Muffler off; Mileage up

[Airman]

haha so taking off my muffler should not hurt the bike?

the bmw dealer said its gonna make it run lean and hot

Your BMW dealer is correct. Modifying air flow through the boxer by removing the muffler, removing the CAT (illegal), employing after market air filters means the engine now receives more air for a given throttle setting. But are you adding more fuel along with this increase in air? Want more performance?.....loose 15 lbs.........want better handling?.....take an advanced rider course or buy some instructor/track time.

def, I question what you're saying here. I have run the CAT without the muffler, CAT with the muffler, stock muffler and Remus y-pipe, and briefly the one configuration I don't recommend, Remus y-pipe with no muffler. (Just to irritate the neighbors) I have never had heating problems. As well, I think quite a number of owners remove the stock muffler because they like the look or want to use the full sized RT left case. Using the stock silencer or the CAT means the sound won't change much. I don't think anyone here is really looking for performance with an exhaust change of this sort.

As far as the airflow, I was interested in San Jose's airbox mod at one time because the airbox does restrict airflow. I don't really believe putting a K&N or whatever air filter will do much. The air box mod cuts a 4" hole in the airbox and increases the size of the airhorn. And guess what ? I asked a guy who did the mod, (he also added the aftermarket exhaust and chip) he only realized performance gains at the top end where he would normally run out of air. 136 mph instead of 124, or something like that. See what I mean ? If the intake is restricted, is the open exhaust actually going to lean out the mixture ? I'm not an expert, but I don't see it.

[rdsmith3]

No, you are not because the TPS is still in the same position and air flow has increased and you have now upset the stoichiometry so carefully adjusted by BMW at the factory. Your engine is now going to run hotter combustion chamber temperatures, exhaust valves and guides are going to run hotter and you may end up doing engine damage (read holes in pistons).

Did the factory intentionally adjust the stoichiometry so that some of the boxer engines experience surging?

[def38]

haha so taking off my muffler should not hurt the bike?

the bmw dealer said its gonna make it run lean and hot

Your BMW dealer is correct. Modifying air flow through the boxer by removing the muffler, removing the CAT (illegal), employing after market air filters means the engine now receives more air for a given throttle setting. But are you adding more fuel along with this increase in air? Want more performance?.....loose 15 lbs.........want better handling?.....take an advanced rider course or buy some instructor/track time.

def, I question what you're saying here. I have run the CAT without the muffler, CAT with the muffler, stock muffler and Remus y-pipe, and briefly the one configuration I don't recommend, Remus y-pipe with no muffler. (Just to irritate the neighbors) I have never had heating problems. As well, I think quite a number of owners remove the stock muffler because they like the look or want to use the full sized RT left case. Using the stock silencer or the CAT means the sound won't change much. I don't think anyone here is really looking for performance with an exhaust change of this sort.

As far as the airflow, I was interested in San Jose's airbox mod at one time because the airbox does restrict airflow. I don't really believe putting a K&N or whatever air filter will do much. The air box mod cuts a 4" hole in the airbox and increases the size of the airhorn. And guess what ? I asked a guy who did the mod, (he also added the aftermarket exhaust and chip) he only realized performance gains at the top end where he would normally run out of air. 136 mph instead of 124, or something like that. See what I mean ? If the intake is restricted, is the open exhaust actually going to lean out the mixture ? I'm not an expert, but I don't see it.

Any time you increase air flow through a properly tuned engine without also increasing fuel, you run the risk of detonation, additional heat and other driveability issues. Our boxers (US delivery) are already running at the edge of overly lean due to EPA requirements.

If you want to modify your bike, I say, have at it......but, beware...there are potential dangers. For those who merely want more noise, the boxer exhaust note is rather boring...why make it more so?

[NoRRmad]

I thought the point of the computer-controlled fuel injection was to adjust the mixture by reading unburned oxygen in the exhaust stream, and adding or reducing fuel to compensate. These adjustments may be due to air temperature, throttle opening, altitude, clogged air filter, missing muffler and so forth, but the computer is just looking for the right number from the oxygen sensor.

