, derived thusly.

The constant 5252 comes from (33,000 ft·lbf/min)/(2π rad./rev.).

NOTE: The 33,000 ft.lbf/min. is the definition of one HP. Why it's divided by 2π rad./rev. I don't recall. But that's the constant you use.

HP = T[ft.lbf.] x RPM/5252

I did mention about the 5252 a couple pages ago. The average horse could lift a 550 pound weight one foot in one second, thereby performing work at the rate of 550 foot pounds per second, or 33,000 foot pounds per minute, for an eight hour shift. That is where the 33,000 ft. lb per minute came from. 550*60= 33,000.

(2π rad./rev.). converts radians to revolutions. There are 2 pi (3.14) or 6.28 radians to 1 rev.

[enkeivette]

That's why I said about 5250.

Are you really trying to say that they cross at about 5250 because it is a number used in the equation but the crossing at 5252 is not a result of that same equation? Ummm.

[Vettezuki]

That's why I said about 5250.

Are you really trying to say that they cross at about 5250 because it is a number used in the equation but the crossing at 5252 is not a result of that same equation? Ummm.

The correct number is 5252. And yeah, the use of a constant in an equation is not a function of the use of itself in said equation. Are we talking about the same thing?

[enkeivette]

The correct number is 5252. And yeah, the use of a constant in an equation is not a function of the use of itself in said equation. Are we talking about the same thing?

True, but the function of this specific equation always yields this specific result (crossing at 5252) does it not?

So would it be wrong to say that they cross at 5252 because of the equation? (The equation which includes all constants and all functions)

[Vettezuki]

True, but the function of this specific equation always yields this specific result (crossing at 5252) does it not?

Yes. (The always part is kind of what an equation is about.)

So would it be wrong to say that they cross at 5252 because of the equation? (The equation which includes all constants and all functions)

No.

Good job framing the questions to get the answer you wanted. :)

I simply meant the proof for why 5252 is a valid constant does not come (directly) from the equation for HP, but that it is used in the equation for HP.

[enkeivette]

Yes. (The always part is kind of what an equation is about.)



No.

Good job framing the questions to get the answer you wanted. :)

I simply meant the proof for why 5252 is a valid constant does not come (directly) from the equation for HP, but that it is used in the equation for HP.

You're an equation for horsepower.

[BADDASSC6]

True, but the function of this specific equation always yields this specific result (crossing at 5252) does it not?

So would it be wrong to say that they cross at 5252 because of the equation? (The equation which includes all constants and all functions)

Power equals force times distance. SInce the motor (crank is rotating) in order to get distance you need to multiply the RPMS time 2 (3.14) R (formula for circumference of a circle). The force is the torque of the motor multiplied by the stroke which is the Radius of the circle and there for cancels out. Plug in the numbers for whatever units you prefer and they will cancel out to 5252 every time.

[enkeivette]

You seem to know more about this stuff than I do. Why didn't you decide to build your own motor on a stand in your living room?

[BADDASSC6]

I have built and ported a aluminum intake manifold in my kitchen sink before. Now I'm all grown up and make good money so I pay people to do that stuff. I not rich or loaded, but I'm past the point in my life where it's appropriate to have a motor in my living room and my career keeps me busy enough that I really don't have the time.

[enkeivette]

I have built and ported a aluminum intake manifold in my kitchen sink before. Now I'm all grown up and make good money so I pay people to do that stuff. I not rich or loaded, but I'm past the point in my life where it's appropriate to have a motor in my living room and my career keeps me busy enough that I really don't have the time.

First of all, good for you. :beer: Second of all, when is it ever not appropriate to have a motor in your living room? :rolleyes:

[BRUTAL64]

Need I remind you guys it is NOT backpressure:bang:

[enkeivette]

Need I remind you guys it is NOT backpressure:bang:

Resistance to flow? Is that what you're going for?

[BRIAN]

Need I remind you guys it is NOT backpressure:bang:
YAY!! I didn't want to be the one to say it then have stupid useless argument against me. Flame jacket Glenn :lmfao:

[Vettezuki]

Need I remind you guys it is NOT backpressure:bang:

I think Throttle Crazy calls it the automotive equivalent blue balls.

[94cobra69ss396]

Need I remind you guys it is NOT backpressure:bang:

Actually it is backpressure. He has 3 psi of positive pressure in the exhaust before the cat.

[Vettezuki]

Actually it is backpressure. He has 3 psi of positive pressure in the exhaust before the cat.

