[quote]ORIGINAL: DarZeelon
A Mac's OS.46FX, ready-to-use, full length pipe, will only be at its best with 10x6 and 10x7 props and with 10-15% nitro fuel.
Change anything and you could be out of the pipe's RPM range.

Dar,
Your statement is very misleading.

When I speak of using a tuned pipe; I'm assuming the user will adjust the header length for best operation. There is plenty of info about how to tune the pipe right here on RCU. Any of the macs 40 - 45 Pipes can be tuned for 10 to 18K RPM with a 46FX depending on the prop.

Since speed is what he is after; forget the 10" props with a 46FX.
Use a 9" or less diameter prop depending on the airframe's thrust requirement for takeoff. Use 15 to 20% Nitro (We're in the good ole USA Right!) Set the tuned pipe for about 17" (glow plug to end of converging cone) and use a prop pitch that limits RPM to about 15K on the ground.
With this setup you will be much faster than with a tuned muffler The tuned mufflers do have a wider band over which they add a boost but the boost they add is much smaller.

Esten....Qoute...

"Since speed is what he is after; forget the 10" props with a 46FX".

I agree....even my long-piped Rossi .45 wouldn't pull a 10-6 APC....[X(]

"Coming on the pipe" means a 2K boost on the top end....(aprox)...
...that will only happen at full throttle, and only if conditions are just right.

Dave.

For the MVVS GAS question, A tuned Exhaust will enhance your performance. Go for it. For everyone else, each has hit upon very valid points and we all learn from them. The biggest point to consider is that a tuned pipe can be adjusted for different applications. Typically this is done by the experts. With joe average, A MACS pipe is used for max speed on 2 stroke glow motors. Other pipes are available for other apps. On gas motors the Tuned exhaust is to make the motor responsive at all power settings with a benefit of a slight rpm increase at WOT.

For the original question that started this thread. MACS makes a pre-tuned pipe/header package for your motor. Try a 10x7 or 10x8 APC with it and go from there.

Macs pretuned pipe is tuned for a 10x6.I get 15500 on my 46fx

There are two different principles here that seem to be a little confused in this thread. A tuned pipe is an exhaust system that will create a standing wave (acoustic) in the pipe at a partiular RPM that results in a reduced pressure at the engine exhaust port. In a properly designed system this back pressure can be below ambient. This reduced back pressure will result in a good increase in power output at that rpm. Since the back pressure can be below ambient, the power may be more than that available when operating with an open port. The pipe is designed to have a fairly broad peak so that the power increase occurs over a band of rpm, but the band is necessarily rather narrow. It is most effective for continious operation at full throttle (i.e. racing).

A tuned exhaust is an exhaust system designed to create acoustic interferance (not a standing wave) over a band of frequences to reduce the noise in the most irritating frequency range. This usually results in less back pressure than a standard muffler which will allow the engine to develop a little more power. The back pressure is always more than ambient, so though the power may be more than with a standard muffler, it is never quite as much as with an open port.

Though they both contain the word tuned, they are two different principles.

LouW,

May I suggest you read Dr. Blair's SAE Paper on Two Cycle Tuned Pipe Design (Dr. Blair invented the two stroke tuned pipe about 1950 at Queens University, Belfast) OR If you can't find that; Gordon Jennings gives a good description of pipe operation in his "Two Stroke Tuners Handbook"

I think you would find them enlightening!

Mostly WOT??
Mhhh I guess you're still in the 80's!

Esten,

I was talking about the pre-tuned, bolt-on, ready to use variety, not the cut-to-length-to-tune-for-your-needs type.

Mac's has both and I was talking about the former. But even the later, once you have tuned it, if you change your prop, you are "back in the woods". The better tuned, mini-pipe type silencers do have the flexibility that a full length pipe doesn't.

Some of these tuned silencers are within only 150-200 RPM off the full length tuned pipe.
Clarence Lee's tests of the MVVS .61 and .77 in RCM revealed such results.

Spicoli, you finally agreed with me on something.

I'm glad LouW chimed in. The standing wave mentioned has the potential to improve scavenging at the tuning frequency due to a reduction of back-pressure or the formation of a partial vacuum. The tuning frequency can be anything the tuner wants, low or high. What I'm unclear on is how things like port timing and throttle position effect scavenging at the pipe tuning frequency. Does conservative timing and partial throttle erase the effect of the standing wave while aggressive timing and WOT can maximize the effect? Seems reasonable.

