Great to have the experts here in the evo section. I do alot of reading here and now am asking for a little advice.

My 98 FXDWG has the Screamin' Eagle mufflers (not the Screamin Eagle IIS) on it and the previous owner had removed the baffles and there is absolutely nothing inside the mufflers. The bike also has the SE carb kit, an SE cam, and the SE air cleaner I do not like the decel pop and would love to have a little more umph at low end. I have read about the old trick of drilling out drag pipes and welding a washer on an eyebolt and installing to decrease reversion of the sound waves. I have purchased some 1/4" eyebolts and am undecided about what size washer to weld on them before installing. Will this help achieve my goal? How much will this quiet the exhaust? How far from the tail end of the mufflers do I need to install them?

Yea I know I can buy some new slip-ons, but money is tight now. Would love to hear the opinions of the 'sperts on here.

Thanks,

Mike

 

I have a few made up that i tinker with. One is in my hand right now. If you want i can send you this one and you'll have some thing to go from. One hole was already in my slip-ons from the baffle. I drilled another one 8" from the end. I use a small plug in the other hole when not in use. It can't be seen unless you look under the bike. These are just for testing purposes that's a spare so you can have it.

 

This should help....

http://www.nightrider.com/biketech/exhaust.htm

 

Thanks Stevelonewolf. What is the od and id of the washer and what size did you find worked the best during your testing?

Thanks 93 FXR! I read that article doing my research. In fact, I have read all the pages nightrider has over the past few years. Very interesting info.

 

The I.D. is .310in/O.D. is .735in. You can slightly turn/angle these as you see fit to get the best tune-a-bility. LoL!

 

You can get a fairly mid range rumble with a 1 inch hole in the pipe. Not too loud but not quiet either. To help with back pressure or noise reduction, buy a fist full of stainless steel woll. stuff it into your pipes & put your restrictor in front of it so it cant be blown out by the exhaust.
If you cant find any in your local stores, hit a machine shop & ask for some from there lathe section. The stainless will not rust out or burn!

 

Tuning the TrueDuals on my RK was done in the left pipe only.I took a 3/8" bolt welded a 2" flat washer on edge to it.Installed it ahead of the muffler inside a 2 1/2" pipe.First start up washer was inline no resistance,took a plug reading,turned washer very little to bring the tune dead on.

 

Well, since you asked, my opinion on the whole "back pressure" thing is this: Back pressure is bad for performance. Studies on a dyno have shown maximum HP with open headers. Internal combustion engines are large air pumps. Less restriction means more air flows through which means more HP. Now, if your fueling system can't handle the air flowing from an open exhaust, that is, the fuel mixture is off and you can't adjust it with your current carb or EFI, then, yeah, I can see that reducing the total engine efficiency by putting something in the exhaust would provide better HP over some areas. However, realize that you're reducing your motor's efficiency to compensate for poor carburation (or fuel mapping) rather than fixing the fuel side to compensate for the open exhaust/more efficient motor. A motor that's too fat or too lean has less than the optimal HP. If you're too lean and you add a washer twisted just so, your motor is less efficient and less air if flowing and now you are not too lean there anymore. That's my theory on back pressure.

Now, my personal observations: I run straight through 2007 OEM mufflers on my bike. It runs just fine, and while a bit louder than my '86 mufflers that were also straight through but built different, I had no need to rejet my carb. I also have a K&N air filter. My wife's bike is a bit different. The 883's are a biatch to get a carb to work right on. She's had open drag pipes on it since about '89 or '90. While small, the cars can certainly hear her coming. I tried several different carbs on the bike. The original was a total POS, but got 90+MPG in the mountains. It was just so lean that it was dangerous to pull out in traffic. The Screaming Eagle carb was too much. Too big for the 883. The S&S Shortie was also too big. And neither were adjustable enough for the dead spot at about 70 MPH. It was a constant battle. Reading the board here, Sporty riders with drag pipes all complain of a dead spot about there, give or take gearing. When I built the Megasquirt EFI system for the bike and tuned it, turning on closed loop (where the computer reads the oxygen sensor and compensates for the current conditions to get it "just right"), the dead spot was gone. In fact, my wife says it's like riding with an automatic transmission: just put it in gear and go. Don't even have to shift, just roll the power on. The people on the Sporty board say that if you put the washer in your open drag pipes, it takes care of the dead spot. I had open drag pipes on my old shovelhead POS and didn't have any problems at all with a Lectron carb, I just had to tune the carb right.

 

Thanks Doc,

My bike actually runs fine. The plugs do not show a lean mixture, for they are a light tan color. It pulls strong at high rpms, but I do not run it up there very often. The decel pop I have is only from high rpms and then completely closing the throttle. From a normal slow down procedure from 70 mph in 5th gear it does not do it. I am just interested in getting more torque in the lower and midrange of the rpms where i spend the most time. Do you think the washers will accomplish a noticeable difference?

Thanks again Doc.

 

Pulled this from the net for some perspective on the subject. It's a good read!
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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