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O.S. 91 SZ-H Heli engine - propped for speed

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Old 02-25-2019, 03:28 AM
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Paolo90
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Default O.S. 91 SZ-H Heli engine - propped for speed

Hi guys.
As I have bought a Phoenix Model Strega MK2 and I have this engine sleeping as spare from my nitro helicopters, I decided to stick a prop on it and do some tests.
Yesterday I did some bench tests using an APC 11x10 propeller, 10% nitro, 10% castor, 10% synthetic and 70% methanol home made fuel, using at the moment the Hatori helicopter muffler, need to buy a tuned pipe and header.


the engine ran great, got 13500 rpms and from idle to full throttle in a hartbeat, incredible performance!!!!

Now I would like to get a tuned pipe for the engine. suggestions? long or short, muffled or non muffled ( novarossi here makes some sexy non-muffled marine 90 pipes that I can buy)

Every tip will be much appreciated.
Thanks to everyone in advance.
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Old 02-28-2019, 07:50 PM
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the Wasp
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thank you for posting, I'm going totally nuts now LOL

I have an OS 70SZ Heli engine I will be putting in a 60 size GP ShoeString, so I have been looking into your question for a long time now,,, the biggest problem is fitting the fuselage..

I would think you would not want to use a tuned cone racing pipe because you want the plane and engine to be fun and consistent, besides that, it sounds to me that you are going to run a low Nitro fuel (10%)

first question, are you going to run the engine upside-down, if yes can you turn your muffler 90 degrees, if you can I will tell you to use that pipe IF it's not to long and will not hit the prop, it wont be pretty but it will perform vary well and be more consistent than a higher preforming pipe, and also easier to tune.

to mount a Heli pipe another way, see my photo, see the OS 90% muffler adapter for 61 to 91 size OS engines,, you can use a 61 to 91 Heli muffler with that adapter as long as the muffler is not to long, and again your engine should not be peaky, you can find used Heli pipe cheap in the Heli forums, just ask..

OS 90% muffler adapter PT# & photo,, here,,
OSMG2598 26625410

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most likely when you see how close a Heli pipe will be to the prop you will not want to run a Heli pipe, if you decide to buy a new racing pipe see my link here, they also have other large Heli mufflers,, or go to RC Groups and look in the engine classifieds or fueled Heli classifieds,,,
http://www.mecoa.com/acc/index.htm

in this link you will find a cheaper quiet pipe here, it's black and will look vary cool hang on the side of your plane,,
https://www.mecoa.biz/shopdisplayproducts.asp?id=202

if you want to talk to guys that know Tuned Pipes well go to the Extreme Speed page on RCU and start a post there !!

BTW,
1) I welded up a test adapter for my OS 61FX plane engine so I can fit a Century Heli 50 size Silent Pipe (it ant silent) on the 61 (see my photo below, yes the plane needs work,,,oops), the engine has had 7 gallons run thew it, but on the bench with that 50 pipe it ran 13,800 with an APC 11x8, that is 2000 more RPM than with the old SlimLine Pitts !! and it accelerated from an idle to peak RPM in an instant,, it was crazy, but I burned out the Pinch in the liner, so now the engine needs a new liner and piston>>>>>>>>> but I have that new, never started 70sz
2) right now I have an OS Hyper 50 with an over size GMS plane muffler on it, it's mounted in a Word Models 46 size P-40 (haven't flown it yet), it's been said the plane will do 70 with a stock AX 46, the stock OS 50sx with stock muffler will turn an APC 11x8 at 11,000 on 15%,, the Hyper 50 with the larger carb an the GMS muffler on 30% sure will zing an APC 11x10,,, I have 3 gallons to burn LOL


good luck, let us no how she goes, we need photos too !!
Jim


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Old 03-02-2019, 10:00 AM
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The NV 90 size pipe or a Macs 90 size muffled pipe would work well too. The difficult part will be the tuning. I would suggest you start off with the header at about 11. This will no doubt be on the long side. You will want to swap the prop for a 12x11. The Strega is a pretty draggy airplane so a small prop high rpm setup is not going to work well. With the 12x11 your goal RPM is going to be around 13k. The header can be shortened until you get to the point that the engine will jump onto the pipe once in the air and as it unloads. If you adjust the header length old school while taking readings on the ground the pipe will actually hold the RPM down as the pipe will be tuned to a lower rpm then what it will see once airborne. I used this method with a Rossi .61 and was able to get 12,500 on the ground with an 11x11. When it unloaded in the air it was like someone hit the NOS button.
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Old 03-02-2019, 10:11 AM
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Paolo90
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Thanks for the replies guys.
I received the novarossi 90 marine non- muffled pipe ( Novarossi code 50905), And early next week I've got the header coming in the mail.

let's see what's the starting lenght of the header then i'm going to experiment, I have no experience with tuned pipes.
So with the 11x10 i'm using a prop that's too little?

thanks for the help.
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Old 03-02-2019, 04:47 PM
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speedracerntrixie
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I would think so, 11x10 was used on .60 size patten airplanes back in the day. Your .90 should have no issues with the 12x11 and the correct pipe length. I would also suggest going to 20% nitro.
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Old 03-07-2019, 12:02 PM
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Originally Posted by speedracerntrixie View Post
I would think so, 11x10 was used on .60 size patten airplanes back in the day. Your .90 should have no issues with the 12x11 and the correct pipe length. I would also suggest going to 20% nitro.
Hi speed, thanks for the inputs.
I received the header and mounted the engine with the pipe set at 11' ( 28 cm) measured from the glow plug to the center of the expansion chamber.
Also I ordered a 12x11 and 12x10 props. Tomorrow I'm going to do some tests just to see how the RPMs react to the pipe with the 11x10, then when I receive the new props some tests again.
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Old 03-07-2019, 11:39 PM
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Great! I look forward to your results.
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