Out of curiosity, what's the actual name of the gauge? :pot_stir:

[BRUTAL64]

Actually it is backpressure. He has 3 psi of positive pressure in the exhaust before the cat.

no it isn't---but...........................:lmfao:

[94cobra69ss396]

no it isn't---but...........................:lmfao:

What would you call it?

Out of curiosity, what's the actual name of the gauge? :pot_stir:

I don't remember the make, maybe Phil will chime in but here is a picture of it that I took when testing my Suburban.

http://www.motorgen.com/pic/data/500/user21799_pic12960_1269743501.jpg

[enkeivette]

I think Glenn's thing is, back pressure would be actual pressure pushing up the exhaust. While you're describing resistance to flow, which is pressure resulting from resistance. Not pressure resulting from some other source.

To develop: the pressure you're measuring is coming from the exhaust gas, while back pressure is not.

[94cobra69ss396]

I think Glenn's thing is, back pressure would be actual pressure pushing up the exhaust. While you're describing resistance to flow, which is pressure resulting from resistance. Not pressure resulting from some other source.

To develop: the pressure you're measuring is coming from the exhaust gas, while back pressure is not.

Back pressure would be any pressure developed behind the retriction.

[Throttle Crazy]

no it isn't---but...........................:lmfao:

definition for "back pressure"

Back pressure usually refers to the pressure exerted on a moving fluid by obstructions or tight bends in the confinement vessel along which it is moving, such as piping or air vents, against its direction of flow. For example, an automotive exhaust muffler with a particularly high number of twists, bends, turns and right angles could be described as having particularly high back pressure [1]. Back pressure, in the exhaust sense of the term, of a four-stroke engine is usually termed as being a "bad thing" for performance; however, in the interest of reducing exhaust sound to levels allowable by public noise ordinances, back pressure can be regulated using systems from simple butterfly valves to fully computer controlled units sensing pressure in the exhaust pipe itself.

In a two-stroke engine however, a certain amount of exhaust backpressure is needed to prevent unburned fuel/air mixture passing right through the cylinders into the exhaust.

Google back pressure guage and get a thousand hits on actual test tools, google "resistance to flow" and check out some hypodermic needles. Try to find a dictionary defintion for "resistance to flow". We already had this conversation. "Resistance to flow" is the cause of the "back pressure". If the exhaust has a restriction or excessive bending that resistance will cause "back pressure" Quit being so fucking stuborn and come up with hard facts or just admit your wrong. I guess if you can't do either then just keep thinking what ever makes you feel special at the end of the day.:bang:

[Throttle Crazy]

Back pressure would be any pressure developed behind the retriction.

Correct! Go home and turn on your garden hose for ten minutes with the nozzle tip closed, shut off the hose valve and try to take the hose off of the valve. I don't think it is "resistance to flow" that hits you in the face. I am pretty sure it is pressure. The nozzle tip creates a "resistance to flow" which causes a measurable back-up of pressure in the hose.

Have you ever seen an EGR valve called a "resistance to flow" EGR valve? NO, but they do make a positive "back pressure" EGR valve that works off of exhaust "back pressure". Toyota used to use alot of exhaust "back pressure" transducers on their EGR systems. AH! but what do they know.

[Throttle Crazy]

I think Glenn's thing is, back pressure would be actual pressure pushing up the exhaust. While you're describing resistance to flow, which is pressure resulting from resistance. Not pressure resulting from some other source.

To develop: the pressure you're measuring is coming from the exhaust gas, while back pressure is not.

Where do you think the pressure is trying to go when the exhaust valves open. I have seen exhaust systems so plugged the engine would not stay running because the exhaust was backing up to the intake manifold and the engine could not create vaccum. I would say that is pushing up the exhaust.

[Vettezuki]

. . .The nozzle tip creates a "resistance to flow" which causes a measurable back-up of pressure in the hose. .

This is basically how I think of it.

[Throttle Crazy]

Just in case you needed more!
http://<a href="http://www.thefreedictionary.com/back+pressure">back pressure</a>

http://<a href="http://medical-dictionary.thefreedictionary.com/back+pressure">back pressure</a>

back pressure
n.
Pressure exerted upstream in the circulation as a result of obstruction to forward flow, as when congestion in the pulmonary circulation results from failure of the left ventricle.

