Kitsch

I was given a new-in-the-box OS 61 SF purchased in 1992. I expect to use this motor with an APC 12x6 or 12.5x6 propeller on a scale plane. Would a Jett Stream muffler make a difference in performance? If so, which one would be appropriate? The bolt holes for the muffler are 42 mm center to center, and I will be using a 15 mm muffler extension in this application.

bob27s

My Feedback: (19)

Simple answer is Yes, we have a muffler

The SF uses the same muffler bolt spacing as current 61FX engine. Both the Jett-stream and Turbo-jett mufflers are available to fit that configuration. Simply specify the engine when you place the order.

15mm extention is quite a bit. Are you sure you need that much? It will affect the tuning and performance of the combination. Less is better. Its not a huge loss, the tuned rpm drops, torque increase a bit..... but with much over a .5" extention we see some diminishing returns.

Tell me a bit more about your applpication. You said it was a scale project. I might be able to provide more info.

The jett-stream will indeed give you a very impressive power boost over a stock exhaust or a pitts type muffler. Gain is 1000-1500 rpm on average, say, with an 11x7 prop. Gets you about 90% of the boost you might find with a full-sized tuned pipe. Similar with the 12x6. Key on the SF and the FX engines is prop selection to achieve peak ground rpm between 12,500 and about 13,500 rpm.

(for reference, a Jett 90L turns a 12x6 about 14,500. OS 91FX with jett stream turns a 12x8 a bit over 13K rpm).

Jett also offers the Turbo-Jett incowl muffler. This too is also tuned and performance boosting. Gives about 80% of the boost found with the jett-stream, but also tends to nudge the torque curve up a bit... so the engine will more easily turn larger props. For anything with a round cowl, or even aerobatic type planes, its definately something to consider.

Kitsch

I plan to use this engine on a T-28 Trojan 61 from BH Models (http://www.rcuniverse.com/product_gu...fm?kit_id=5133). I just received this ARF model today, so I looked in the instruction manual at space allocated for the muffler. It appears that the cowl and firewall area are designed around a standard OEM 2 cycle muffler; there doesn't appear to be space for the longer Jett-Stream muffler. Can you tell me if the Turbo-Jett muffler would fit inside this cowl?

Also, the OS 61 SF has a 24mm stroke and 23mm bore, which suggests it is designed for slightly larger props and lower RPMs compared to an OS 61 FX. Will this affect the advantage of using a Jett muffler?

bob27s

My Feedback: (19)

Unless it is one of the pumper SF engines or one of the Hanno varients, it will run just fine. (also double check the info on the SF engine ..... the pumped ABC pattern engines were a little different than the ringed SF sport engines)

With a jett-stream you would want to prop the engine for between 12,000 and 13,000 rpm. 12x6 would be right in the sweet spot

Dimensions for the turbo-jett mufflers are on the website, along with a few example photos.
http://www.jettengineering.com/accy/turbo.html

Your application/installation would be most like the Corsair shown there. If you can compair your T-28 cowl to that of the .60 size corsair... that will be a useful reference.

The turbo-jett, by its nature, works in a lower rpm band. The 10cc muffler and the 15cc muffler work well from 10,000 rpm up to around 13,000 rpm - broader range to work with. You will not see as much of an rpm gain with the turbo-jett, but what you will see is that it will turn a bit more prop at that same peak rpm. For example, jump from a 12x6 on a stock muffler to a 12x7 or 12x8 with the turbo, and hold the same 12,000 rpm.

Looking at the BH T-28 manual on the hobby people website....... it is hard to tell if the turbo-jett would fit 100% - but it looks close.

For the jett-stream, they at least got your started with a muffler channel. Probably would not require much work to make the channel a little bit longer.