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  1. #1

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    Magnum 91 XLS 2-Stroke power?

    Anyone have any accurate tach readings on this engine? Coudn't find anything in the tachometer readings section. Would like to know what the engine will turn a 13x6MAS specifically. Include fuel used and air temps also if possible. Thanks!

  2. #2

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    RE: Magnum 91 XLS 2-Stroke power?

    G'day Mate,
    I hope a 91 would turn a bigger prop than that, I use a 13x6 on my 61FX, try a 14x6 or 15x6, & don't worry about the revs, just fly it, the performance of the plane will tell the tale, not the expected revs of the motor.
    Good Flyin Mate. Keep Thyne Airspeed up, lest the ground arise & smite thee,
    Allan.

  3. #3

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    RE: Magnum 91 XLS 2-Stroke power?

    Wanting the information to compare it to an ST G90.

  4. #4

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    RE: Magnum 91 XLS 2-Stroke power?

    G'day Mate,
    If you want a great motor, get an OS 91FX, but if a bit expensive, then I'd recommend the Magnum over the Super Tigre.
    They just run better & no need to change the carby.
    Good Flyin Mate. Keep Thyne Airspeed up, lest the ground arise & smite thee,
    Allan.

  5. #5
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    RE: Magnum 91 XLS 2-Stroke power?

    ....the Magnum, the ASP, and the OS .91 two strokes are all just bored out 60 engines.
    They do not pull big props in any way, shape, or form....you will have to find this out
    for yourself.

    As an example....the ASP .91 (same as the Magnum .91) calls for one prop in the
    instructions....guess what ? A 12-6....but wazzup with that....that's a 60 sized prop !!

    Yep....in fact....I have a n OS .61 RF engine that pulls a bigger prop, a 12-8.

    If you think those engines are going to pull big props, you will be disappointed. [:@]

    The S.T. is a better bet for bigger props.

    FBD.
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    An engineer says.... "That will not work".
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    Old age and treachery will overcome youth and skill.
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  6. #6

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    RE: Magnum 91 XLS 2-Stroke power?

    My 91 fx turns APC 13-6 at 12800 rpm and MAS Scimitar 14-6 and 13-8 both at 12500 rpm, MAS K-series 14-6 at 11700 rpm. I have fitted a Tower 75 muffler and a OS 60L carb. When stock the engine would almost not run at all due to the old and crappy version of remote needle. My guess is that the newer 91 fx will turn MAS 13-6 at around 12500 and I wouldn`t be surprised if the Magnum is a little stronger

  7. #7

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    RE: Magnum 91 XLS 2-Stroke power?

    G'day Mate,
    OS recomended prop size for 91FX, starts at a 15x8 & goes up to a 17x6.
    http://www.os-engines.co.jp/english/...e/proindex.htm
    Magnum recomend a 14x6 for break in & a 14x8 up to a 15x6 after that.
    I know what you are gunna say, that's too much prop, it won't scream it's head off, & so on, but they work well no matter what you say.
    Good Flyin Mate. Keep Thyne Airspeed up, lest the ground arise & smite thee,
    Allan.

  8. #8
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    RE: Magnum 91 XLS 2-Stroke power?

    alan....don't believe everything you read....I know this for a fact....a buddy of mine
    had an OS .91 FX....what a dog....

    ....it will not pull a 14-8 in any way shape, or form, in fact....it wouldn't even pull a
    13-8.....try one for yourself....I ain't makin' this stuff up.

    Probably one of the main reasons the OS .91FX was a dud was the fact they were
    over propped....big time. Over heating did not help the reputation of the engine.

    It is nothing but a bored out 60 engine. I have the "clone version" of this engine.

    Thust me Alan.....they won't pull crap for a prop....

    FBD.
    An engineer says.... "That will not work".
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  9. #9
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    RE: Magnum 91 XLS 2-Stroke power?

