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Straight Pipe Myths

45K views 42 replies 36 participants last post by  Troy Jollimore 
#1 ·
Gentlemen I have been searching the forums trying to see if anyone had any problems with running straight pipes on their bike.

I see a lot of posts with people talking about having done it and it sounds like they didn't have any problems with back pressure. I was always under the impression that some engines don't work well with no back pressure I was wondering what the case would be.

I recently bought an 1100 Shadow and the previous owner put somewhat of a custom exhaust on the bike I want to remove some of it because they are too long and I can't use the helmet holder. I don't think it currently has a baffle but I am super new to bikes and can't really tell. I just want to remove the end of the pipes and have them end about six inches behind the controls. Will I lose performance?

Thanks guys!
 
#2 ·
Back pressure is key to optimal performance. You can jet around the loss of back pressure and keep things running true (no popping/backfiring) but you may lose a few hp as a result.

The results of that loss should be relatively insignificant, visible only on dyno's and at the drag strip where 100th's of a second count. If you don't intend to drag race your bike then go for whatever looks and sounds like you want it.
 
#4 ·
Heat trashes the neoprene layer on helmets like nothing else. So even if you can keep it off the exhaust having it within a few inches and/or in direct sunlight is likely to shorten the lifespan dramatically.

If that's your main concern just get a cable lock: Amazon.com: Cable Bike Locks: Cycling & Wheel Sports: Sports & Outdoors and loop it through the helmet lock then set the helmet on the sissy bar ( and covered or parked out of direct sunlight) or whatever. My bike currently has no means of storage so I have one of those, a pair of sunglasses, and a bungee net in a fork bag.

Way cheaper and easier than modifying the exhaust anyway...
 
#9 ·
The issue with straight pipes is not backpressure. It flow. The way the exhaust moves through the pipes is setup to assist in scavenging the used exhaust gasses from the cylinder. Plugging off the end of the pipe will increase the overall pressure in the pipe but will restrict flow. You cannot do one without the other (google Bernouli).
The flow through the pipe actually creates a vacuum at the exhaust valve to "Suck" the exhaust out of the cylinder and as a result suck the new fuel and air in. When that balance is changed the cylinder does not scavenge correctly and the bike will run poorly as a result for no other reason than a percentage of the charge in the cylinder is burnt already. Stock pipes and performance headers are "tuned" to keep this flow correct. Poorly designed or cheap pipes are just that...straight pipes. You may be able to hide the problem but your performance will suffer as a result.
Remember an exhaust is more than just a tube with crap stuffed in it.
Pressures within the exhaust stream need to be set by the design of the pipe at specific points in the pipe (the pressure is not equal at every point in the path) This is done by increasing and decreasing the diameter of the pipe slightly (called convergence and divergence. Again google Bernouli).
While these changes are not readily visible to the casual observer they are there.
 
#12 ·
If you have a windshield, you can relocate the stock helmet lock to the windshield mount like I did.

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#13 ·
Much ado about nothing. If performance was the biggest worry, you probably wouldn't be riding a cruiser. People customize for many reasons, but I've yet to see performance as a primary catalyst to modify...only to dissuade others from personalizing. Like others have suggested, you might not even notice the difference. In that case, I say don't let the worry of performance loss be what stops you from doing what you'd like to do.
 
#15 ·
To add to the back pressure debate I found this very informative article that explains the back pressure myth which will affect my exhaust design..

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).
 
#18 ·
Quote:

"We are going to state this very clearly...Backpressure does not increase horsepower. Period.

An often heard statement from the well-informed is "You need a bit of backpressure for an exhaust to work". Usually this comes from someone who is not a tuner or someone who is faced with a situation where he does not have the tools or means to adjust things. Anything you do in the exhaust will change the flows, the pressures, or velocities somewhat. The correct scenario is that the exhaust has to be properly designed and then you optimize the jetting, ecu data inputs, camshafts, port dimensions, valve sizes and the like. The exhaust has to be designed for the intended use."


More here > RB Racing LSR 2-1 Exhaust Technology
 
#28 ·
You don't want to put it that far up in the pipe. These work best mounted somewhere near the end of the exhaust, but do work further up in the pipe. I made my own Thunder Monster Baffles and mounted them right after the head pipe in the muffler itself and then tuned for my application. You will notice more torque with some sort of baffle compared to open pipes.

http://www.hondashadow.net/forum/72-technical-discussion/137386-homemade-baffles.html
 
#22 ·
On my "04" Sabre, I pulled the back baffles out. Then on a suggestion blocked off the crossover pipe. All that did was make the bike sound like an old Chevy six with straight pipes! Very BLATTY, to me it sounded like crap, so I removed the blockage on the crossover and the bike sounded much smoother. So instead of each cylinder sounding off at different times, the exhaust was mixed and made it sound much smoother, I like that! Some people say that our Shadows (All sizes) sound like a sewing machine if you do not spend $XXX amount of large dollars plus re-jetting, synching, and fine tuning (TIME as well as $XXXX). But I beg the question to be answered: Who has EVER heard a 600cc to 1300cc SEWING MACHINE? Hell, ever see one? HUH???? Love the sound and the ride. Our Shadows are just about perfect for their intended perposes!:grin: Mod all you want and have a great time doing it, just don't expect any HUGE gain in HORSEPOWER! A little? sure! You will have to work for it! :grin:
 