[Airman]

I thought the point of the computer-controlled fuel injection was to adjust the mixture by reading unburned oxygen in the exhaust stream, and adding or reducing fuel to compensate. These adjustments may be due to air temperature, throttle opening, altitude, clogged air filter, missing muffler and so forth, but the computer is just looking for the right number from the oxygen sensor.

I kind of had that idea myself. That raises the issue of what happenes at altitude. One of the brothers suggested that the bike would run leaner at a higher elevation. Does it ? Or does the computer compensate, and to what effect? If this were true, owners in Denver would have excellent MPG numbers, right ?

[Beemeridian]

11

[owldaddy]

I have been running with only a chrome tip for 3 years, Thanks DJ. I do think I lost 1 or 2 mpg, I always thought the bike ran pretty good, I knew I had a lean stumble at wot. I believe that the bike always ran that way, even new. As I understand the fuel mapping, it is set lean and thats just the way it is gonna be. I finally got a Techlusion fitted last week, I am keeping an eye on the plugs, and the mpg, so far they look real good, but that is with only 2 tanks. I can tell you for sure though, this bike now runs like it should from the factory. no surging, fewer backfires, no hickups, better throttle response, a lot more fun to ride, last tank was 42 mpg, with some spirited riding on twisty roads. That is about normal for what I have gotten in the past with this type of riding. I might have been able to get 44 when the bike was new, so I think that losing the muffler may have it's cost, but being able to fit a full size bag is a big advantage, and when the bags are off you have that view, I just love looking at that wheel.......
Don

[MattPie]

I thought the point of the computer-controlled fuel injection was to adjust the mixture by reading unburned oxygen in the exhaust stream, and adding or reducing fuel to compensate. These adjustments may be due to air temperature, throttle opening, altitude, clogged air filter, missing muffler and so forth, but the computer is just looking for the right number from the oxygen sensor.

I kind of had that idea myself. That raises the issue of what happenes at altitude. One of the brothers suggested that the bike would run leaner at a higher elevation. Does it ? Or does the computer compensate, and to what effect? If this were true, owners in Denver would have excellent MPG numbers, right ?

I won't say for sure, but I believe Def's supposition relies on two things: the muffler is bottleneck and the engine gets the same amount of fuel at a given RPM or throttle opening. I do not believe either of those conditions are true on our bikes. I'm fairly sure the cat is the bottleneck (or is matched to be just as restrictive as the muffler). And, as I understand it, the FI compensates the mixture via the O2 sensor. If you pull the CCP out which (I THINK) takes the O2 sensor out of the loop all bets are off, although it seems like the bike runs richer with the CCP out so it should be safe.

FWIW, I took my can off for awhile when I first got my bike. My local HD-riding and wrenching friend made a similar statement as Def's. I put the can back on since I didn't want any hassles with the warranty and the bike wasn't paid off. I've been thinking of pulling it again lately, since it did sound nice and I wouldn't mind the larger left case.

[Airman]

I thought the point of the computer-controlled fuel injection was to adjust the mixture by reading unburned oxygen in the exhaust stream, and adding or reducing fuel to compensate. These adjustments may be due to air temperature, throttle opening, altitude, clogged air filter, missing muffler and so forth, but the computer is just looking for the right number from the oxygen sensor.

I kind of had that idea myself. That raises the issue of what happenes at altitude. One of the brothers suggested that the bike would run leaner at a higher elevation. Does it ? Or does the computer compensate, and to what effect? If this were true, owners in Denver would have excellent MPG numbers, right ?

I won't say for sure, but I believe Def's supposition relies on two things: the muffler is bottleneck and the engine gets the same amount of fuel at a given RPM or throttle opening. I do not believe either of those conditions are true on our bikes. I'm fairly sure the cat is the bottleneck (or is matched to be just as restrictive as the muffler). And, as I understand it, the FI compensates the mixture via the O2 sensor. If you pull the CCP out which (I THINK) takes the O2 sensor out of the loop all bets are off, although it seems like the bike runs richer with the CCP out so it should be safe.

