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Old 12-01-2001, 09:47 PM   #1
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Backpressure-

A lot of people have different thoughts on backpressure, and often confuse it with Velocity and Delta Pressure...
I will now post a colaboration of posts from Purehonda.com

"THE MYTH OF BACKPRESSURE"

Öis probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most impotant ones you need to be aware of to understand the things I'm about to say:

BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.

One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is itís pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that Iím no mechanical engineer, simply an enthusiast who done all the reading he can. I donít claim that this information is the absolute truth, just that it makes sense in my eyes.

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).

The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to acheive optiaml gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.

These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; becuase is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhuast gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).

This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburnt mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. if the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuelís knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow.

Itís all about finding a compromise to work at both high and low RPM on most cars, but thatís a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, hope this helps, peace. "

-here's a reply to the above post-

"I've been seeing a resurgence of the backpressure misnomer, but didn't have the time or inclination to write it up. So, again, thanks.

There is one thing I'd like to add to texan's work:
Exhaust Scavenging
In essence, this is the opposite of the exhaust reversion that texan describes.

Reversion: at the beginning of the intake stroke during cam overlap, exaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge.

Scavenging: at the beginning of the intake stroke during cam overlap, the momentum of the exiting exhaust gasses creates a brief vacuum (positive delta P) in the header, pulling out the remaining exhaust gases from the combustion chamber, and allowing the new air/fuel charge to be full-strength.

Scavenging is also the reason for differently shaped headers (4-2-1, 4-1) and collectors. We use the momentum of exiting exhaust from one cylinder to scavenge exhaust from another that is next in the firing order! The different shapes allow for this to happen at different airflow velocities thus at different RPM bands.

Scavenging takes advantage of the momentum of the exiting gasses. In essence, the fast moving exhaust pulse pulls a vacuum behind it. Momentum is mass times velocity. So not only do we need to keep the velocity high to prevent reversion - but it greatly improves the scavenging effect.

Thus we have a balancing act (as others have pointed out). We want to minimize friction to lower the backpressure as much as possible - larger pipes have less friction because they have less surface area per unit volume. But we want to increase the delta P as much as possible to prevent reversion and increase scavenging effects - smaller pipes increase delta P because they increase velocity.

There are lots of tricks to try to widen the useful RPM band (stepped headers) or to increase the overall effiency (ceramic coated exhausts), but it's still subject to this basic tradeoff:
Friction vs. Velocity
AKA: Backpressure vs. Delta Pressure
You want low friction and high velocity.
You want low backpressure and high positive delta pressure. "


Credit given to Texan and Fritz for their info on this topic.
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Old 12-01-2001, 09:48 PM   #2
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"Needing Backpressure - Myth or Reality?

The goal of any exhaust system is to efficiently remove burnt gases from the combustion chamber, prevent reversion at overlap, and by enhancing exhaust gas velocity leaving the chamber, create a vacuum to help draw or scavenge in more intake charge volume at cam overlap.

The key is maintaining exhaust gas velocity or energy as the gases leave the exhaust port when the exhaust valve opens.

So as the exhaust gas leaves the exhaust port in a 4 stroke engine , it creates a series of pressure waves travelling at the speed of sound that move towards the exhaust tip (or forwards) and then some reflects back. Like the water waves coming onto the beach, forward and back, forward and back. The main overall direction is forwards but there is some reflection back to the exhaust port (reversion).

Simple enough...everyone knows this. So what's new and groovy?

The problem is at cam overlap (when both the exhaust valve and intake valve are both partially open and when the pressure in the chamber is greater than in the intake port).

If a high pressure wave is reflecting back and arrives at the exhaust port at the wrong time (i.e. when burnt gases still need to leave), it blocks the flow out. You see these instances when a high pressure wave is reflected back at the wrong time as dips in the torque curve AT REGULAR INTERVALS (usually in the midrange rpms).

If a low pressure wave is reflecting back at the correct time at the exhaust port it actually helps pull burnt gases out of the chamber and also helps pull in more intake air/fuel at overlap. You see these favourable low pressure reflected waves occurring on your torque curve as small torque increases AT REGULAR INTERVALS.

