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What would good numbers be for Magnum 91 4s

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What would good numbers be for Magnum 91 4s

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Old 03-11-2010, 08:23 AM
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goirish
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Default What would good numbers be for Magnum 91 4s

I put a Magnum 91 4s on my super stick. Swinging a 13/6 MAS prop. Idle 2650, WOT 10,500. Is this in the ball park. Great transition. When I take the glow igniter off I lose a couple hundred RPM's. I am not using a 4s plug. Probably some plug I found out in the field
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Old 03-11-2010, 08:25 AM
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Default RE: What would good numbers be for Magnum 91 4s

OOOOOOPS....The prop is a APC 13/7
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Old 03-11-2010, 08:28 AM
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Default RE: What would good numbers be for Magnum 91 4s

Iseem to recall 9800 on a 13X8 as a good benchmark. I've also been toldthe OS F is the plug to use, especially if you are having trouble with the engine. This will help with the RPM loss, although mine slows a little when Itake the driver off. I've run almost a gallon through it so far, and Ihaven't adjusted the low speed needle yet. Haven't needed to, really. Good luck with it. Jim
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Old 03-11-2010, 08:31 AM
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Default RE: What would good numbers be for Magnum 91 4s

This engine is broken in. Had it on my SeaDancer for a couple of years. Just never got to the lake that often. It should make this stick haul butt.
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Old 03-11-2010, 08:33 AM
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Default RE: What would good numbers be for Magnum 91 4s

I have another Magnum 91 4s that I am putting on my Dirty Birdy, I understand that is as large as I can go for SPA guidelines.
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Old 03-11-2010, 08:34 AM
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Default RE: What would good numbers be for Magnum 91 4s

Thats really good for running any old plug in it. . If this is a new engine , it is really, really goood. As a rule of thumb , I prop my 4 strokes to never exceed 10.000 rpm. Try a 14x6 or 8 and see how it does if it's broke, in.
I bought one of these a few months ago and after 1/2 gallon break in was getting about 9800 rpm's 14x6 APC.and the idle was so low and steady , I did'nt tach. it. Picked up a BTE Venture this winter and was going to use a Saito GK 1.00 I just rebuilt, but if the plane needs nose weight , it's getting the magnum .
Glad to hear yours runs so well after the run around you got on your .70.
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Old 03-11-2010, 08:41 AM
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Default RE: What would good numbers be for Magnum 91 4s

Never had a problem with the Magnum engines, Just the other situation. This engine is broken in and I am running 15% S & W fuel. Geeze, I like that fuel. Our club bought a 55 gal drum and the end cost per gallon was about 9.00.
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Old 03-11-2010, 09:09 AM
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Default RE: What would good numbers be for Magnum 91 4s

I'm running a 14X6 on mine, getting 9700 on it. Ireally love this engine. I've never had trouble with it. 15% Cool power and an OS F plug. Istarted with a 13X8, but the plane was really fast, Iwanted to slow it down a little.
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Old 03-11-2010, 12:24 PM
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Default RE: What would good numbers be for Magnum 91 4s

Anything from the mid 9s to the low 10s is about normal. Not sure what they call the working rpm range but never go over that!! The rpm drop when you pull the driver is sort of how I tune the low end. The drop is telling you that it's a bit fat on the low end. I lean it out until I loose the drop then faten it back up until it just drops a bit and that seems about right. A nice new OS type F would be a good idea. Just an FYI, I have one old YS 1.20 that drops hundreds of rpms when I pull the driver and it runs perfect. If I try to lean the low end anymore it doesn't run for crap. This is an old NC model and you tune the low end with the regulator screw, there is no low end needle at all so it this engine doesn't count. Your idle is a bit high but as long as it's working and you are happy with it then it's fine.
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Old 03-11-2010, 01:21 PM
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Default RE: What would good numbers be for Magnum 91 4s

Thanks Gene

I will put a 4 stroke plug in it and do what you suggested about leaning the low end until it does not drop when you take the driver off. Then (as you say) fatten it up a little. Let you know how that works. Thanks
Gene (the other Gene)
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Old 03-11-2010, 06:22 PM
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Default RE: What would good numbers be for Magnum 91 4s

