Muffler effects?

What is the effect of increased muffler back pressure on the 4-2-4 break?

>What is the effect of increased muffler back pressure on the
>4-2-4 break?

The effect is to make the break both harder and earlier. Assuming no other changes, of course, a couple of which can negate the effects of simply increasing muffler back pressure.

Or is it the other way 'round? Where is current Bad Boy Stunt column? I think was addressed, but can't seem to remember for certain...

Hello Dan, I,ve got a couple of tongue mufflers with about 12 holes, or so, drilled in them; four of which are threaded with 4-40 machine screw threads. Are these four holes threaded just for the purpose of adjusting the back pressure? The only thing I've noticed is a difference in noise the engine produces. I like quiet engines so I just plugged all four holes. The engine seems "happy" this way so I just left them plugged.

Thanks, Bill Ervin

>Hello Dan, I,ve got a couple of tongue mufflers with about
>12 holes, or so, drilled in them; four of which are threaded
>with 4-40 machine screw threads. Are these four holes
>threaded just for the purpose of adjusting the back
>pressure? The only thing I've noticed is a difference in
>noise the engine produces. I like quiet engines so I just
>plugged all four holes. The engine seems "happy" this way
>so I just left them plugged.
>
> Thanks, Bill Ervin

Yes, noting that increasing or decreasing back pressure can not only change the way engine runs, it gives other options as well.

I've got a couple tongue mufflers which were originally used on Fox 35s, then converted to use on one of "those" engines. They both have all the holes tapped 4-40, which is a pretty dumb thing to do if you think about it.

You will find the mathematics of the situation to not relate well to real-world plugging and unplugging of holes. In other words, unless you are well along in a testing program, don't just plug one hole at a time and expect to see much difference. Fly with all the holes open. Then fly with 4 to 6 of them plugged. That will make a difference, and then you can determine if you want something in between. Or not, as the case may be.

When I was doing my testing of mufflers with the modified 53 Magnum, I was finding an increase of about 100 rpm in the maximum rpm of the engine for each hole I added--up to a point. I had previously been doing mufflers with 12 holes and ended up with 15 for this very reason. Now if your maximum rpm is 9000, say, and you are running onlyu 7800 with a certain prop that extra 100 rpm may not make much of a difference. If you are screaming the engine to its max, it very well can. So it makes a lot of difference what type of engine you are using and how you are running it. We use the same basic tongue muffler on the 35s and 40s as we do on the 53s and 60s, so it can make a lot more difference on a larger engine also, compared to a smaller one.

As for all of the holes being tapped, you obviously don't want to plug all of the holes. But on the other hand, this give the option of plugging the outer holes, the inner holes, or some combination in between. You really want a variety of hole lengths as this will give the greater variety of sounds--each hole length producing its own sound. If you had one big hole at one length, you would have one loud noise at one frequency. If you have two smaller holes of different lengths, you have two frequencies blending together. The more holes of differing lengths, the more pleasant the tone becomes until, carried to its extreme you would end up with "white noise" or no noise at all (everybody calcelling each other out.)

This plugging and unplugging of muffler holes affects the length of your run for a given fuel amount?? This is a question??

>This plugging and unplugging of muffler holes affects the
>length of your run for a given fuel amount?? This is a
>question??

Yes. This is an answer!

Seriously, more muffler pressure has a similar effect of going to a smaller venturi. Less muffler pressure has a similar effect of going to a larger venturi. If you have a highly restrictive muffler and the engine runs well on a certain prop, if you go to a less restrictive muffler you may need to go to a smaller venturi (or bigger prop).

But more muffler pressure chokes the engine and should cause you to burn less fuel (and produce less power).