Now here's the first bone of contention and a source of debate between exhaust makers.

1. Is a reflected high pressure wave always bad?

Most of the experienced people I speak to and read on the various boards say YES! You never want backpressure and you want it as low as possible for as long as possible. The low backpressure assists in maintaining that high exhaust gas velocity. They then design anti-reversion chambers and/or place steps (increases in diameter at various proprietary points along the length of the header) to prevent the reflected waves from travelling back to the head.

There are also some pretty smart people who believe slightly differently ...They believe that if you have a high pressure reflected wave arriving a few milliseconds before exhaust valve closure, you prevent the loss of intake air:fuel out the exhaust valve at cam overlap. The exhaust backpressure at this crankshaft degree in the exhaust stroke prevents leaking out or bleeding out of you intake charge into the header and ensures all of it goes into the chamber for combustion.

However, these people do NOT use the exhaust diameter as a way to create this backpressure. That would be too crude or less precise, since the backpressure would exist at all times and they only want this backpressure over the few crankshaft degrees when the exhaust valve is just about to close ,when the intake valve is opening further, and the piston has reached TDC and starts downward for the intake stroke. Using an exhaust just to have backpressure then is like cutting butter with a chain saw.

The people who agree with this will often tell you that combustion chamber and intake port pressures are higher than the pressure in the exhaust just before exhaust valve closure . So some intake flow into the chamber can get pushed out the closing exhaust valve by the higher combustion chamber pressures.

So all you guys that say backpressure is a good thing...I don't believe so...not at all crankshaft degrees which is what you get with a restrictive diameter exhaust. You don't want to have too big a diameter (actually it's cross-sectional area) that will slow or kill velocity or energy. But no backpressure most (99%) of the time is good.

2. How do we get low pressure waves and high pressure wave to arrive at the correct time?

The conventional way to get the exhaust gas harmonic to do this dance of low pressure to pull in more intake charge and high pressure to prevent bleeding off all at the right time is by changing the tube layout on the header: using lengths, diameters, collectors with various merge angles. But these are limited to one harmonic or exhaust gas speed.

So some Japanese engineers at Yamaha (figures, it's always some genius engineer at some bike manufacturer that comes up with these wild ideas) thought: "What if you have an exhaust throttle valve (located in the header collector or at the entrance to the secondary tubes in the first merge collector) that could control the pressure wave behaviour?".

The throttle valve angle would vary as the speed of the exhaust gases changed to control the reflected waves. In an 11,000 rpm bike, the valve opens progressively as the rpms climb as the tubes are "in step" with the engine harmonics and less reflected waves occur but at around 7000 rpm, the valve is closed down to 40-60% of wide open when the harmonic is "out of step" with the engine and at 8500 rpm the exhaust throttle valve is progressively opened. How much to change the throttle angle is based on crankshaft angle input or ignition signal input to an ECU with then controls the throttle valve angle knowing the harmonics of the engine.

We see these in the Mercedes McLaren F1 car. If you think this is somebody's Frankenstein pipe dream then guess again. The new Suzuki GSXR1000, Honda Fireblade, and Yamaha R1 already have these. And those are today's street bikes! Can the new RSX and Civic Si be that far away from the next stage forward for more power? The impetus will not be performance oriented but the drive to bring this to the market place will likely be more practical, as this throttle valve (the first one was called the Exup or Exhaust Ultimate Power by Yamaha in the late 80's) gains better emissions and lower exhaust noise (less pollution is good ...admit it Kyoto is the right thing to do).

So the new toy for exhaust makers will be like variable valve timing and variable cam timing...the mating of electronics to optimise exhaust harmonics at each rpm as the harmonics change with the rpms climbing. It won't be just cut and try any longer...it will be cut try and reprogram. Welcome to the new millenium. "

Credit Given to Tuan at SHO. Happy now?
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Last edited by Prelussion; 12-04-2001 at 05:39 PM.
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Old 12-01-2001, 11:08 PM   #3
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Thanks for stating that early in the forming of this forum man.

