Johnnylightning

Hi Bob, I've got a Jett 90L that's going into a Diablotin Super FG and I'm afraid that my transition might not be crisp and instantaneous enough for 3D. I've bench-ran the motor w/ an APC 14X4W and that's when I discovered that the transition was rather sluggish. I've ordered a couple of APC 13X4W's and a 3 blade Bolly 13X4.5 ($$$) to hopefully improve on the spool-up. Have I done the right thing? Please help. Maiden will be on a week from today and the only thing that's got me worried is this issue. Thanks!

bob27s

My Feedback: (19)

Hi,

Thanks for writing.....

The standard SJ-90L is not a very good choice for 3D

First, some background.

The SJ-90L is an RPM engine, and with the standard Jett-Stream muffler should be set up (prop selection) to run at 14K rpm or higher... preferably closer to 15K on the ground. The engines are shipped from Jett with the low range needle pre-set for perfect transition using a 12x6 prop in this RPM range.

Assuming the props you have are hitting the 14K and over range, the transition issues you are seeing may be due to the larger prop (likely a thicker blade section), and you may have to lean out the low/mid range needle a bit. Instructions are on the Jettengineering.com web site.

In summary, turn the low end needle in about 1/8 of a turn at a time until you get perfect transition, and a smooth idle. Only do this after you have at least 20 minutes of run time on the engine, preferably in a test stand. Let the engine run a full power, lean the top end to peak, back off 400-500 rpm. Then pull to idle. Let it sit at idle for 20-30 seconds. Then open the throttle quickly. If it loads up and then accellerates, you have to turn the low end needle in a bit more. Then repeat the process. Always let the engine run at full throttle for a few seconds (to clear it out) before pulling back to idle and checking.

Additionally, the tuned effect of the Jett-Stream muffler creates a step in the power curve as the engine comes on and off of the muffler. This happens just above mid-range, and if the engine is over-propped, it can create some of the transistion issues you are seeing. It is simply the nature of a tuned exhaust system.

The SJ-90T you may hear about is basically the same engine, with a longer 120 muffler so it is tuned for more around 11-12K rpm (larger 14x6 prop, slower).

Using the Turbo-Jett in-cowl muffler on the engine will help with the pipe-step situation, as it is a much broader-band tunning device.

So how has Jett addressed the 3D issue?

In 2002, Jett first introduced the BSE100. This engine was designed from the ground up to run at lower RPM, primarily with the Turbo-Jett muffler. This engine is AWESOME for 3D work, and

I fly one in my AeroWorks 90 size Edge 540. Turns 15x8 at about 9600 rpm. Great midrange torque, smooth transition. And like the 90L, it is in a '60' size package.

This year at the 2003 Toledo Show, Dub introduced the BSE120L. Same size as the 90L and 100L, with additional power. This is the ultimate "60" size 3D engine.

Let me know how things work out

Bob Brassell

Johnnylightning

Hi Bob, thanks so much for your prompt reply. I believe that the muffler that I was supplied with was actually the long (1.20) size. But, now I'm afraid, according to your post, that the APC 13X4W will be "underpropped" and that I'm gonna' have problems on the low, mid AND high. I'll try the APC 13X4W and then I'll post the results. Thanks again...Steven

bob27s

My Feedback: (19)

I would not worry as much about being under-propped on the the high end. The engine/muffler will be ok turning a bit faster.

Let me know how things work out.

Bob

Johnnylightning

the .90L ran a heck of alot better w/ the APC 13x4W. Transition was pretty crisp (however, could use a little improvement), high was super-clean & idle was remarkable (about 400rpms, really). Unfortunately, the aircraft plunged to it's death, on it's maiden voyage, due to a faulty Li battery. But, I now see exactly what you mean about the "piping" issue. I was wondering...that if I was to change the stock Jett exhaust to an after-market muff(Macks/Tower/MVVS), if this would eliminate this "get-on-the-pipe" issue. Do you know if anyone has tried other mufflers or are Jett engines specially timed/ported to run only w/ the stock pipe?

bob27s

My Feedback: (19)

Hi.......

Sorry to hear you lost the plane. I hate when little crap like that takes the plane....

At least you found a better prop match for the engine.

For the future, that small amount of remaining transition issue can be a glow plug issue. 90% of the time, a good ole K&B-1L plug works great. When you lug it down in mid range, you may need a cooler plug. If you can get a hold of them, get a Rossi 4, Rossi 5 and Rossi 6 plug. The K&B is pretty much the same heat range as the R-4. Try the 5 and 6, and see if that makes a difference.

One other thing that we have been experimenting with (privately) was using lower oil fuel in the larger engines. I've been running my SJ-120 on 4-C fuel (10% nitro, 15% oil). The change eliminated a transition issue I was having with a larger prop. Might be worth trying in the 90... I will put it on my list.

We have run the engine on other mufflers. Bolt pattern matches a ST-75 if you wish to experiement a bit.

Clearly, it produces the best power with the Jett-Stream muffler, and produces nearly the same power with the Turbo-Jett in-cowl muffler, but with a much broader power band.

If need be, Dub can provide you with an un-tuned version of the muffler, constructed without the tuning devices installed. These have been used in the past, and have eventually led to the development of.......

Jett's new non-tuned light-weight muffler for the 76-120L class engines, similar to the one provided with the BSE65 3D engine (shown). They are currently available for "30" size and "46" size engines (all types). This "60" size muffler is still in development, but will be available in a few months. These mufflers weigh next to nothing, and provide a slight amout of power boost at top-end power...but on average are very slightly louder than 'most' stock mufflers.

Johnnylightning

Hi Bob, any word yet on the availability of the new non-tuned light-weight muffler for the 76-120L class engines?

bob27s

My Feedback: (19)

Not completed yet.

Dub is off at the NATS this week, and he has spent a week or two prepairing... so things have gotten a bit behind.

I will check with him when he returns...

Bob

Johnnylightning

Thanks again for the quick reply, Bob. Hope to hear from you soon. Oh, and good luck @ the NATS!

tknobby

bob27s,

Great thread.

I have the 90LT I will be using on my new Excite 90 for 3D flying and was wondering if the new lightweight muffler was available for this engine. I have the 120 sized muffler now but am interested in the lighter weight and the non-pipe effect this muffler will provide.

Thanks,

Tom...

bob27s

My Feedback: (19)

Hi Tom...

sorry for the delay in reply...

Dub is working on lite mufflers for the big block engines, but right now they are still in development.

You can get the 'standard' jett-stream muffler provided as a non-tuned muffler. It is quiet, and has no pipe effect.

Bob

Johnnylightning

Hi Bob, thanks for keeping us up-to date w/ the production of the lite muffler. I've heard of the Jett-Stream muffler as an option to the piped version, but I think I'll wait for the new Lite-Muffler. Please keep us posted! Thanks again...Steven

tknobby

Hi Bob, thanks for the reply.

I really want the lighter weight of the Lite muffler. Will Dub be testing any prototypes? I'd be willing to try one and report feedback.

Tom...