    If anybody wants a good and cheapest 91 2 stroke,then take a look at:
    http://www.rcuniverse.com/forum/m_4562154/tm.htm
    Especially Ed Moorman s comments on SK 90
    Less Prop is 15-6 for it.
    GlowHead Brotherhood #31

  10. #10

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    RE: Magnum 91 XLS 2-Stroke power?

    17-6??? wooohoooo!!! fry that sucker

  11. #11

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    RE: Magnum 91 XLS 2-Stroke power?

    G'day Dave,
    Our Club President has an OS 91FX, in an Edge 540, he was flying it today, & he is using a 15x6APC, runs sweet, he has also tried a 16x6, ran nice , 15x6 suited his flying style, they are definately not dogs, even if they are, just bored out 60's, they are still 15cc's & as you guys like to say, there is no substitute for cubic inches.
    Maybe it has to do with tuning, or installation, whatever it is, the OS 91FX is a great motor.
    And I'm sure the Magnum is good too.
    Good Flyin Mate. Keep Thyne Airspeed up, lest the ground arise & smite thee,
    Allan.

  12. #12
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    RE: Magnum 91 XLS 2-Stroke power?

    I am going the oppisite direction with mine. I have Ultra thrust (Short racing) muffler which will be installed on mine turning a APC 12-8. I hoping for 13,000+++ static. It is going in a WM Midget Mustang.

  13. #13
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    RE: Magnum 91 XLS 2-Stroke power?

    I've been around all three of the engines, the OS, the Magnum, and my ASP. I think
    my ASP is the strongest, because it is the high compression Euro model. It runs on
    5% nitro, it detonated like all heck on 15% and a light 13-7 prop. [:@]

    In fact, I was surprised I didn't damage the engine, I never heard an engine knock
    like that, and it blew two plugs on the first two runs....a Fox Miracle plug, and a fairly
    cold R4 Rossi plug. I figured out the problem, and switched to 5% Power Master fuel,
    and a K&B 1L plug, and a 12-7 wood Zinger. It was still detonating (believe it or not)
    on the low end a tad, and I had to richen the idle a touch to make it stop.

    It does run quite strong, I know stronger than the OS .91. A Guy had one in a Cermark
    F-16 prop jet. He had the Macs mini pipe on it, running on 15% nitro. The thing just
    wouldn't pull any revs. Finally I put a 13-6 prop on it, and it was still a Dog. The guy ran
    the engine about 4 or five tries....couldn't get any performance out of it....pulled the
    engine off and sold it, and gave me the plane for some work I did for him on a couple
    of his planes. We couldn't have been more disappointed in the performance (or lack of)
    from the OS. He had over a thousand dollars in that plane.

    A Tower .75 will whoop the pants off the OS .91 for speed and performance, no doubt.

    I saw another Guy with the Magnum .91 in a Extra 300 (a plane of that type) brand new.
    I was helping him. He had one dead stick on take off....the engine just quit at about 50 feet.
    He had it cowled....too much prop....with a Pitts muffler....and running on 15% Cruel Power
    (green death) fuel. You can guess where this is going.

    The second flight it died again....same 50 feet. The third flight he wanted me to try it, so I did.
    I set the needle a tad richer than normal....smooth take-off....engines quits again, same thing.
    This time....three deadsticks....the engine was devoid of compression....shot. [X(]

    The owner was not pleased....he left....said he was going to take the engine back to Hobby
    People....I never saw him again.

    This is why I say to forget the big props on those engines. No one I know has had any luck with
    big props. I'm running a 12-7 on mine....it calls for a 12-6....

    ....yes, I know that is a 60 sized prop.

    FBD.
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    An engineer says.... "That will not work".
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    Old age and treachery will overcome youth and skill.
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  14. #14

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    RE: Magnum 91 XLS 2-Stroke power?

    I have had a Magnum 91XLS for about 3 years. It has always been easy hand starting, easy to tune and lots of power. For the past 5 months, I have been running it on a Seagull 540 Edge (vII) turning a 15X6 prop (presently Pro Zinger) on 5% Coolpower. I think this is probably the upper range of prop for this engine, but it flys the plane terrifically. I am totally pleased with the performance.
    Sometimes, things are exactly as they appear to be.

