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Exhaust Extensions..... RPM???

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Old 03-04-2012, 05:32 PM
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_Tommy D
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Default Exhaust Extensions..... RPM???

Guys

I'm finding in many cases I need to add muffler extension/spacers to use performance style mufflers. My question is does the addition of these spacers hurt the RPMs I'm trying to increase?

Thanks

Tommy D
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Old 03-04-2012, 07:24 PM
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Default RE: Exhaust Extensions..... RPM???

Good question. I am curious also as my current build requires my to double the distance to the muffler to clear the cowl.
I don't know the answer, but somebody will likely chime in.
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Old 03-04-2012, 07:54 PM
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Default RE: Exhaust Extensions..... RPM???

Normally the muffler extensions do not hurt the RPMs put out by the engine. With regular mufflers, there is no gain or loss of RPMs using extensions.
With a tuned muffler it could cause it to lower it tuning point slightly, but you probably will not be able to notice the difference, as it will be in the varying RPM readings you get when you check the RPMs using a tachmometer. If you could measure it, you are looking at about a 25 rpms gain as the extension increases the muffler volume slightly, so you really have a slight improvement. With a tuned pipe you could see a difference, but since you move the tuned pipe in or out, it would negate any effects from the extension.
You could cause a problem if the exhaust outlet through the extension is much smaller than the exhaust outlet on the muffler and engine. Ideally you want a smooth transition for the exhaust to flow through into the muffler. But you should be able to use some handfiles to fix that problem if you have it.

Years ago, someone found that using a long exhaust pipe tube off the end outlet of a muffler could cause 50 RPMs per inch loss or 100 rpms per inch loss. It really was affected by the tube diameter, where a smaller diameter tube was more restrictive than a larger diameter tube. Thus with a tube that just fits over the muffler exhaust outlet could cause a 100 rpms loss per inch, so if you had a 12 inch tube off the engine, it would cause about a 1200 rpms loss. The short angled silicon muffler deflector tubes guys put on some engines actually reduce engine noise slightly and cause about a 100 rpms drop in performance.

Also some mufflers are pretty restrictive already, you could loose as much as 2,500 to 3,000 rpms loss with some mufflers versus a open exhaust or no muffler. So adding extensions, deflectors, etc. may not really subtract all that much more performance from a particular engine setup.


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Old 03-05-2012, 04:44 AM
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Default RE: Exhaust Extensions..... RPM???

Thanks earlwb for your reply, and thank youTommy D for aasking the question.
I did not think about what the effect of my extension would be untilTommy D posed his question.
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Old 03-05-2012, 09:31 AM
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Default RE: Exhaust Extensions..... RPM???

I use a lot of the little rubber exhaust deflectors sold by DuBro and Tower Hobbies. I've never noticed any power loss but even if there is a loss of just a few rpms it's worth it to me to help keep the goo off of the plane. Some mufflers don't have the groove to help keep that deflector from coming off. You can build up a little ring with JB weld on the end of the muffler real easy that will keep them on there. Be sure to clean all the oil off with acetone and use strong zip ties..... and use a little high temp (red) silicone rubber too.

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Old 03-05-2012, 09:33 AM
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Default RE: Exhaust Extensions..... RPM???

The best thing to do is to check RPM with a tach and make changes to lengths of header ext, or tailpipe ext. Restrictions usually slow things down but different lengths can improve things.Tailpipe extensions are usually bigger than the actual hole size and can be afair bit longer to a point, but again check length in stages with a tach.On tuned pipes, a smaller outlet dia. can make it come on to the pipe easier, and doesn't really restrict the revs. It may run hotter because of this. If more power is needed with a pipe or tuned muffler, exhaust timing can be changed to take greater advantage of this, at the expense of idling and mixture problems. To keep tailpipe extensions on, a short 2-56 screw with a round head can be threaded into the rear of the tailpipe and a tie wrap used to hold it on as well. The castings are usually tapered the wrong way to make the castings release properly.
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Old 03-05-2012, 01:44 PM
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Default RE: Exhaust Extensions..... RPM???

I think it is a really complex issue.  Logic leads us to assume more length is more restrictive. 

But then there are tuning issues that can enhance engine performance too and tuned  pipes are not short by a long shot.  All I do know is I have observed a buddy that used an extensive extention slightly larger in diameter than the stinger out of his muffler.  He did not suffer a great deal of degradation.  In fact, it would have been hard to measure much difference.  And he had added about 10 inches to the back of the muffler. 

So if you are concerned with losing 10 to 100 RPM, I would suggest you do not do anything.  If on the other hand, 100 RPM does not make any difference to how your airplane performs for you, go for it.
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Old 03-05-2012, 02:11 PM
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Default RE: Exhaust Extensions..... RPM???

+1
A lot depends on the muffler though. Most non-tuned systems however are very tolerant of extensions.
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Old 03-05-2012, 03:44 PM
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Default RE: Exhaust Extensions..... RPM???


Quote:
ORIGINAL: _Tommy D

Guys

I'm finding in many cases I need to add muffler extension/spacers to use performance style mufflers. My question is does the addition of these spacers hurt the RPMs I'm trying to increase?

Thanks

Tommy D
Umm, are we talking about 'spacers' as in spacing out the distance between the engine and the tuned muffler?

If so then the longer the header section the lower the operating rpm band will be due to the time it takes to travel from the plugs centre to the first reflective wall in the pipe and back again has to match the engines exhaust opening period.

Any extension past the first reflective wall such as a longer stinger will do similar but to a lesser effect. but is a DuBro style silicon extension really a stinger extension? I don't think so as its internal diameter changes and it tends to simply become more resistance to the gas flow upping the back pressure.

Cheers.

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Old 03-05-2012, 07:13 PM
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Default RE: Exhaust Extensions..... RPM???

Experiment.  A longer extension will mean a lower rpm and verse visa... if it was even tuned close in the first place. Prop selection could be the answer to any changes.  Use a tach. on every change with a couple of desired props and note the difference.  Likely there will be very little difference with mild extensions.
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