The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

--------------------------------------------------------------------------------
back pressure
Etymology: AS, baec + L, premere, to press
pressure that builds in a vessel or a cavity as fluid accumulates. The pressure increases and extends backward if the normal mechanism for egress or passage of the fluid is not restored.
Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier.

back pressure
n
1. (Engineering / Mechanical Engineering) Engineering
a. the pressure that opposes the motion of a piston on its exhaust stroke in an internal-combustion engine
b. the exhaust pressure in external combustion engines
2. (Medicine / Pathology) Med the local pressure that builds up when fluid flow is obstructed in the cardiovascular or urinary systems
Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003

[enkeivette]

Where do you think the pressure is trying to go when the exhaust valves open. I have seen exhaust systems so plugged the engine would not stay running because the exhaust was backing up to the intake manifold and the engine could not create vaccum. I would say that is pushing up the exhaust.

I would agree with you that back pressure has multiple definitions and that it can be either upward flow, or a restriction. I was just explaining what I thought Glenn's position was.

When you create pressue, it is going to try to escape in all directions favoring the path of least resistance. So when you press your lips on a piece of glass and blow, the glass isn't pushing air into your mouth, but, pressure is trying to move back into your mouth.


So yeah, I would call it back pressure.

[BRUTAL64]

I think Glenn's thing is, back pressure would be actual pressure pushing up the exhaust. While you're describing resistance to flow, which is pressure resulting from resistance. Not pressure resulting from some other source.

To develop: the pressure you're measuring is coming from the exhaust gas, while back pressure is not.

perfect!!!!!!!:bigthumbsup::bigthumbsup: :bigthumbsup:

[Throttle Crazy]

I think Glenn's thing is, back pressure would be actual pressure pushing up the exhaust. While you're describing resistance to flow, which is pressure resulting from resistance. Not pressure resulting from some other source.

To develop: the pressure you're measuring is coming from the exhaust gas, while back pressure is not.

If it was pressure traveling directly against the exhaust from another source it would be by definition an opposing pressure not a back pressure. One pressure would not need the other to exist.
Main Entry: op·pose
Pronunciation: \ə-ˈpōz\
Function: transitive verb
Inflected Form(s): op·posed; op·pos·ing
Etymology: French opposer, from Latin opponere (perfect indicative opposui), from ob- against + ponere to place — more at ob-, position
Date: 1579
1 : to place over against something so as to provide resistance, counterbalance, or contrast
2 : to place opposite or against something
3 : to offer resistance.

[BRUTAL64]

If it was pressure traveling directly against the exhaust from another source it would be by definition an opposing pressure not a back pressure. One pressure would not need the other to exist.
Main Entry: op·pose
Pronunciation: ə-ˈpōz
Function: transitive verb
Inflected Form(s): op·posed; op·pos·ing
Etymology: French opposer, from Latin opponere (perfect indicative opposui), from ob- against + ponere to place — more at ob-, position
Date: 1579
1 : to place over against something so as to provide resistance, counterbalance, or contrast
2 : to place opposite or against something
3 : to offer resistance.

What do the French know?:p

[94cobra69ss396]

What do the French know?:p

So you still haven't said what you would call it. It is not just a resistance to flow because a resistance to flow wouldn't necessarily create back pressure. A 90° bend in the exhaust pipe would be a resistance to flow but may not create back pressure.

[enkeivette]

If it was pressure traveling directly against the exhaust from another source it would be by definition an opposing pressure not a back pressure. One pressure would not need the other to exist.
Main Entry: op·pose
Pronunciation: ə-ˈpōz
Function: transitive verb
Inflected Form(s): op·posed; op·pos·ing
Etymology: French opposer, from Latin opponere (perfect indicative opposui), from ob- against + ponere to place — more at ob-, position
Date: 1579
1 : to place over against something so as to provide resistance, counterbalance, or contrast
2 : to place opposite or against something
3 : to offer resistance.

When did I ever say "from another source?"

I would agree with you that back pressure has multiple definitions and that it can be either upward flow, or a restriction. I was just explaining what I thought Glenn's position was.

When you create pressue, it is going to try to escape in all directions favoring the path of least resistance. So when you press your lips on a piece of glass and blow, the glass isn't pushing air into your mouth, but, pressure is trying to move back into your mouth.


So yeah, I would call it back pressure.


back pressure
n.
Pressure exerted upstream in the circulation as a result of obstruction to forward flow, as when congestion in the pulmonary circulation results from failure of the left ventricle.