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    RE: Magnum 91 XLS 2-Stroke power?

    Never in the history of RC has one engine (the FX .91) been the subject of so many internet myths!

    I have seen about half a dozen at our field and have operated two others, so here's what I've seen...

    Let's start with a couple...

    1) It's a .60 size bored out to .91 so it's a dog
    Try telling Dub Jett or any of his customers that a Jett .90L or .90 LX engine (a .60 sized block bored out to .90) is a dog. I am sure this concept will be a source of great mirth for these guys. The OS VR DF .91 - there's another ".60 block bored out to .91" - must be a dog too! It's time to move on from this myth guys - seriously. Some .91 engines in this form factor provide just adequate performance and some others are performance superstars - this old cover-all myth just does not apply anymore.

    2) ....it will not pull a 14-8 in any way shape, or form, in fact....it wouldn't even pull a 13-8.....try one for yourself....I ain't makin' this stuff up.

    Well, here's some tach figures for your edification.

    OS .91 FX, immediately after break-in, stock muffler...

    APC 14x8 - 9700 rpm
    APC 13x10 - 10,000 rpm
    APC 14x10 - 9,000 rpm

    The APC 13x8 spins in the early 11s and the APC 12x8 (and a 13x6) spins at just over 12k.

    This performance exceeds that of all the .75 two-strokes we've ever tested (ST, Tower, GMS) by a healthy margin. It also exceeds any four-stroke (except the YS 1.10) under 1.20 we've ever tested.

    Put a tuned muffler or a UT/Jett style muffler (of the appropriate length for the load) on the .91 and you can add about 800-1000 rpm to these numbers.

    Sure, there have been a few internet based accounts of problems with early remote needles with these engines, but having seen 7 or 8 of these engines in regular service in our club, I have yet to see a poor performing, unreliable one.

  16. #16
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    RE: Magnum 91 XLS 2-Stroke power?

    FBD,
    Did you ever try adding a head shim to lower the compression if yours was detonating? I will agreee with you on the Tower .75 being strong, but I do not think it will run with a Maggnum .91XLS or OS .91FX. I have seen several of both the Magnum and the OS (old style) and they appear to run really good. The problem is, I have never seen them on little props like 12-8's. All of the ones I have seen have been running Pitts mufflers with 14-6's and 15-6's. I am uncorking my .91XLS with a (SHORT) Ultra Thrust muffler and a APC 12-8. Performance specialties claims 13,000+++ on a good OS .91fx. It should make the Midget Mustang haul A**.

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    RE: Magnum 91 XLS 2-Stroke power?

    Motorman,

    Be sure to get back to us with your findings. If the Magnum you are referring to is the one that is basically a clone of the FX .91, it should perform pretty much the same as the FX .91 I would imagine.

    13,000 rpm with an APC 12x8 should be within reach of a good .91 two-stroke on a tuned pipe.

  18. #18

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    RE: Magnum 91 XLS 2-Stroke power?

    Well it looks like this thread got hi-jacked as usual. Anyway, thanks to Jim Dines and Motorman I at least got a couple of hints. Would like to know what you guys have seen as far as this particular engine, MAGNUM 91, and its ability to pull different props. As far as the almighty OS 91FX goes.....my Super Tigre G90 on a draggy Ultra Stick will keep up with the OS on a Matt Chapman Cap 580, both engines pulling identical props and filled with 16 OZ tanks. Check my post on this in the tachometer section for the ST G90. Thats the primary reason for the question, my G90 got crashed and has lost power and was looking at the non-ringed Magnum as a possible sneaky replacement! The engine is an ABC engine and should pull some good numbers.

  19. #19
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    RE: Magnum 91 XLS 2-Stroke power?

