Magnum 15XLS Carb Modification for SSC

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Blue Note
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Magnum 15XLS Carb Modification for SSC

Post by Blue Note »

Note (Keith Jones):- The original post seems to have got scrambled during the web-site transition, so for the full text of this report, please look at Chris' re-post dated Feb 8th 2010 further down this thread.
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Ed Kettler
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Post by Ed Kettler »

Excellent work Chris. Any idea on the cost to modify the carb and provide a set of venturis?
gsjames
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Post by gsjames »

Of course, we could just change the rules to allow us to go to 18,000 if the LA'a and the electric guys would agree.
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Blue Note
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Post by Blue Note »

Ed Kettler wrote:Excellent work Chris. Any idea on the cost to modify the carb and provide a set of venturis?
I got the impression that they are planning to release a different carb kit all together. I'm not sure if they may offer some kind of trade in allowance or modification provisions, or of if will be a carb package option from a given point on. As far as costs, Mike gave a couple figured quoted in the original post. My biggest concern if they go that way is how it might effect the total cost of the engine setup for SSC, and as it relates to the restrictions.
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Post by Blue Note »

gsjames wrote:Of course, we could just change the rules to allow us to go to 18,000 if the LA'a and the electric guys would agree.
That would be my preference, but this method is an alternative. Global Hobbies has been great to work with and do indeed seem to be considering our needs. Unfortunately, there are probably a lot of "chiefs " in that camp, so the ultimate solution may be a while in the making.
sgilkey
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Post by sgilkey »

Chris, that's very interesting, thanks! the ease with which those venturi inserts can be popped in might be very nice! I for one long ago gave up trying to get as close as possible to max rpm, the change in temp, atmospheric conditions, etc from round to round can make it a constant chase and swapping head shims, etc usually takes too long so I just look for something reasonable in the 17.2 range and say "good enough". That's why I like the FP, since it does not over-rev and you don't have to choke it off to make rpm. The XLS is a different story and you have to restrict it somehow. I only have one and haven't yet used it in competition for that reason, don't want the hassle. Some day I will try it.

The ability to get easily closer to 17.5k would be nice, in SSC that certainly makes a performance difference.

Several years ago I made a variable exhaust restrictor and posted test results. I wanted something quick and easy to tune. Basically I tapped the side of the tailpipe for a 4-40 bolt and inserted it with a jam nut. You could screw the bolt in and out and snug down the jam nut to vary the restriction. It worked but the action was not linear, you didn't get 100 rpm per turn or something convenient like that, but the nice thing is that it's fast to change, and could even be done while the engine was running. Another approach would be to sleeve the tailpipe using K&S tubing, if you need more restriction just keep inserting the next size smaller tube since all sequential sizes fit inside each other. secure the restrictor sleeves with a small cotter pin or similar. Additional advantage of the exh restrictor is that it improves tank pressure.

Closing the throttle barrel to limit rpm can sometimes lead to difficulty getting a good needle setting- the ground setting does not seem to be optimal for the air, sometimes. HOpefully these carb restrictors will not do that, but since they only restrict the top casting, and not the carb barrel where the spraybar is, that might be worth checking (i.e. in-flight performance check vs. bench testing only).

What somebody needs to invent is a variable intake restrictor that works like the iris on a cammera lens, just dial in the restriction!!! (and make it impervious to congealed fuel, dirt, grass clippings, streamer fragments, and sticky stuff!!!) C'mon Chris, anybody who can play blues guitar can invent one of those in his sleep!!!

Great report and very interesting, thanks for the work!!!
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Post by montague »

I honestly think I was one of the first guys running the .15XLS in competition, I bought a couple of them right when they came out, since I'd been running the .15XL for years and years, and always thought it better than the LA.

To detune my .15 XLS, I have a bit of aluminum tubing JB Welded into the exhaust opening, and 3 head shims. This is usually enough to get down to legal, and has the added benifit of better tank pressure for better fuel draw in high-G turns.

(I noticed right away that the stock XLS does not draw fuel nearly as well as the older XL, and I assume the LA draws better as well. I also shimmed my fuel tanks up a little with the XLS to avoid leaning out in tight turns).

The carb inserts sound great, and combined with a reduced exhaust outlet sound like they'd be perfect.
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Blue Note
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Post by Blue Note »

Part of the reason of doing these tests on the 15XLS is to also determine if it is a viable consideration for the upcoming 25XLS. With the unlimited RPM of Open B compared to the regulated RPM of Limited B, a similar difficulty may arise to be compliant with the rules in regard to RPM limits. If you notice from the spreadsheet, the larger inserts actually yield a performance increase, which can also be useful for some cases. I'm mostly trying to help Magnum to be aware of some of the considerations combat pilots have to deal with if doing any kind of "official" competition.

Have any of you monitored the change in running temperature when restricting the exhaust? As I have hesitated going that route for said reason, I would be interested in you findings.

What about glow plug performance with a given number of head shims and the RPM differences from stock? I hate to repeat these tests if they have already been conducted and documented.
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Post by montague »

I haven't noticed any problems due to temp from the exhaust restriction. But I haven't done anything close to scientific testing, I don't even have a measurement of head temp. It's a small restriction though, just one bit of tubing. I'll have to get some measurements and pictures for you.

I'm also running 3 head shims. Plug life is normal, no issues, haven't replaced a plug this season, but I use K&B HP plugs which I think are a bit more durable than the OS plugs, and a bit colder.

Before going with the exhaust restriction, I'd tried as many as 6 head shims in the engine. At 6 shims, the engine became really hard to keep stable. With 3 shims, It's as easy to tune as stock.