FWIW, I took my can off for awhile when I first got my bike. My local HD-riding and wrenching friend made a similar statement as Def's. I put the can back on since I didn't want any hassles with the warranty and the bike wasn't paid off. I've been thinking of pulling it again lately, since it did sound nice and I wouldn't mind the larger left case.

An HD rider who tells you not to remove the muffler ? The man's a true outlaw. I just took the CCP back out. I had it in for a week, but I had forgotten how bad the surging was. I removed it again. Whatever it's original design was, the actual effect is bad. Herky-jerky at all rpms below 3500, and even a little above that.

[Max Continuous]

To add my 2 cents, I tend to agree with what def38 is saying, which is simply, by adding more airflow, the fuel mixture will change. The bike can compensate to a certain degree but without knowing what the actual tolerances are, you really have no idea what is happening inside and whether or not you have leaned it out too much. For instance, it may work great at sea level, providing years of reliable service, then a ride to a higher altitude may be just enough to lean it out beyond the limits of what the O2 sensors can adjust to. Now you run the risk of bending a valve or doing some major damage, all for a cooler sound and basically, no performance increase. (This assumes that there is in fact a limit to the amount which the bike can compensate for, which I am sure there is )

Leaning out a fuel mixture just by a small amount adds incredible heat. Anyone on here who has flown a piston powered airplane, which has a manual mixture control has seen the exhaust gas temp skyrocket with just a few turns. Many frustrated mechanics have had to rebuild engines because a pilot has leaned the engine out too much due to a climb to higher altitude without richening the mixture from the previous setting. It would be really interesting to put a thermal probe on the exhaust manifold of our bike and actually measure the temp in both configurations. But unless you know what the actual limits are, that doesn't help much either. I am not sure how much extra airflow the O2 sensor on 1150 can control or adjust for but I really don't see an incredible advantage by changing the stock exhaust set up, other than weight loss but the 1150 is already a relatively heavy bike anyway.

FWIW, I consistently get 300Kms ( 187 miles ) until reserve, which occurs with about 4 liters (1 gallon) left in the tank on a stock set up.

[Sunbeemer]

I'm confused by the need to richen the mixture at higher altitudes where the air is thinner. This has been mentioned twice in this thread, and it seems to me that the fuel supply in an airplane at altitude would need to be reduced (leaned out) to keep the air-fuel ratio at 14:1, since with fewer air molecules at higher elevations, you'd need to add less fuel to maintain this stoichiometric ratio, right?

However, our fancy, computer-controlled machines are supposed to automatically regulate the mixture according to various sensors mentioned, so I wouldn't think anything done to the exhaust system would affect this ratio, since the computer would see the change in exhaust gas composition (from the O2 sensor) and compensate for it, at least as much as it could...

That's not to say that reducing backpressure on the exhaust valves by removing muffler components wouldn't allow more hot gas to bleed by them and burn them prematurely. A tuned exhaust system, working in conjunction with tuned intake runners, uses pulsed backpressure to enhance the flow of gases through the cylinder for a clean sweep and a better fuel charge.

[Max Continuous]

Sunbeemer

Yes, you are correct. I forgot to take my dyslexia meds today! Sorry for the confusion. Guys do get into trouble by changing altitudes and not readjusting the mixture however, usually by climbing to a higher altitude at a higher power setting without first richening to compensate for the climb power setting and then leaning it back out when the cruise setting is re-established.

The real point I was trying to make I guess was that while the O2 sensors will compensate for various mixtures, what are its' limits? Just like a shock absorber can be adjusted for riding style and road conditions, it only has so much travel. Is removing a can altogether within the acceptable limits on our bike? I honestly don't know and judging from all the varying opinions on here, I would say most don't actually know for sure.