    I have some bad news for Harry Lagman....they must have some real dog engines
    down in his neighborhood if he thinks the OS 91FX has good power....

    OS .91 FX, immediately after break-in, stock muffler...

    APC 14x8 - 9700 rpm
    Look down to the specs on the old OS 61 pattern engine with the same 14-8 prop.

    9691. Let's round it up to 9700 RPM shall we ? The big bad FX won't even
    out pull a good 61....not even with a 50% advantage in displacement, the Fx has
    no more power than the .61.

    By the way, a 14-8 is an insane prop for a .61, and the old OS will hang with the
    newest dog in the OS line-up....the .91 FX. The other dog in the new OS line-up
    is of course, the .61 FX which came in dead last in the 60 size engine shoot-out
    awhile back.

    No small wonder when you bore out the 61 dog to a 91, you get the 91 FX Dog.
    Not too hard to figure that one out, huh ?

    BTW, I didn't say anything at all about Jett engines.

    FBD.
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    An engineer says.... "That will not work".
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    Old age and treachery will overcome youth and skill.
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  20. #20

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    RE: Magnum 91 XLS 2-Stroke power?


    ORIGINAL: Flyboy Dave

    I have some bad news for Harry Lagman....they must have some real dog engines
    down in his neighborhood if he thinks the OS 91FX has good power....
    No, not really...

    What we do is compare apples to apples.

    You are comparing the FX .91's tach figures with a .61 fitted with a full length tuned pipe running a prop many of us have never heard of.

    Would you care to run up a Hanno .61 with a muffler (non tuned) and an APC 14x8 for a valid comparison? In fact, run up the Hanno *with* the tuned pipe and see what it spins the APC 14x8 at.

    Here's a guess: about 8000 rpm.

  21. #21
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    RE: Magnum 91 XLS 2-Stroke power?

    .....I guess Harry doesn't know there is no such thing as a muffler for an OS Hanno....

    ....the only way they come is with a red anodized pipe. Stock for stock, the new OS 91FX
    won't even outrun the old OS .61 pattern motor.

    Talk about a dog motor....put the FX up against the old OS .90 FSR and see what happens.

    I don't think two FX's hooked together would stay with the old 90 FSR.
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    An engineer says.... "That will not work".
    A mechanic says..."Oh yeah, watch this".
    Old age and treachery will overcome youth and skill.
    Revver Bro #4.

  22. #22

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    RE: Magnum 91 XLS 2-Stroke power?


    ORIGINAL: Flyboy Dave


    Talk about a dog motor....put the FX up against the old OS .90 FSR and see what happens.
    Got any tach figures to back up that claim?

  23. #23
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    RE: Magnum 91 XLS 2-Stroke power?

    G'day Mate,
    OS recomended prop size for 91FX, starts at a 15x8 & goes up to a 17x6.
    http://www.os-engines.co.jp/english/...e/proindex.htm
    Magnum recomend a 14x6 for break in & a 14x8 up to a 15x6 after that.
    I know what you are gunna say, that's too much prop, it won't scream it's head off, & so on, but they work well no matter what you say.
    Allan I,ve seen this listing before and I think it is wrong compared to older ones I 've seen.

    Look at your chart again ---The starting prop for OS108 is 14x8 1 inch smaller and starting props for 4 stroke 91 is 11 inch. Hey maybe they don't want to turn much RPM ,so as not to peel liner. I think Magnum has more realistic prop size listings.
    Hey Buddy -That was the most spectacular crash I\'ve ever seen , That rolling cart wheel with parts slinging out was cool...

  24. #24

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    RE: Magnum 91 XLS 2-Stroke power?

    14" and 15" props seem quite large for a 91 2 stroke, unless stealth mode is required, perhaps for noise regulations. Those are 4 stroke props surely...? Anyway, each to his own.

  25. #25

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    RE: Magnum 91 XLS 2-Stroke power?

    Can't say much for OS 2 cycle, but I'm with FBD and Speedster on the prop issue.


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