Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

OPS .21

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Old 03-09-2006, 02:50 AM
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willflyforfood
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Default OPS .21

Hey guys I have an OPS side exhaust .21 with a tuned pipe that I am building a speed plane for. I was just curious as to prop / nitro reccomendations, I have never dealt with OPS engines before, I was thinking 7x7 prop, and 15 or 20 % nitro. How does that sound?
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Old 03-09-2006, 02:16 PM
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Default RE: OPS .21

Sounds pretty good although I'm not sure how it will handle the nitro content.

In Europe we use this motor in club 20 racing with 5% nitro, APC 7x6 or 7x6 Wide blade and an Irvine magic muffler.

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Old 03-09-2006, 04:42 PM
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Default RE: OPS .21

sounds good i'll play with it and see, what rpms are you getting?
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Old 03-10-2006, 01:17 PM
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Default RE: OPS .21

On my old and a bit tired OPS I got about 20 Krpm. 21-22 Krpm should be possible.

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Old 03-26-2006, 10:00 PM
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Default RE: OPS .21

Hey guys I tached the OPS today at 19,500 rpms with the 7x6 apc w/ 15 % powermaster and pipe, but I am curious as to how I am supposed to set it up and know when it is on the pipe, do I need to get a longer piece of tubing and just cut it shorter and shorter till it peaks or go ahead and start cutting off of the pipe from where i am now, it doesnt sound like it getting " on the pipe ".
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Old 03-26-2006, 10:19 PM
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Default RE: OPS .21

WFFF, first get a baseline rpm with open exhaust. It is also helpful to have head shims [or the ability to cut your own head chambers], to get the best compression worked out. Then I get a reading with just the header on. Now it's time to get a reading with the full length system. As you take your reading, lengthen the system slightly and watch the tach, hopefully the rpms drop, if the rpms go up, then the system is too short [or small]. Chances are very good a new, properly engineered system is too long. The painstaking steps come next, hack off a 1/4 inch from the header and take another reading, you should see either improvement, or no worsening. Don.t use a tubing cutter because it leaves an internal flare after the cut is made, use a cut off wheel or hacksaw. Keep going in 1/4 inch increments until the rpm improvements end, you will know when the engine hits resonance, it is an explosive jump in rpm. If you don't get that jump, either the prop is too big or the engine has a problem. Be ready to open up the needle to allow for the extra fuel demand, this is a juggling match on a throttled engine, much better off running a fixed venturi with a fuel shut off.
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Old 03-27-2006, 02:39 AM
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Default RE: OPS .21

Thanks CP, but how do I get back pressure to the tank running just the header with no muffler tap? It also has a perry carb which I am very unfamiliar with, but I got it to run well, just not throttling worth a darn. I was told the large wheel is the "coarse" adjustment by one guy and the "low end" by another but that doesnt make sense after examining it. Any advice there would help also, long as i can get the high end doin what i want i dont really care how great if any throttle response there is. I dont know anything about these carbs but it does seem to work well, all I need it to do is smoke the turkey and shut it off, the inbetween is irrelivant.
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Old 03-27-2006, 09:36 AM
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Default RE: OPS .21

Pressure can be obtained from drilling the side of the headers' flange and installing your own pressure fitting. Fox, Prather, K&B and others sell tap fittings. Backplate pressure works with a check valve, also. I don't use a pressure fitting with a throttled OS .15 and it behaves perfectly, so it is a case to case kind of thing, whether you need a pressure line or not.
Sorry, I've never messed with a Perry carb, you could do a search.........just make sure you understand how it works so you don't accidentally run the engine too lean in the idle or mid range. Running the engine lean on the high speed needle isn't the only way to fry an engine.
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Old 03-27-2006, 12:28 PM
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Default RE: OPS .21

The "large wheel" is the low speed adjustment. It should have a slot with a "+" on one side and a "-" on the other. Use a small screw driver in the slot to turn it, "+" direction for richer and "-" direction for leaner. 1/16 of a turn makes a huge difference. The best way I have found to set low speed needles is to get the engine started and idling then remove the glow plug battery. Listen for a change in rpm. If it goes up the low speed is lean, conversely if it goes down its rich.

The best place for the pressure tap on a tuned pipe is in the fattest section, this is however not the "easiest" place to put it.
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Old 03-27-2006, 12:48 PM
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Default RE: OPS .21

well I sure appreciate it guys that will help a lot, this engine should do pretty well once i get the kinks worked out of it.
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