#24 ·
It's not that people use performance to dissuade modding, it's that invariably folks come back later wanting to know how to increase performance. You won't get the same performance from straight pipes an lollipops as you will with a properly designed exhaust. That said, I ran straight pipes on my bike for a while, just welded about eight inches of exhaust pipe to the headers, with a big thumb screw in the end. Very blatty indeed. Now I run a Mac exhaust


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#30 ·
long pipes and helmits

I've put a pair of Cobra Low-boy shotguns on my 03 750 ACE, Rejetted to specs, and re-synced, I still get a bit of pop now and them especially when letting off the throtttle quickly, but nothing bad, with no loss of performance. As far as the helmit lock, I moved mine off the fender strut, and relocated it to the brackets on my sissy bar, no biggie, drill two holes.

Life is nothing but a long list of compromises.....

Good Luck
 
#31 ·
2 things...

1. I kept the front pipes and used 2" chrome extensions to make my rear pieces. I use nothing to increase 'back pressure'. But then im not the average rider and know a thing or two about tuning and such making it all work. Simply: YMMV.

2. On the finger over the hose outlet analogy... youre wrong. It doesnt make the water flow faster, you simply increase the outlet PRESSURE. Insinuating that the flow is somehow increased implies you have control over the water companies supply flow and you dont. This is why the supply line to your house is usually 5/8" and internal pipes are 3/8", to increase PRESSURE not aid in flow speed.

Think of it this way, take a drink of water and hold it in your mouth, now open your mouth ALL the way and let it run out. Now, do the same thing, but purse your lips like you are going to whistle and push the water out with your tongue. Is the water going faster or farther? Is actually going slower because its under pressure now.

Back pressure is needed to help the valves perform properly and to help the exhaust pulses 'pulse' as they should. You dont need a lot, but you do need some.
 
#32 ·
I have noticed a leak at the back end of my engine - at the top of the block. not really bad (yet) but drips on the front sproket cover when on side stand. I have 60,000 miles and just recently did the hole saw with 1-1/8". Would the two be related, or just the age of the engine needing bolts retorqued?
 
#33 ·
MrSteej gave an excellent answer to back pressure. Just an additional item, two straight pipes may not increase performance and in fact, if checked on a dyno, may contribute to reduced efficiency.

Two-into-one pipes ala Vance and Hines Pro Pipe exhaust, if properly engineered can increase efficiency through the "pulse effect" as both cylinders are feeding into one exhaust doubling the pulses thus increasing extraction effectiveness. The exhaust should travel uninterupted through the header portion of the pipe to the muffler section. There the pulses are doubled in the single resonator increasing extraction efficiency. This is why stock pipes have a crossover pipe when equipped with two resonators/mufflers.

One should not adjust the length of the header pipes unless capable of calculating pipe tuning parameters then able to test them on an engine dyno. If you measure the length of the two pipes you may find they are different lengths. This results in the 'pulse' from each cylinder arriving at the collection pipe at the correct time to increase extraction efficiency.

Check out any high performance engine exhaust system and note the headers (for example, a V-8) usually go four into two or one exhaust pipe. Each Header pipe length has been engineered to a particular length for maximum effectiveness. This dramatically increases extraction. For most participants on this board, two straight pipes may be loud yet won't aid performance and can reduce fuel efficiency.
 
#34 ·
If the original post claimed that there were aftermarket straight pipes already on the bike why has there not been more discussion of aftermarket baffles. I installed a set on a buddies shadow 500 and it made a world of difference as far as popping on decell and on the quality of the sound out of the pipes. Additionally these can be purchased from reputable companies which have actually been designed by engineers to reduce the raspy tone, still allow for ample gas flow while providing the necessary back pressure to ensure optimal performance. If you were to make the necessary cuts to the pipe length and then purchase the appropriate sized baffle, would that not provide the best of both worlds, increased quality of sound and an affordable solution to your back pressure concerns?
 
#35 ·
If the original post claimed that there were aftermarket straight pipes already on the bike why has there not been more discussion of aftermarket baffles. I installed a set on a buddies shadow 500 and it made a world of difference as far as popping on decell and on the quality of the sound out of the pipes. Additionally these can be purchased from reputable companies which have actually been designed by engineers to reduce the raspy tone, still allow for ample gas flow while providing the necessary back pressure to ensure optimal performance. If you were to make the necessary cuts to the pipe length and then purchase the appropriate sized baffle, would that not provide the best of both worlds, increased quality of sound and an affordable solution to your back pressure concerns?

You lost me here; "Reduce the raspy tone " ? I thought the idea was to increase the tone not reduce it ?
 
#36 · (Edited)
I always thought that backpressure was a two stroke engine issue. The idea is this: because of the way it is designed, there is a moment where the intake port and the exhaust port are open at the same time. Backpressure will prevent the fuel/air mixture to get out to the exhaust before the combustion cycle. Not sure what that would benefit a four stroke engine.
Here, check out this animated gif:

 
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