bob27s
Posts: 4710
Joined: 4/9/2002
From: Cleveland,
OH, USA
Status: offline
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Hi Jim...
Thank you for writing. Off hand, I do not see anything that is out of line or a problem. Let me see if I can help.
Short answer first. - Need a little more data
You will see an increase of approxmately 800-1000 rpms from performance with the stock muffler, depending on the prop. The 40 size Turbo-Jett is tuned to operate over 13,500, and works even better as you approach 16,000. First, the engine must turn AT LEAST 13,000 on the ground for the Turbo-Jett to work well.
To do this, the prop selection determines RPM. Smaller prop = more RPM, which is better. The Turbo-Jett is more broad-ranged than the Jett-Stream, and you will be able to turn up to 11x5 with a little care. In trade, it does not boost RPMs as much as the Jett-Stream. A 12x4 is about the limit I have found with .46 size engines.
Of note, the turbo-jett provides more muffler pressure than the stock muffler, so this should assist with fuel delivery.
It was not clear if your test was in an aircraft or on a solid test stand. Please install the engine on a test stand for evaluation. There are too many variable in an aircraft installation. A test stand will eliminate most of them.
Please provide RPM with the stock (factory) muffler on the test stand, and then with the Turbo-Jett. Try first a 9x6 APC and the 10x6 APC.
Do not get the engine lean. Find peak. Back off 2-300 rpm. Shut the engine off, and then let it cool down for 5 minutes. Restart the engine, and get your RPM reading there.
My own OS46FX and TT46 Pro both turn up to 13,700 - 14,000 rpm peak with the stock muffler, APC 10x6 and 15% Omega. Either is comfortable for flying and slightly rich running at 13,500 with the factory muffler. I think Dub's test numbers showed a peak of 14,200 on the OS46/os muffler, so his data is in the same ball park. Other OS and TT engines I have set up for people had similar performance.
My point to this question is, that if your engine is for some reason not achieving performance near this level with the factory muffler, there may be a fundimental problem with the engine itself. Thus my request for your test. Just as a baseline.
For reference, I have personally flown the following engine/prop combinations with the Turbo-Jett. Keeping in mind that my RPM data almost consistantly runs 300 rpm below what Dub gets. These are also peak RPM's, and I typically back off 300 rpm for flight.
Jett SJ-46 with
9x6 = 16,100 10x6=15,200 11x5=14,600 12x4=14,100
All props unload quite a bit in the air.
OS46 with
10x6=14,800 11x5=14,200 11x6=12,800 12x4=13,600
TT46 with
10x6=14,800 11x5=14,300 11x6=12,700 12x4=13,500
It is also worth noting, that, in general, I find that my test stand RPM's are on average 100 rpm higer than what I see as installed in an aircraft.
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So, if your OS46 with the 12x4 is peaking around the 13K range, that is pretty close, and it will likely unload nicely in the air ( I fly this on combination on my P-51 with the SJ-46). But a 10x6 should easily make it over 14,500 under any conditions.
Let me know about the test stand, and stock muffler RPM and we can work from there.
Bob
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OS46FX with Turbo Jett Muffler - 
OS46FX with Turbo Jett Muffler - 