ORIGINAL: goirish

Thanks Gene

I will put a 4 stroke plug in it and do what you suggested about leaning the low end until it does not drop when you take the driver off. Then (as you say) fatten it up a little. Let you know how that works. Thanks
Gene (the other Gene)
i tune for instant transition, just like a 2 stroke, seems to work fine for me, ill try this method to see if its any better

as for rpm's
im getting 9900-10000 on a apc 13x8 on 5% nitro the engine prob has 3-4 gallons through it
im useing a thunderpower plug
idles around 2200
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Old 03-12-2010, 09:20 AM
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Default RE: What would good numbers be for Magnum 91 4s

Im going to run my first tank of 15% through my 91 this weekend. Maybe 2 tanks. Per the instructions from Magnum I will be running a 14X6 APC prop. The 15% is also per the book. I will keep you guys posted. Wish me luck.

Oh and thanks for starting this thread. Absolutly great info to have before starting an engine with my limited experiance.
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Old 03-12-2010, 09:28 AM
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Default RE: What would good numbers be for Magnum 91 4s

If you break it in like they suggest, you will have a good engine. BTW the Tide Did Roll!!! You had an old Michigan State Coach that wasn't good enought to coach Mich. Stqte. Hahaha the joke is on them.
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Old 03-12-2010, 09:45 AM
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Default RE: What would good numbers be for Magnum 91 4s

If the RPM drops at idle when you take off the igniter, it's probably fat on the low end like Gene said, but if you're losing RPM at full bore, I'd guess it's the plug.

4-stroke engines really like 4-stroke plugs
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Old 03-12-2010, 10:00 AM
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Default RE: What would good numbers be for Magnum 91 4s

I just hope we can hang on to this guy. maybe he will be like Spurrior who says S. Carolina is his last job.
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Old 03-12-2010, 10:03 AM
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Default RE: What would good numbers be for Magnum 91 4s

He is a good one that is for sure. I hope Mr. Kelly for the good old Irish will bring them back to national status. Hence, my screen name (goirish)
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Old 03-12-2010, 10:38 AM
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Default RE: What would good numbers be for Magnum 91 4s

I keep hearing about admission standards that  keep the guys that are keeping the big brutes out of ND.  Its too bad really that all the schools dont adhere to a high standard.  And I would love to add ND to our schedule.  We have Penn State this year.  That will be a game for the whole country to enjoy.  Good luck with your new coach!
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Old 03-12-2010, 11:38 PM
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Default RE: What would good numbers be for Magnum 91 4s

Its around 1030 pm here and I just finished running the second tank of 15% thru my 91 4s.  My tach appears to be broken so I do not know what my numbers are.  But man that thing sounded good!   
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Old 03-16-2010, 05:06 PM
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Default RE: What would good numbers be for Magnum 91 4s

I have run everything from a 13X8 to a 15X6 on my Magnum .91. I have only used 4S plugs on it and 15% nitro with at least 18% lubricant. I never like to see it turn anything near 10,000 on the ground, as someone else mentioned earlier. We are about 600 to 800 feet above sea level here in San Antonio and it does get quite warm in the summer.

Mine is mounted on a Pheonix Decathalon and is just enough overpowered to do anything the plane is capable of. I am using an APC 14X6 on it due to clearance restrictions and the plane slows up nice on final and pulls off the ground in about 20 feet or so for takeoff.

I also have a Magnum 1.20 that is equally awesome and reliable.

The key to most 4s engines is getting the bottom end set up right and never run too lean on the top end. You should always see a bit of a smoke trail on a clear day, or you may be too lean.

Good luck with your engine.
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Old 03-16-2010, 06:13 PM
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Default RE: What would good numbers be for Magnum 91 4s

about lower 9k with a 15x4w apc would be about right
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Old 04-08-2010, 04:17 AM
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Default RE: What would good numbers be for Magnum 91 4s

I'm running a 16x4w on a 91 magnum 4s at 9700-9900 rpm this prop works wonders tryed a 15x4 w and 15x8 apc but the 16x4w i had on seagull decat- now its on a aeroworks 46-60 edge540t hovers at 1\2 thottle and has unlimted vertical.
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