For instance, most of the after market slip on mufflers like Leo Vince or Akrapovic increase airflow by reducing back pressure and claim an increase in the 3-4hp range without having to re-map. They have accomplished this with testing and engineering obviously. When some of us, as amateurs start to remove items, or bypass systems without the proper ability to actually measure what effect our mod has done, or have an accurate awareness of any long term effect, then we are accepting a certain amount of risk by doing so. For me personally, on this particular bike, I don't really see enough benefit by removing or replacing the muffler, or the entire exhaust system for that matter, to warrant the possible risk.

I think that is what def38 was getting at in his previous post. This a great topic and interesting thread BTW.

[MattPie]

An HD rider who tells you not to remove the muffler ? The man's a true outlaw. I just took the CCP back out. I had it in for a week, but I had forgotten how bad the surging was. I removed it again. Whatever it's original design was, the actual effect is bad. Herky-jerky at all rpms below 3500, and even a little above that.

Actually, he said you need to re-jet the carburettor or update the fuelling chip, if I recall. His bike was quick from what I hear and not loud, but I never rode with him. He was a bit of a American-only snob. The other HD guys in that office had no issues with me.

[MattPie]

(This assumes that there is in fact a limit to the amount which the bike can compensate for, which I am sure there is )

The fuel injectors can only flow so much fuel (they're usually rated in 'cc/min'), so eventually they'll run out of capacity.

According to this, 2 injectors need to flow 279 cc/min to make 85HP. Anyone know what the R1150R injectors are rated at?
http://www.injector.com/injectorselection.php

Listing of Bosch FI part numbers:
http://users.erols.com/srweiss/tableifc.htm#BOSCH

Thinking about it, since no one seems to be claiming any performance gains it would seem that there isn't much (or any) increase in HP when you take the muffler off. Based on the above calculator, the same power requires the same fuel flow, so there's no issue here. The FI still needs to be able to deal with whatever slight changes happen.

[priapismic]

I've been following this thread with interest, because as I said in my earlier post, I've run both with and without the stock muffler, and while I like the sound and the reduced weight of running with just a short tip, I wonder if I'm doing some subliminal damage long-term. I understand what Def38 is saying about running lean, but as others have pointed out, our computer-contolled fuel injection system should be able to sense and compensate for increased airflow by richening the mixture.

I decided I wanted to escalate this issue outside the realm of our esteemed experts on this board and ask someone who should know something about fuel mixtures, etc., so I wrote an e-mail to Chris Sidah, the Tech editor for Rider magazine, and a pretty well-known resource and BMW wrench. A recent letter in his magazine column inquired about exhaust modifications and airflow/mixture considerations (he basically answered that mufflers are necessary to create back pressure to deal with valve overlap), so as a follow-up to that, I sent him the following message:

As a follow-up to your answer to "Ken" regarding rejetting and exhaust modifications (July 2008), and your detailed answer about valve overlap and back pressure: would the rejetting issue even exist on a fuel injected machine?

There is a fervent discussion going on at the BMW Roadster/Rockster internet forum (http://www.R1150R.net) regarding removal of the stock muffler, and running with just a chrome tip coming off the catalytic converter. Some on the board are of the opinion that the cat con has sufficient back pressure by itself, and that the factory muffler is superflous. And, in support of their argument that the muffler's removal will NOT result in a lean condition, they reason that the computer controlled ECU (which manages the fuel injection system) will compensate for any muffler removal and it's subsequent lean condition. Others on the board are just as adamant that muffler removal WILL result in a lean condition, generating excessive heat and insufficent back pressure.

What are your thoughts about fuel injection systems, cat cons, and muffler removal? A lot of people on the web are anxious to hear your verdict.

