Looking for the correct tuned pipe for my .91fx. I was looking at Macs and they have a 10mp for .61 and a 15mp for .91-1.20
When I look at the tuning info they have I see that they are tuning the .61's at 14,500rpm with the 10mp and the 1.20's at 10,500 with the 15mp.
So if I put the 10mp on my .91 it should tune at 9,700 and the 15mp should tune at 13,800. If I want to use a larger prop and tune my motor for grunt I would think that I should use the 10mp pipe. Right now the motor turns a 15 x 6 apc 10,500 with stock muffler and removed baffle and 11,700 with an Ultra thrust. I would hope that if I used the 10mp tuned with gradual back angle I would be able to run a 16 x 6 or 16 x5 close to the 10,000 rpm range. I have been trying to learn about pipes so someone that knows them please advise me if I am thinking correctly.

GPSpacewalker.
My experience with the 91FX is as follows. This engine is timed for "Sport" use. The exhaust port timing on this engine is very sedate at Aprox' 145 Degrees. It is exceptionally hard to get this engine to work on pipe correctly. You really need an exhaust port timing of around 155 Degrees to get it to work well.
Your best bet is to run an APC 13 x 10 with 10% Nitro. This will Tach at around 9100.

Coo-ee
CP

Thank you for the advise. Does that mean that I am correct in thinking that I should use the 61 sized (10mp) tuned pipe that is made to run at 14,500 with a .61 and not the advertised 15mp pipe that is made to run 10,500 with a 1.20? Also, when you say that this motor will not run on a pipe correctly do you mean that it will not develop good H.P. on the 15mp pipe revving at 13,800? If I choose a pipe that has a lesser included angle on the back of the pipe, that means that the pipe will be less peaky--correct? -also it probably will develop less peak impact??

Good evening,

I'm running a OS 91 fx with the macs 1062 pipe and a KM header.. Prop APC 15x10 prop at 8900 rpm or the 14x10 at 10,000rpm with 25 % cool power and the OS F plug, fuel tank is on the Cg and no pump or header tank just pipe pressure.. This moves the 8 pound pattern plane very nice.. great vertical... Your pipe lenght should be about 21 inches to the first baffel.. Some times you may have to add a head shim or two like .007 works good....My engine is all stock !!!

Thank you very much for the info. How come you are not running an 1160; is it because it is to loud? . I see that the 1062 is for the .61 to .75 long stroke. What is different between the 1062 and the 1060? The spects show the same weight and length. Is it cone shape? Nice looking header, where did you get it and how long is it?
Wow 8900 with a 15 x10!

The 1160 is a non muffled pipe, so noise is a issue.. the 1062 long stroke pipes baffle is further back. The 1090 is longer and more money.. both work well... The headre is a Karl Mueller header which is very well made header and is the only i will use... If you need one I may have access to one at $47.00 plus shipping. Let me know..This engine is a power house and people are coming back around to two stroke for pattern.. I think if OS made this a pumped engine it would be a kick butt setup..
I uploaded a picture of the header for you..


Scott

Hey Scott

I'm also interested in the KM header-not for the 91Fx but for some rear exhaust
engines that I need an S-shaped header to route the pipe the same direction as a side exhaust motor. Does Karl Mueller have a website? address? phone number? where he can be reached?

Many Thanks--Windwalker

Scott,

I have no need for the header at this point, but may end up getting a pattern plane eventually. Thank you very much for the header picture with the scale next to it--it looks like your header is quite long. Have you ever tried anything other than a 15 x 10 or 14 x 10?

Thanks,

Steve

windwalker,

I will see what info I can get for you.. Give me a few days I'm at my full time job this weekend...What engines and drops/rises and shapes are you looking for ?

scott

GPSacewalker,

I"m going to try a 14x11 also I have been talking with APC about a 4 blade prop.. A fair amount of the guy around here have been using 4 bladed props...may not get as much thrust but plane flies slower...The Viper series is a very good way to get into pattern the 91 size is around 495.00 and with the use of a 2 stroke you save alot right there.. If you want more info on the viper let me know...

PatternPilot you wrote that you use OS F type plug. I think this is for four cycles, is there a good reason to use it in a two cycle ?

Obolel,

We have found that it last longer and holds up to the Nitro better, I have tried the OS number 8 I think it was ( the plug that they reccomend ? ) and I had proplems with it, In the early stages of setting up the engine I did have the Iron bay models fuel regulatior in place for the first 18 flight during testing and we would go thru a plug about evert 2 flights , then the night before the contest we where showing this at, almost lost the plane from the regulator, in a mad scramble we went to basics and the first round of the contest was also a test flight.. The set up is: OS91 fx with a KM header and macs 1062 pipe with 25% nitro and OS F plug and and a 15x10 or 14x10 APC prop, fuel tank is on the CG and ONLY pipe pressure and NO head shims for me.. Also I have use some other plugs since and I just keep going back to the OS F plug.
The photo is Viper OS project 1 and 2. Mine is OS Viper # 1 ( red/white/blue)

scott

Steve,

sent you some pm also

scott

With over 22 years in RC boats I would like to add to this discussion.