I considered putting one on my 1.60FX like the pattern boys but decided the benefit was slim compared to the cost.

Macs pre-tuned pipes are for a specific engine, not a specific prop! You will get better pull on that pipe usung a 10x8. If you really want to wind the motor out and get every ounce out of it then use a 9x9 or 9x10. Tuned pipes are not tuned to props....

Deputydog,
A Macs pretuned is not tuned to a specific engine.The manual that came with mine said it is good for about a dozen 45-48 size engines as long as you use a 10x6
.
Why would the manual tell you to use a 10x6?

Let's see...what was the question Joe asked anyway???
Joe I have used tuned pipes for yrs. because there is some increase at some rpm which depends on pipe length(and tuning a pipe for a particular small range of props is not that hard) but I mostly use the muffled pipes from Macs because they sound better, are guiter, and keep my plane one hell of a lot cleaner.

When the ehaust pulse hits the back of the pipe it bounces back into the cyl. essentially supercharging the engine.

The theoretical way a tuned pipe/silencer works, has been explained again and again in RCU.
I wrote a full explanation four, or five months ago.

You could search the old RCU pages for these explanations.


The matter is: a high RPM tuned exhaust system allows the engine to combust 30-50% more fuel-air mixture, than it would with a standard, expansion chamber muffler (15-30 more than with an open exhaust).

The engine thus can produce more horsepower to the same ratio.

The gain in horsepower is translated to the third power, in RPM terms.

If an engine's RPM with a given prop is raised, say, from 12,600 to 14,400, the horsepower gain is not 14%, but 49%.


A high torque, low RPM pipe, like the Bolly pipe in the MAN engine review for the ST .90, allowed the engine to make the same maximum horsepower at 20% lower RPM. This means a torque (and BMEP) gain of 25%, at the respective RPM of maximum HP.
Very respectable, from where I am looking.

So, If I am looking to maximize the performance of a MVVS 2.15 gas engine that will be used primarily in a 3d application on a profile plane. What pipe/muffler should I be looking for?

Crash,

You need the #3268 MVVS full length tuned pipe, for the 35cc engine, with the #3269 adapter.

You can just bolt it on.

Is the 3268S the same thing?

Yes, Crash.

It is just a matter of names and numbers; "S" signifying "Side".

Just buy a tower hobbies .46 muffler. They are excellent on the .46fx.

thanks for all the responses. i still dont know what to do. there are so many opinions everybody gave. Guess ill have to do so more research

look at all the modern mx bikes and how they use real pipes and gnarly port timing to make loads of power on the top end. everyone knows that this is a recipe for a torque-less low end and sick transition with any kind of load, right? they overcome this shortcoming by using a "power valve" to effectively lower the exhaust timing by restricting it at lower throttle settings. now they have loads of torque down low, lightning quick transition and a powerful top end.

reminds me of the exhaust throttles i played around with on boat engines years ago.

dave

Joe,

From all the replies you must understand that a tuned exhaust system does make a big difference.
Its effect is, of course, at mid-high to high RPM.

If you select a correct tuned silencer system (and adjust the low end needle properly), the idle and transition will remain great.

So the only major down side of such a system is cost (the weight penalty should be very slight).

If you select a Jett-Stream, or an Ultrathrust, the toll will be highest, the Tower/Hobbico muffler will be the the most cost effective. All other possibilities are somewhere in between.

Spicoli;
Maybe we are talking 2 different things! The black pretuned mufflers they sell can be used on many different engines. I'm talking about a tuned pipe and header. Typically you have to cut the header to tune the pipe to the engine. Macs offers a pretuned package that is for specific engines due to the header fitting that engine. Granted an OS will probably fit a magnum but it wont fit a Fox.
You go nowhere with a 10x6 prop! Normally a pipe on glow motors is for speed and you want hi pitch props to take advantage of it. Most speed plane use low diameter, hi pitch props. Sorry if there was some confusion.

Deputydog,
I have the pretuned pipe and header.The manual calls for a 10x6.It is not for a specific engine.They list about a dozen motors and only one prop.They are pretuned for a certain rpm which a 10x6 gives you.Using a higher pitch will work as long as it has the same load as a 10x6
Do you have one of their pretuned tuned pipes?
Did you read the manual?
The macs muffler is not tuned for anything.