What about noise levels. How does a tongue muffler compare with an engines standard muffler? has anyone done any back to back measurements?
They are rarely seen in the UK and I wondered if that was because of an increase in the noise and the weight saving and manufacturing cost would seem to be an advantage

Regards

Mike

>What about noise levels. How does a tongue muffler compare
>with an engines standard muffler? has anyone done any back
>to back measurements?
>They are rarely seen in the UK and I wondered if that was
>because of an increase in the noise and the weight saving
>and manufacturing cost would seem to be an advantage

I did some testing with the db meter and they pass the FAI standards. Matter of fact, a few years back I was comparing the Tongue Matt was using to the pipe ships at the FAI trials and the pipes were louder. It all depends on the type of muffler, obviuisly. If you have more expansion chambers in an expansion chamber muffler and a smaller outlet, then this is going to be the quietest. It will also be the most restrictive unless made large. The pipes were louder because, even though they had a larger expansion chamber, they also had a larger outlet. However...

(and here's the big caveat. The expansion chamber muffler will sound more pleasant because it will give a deeper tone. A shriller noise always appears to be more annoying to the human ear, even though it may even be of less volume (quieter) than a comparable lower pitch tone. The pipe engine, running considerably faster, can still give off a lower pitch tone than a side exhaust tongue muffler engine turning a prop at a lower rpm because of the "tuba effect."

LAST EDITED ON Nov-17-04 AT 08:31 PM (CST)
 
>>This plugging and unplugging of muffler holes affects the
>>length of your run for a given fuel amount?? This is a
>>question??
>
>Yes. This is an answer!
>
>Seriously, more muffler pressure has a similar effect of
>going to a smaller venturi. Less muffler pressure has a
>similar effect of going to a larger venturi. If you have a
>highly restrictive muffler and the engine runs well on a
>certain prop, if you go to a less restrictive muffler you
>may need to go to a smaller venturi (or bigger prop).
>
>But more muffler pressure chokes the engine and should cause
>you to burn less fuel (and produce less power).

Agreed. I had a Fox 40 that produced decent power, but was hard to needle due to low fuel consumption (my opinion) with the muffler that I was using. I was using about 2.5 oz for ~9 minutes of run time. Ended up enlarging the holes in the muffler (Adamisin style tongue-very good and light by the way) and started getting about 4 oz for 7 minutes. Much easier to needle and markedly more power. I keep this in mind when I am running my engines and use it as a tuning tool.

LAST EDITED ON Nov-17-04 AT 08:06 AM (CST)
 
When we first started using OS 32 and Weber 32 in Classic ships I had a very good point taught to me about back pressure.

One of the "tuners" with a lot of experience in this type engine running 4-2 at higher than the then expected rpm said that increased back pressure "fools" the engine into believing it is lower timed. The charge stays around a little longer.

"Usable" power is not too greatly affected since these engines produce quite a bit more than the OS 35S and Fox 35, anyway.

It all came about from one of the great pilots telling us to block off a lot of the holes on the OS 32 tongue (we ended up blocking 4 holes, plus one with pressure outlet). Engine ran much better and predictably. The first OS 32 we ran this way gave Aaron a great boost in his overall performance.

Bill <><

Leonard,
I thought that reducing back pressure, therby reducing combustion heat, allowed the engine to four cycle. Increasing Back pressure and increasing combustion heat, made it harder to get it to four cycle. Is this correct?

Hi Steve,
Don't know what Leonard is going to say, but I do know that with the OS 32F and Weber 32, each having the blowdown slightly reduced and the heads given a slight hemi shape, run a MUCH more consistent 4-2 break with a lot of back pressure. The Weber is breaking at about 11,000 with a 3.75 pitch prop, while the OS is a little lower on the scale and uses a 4 to 4.5 pitch.
I have no claims to being an engine expert, but I can say that this set up works. These tips came from a excellent engine man who has set up a several of them, and two CLPA world champions.
........"your milage may vary".
Bill <><

I would be reluctant to say there is a general rule on this. First off, remember the first Fox 35 and OS Max engines did not have mufflers. Seemed to work OK then. We have hot plugs and cold plugs. If you use a hot plug you don't need muffler pressure to keep the plug lit. If you are having problems, check your plug.

Now, that said, if you are finding that your engine is running more consistently with more back pressure, go back to my original post. The "problem" may, indeed, be the fact that your venturi is too large. With a little smaller venturi you may find you get the same, consistent, run with a less restrictive muffler.

>I would be reluctant to say there is a general rule on this.

I'm not. Run a Fox 35 open-exhaust. Run it with a tongue muffler. Run it with a Big Art tube-type muffler. Run it with a stock Fox muffler.