Your definition, back pressure - upward flow.

Think about my example. When you blow against glass, there is pressure being exerted into your mouth. Obviously, not from another source. But "pressure traveling directly against" your mouth? Yes.

[enkeivette]

I think Glenn's thing is, back pressure would be actual pressure pushing up the exhaust. While you're describing resistance to flow, which is pressure resulting from resistance. Not pressure resulting from some other source.

To develop: the pressure you're measuring is coming from the exhaust gas, while back pressure is not.

If you're referring to my first post, once again, I was explaining what I believed Glenn's stance was. Not my own.

[Throttle Crazy]

If you're referring to my first post, once again, I was explaining what I believed Glenn's stance was. Not my own.

I know, I understand you get what I am saying and agree. That last post was directed more toward Glens last comment (perfect!)of that same post. sorry I should have just quoted his post, but then yours would not have been there.

[enkeivette]

Oh, sorry to be so defensive. I'm in final taking argumentative dick mode right now. More so than normal.

You know what's really interesting, in the loud speaker world, when you hit the resonant frequency of a port, it will actually push air out as the speaker tries to suck air in, and it will suck air in as the speaker tries to push air out.

I wonder if this happens with exhaust gas when the resonant frequency of the pipe is hit?!

[Vettezuki]

. . .
You know what's really interesting, in the loud speaker world, when you hit the resonant frequency of a port, it will actually push air out as the speaker tries to suck air in, and it will suck air in as the speaker tries to push air out.

I wonder if this happens with exhaust gas when the resonant frequency of the pipe is hit?!

I'm 99% sure this is not a function of resonant frequency. The purpose of the ports in cabs is precisely to allow woofer cones to move more freely. I think you could measure a constantly variable pressure across a port proportional to the displacement of the cone regardless of freq.

[enkeivette]

I'm 99% sure this is not a function of resonant frequency. The purpose of the ports in cabs is precisely to allow woofer cones to move more freely. I think you could measure a constantly variable pressure across a port proportional to the displacement of the cone regardless of freq.

Indulge your 1% My Dad is a loudspeaker engineer. Not to mention I worked for his company as a tech for a few years. I grew up cutting port lengths to find the resonant frequencies of boxes. He's explained it many times to me. I am 100% certain. Actually when you hit the resonant frequency, the driver almost completely stops moving. The air in the box becomes "excited" and pushes and sucks air like crazy. It feels like a high power fan and the sub is barely moving (when it should be going crazy at around 30hz), pretty cool to see. This can also be done with passive-radiators.

If the port was just to allow the speaker to move more freely, it would cause cancellation. Think about it. Why do we put speakers in boxes (baffles) in the first place? To prevent cancellation.

Acutually, below the port frequency, the frequency response drops off rapidly. If you don't have enough box volume to have a port of sufficient length, its better to have the box sealed. Porting a box is typically done at the 3db down point and gives you a bump up until that point, at which point the frequency response will fall off faster after that point than it would had the box remained sealed.

[Vettezuki]

Indulge your 1% My Dad is a loudspeaker engineer. Not to mention I worked for his company as a tech for a few years. I grew up cutting port lengths to find the resonant frequencies of boxes. He's explained it many times to me. I am 100% certain. Actually when you hit the resonant frequency, the driver almost completely stops moving. The air in the box becomes "excited" and pushes and sucks air like crazy. It feels like a high power fan and the sub is barely moving (when it should be going crazy at around 30hz), pretty cool to see. This can also be done with passive-radiators.

If the port was just to allow the speaker to move more freely, it would cause cancellation. Think about it. Why do we put speakers in boxes (baffles) in the first place? To prevent cancellation.

Acutually, below the port frequency, the frequency response drops off rapidly. If you don't have enough box volume to have a port of sufficient length, its better to have the box sealed. Porting a box is typically done at the 3db down point and gives you a bump up until that point, at which point the frequency response will fall off faster after that point than it would had the box remained sealed.

Cool, I learned something. :drink:

PS I'm not a speaker/cab designer.

[enkeivette]

Cool, I learned something. :drink:

PS I'm not a speaker/cab designer.

Are we not allowed to say ****** on this forum?

[BRUTAL64]

So you still haven't said what you would call it. It is not just a resistance to flow because a resistance to flow wouldn't necessarily create back pressure. A 90° bend in the exhaust pipe would be a resistance to flow but may not create back pressure.

Old school tech (late sixties early seventies) called it resistance to flow.

