running a 46 fx with a 11/6 macs pipe cut to specs,??????????(this is my first pipe), start eng runs great, full power not so great,it seems like it takes about 4 or 5 secs to come up to full power,is this norm,i have tryed diff needle setting but the same thing happens, fuel tank is set right,fuel tubings is as short as it can be and good press of the pipe.ok that the heck is it or is this norm..........any help would be great

What do you mean "cut to specs"? If you purchased a Macs "pre-tuned" header and pipe combo than your prop is way too big for this engine/pipe combination. You need to run a 10X6. If you bought the header and pipe separately then you should only cut 1/4 inch off the header at a time until the engine stops gaining rpm and still has a good "needle" and clean aceleration. Of course, this all depends on having your carb tuned correctly. The problem could be as simple as being too rich on your low-speed needle.

A 10-6? Are you serious? I thought a 10-6 on a 46 would over rev it. Maybe Im wrong...............John

A 2 stroke engine's transition is dependent on the low speed mixture. Verify it is correctly adjusted.

Also, when running an engine with a standard muffler and you lean it from very rich (4 cycling), to rich 2 cycling, you still have a long way to go, sometimes up to one more turn of the needle, before the engine reaches peak RPM.

When running the same engine with a pipe (or with a tuned muffler) it needs to be richer than it was with the regular muffler.
When the pipe is the correct length and the engine is running very rich (4 cycling), it is not yet on the pipe. As you close the needle and the engine breaks into 2 cycle operation, you can usually have only 1/8 to 1/4 of a turn, sometimes even none at all, to make it peak "on the pipe".

When running on the pipe, the engine uses more fuel and air. That formerly rich 2 cycle needle position equals, or is very close to, peaked piped running position.

If your total tuned length is a little too short, the engine can "get stuck" in rich 2 cycle, just shy of the RPM it needs to get on the pipe. It will get on the pipe if leaned, but will likely be too lean.

I do agree with Handyman that if you are using an 11x6 prop, the total tuned length may be too short, as it is probably peaked for a 10x6, or a 10x7.

Try running the engine with a 10x6, or a 10x7 and see if you get the response you want. If you do, but you want to use the 11x6 prop for better thrust, lengthen the header (longer, thick gauge silicone coupling will do) and mount the pipe further back, to get the longer total tuned length you need.

The lenght difference you need may be a 1/4 inch, or more.

Sincerely,

chuck is correct! you have put too much prop on your engine. Get a 10x6 and you will see the difference.

Don't be afraid to rev your .46. It ain't "over revving" until your pushing well over 17,000 rpm. It's the prop that gets maxed out before the engine. A 10 inch prop will sling a blade before anything lets go in the engine.

Chuck, Jmulder,

The RPM limits on props depends on their producer:

Master Airscrew limit their prop RPM to 166,000/diameter in inches
(a 10" prop is limited to 16,600 RPM...).

Bolly limit their Clubman series props to a calculated tip speed of 475 MPH (do your own math... 15,234 RPM for a 10.5" prop).

APC limit their props to 0.75 Mach tip speed, 190,000/diameter in inches (a 10" prop is limited to 19,000 RPM...).

Even when fitted with a Jett-Stream muffler, on 15% nitro, the 46FX could only manage 15,900 ((c) www.jettengineering.com).
The borderline for Bolly is offset by the fact that their props are all +.5", so you will not use their 10" size, only 10.5", or 9.5", the later with complete safety as its limit is higher.

I have never seen a blade thrown off a previously sound prop, by any engine. It is the prop manufacturer's insurer speaking here.

For those of us who keep on using crashed and badly nicked and mangled props; this is fair warning. A thrown blade can kill, or cause serious injury.

Sincerely,

Don't forget, props unload quite bit in the air. Up to 2,000 rpm or more on a highly tuned engine.

I wasnt thinking about the prop flyin off I was thinking about how long my engine will last running that hard............John

ChuckN, JMulder,

Since most of us poor modeler do not have access to telemetry equipment, we all measure the RPM on the ground.

Every avid modeler and prop manufacturer knows that the unloading in the air increases the RPM, sometimes by 2,000.
I believe the prop manufacturers take this into account, when setting the limit. They simply limit the RPM to about 30% less than the prop was engineered for, just to play it safe.

It would be interesting to do a controlled experiment, to see when a prop will actually let go. This will not be done on a model engine, but rather on an electric centrifuge motor....

For some reason I believe an APC 10" will withstand over 30,000 RPM.... Supersonic speeds at the blade tips.

JMulder; 17,000 RPM is about 11 meters-a-second maximum piston speed.
No problem with engine life.

Sincerely,

Yup! 17,000rpm is not a lot of piston speed. The only thing that will wear out a motor prematurely with a tuned pipe is setting the needle too lean.

Not sure I'd trust any prop that went completely supersonic anywhere along the blade. The shock waves can be very violent. Aircraft wings will twist up (control reversal) if they aren't designed for supersonic flight and the airplane exceeds Mach 1. I'm not sure there any props designed to operate in that realm. Besides, what engine could spin a 10 inch prop 30,000rpm? You would probably need about 15hp

ChuckN,

I was meaning it in a theoretical sense.

If you have listened to any of the larger crop dusters/sprayers, those with large radial engines, as they fly by you. The sawmill sound you hear is their blade tips going supersonic.