You will see a specific trend here, and it's not toward a better and better transition to four-stroke running.

Are there specific exceptions to the rule? Maybe, although I do not know of any. And it is quite likely that any experimentation which refutes the general rule was done with an engine which was not "right" in the first place. Which I why I suggest testing with an open-exhaust Fox 35. They might not be everybody's favorite motor, but there is no question how well they run with open exhaust.

>>I would be reluctant to say there is a general rule on this.
>
>I'm not. Run a Fox 35 open-exhaust. Run it with a tongue
>muffler. Run it with a Big Art tube-type muffler. Run it
>with a stock Fox muffler.
>
>You will see a specific trend here, and it's not toward a
>better and better transition to four-stroke running.
>
>Are there specific exceptions to the rule? Maybe, although I
>do not know of any.

Oh Danny Boy, you do know of an exception to this rule, but probably don't realize it. I guess it goes back to "all rules are meant to be broken".

>And it is quite likely that any
>experimentation which refutes the general rule was done with
>an engine which was not "right" in the first place.

Being that Randy Smith usually builds pretty good engines, I can't believe that the two OS 32Fs I was referring to that he built were both bad. And they weren't messed with otherwise.

> Which I
>why I suggest testing with an open-exhaust Fox 35. They
>might not be everybody's favorite motor, but there is no
>question how well they run with open exhaust.

Could be that because the Fox was designed for no muffler and there have been no changes to that design, or maybe the juxtaposition of the planets is all screwed up...........

>
>Dan

The one you are probably not aware of is the one a certain Mr. Werwage used to win VSC at least once, and I have another one set up exactly like it. For whatever reason, they like back pressure, and it's sorta like the 20FP/Magnum 36 set ups. It wasn't broke so I didn't try to fix it.

With all due respect,
Bill <><

>>>I would be reluctant to say there is a general rule on this.
>>
>>I'm not. Run a Fox 35 open-exhaust. Run it with a tongue
>>muffler. Run it with a Big Art tube-type muffler. Run it
>>with a stock Fox muffler.
>>
>>You will see a specific trend here, and it's not toward a
>>better and better transition to four-stroke running.
>>
>>Are there specific exceptions to the rule? Maybe, although I
>>do not know of any.
>
>Oh Danny Boy, you do know of an exception to this rule, but
>probably don't realize it. I guess it goes back to "all
>rules are meant to be broken".

Bill,

Good try. No, really. The question posed had to do with increased back pressure and a 4-2-4 run. The piece which you obliquely mention does not run in this manner; instead it only cycles from a "wet" to a "less-wet" condition. If one hears such an engine break into a four-stroke, it is well and truly not "right" to begin with.

>
>>And it is quite likely that any
>>experimentation which refutes the general rule was done with
>>an engine which was not "right" in the first place.
>
>Being that Randy Smith usually builds pretty good engines, I
>can't believe that the two OS 32Fs I was referring to that
>he built were both bad. And they weren't messed with
>otherwise.
>
>> Which I
>>why I suggest testing with an open-exhaust Fox 35. They
>>might not be everybody's favorite motor, but there is no
>>question how well they run with open exhaust.
>
>Could be that because the Fox was designed for no muffler
>and there have been no changes to that design, or maybe the
>juxtaposition of the planets is all screwed up...........
>
>>
>>Dan
>
>The one you are probably not aware of is the one a certain
>Mr. Werwage used to win VSC at least once, and I have
>another one set up exactly like it. For whatever reason,
>they like back pressure, and it's sorta like the 20FP/Magnum
>36 set ups. It wasn't broke so I didn't try to fix it.

That's a different discussion. Lots of engines "like" back pressure, the two you mention being real good examples. However, again we are talking about increased back pressure, increased from some sort of norm which is known to work. On the former of the two engines you cite, slip one of those silicone exhaust extensions on the muffler, fly engine back to back with extension off and then back on. Then we'll talk again!

My best,

Dan

>
>With all due respect,
>Bill <><
>
>When the character of a man is not clear to you, look at his
>friends.
>Japanese Proverb