But because of the resistance to flow will cause pressure behind the resistance then I see the issue---not a problem.

If you want the tern "back pressure" to mean the same as resistance to flow, that's fine by me. I'm always easy to get along with----:lmfao::motorsmile:
:bigthumbsup:

[Vettezuki]

Are we not allowed to say ****** on this forum?

No.

[94cobra69ss396]

Old school tech (late sixties early seventies) called it resistance to flow.

But because of the resistance to flow will cause pressure behind the resistance then I see the issue---not a problem.

If you want the tern "back pressure" to mean the same as resistance to flow, that's fine by me. I'm always easy to get along with----:lmfao::motorsmile:
:bigthumbsup:

But they are not the same thing. You can have a resistance to flow without having back pressure but you can't have back pressure without having a resistance to flow. Phil and I were talking to my dad about scavenging the other night (they were explaining the sound wave/tuning part to me) and even my old time hot rodder dad who's 59 called pressure in the exhaust back pressure and resistance in the exhaust resistance.

[94cobra69ss396]

I have to add more. You say that "because of the resistance to flow will cause pressure behind the resistance..." but this is not true. Just because there is resistance doesn't mean there is back pressure. Take a 90 degree bend in the pipe. That bend is a restriction or a resistance to flow. However, it may not cause pressure in the exhaust and so there wouldn't be any back pressure. Another example is my Suburban's exhaust. It has many bends that are restrictions as well as cats and a muffler. These all cause resistance to flow. But when I hooked up the back pressure gauge only the side of the exhaust that had the plugged cat had back pressure. The side with the good cat had 0psi. So it has resistance to flow without having back pressure.

[enkeivette]

Damn, I really need to get that high flow cat welded in my Neon. I miss my metal saw :(

[Vettezuki]

I have to add more. You say that "because of the resistance to flow will cause pressure behind the resistance..." but this is not true. Just because there is resistance doesn't mean there is back pressure. Take a 90 degree bend in the pipe. That bend is a restriction or a resistance to flow. However, it may not cause pressure in the exhaust and so there wouldn't be any back pressure. Another example is my Suburban's exhaust. It has many bends that are restrictions as well as cats and a muffler. These all cause resistance to flow. But when I hooked up the back pressure gauge only the side of the exhaust that had the plugged cat had back pressure. The side with the good cat had 0psi. So it has resistance to flow without having back pressure.

I think this was summed up with
- You can have resistance to flow without back pressure.
- You can not have back pressure without resistance to flow.

[94cobra69ss396]

I think this was summed up with
- You can have resistance to flow without back pressure.
- You can not have back pressure without resistance to flow.

I know, I was just trying to get the point across. You know how those old people are, sometimes you have to draw them a picture in crayon. :sm_up_there:

[Throttle Crazy]

I think Throttle Crazy calls it the automotive equivalent blue balls.
Sorry could'nt find my crayons!
http://www.altpenis.com/penis_news/img/blue_balls.jpg
"Blue balls derive their name not from the sorrow their condition inspires, but from the color of your testicles. That's right, depending on the severity of the condition your balls really can take on a bluish tinge, although technically it's not your testicles themselves but the skin of the scrotum that turns blue."

"What causes this condition is vasocongestion, which is another way of saying more blood is going into an area than is coming out(Back Pressure). During an extended period of foreplay the brain sends out signals along the parasympathetic nerve system to the neurovascular bundles leading into the genital area. These signals cause a chemical chain reaction that ultimately causes the smooth muscles of the artery walls to relax, allowing blood to flood in. At the same time the veins become constricted, leading to a net increase in blood pressure(Back Pressure). This is not a bad thing. Vasocongestion is essential for penile tumescence and ultimately that raging hard-on you know and love."

"But if orgasm doesn't occur, no signals are sent through the nerve system. The arteries don't close, the veins don't open and blood flow remains restricted. After a while, the stagnant blood becomes de-oxygenated and as you might remember from your grade 6 science class, blood lacking in oxygen becomes darker. Seen from outside the skin it appears blue - hence blue veins and in this case, blue balls.":rolling:
http://gibbleguts.net/funnypictures/blueballs.jpg

[Throttle Crazy]

I know, I was just trying to get the point across. You know how those old people are, sometimes you have to draw them a picture in crayon. :sm_up_there:
I like this one!
http://images.elfwood.com/art/o/p/oppaque/blue_balls.jpg