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Discussion Starter · #1 ·
I plan on testing this but wanted to hear what your opinions on the matter are first. I've had open Harley silencers on my bike for 2 years now, with standard jets in my carbs and standard air filter. I had a KN filter for a while and changed it this year and I can't say I've noticed any difference at all, if anything maybe a slight gain in MPG's. I did the DIY adjustable baffle thing but it's still a bit loud for my tastes and I'm averaging about 42-45 mpg's currently. I found a set of drop in baffles that should fit the harley cans for only $10, so as soon as they arrive I plan on installing them. I may or may not also add some muffler wrap when I install it. What do 'normal' Harley cans look like inside? In other words, what did the person who had mine before I bought them do to them?



Final question: do you think increasing the back pressure on the exhaust will give me any change in either performance and/or mpg's?
 

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I don't think you will improve much.That mileage is around the official MPG for a CX500.The figure often banded about of 50 Mpg was for the UK/European CX500s which equates to roughly 42 Mpg US.Of course a lot depends on riding style,terrain,maintenance and tyre pressures kept correct.

No harm in experimenting though as you never know
 

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I wish someone would just test the pressure/CFM for the intake and exhaust on these bikes, that would settle a lot of arguments. It's hard to say "pods will ruin the bike" when you can look at a graph and say "Oh look they match the intake of the stock airbox".
 

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Backpressure?

I ferkin` hate that term.

I read this on the internets the other day, i don`t know where it comes from but i agree with everything said;



"Backpressure: The myth and why it's wrong.



I. Introduction

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding

of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need backpressure" when discussing exhaust

upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.



II. Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream;

exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more

pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the

resistance to positive flow of the exhaust stream.



III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to

see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why

isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o

a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover

part of the opening, the water will flow out at a much much faster rate.



The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the

highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the

pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the

velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable

exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which

is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want

your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust

velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a

narrower pipe than if your powerband is located at 8-9000RPM.



Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change

in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to

maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust

volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler

has two paths; one path is closed at low RPM but both are open at high RPM.



IV. So how did this myth come to be?

I often wonder how the myth "Engines need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as

pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner

notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust

eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he

killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a

much narrower pipe - in that way he would not have lost all his flow velocity.



V. So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse

scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area

acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low

pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle.

This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please"

messages written in the thickly collected dust on the rear door(s)."
 

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Good post, Eurovee. I agree with your and the writers thoughts on this backpressure concept. This has been chased around the tree so many times that it has come to being accepted as gospel. The only problem is that of the 2600 on this fourm, maybe 1% will read it. Thats 26 people, of which half will believe what it is saying. The thirteen that accept this are mostly over 40, and their memory is nebulous. By next week, only 4 will remember that they read something about backpressure and 2 of them will remember what it said. So that leaves you and one other guy to keep us on track.
 

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Discussion Starter · #6 ·
Appreciate the replies so far, and Euro that was an interesting read and makes sense, but at the same time it still raises some questions for me. If the exhaust is all about the speed of exhaust, does an ungutted silencer have higher exhaust speed? My biggest goal in adding the baffles is to quiet things down a little, I love the sound of my harley cans but just don't want quite so much of it. At least with the ones I plan on using I'll be able to remove them easily if they don't work out, so I'll eventually post back my results. I guess the biggest thing to take away from Euro's post is maybe that Honda did a good job engineering these bikes to start with, and when we start making changes without understanding what we are really doing we often end up causing our own problems
 

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I would like to see some science/math to back up that article.



Think of a spray nozzle on a hose. As it is closed it has the maximum pressure. You can't have a higher pressure because it is completely stopping all flow.

As you release the nozzle the distance the water sprays increases, until you hit the sweet spot, then it starts to drop until you have just the plain hose with no nozzle.

On the bikes the exhaust is the nozzle, and the engine is the hose. It has a sweet spot.



So how do you hit that sweet spot? According to that article you modify your velocity of course! and how do you do that? You can't. Not directly. (well maybe a reverse turbo charger?). But you can modify the pressure that builds in the exhuast/h-box/headers. That is what is referred to as the backpressure.
 

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I would like to see some science/math to back up that article.


I posted an article in a recent H-Box thread with the science behind it, If I have time I'll go look for it. The science is sound.



The big issue with the cx/gl, is the time between pulses. One pulse pulls the next, and with only 2 cyls there is a relatively long cycle between pulses per cyl. The H-Box crosses over and mixes the pulse from each cyl so that it pulls the opposite cyl.



So how do you hit that sweet spot?


You leave the factory intake and exhaust system the way Mr Honda intended for best overall performance.
 

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Loved the post Eurovee, as it was explained in a manner most will understand. The hose example was a good one, as the principles and math are exacly the same, just a different medium (I daily have to perform water hydraulic calculations for hydrant and fire hose flow).



I am not sure how much effect the actual mufflers would have (on a bike with the H-Box in place), but will find out if I get my bike charging system running, as I just replaced my "punched out stock mufflers" with a set of dyna mufflers from a harley softail. Bike ran well since i restored it, but the open pipes were just to obnoxious for even me, making me feel like an ass if I gunned it anywhere but a farmland area.



Can wait to get my first "real muffler" ride in :-(
 

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Backpressure?

I ferkin` hate that term.

I read this on the internets the other day, i don`t know where it comes from but i agree with everything said;



"Backpressure: The myth and why it's wrong.



I. Introduction

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding

of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need backpressure" when discussing exhaust

upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.



II. Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream;

exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more

pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the

resistance to positive flow of the exhaust stream.



III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to

see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why

isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o

a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover

part of the opening, the water will flow out at a much much faster rate.



The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the

highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the

pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the

velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable

exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which

is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want

your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust

velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a

narrower pipe than if your powerband is located at 8-9000RPM.



Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change

in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to

maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust

volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler

has two paths; one path is closed at low RPM but both are open at high RPM.



IV. So how did this myth come to be?

I often wonder how the myth "Engines need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as

pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner

notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust

eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he

killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a

much narrower pipe - in that way he would not have lost all his flow velocity.



V. So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse

scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area

acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low

pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle.

This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please"

messages written in the thickly collected dust on the rear door(s)."
 

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Hello all,

Everything Evro relate to in this post, My feelings are, that he is right. The one thing he didn't talk about is the amount of

air these air pumps move thru these exhaust systems. The size and the lengh of the hedder pipes have a great deal to do with

the cx500 performance.But they do not move a great deal of air so the performance part of the exhaust system ends in the H-box.

Now if these air pumps move a lot of air, it could not get thru those one/half holes in the end of the mufflers.Now Honda designed

their muffler where the flow, changes direction, several times before exiting the end of the muffler. So the mufflers you use are

base on you ear. Now here is something for dicussion (did I spell that right).On my bike I am going to use two,two/inch core

16 inch long bullits (no nipples welded in there ends) to this I am going to weld a four inch piece of two inch exhaust tubing

and that in turn will be attached to the H-box. now for a little backpressure I am going to add a two inch washer with 3/4 inch

hole in it to the exit end of the muffler. Will this be loud are some what mellow. The old mufflers that where rust out on the

inside and straight thru. And werenot loud

Thanks a lot OLDBIE
 

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. The size and the lengh of the hedder pipes have a great deal to do with

the cx500 performance.

Thanks a lot OLDBIE


I also think they do...

The CX collector box, or `power chamber` as Honda called it is a handy place (or was that it`s sole intended purpose?) for the headers to be extended without adding length to the overall system.

If you look at the drawing they turn at right angles inside the collector to provide the correct `tuned length`.







It`s also interesting to see this early CX prototype is fitted with looooong experimental headers;







I had this in mind (and the `backpressure` text i posted above) when i made the exhaust for the CX-P`s new engine.

I`m not saying that this exhaust was in any way `designed`, `tuned` or anything bullshitty like that but i do know two basic facts: long(er) headers keep exhaust gasses flowing quickly, promote cylinder scavenging and keep the power lower down the rev-range. Small(er) diameter headers (the stainless-steel tubes my exhaust is made from are a few mm smaller in internal diameter than std) also keep exhaust gasses flowing quickly, promote cylinder scavenging and keep the power lower down the rev-range.

The new engine goes really well, so the weirdy exhaust certainly hasn`t done any harm..........

( the can was off a CBR600rr underseat model)



 

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Header size and length are designed for scavanging, which most refer to as back pressure. Here is a simplified pricipal.



If you have NO exhaust system (cylinder exhaust port open to the air) then there is no "restrictive presure on the exhaust". So, what you get is MOST of the exhaust gasses pumped out (some stays, which is the portion of head volume at TDC). When the intake valves open and draw in air/fuel, it mixes with the remaining exhaust gasses. This gives a mix of air/fuel AND X% products of combustion (actual percentage is the difference between cylider volume at TDC and cylinder volume at the pistons bottom of travel. Simple enough.



If you stick a header pipe on that is very small in diameter, the "restrictive pressure " on the exhaust gasses iv very large, and even when the piston is at TDC there is a "positive pressure" inside the cylinder (pressurized exhaust gasses which = More exhaust gas at atmospheric pressure).



These are the two extreames, but between the values is a "sweet spot". This "sweet spot" is a ballance between "restrictive pressure" that causes a unique thing to happen ... development of a high pressure wave front. The key point is BEHIND the wave front is an area of VERY low pressure, which in essense creates a vacuum state in the cylinder (negitive pressurized ehaust gasses which = LESS exhaust gas at atmospheric pressure). If you have trouble understanding the concept of the the low pressure behind the wave front, go to you tube and look up a video of an atomic test. You see the high pressure front expand out flattening things ... THEN ... you see stuff pulled back the opposite direction (by a huge low pressure area). This low pressure is what actually creates the "mushroom cloud" shape



What you end up with is a higher % of air/fuel compared to residual exhaust gasses. This, along with the concept of creating a "standing wave" (by "tunning" the exhaust system) is what the designers shoot for when the exhaust system is designed, yeilding inproved performance, fuel efficiency and reduced emmisions.
 

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To add to this what the collecter is ,is basically what drag racers call an exhaust termination box.The box should have 8 times the volume of on cylinder.This tricks the engine into thinking the header pipe is dumping into open air.The length of the primary pipe is very important as it changes the TIMING of when for lack of a better term the exhaust slug arrives back at the exhaust valve.

This slug creates a low pressure area at the exhaust valve and if it arrives at the proper time when both the intake and exhaust valves are off their seats during over lap the low pressure area helps filling the cylinder with fresh air fuel mix.It really works exactly like adding positive pressure to the intake tract just like adding a turbo just on a much smaller scale.

The muffler used after the h box should really not make any difference as long as it is not to restrictive as the tuned part of the exhaust ends in the h-box, the exhaust wave reflects from the end of the header tube.Primary pipe diameter and length are critical for this tuning to work to big a diameter or to short a primary and the scavenging effect happens at to high an rpm to do any good.To long or small a diameter hits it's sweet spot to early and kills power at higher rpm's by being to restrictive.

The tuned pulses only help at certain rpms because the speed of the exhaust slug return is fairly constant and will only arrive at the proper time at a few points in the rpm range so it is important to decide where you need the help.Normally it is done to help below the tourqe peak to improve low end power and response.

Now to boil all this down in my opinion Honda got it right don't fix what isn't broke.
 
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