Now, if Fritz would get here and "stickify" it...

peace
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Old 12-03-2001, 09:05 AM   #4
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You're not going to give any credit on those writings? Ok, I will: the first part of the first post is mine, the second part of the first post is fritz's, but whose wrote the second post? It's been awhile since I've been to Purehonda.
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Old 12-03-2001, 09:47 AM   #5
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Quote:
Originally posted by texan
...but whose wrote the second post? ...
It was Originally posted by Tuan at SHO.org

Maybe Prelussion or a mod can update the original post to include credits?
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Old 12-03-2001, 10:52 AM   #6
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Ok, thought I recognized that.
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Old 12-03-2001, 11:17 AM   #7
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Quote:
Originally posted by texan
Ok, thought I recognized that.
There you go Texan. I made an additional comment at the bottom of the first post giving credit.
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Old 12-04-2001, 05:41 PM   #8
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Well, if people paid attention i didn't exactly give credit to texan, but it was noted that the second part of my post said it was in reply to the post above, and fritz noted the post by texan....but whatever sorry i didn't come back soon enough to change it good post though, you prolly know more than i do.
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Old 12-09-2001, 12:46 PM   #9
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good post....now when people tell me I'm stupid for telling them that zero backpressure is a good thing I can just direct them to this post instead of typing it out every time

can you believe that one guy even told me that his car would not move if he had zero exhaust backpressure
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Old 12-10-2001, 12:52 PM   #10
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Quote:
Originally posted by madtownhonda
can you believe that one guy even told me that his car would not move if he had zero exhaust backpressure
Did he have a jet car?
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Old 12-14-2001, 05:38 PM   #11
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Thumbs up

an excelent post. a little drawn out and not enough emphasis on cylinder scavaging and its effects, but over all ony of the best post's i've read:flash:
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Old 01-02-2002, 08:21 PM   #12
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82civic wagon, about your sig.......it's spelled punani, and it's pu in sai....just clearing things. peace.
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Old 01-30-2002, 12:09 PM   #13
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hehe..I'm a mechanical engineer . Actually, I'm kind of a rocket scientist too, since the company I work for designs/makes rocket engines. Anyway, I've always been trying to apply what I've learned in school to cars, since I dont really know much about cars. But everything you posted about pressure and everything is correct by fluid dynamics. Its actually called bernoulli's equation if anyone wants to look it up. It covers flow from one point to another and pressure and velocity are proportional at both ends (pressure to velocity^2). So any increase in one variable is a decrease in another. keep in mind there are a lot of head losses due to pressure drops,etc, bernoulli's is an ideal case. Thanks for the info, tho, you guys rock.
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Old 02-06-2002, 09:24 PM   #14
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hey! so by what you explained there, do you think it's only wise to go with 3 inch exhaust piping only if my engine was fully blown pushin about 10psi boost or more??
I'm going to get some custom piping done and i was thinkin to go with 2.5 inch tubing, with my 92 lude si, and i don't have a turbo in yet, but would it be wise to go with a smaller 2" or so until i have a turbo in? Also do you know what an effect a 3 angle valve job would have on backpressure if any? I got that done recently all in part of the prep for the turbo install! Any feed back would be greatly appriciated! Thanks!
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Old 02-06-2002, 11:40 PM   #15
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there are a lot of numbers to crunch right there, and obviously automobile exhaust is not ideal (friction, etc), I was just confirming what others said. I guess I would look at what aftermarket companies have on their specs, because they are the ones who run the tests and hopefully design for optimum hp/torque gains and not just being loud. I'm sure mimicking there pipe diameter and choosing a smooth shape for your piping will yield you good results. Like I said, just cuz I know a few equations doesnt mean I know the best thing for your car or any car. But I definitely wouldnt want my exhaust hindering my overall output by not having enough flow. Remember that turbo uses exhaust gases, so I'd figure that it might not change the total mass flow, if anything it would decrease it. Hmm...shit dude, I dunno... . Good luck - hopefully someone else can give you a better answer.
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