I wasn't expecting a personal reply, but lo and behold, Chris replied to me the same day he got my message. Here is his abbreviated reply:

"The oxygen sensor plays the most important role in determining mixture. The cat, if I remember correctly, is a matrix type and would keep any pulse or pressure waves from the muffler affecting cylinder charge. Run the stubby pipe. I'll answer more in depth in the October issue.
Chris Sidah"

Now, that short answer tells me:

- don't mess with the O2 sensor by pulling the CCP plug and changing the closed loop system to an open loop, and
- those of us who guessed that the ECU would compensate for increased airflow by richening the fuel mixture are on the right track, and
- there would appear to be adequate back pressure generated by the Cat to allow the proper valve overlap, and
- running a short tip on the cat con in lieu of the stock muffler probably won't hurt.

I, for one, can't wait to see Chris's detailed answer in an upcoming issue of Rider !

[DJ Downunder]

I think a lot of people don't realize just how big our cat is..It's a whopper.. ..and it does work like a big muffler..(IMO).

I have checked with at least three different BMW mechanics about what I'm running and none of them have showed any real concerns.

Some even said..What a great idea..and they say it should be not much different to running a sports type muffler with no cat...I don't get much popping and backfiring when I back off.

But I have often wondered about some exhausts having a tuned length..Some bikes even have the exhaust snaking around to get the length right.

I have also noticed that a short stubby exhaust is becoming the latest thing with many new bikes..They do make a lot of sense.

Maybe we are the trend setters with this idea..

It's all interesting stuff..that's for sure.

DJ

[Dr. Strangelove]

Just had the a tire change at my local dealer and while his comments do not address the lean/hot discussion, he had no problem with the stubby and when he took it for a test ride, said the bike was running "tip top." He was impressed how it ran. I had done the TBS and valve adjustment and noticed a big improvement, and it was nice to have independent confirmation.

Just got back from a ride to College Station--450 miles away; I got nearly 50 on a couple of tanks, maybe 3; I got 45 the rest of the time--spirited riding on backroads around CS and the lowest I got was 41 and that was full on hooligan which previously led to 38.

The "gas" there had 10% ethanol I am told and PLus was 4.10/gallon.

She perfomed beautifully on the trip, never a hesitation, no popping and all the power I wanted.

John

[CycleRob]

That is correct about the fuel mixture being leaned out, but if it like most EFI systems (I'm not 100% sure that BMW's is), it is only at idle and near/at full throttle. THAT could be a problem if there's a sustained full throttle run. During part throttle cruise, which is most of the time, the O2 sensor "trims" the fuel delivery (leaner/richer) by reading the Oxygen remaining in the exhaust gases and reporting the instantaneous reading to the Motronic.

About the "Muffler Off; Mileage Up" - - - - I'm with KHaynes's reasoning where the louder exhaust has you more aware of the throttle, essentially training you to keep it light-n-steady while providing a visceral satisfaction that substitutes for heavy handed throttle moves more likely with the stock muffler. To that I'll add the 14lb weight loss from no muffler and the leaning out effect it has even at your high cruise speeds, also without the CCP in place.

I tried the no muffler ride when I delivered my cut open muffler for the rewelding closures after I completely gutted it. Riding with no muffler was too loud by a tiny bit.

My latest muffler mod work was just uploaded in another post.

.

[BigC]

I can't prove or disprove the increase in fuel economy with the muffler removed. The tip provides a grin factor that I'm young enough to appreciate The weight reduction is welcome as well. Although if I went on a diet I'm sure I could make up for more than the muffler...

After reading this thread I though it might be interesting to see how the bike ran without the CCP in place. I went out to remove it and found it MIA... Obviously the previous owner thought it wasn't necessary. When I bought the bike it had a stock air filter and it would hesitate badly when cracking the throttle either at idle or under load. I went against the grain here and put a K&N filter in the bike and the hesitation went away and it now runs great. I'm NOT advocating deviating from the stock air filter. I would say that the running in open loop (no CCP) with less restriction in the intake made my bike run smoother.

[GypsyRR]

While you guys discuss..........


It does look cool without a pipe, and you can see the Keith Haynes roundel better now too!!