Sincerely,

I understand. Didn't mean to sound like I was contradicting you.

"Supersonic" props are an interesting thing. Loud prop blades (I've heard plenty of 'em at Osh Kosh ) are actually going supersonic over just small part of the chord. Not the whole blade chord (width from leading edge to trailing edge). It's the fattest part of the blade on the curved side that goes supersonic first. If the whole width of a prop blade went supersonic anywhere along it's length you would see catastrophic failure.

I did not think you were, no offence taken.

What you are saying is true, but then the rest of the blade tip is transsonic; .98-.99 Mach.
BTW, the prop blade profile is inefficient at supersonic speeds, so it take a lot more power to spin it, but that would not result is any loss of control, just the "sawmill" noise.

Sincerely,

Make sure you are running a cooler plug than you would normally use for sport/muffler use. A lot of people think they can get away with the same Fox R/C plug they have always used and this isn't so. Try a Rossi R5 or 6 or even an OS 8. Plugs make a world of difference once you introduce a pipe into the setup. I would also suggest staying away from 15% or higher nitro fuel in your .46FX.

John

Hi!
Yes stay away from 15% nitro fuel if you use a standard OS .46 engine a 9x7 or 10x6 APC prop and a fullenght pipe.
The pipe will increase compression and this engine is not going to produce the power you want or expect if you don't lower the compression ratio (adding a 0,10mm head shim) or reducing nitro to say 5-10%.
And don't remember to use a cold plug like a Rossi 6-8.
Easiest way to have the engine produce more power than a standard engine does is to get a pre-tuned pipe from Dave Shadel (US YS importer).


Regards!
Jan K

Will using 15% fuel, and an extra head shim generate more power, or just work around problems with the high nitro fuel + pipe combo?

Very timely info here. I will test a Macs quiet pipe for a 46FX any day now, and have also been studying a prop tip speed chart to try to figure out a good prop size that will deliver the desired result (good thrust, not high speed, for an aerobatic plane) with out generating excessively high prop tip speed. I think 11x5 is best. It will run (I expect) near 14500 rpm. That will leave the tip mach number just above 0.6, which I expect will be okay (not too much noise).

I'll find out soon enough how quiet it is. In the mean time, I'm wondering: Is it possible for a quiet pipe such as Mac's to be effective enough at reducing exhaust noise that the major noise source then becomes the propeller?

All very interesting. I think my hobby has become testing mufflers

It's doubtful you will see much power difference by adding head shims and nitro vs. using low nitro and stock head shims. Downunder says he shaves the head to raise the compression when using FAI fuel and can get as much power out of his engines as those of us using nitro.

Personally, I'd go the cheaper route and run lower nitro while leaving the shims stock if at all possible. For me, cheap is king!

John

Thanks for the advice.


Do you think it is an issue with a "quiet pipe". I believe the pipe is a bit milder than a regular tuned pipe. I think it has a flat plate baffle, rather than a converging aft section. I read that on Bolly's web site, not on Macs.

I recently entered a message similar to this one, in another thread.

A fixed length, encased mini-pipe type, tuned muffler is an obviously easier alternative to a full length dual-cone tuned pipe. It will be within about 200 RPM off the "unabridged" version.

Since it is also less costly to manufacture, it is cheaper too, right??? WRONG!

If the Nelson Ultrathrust (TM) is worth $65, then a ball bearing, sport .40 engine (ASP, OS, Magnum, ST and the like) is worth at least $1,000, in terms of production effort, parts and assembly.

I am sure Mr. Nelson has his good reputation to maintain and I am sure a lot of research (?) went into getting all the sizes, diameters, lengths and materials right (knowing the exhaust gas temperature range, displacement range, target RPM range and exhaust port size range, will give you a spot-on estimate...).

Had Mr. Nelson sold this muffler for $25 a pop, he would be selling 20 mufflers for each one he sells now. He would make a much larger profit.
If he moved production elsewhere, he could sell them for $15, sell even more and make still more profit, without cutting down on quality. Nobody will use the standard mufflers any more.

...And we would all be getting our money's worth in performance.

The same goes for Jett Engineering's Jett-Stream (TM) mufflers, although they cost $15 less than the Nelson, to begin with.

Just go into: http://www.mvvs-nl.com to see the MVVS #3248, for less than $23. It is not a full bolt on, but it adds 2,200 RPM to the standard muffler equipped MVVS .49, on an 11x6 MA prop; my own repeated observations.

BTW, both the Nelson and the Jett are excellent, power boosting .40-.50 size mufflers, which are tuned for RPM achievable with 10x6, or 9x7 props. If you opt for more usable 10x7, 10x8, 11x6, or 11x7 prop sizes, the boost you get may be much more modest, if any at all. Some adjustability is good after all.

It is a Rolls Royce syndrome; the more expensive it is, the more people will look at it and the less will actually buy it.

Sincerely,

I think everyone would still use standard mufflers, 'cause those things are loud.

Nelson, Jett, Rolls Royce, et. al. can charge what ever the heck they like.... indeed if they're not interested in increasing supply then raising the price until demand meets their capacity is perfectly logical.

Dar, if you can flood the market with a $25 version, I'll buy two. When you do that, can you make it about the same size as the Ultrathrust/Jett but add a bit more noise supression. Of course, some performance loss would be okay to accomplish that.