1. If and engine's exhaust timing is not at least 160 degrees or more a pipe with nitro of 25% or more will not do much of anything but make needle settings hard to hold.

2. If the exhaust port timing is changed to anything beyond the 160-170 degree range THINGS WILL PICK UP IMMEDIATELY with a tuned pipe BUT intake timing may need to be addressed also. Sometimes a larger carb can help some.

3. We NEVER put a smaller pipe on a larger engine. It is just not a practice that will get you much with higher nitro. We DID put larger pipes on smaller engines and set them at a longer distance to center of pipe. It will however effect needle settings.
4. Check the timing first , if it is below 160, get that changed, use 25%nitro, and a pipe setting of around 12 inches and work back from there.

I have an ASP 91 generation 3 Sport Marine that I changed the timing on, put on a marine nitro quiet pipe, and 20% nitro with a head clearance of .010 and it runs as well as an old OS 91 Marine I used to have.

Just something to think about.
Regards Curt

One other thing I forgot to add, without a higher nitro head button that usually has a deeper combustion concave, the nitro will be pretty hard on glow plugs, and idle. The head button design is VERY important as you raise the nitro/pipe factors into an engine.

Regards
Curt

crrcboatz,

Nice to read a post from someone thst understands what makes 2 cycle glow perform

Curt,

I believe it was determined that the duration of the blow-down period, is more consequential to the ability of the engine to make a gain from a tuned pipe, than is simply the exhaust timing.

The reason a long exhaust duration works well is because if the intake bypass duration is not also lengthened, the blow-down period is.

This gives the pipe enough time to cram more of the fresh mixture, back into the cylinder, without this extra charge escaping back to the crankcase. A longer duration exhaust is necessary for breathing, at high RPM.

If a pipe is intended to boost engine power to 16,000 RPM and northward, you do need 160 degrees and more exhaust duration.

For high boost at 9,000 RPM, 145-150 degrees can be very usable, given the intake bypass period is short enough for a usable blow-down.

The pipe that you need for this is longer, of course.

I'm looking someone who makes S-shaped headers for RE engines so that if the engine is upright the pipe runs down the side of the fuse between the canopy and the wing--or if the engine is mounted on it's side the pipe runs under the fuse; the same direction a SE engine w/regular header would, with the clearance between 1.25" and 2.0" (hope this description makes sense, if not let me know & I'll try to explain further)

Fly Hard, Land Soft
Windwalker

Mr. DarZeelon,

Please inform.. Do you mean that length from the piston to the middle of the convergent cone is longer for a motor that tunes at 9,000 than at 16,000? Please look at my first post. How important is residence volume in the equation --10mp pipe compared to 15mp pipe? Also, does a greater included angle of the convergent cone work better will lower exhaust timing? The ultra-thrust muffler had a great impact on performance of this engine in the 10,000-12,000 rpm range. How does this pipe work? Is it a 1/2 or1/4 wave pipe? Thank you for your help. How about Jett in cowl mufflers?

Thanks,
Steve

Curt,


This was not the main issue of my post. The blow-down period was, saying exhaust duration is not alone here.

As to your question; I was saying that a tuned pipe of a given design (divergent-convergent cones F.E.) must be longer for low RPM than it needs to be for high RPM.

Please see this web page: http://www.mh-aerotools.de/airfoils/...pe_main_en.htm .


GPSpacewalker,

What you wrote about the Nelson UltraThrust surprizes me.
Its Jett counterpart, the Jettstream, is admitedly biased toward the 15K-18K RPM range.

When mounted on Jett .40-.50 engines, its performance with larger 10x8 to 11x7 props, could at best be considered average.

In his tech-sheets on tuned pipes, Dub Jett wrote that on the Jett .46 with a 10x6 prop, a full length tuned pipe, may realize a gain of 300 RPM over the already tuned Jettstream. This, while on larger props, about 1,500 gain is realistic, when changing to the full length pipe. With the Jettstream this prop is turned at only 12K.

This would be a direct indication that the Jettstream is not tuned to resonate in the 12K-13K RPM range.
It surprizes me that that you wrote that the Nelson UltraThrust is.

Thank you for the info. Very good links.

Steve

DeZeen,

I did not make any statements regarding any of the statements you made in your last post. You must be mistaken.My experience is with boats not aero. motors. Your comments about blow down, etc are not of interest to me. I rely on my experience and background of the marine motors I have worked with over the years. I simply suggest that much of that could apply to aero motors also.

regards Curt

Curt,
IT DOES APPLY! Same laws of Physics apply to Airlpanes, Boats, Cars, Motorcycles, and everything else.

Thanks for all the info! I still need to know more-so if you run into info please send it my way.

Thanks,

Steve

crrcboatz - The requirements from a boat engine and an aero engine are very diffrent. You guys need max top end and spin 20K+. We fly at 50-60% throttle 90% of the time. Midrange and fast transition from idle and back are key for us. Unless you are talking extreme speed planes.