With 3 shims and the exhaust, on some days I still have to run 5% nitro or use the trottle ATV though, the engine is still able to go over limit. I just don't have to close the throttle much, which really helps the in-air performance.

(Of course, if the air is too good, I put the XLS powered planes aside and bring out the XL powered planes. Those old engines never hit 17.5 on the ground but unload like crazy in the air, way more than the XLS or the LA). My "dream" SSC engine would be an old XL body with an XLS carb with a venturi about half way between the XLS and XL. I suspect that would still "bog" on the ground like the XL does, but unload even more.

All the work you're doing is great stuff.
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Post by sgilkey »

Gary James did a ton of plug testing in a 25FX and posted the results somewhere, can't remember where I saw it. Interesting stuff. He gave the merlin glow-bee style the best rpm, the K&B 1L close behind. Personally I only use the OS A3, 8, or K&B 1L. In my experience the K&B does indeed give a couple hundred more rpm in my SSC engines. It also throttles well. THe only other plug I have tried for combat is the Fox 8 which works fine but is rather cold and does not throttle well. I generally use the OS plugs as they are very durable and throttle well and just use the K&B if I have a particularly wheezy FP, some of them have a hard time even hitting 17k. I used to think that was "bad" but have learned to live with it. Seeing some guys with better speed and climb can get frustrating at times but the FPs are consistent and reliable and low-hassle and even in SSC where as-close-as-possible-to-17.5k is a definite benefit, so it predictability, reliability, and low hassle!

Biggest problem with the FP is the rod- older versions have no bronze bushing at either end and they will sometimes gall and the only indication you get is the engine acts tired. OS recently cancelled the FP rod as a service part- thanks, guys! I have found the LA rod "fits" (same center-center distance, wrist pin, and crank pin diameters) but needs some slight grinding to clear the case on the bottom end. The double-bushed LA rod is the same as the CV rod, which is good for north of 30k rpm at 12% oil in r/c cars so it should be good for SSC (at least until I grind the meat off the bottom end....). I just reworked my first LA rod and have yet to test in an engine, that'll have to wait till spring!

I never got the Mag XL to unload like Kirk's. His always ran much better than mine. I like the MagXL and used it a lot until we started flying Avengers, still use them in some of Eric's Piranhas. They just don't unload like Kirk's. THat could be due to compression- I have mine set up with only one thin head shim and I have been told that lower compression engines "unload" better but I have never experimented with it to find out.
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Post by Blue Note »

Thanks for the additional input, guys. It is this kind of information that can help take some of the guess work out of things for newbies, thus reducing the frustration and hopefully increase participation. Club combat may be pretty lax when it comes to rules, but being able to be compliant with the "official" requirements is vital if you plan to branch out. I try to promote "legal" combat as much as possible to avoid having to break people of bad habits down the road, including myself.
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Post by gsjames »

sgilkey wrote:Gary James did a ton of plug testing in a 25FX and posted the results somewhere, can't remember where I saw it.
I put it up on the RC Universe Q-25 pylon racing thread. It also turned out that the OS A3 was the SLOWEST plug that I tested in the 25FX, Sooooooo, that's what I'm currently using in my Magnum 15 XLS's because I need to slow them down. I got some head shims for them today and hopefully will be able to run some head shim tests tomorrow if it's not too cold.
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Post by montague »

I still have been too lame to get a picture of the exhaust mod I have in place on my XLS, but I did measure the tubing insert. It appears to be a bit of .25" o.d. tubing. I know I got it at the local hobby store, so it's K&S tubing, nothing special or hard to find (I think). The i.d. was something like .23" I or something close to that, I think.

About the XL unloading, I usually ran those engines with just one head shim as well, and the K&B HP plug. Usually 15% nitro Omega fuel. The biggest thing with those XL was getting the carb right, and not having the needle too lean, they are such a pain that way, but you knew that. What's funny is that when I first started pulling one of the shims from my XL engines, I didn't noticed that the two shims were different sizes, and the Global Hobbies website doesn't list both. So I think some of my engines had one, and some had the other. I've also tried running with out a shim, that didn't work at all, the finish on the head and top of cylinder is not good enough to seal.

I think the unloading might have a bunch to do with airframe drag as much as anything. A long while back I'd noticed that the Battle Floyd I stared out flying in A-class around here would go a TON faster on the same engine/prop if you just tapered the back of the fuse. I was also one of the first to go to a rather thin airfoil in SSC, even if I do have a non-tapered "plank" wing, and still have the servo sticking out of the top. Compared to the Battle Axe and some Avenger setups, where there is almsot always something with a big, square back-end creating a vacume, I think it's a factor.
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Post by Blue Note »

Thanks for the input, guys. I suspect the real testing will have to wait until the temperature and humidity rise again, but having all my ducks in a row before hand will help. Hopefully, I'll have the opportunity to check out the Magnum 25XLS early spring for the other classes. Time will tell.
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Re: Magnum 15XLS Carb Modification for SSC

Post by midair02 »

Guess what I got in the mail.

Image

Image

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This came with my new engine. Now to order some inserts.


http://www.hobbypeople.net

280022 S15801X Magnum Carburetor Assembly - XLS 15 w/Insert $23.95

280023 S15869-5.5 Magnum Insert - XLS 15 Std 5.5 (for S15801X) $4.00

280027 Magnum Insert Set - XLS 15 (3.2, 3.8, 4.2, 4.7) $12.99
William "Dominator" Drumm III
RCCA Secretary/Treasurer #713
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