Ernie Misner

I finally got to the G90 with mousse can test today and it blew me away. I used the same 3 props that I used the other day and with the 14x4W APC, gained 700 rpm, and with the 13x6 APC gained 800 rpm. In the previous test the muffler had the baffle removed.

The biggest difference with the mousse can today is the WAY it ran. The throttle response is really fast and clean. It may still have been barely rich in the midrange with the mousse can, but when going from 1/2 throttle to full, it was right now with no hesitation or having to clean itself out like with the stock muffler. I was amazed and had a big smile on my face. Looks like it would work just fine for 3D to me. Next week I will get it in the air and we'll see.

The first runs were with the Mac's header at the full 9" length. I cut 1" off and tried it again. There was a slight increase in rpms with the two smaller props, about 300. I was tempted to keep shortening the header and testing, but it ran so good that I just left it at 8". At this length it is ALWAYS on the pipe with instantaneous throttle response, almost like my little Webra. It became obvious that the stock muffler (with baffle removed) is a very, very short tuned pipe of sorts. At WOT it ran good but was pretty sick at the lower rpms compared to the header and mousse can. With the can, the needle had a nice broad range, not touchy at all which felt different. Anyway, here's the figures with stock muffler and then with the mousse can. The can is a little larger than 46 size, about 2" by 7".

13x6 MA 12.600 ......... with mousse can 13,400
14x4 W APC 12,000 ........ with mousse can 12.700
15x4 W APC 10,600 ........ with mousse can 11,000

Ernie

Flyboy Dave

My Feedback: (21)

Ernie....

....what is the diameter of the outlet you soldered onto the mouse can, compared
to the stock outlet on the original pipe ?

Dave.

rainedave

My Feedback: (1)

That's impressive. Is it much quiter than the stock muffler? Also, is that a Tower engine with a Super Tigre carb in the background?

Ernie Misner

Dave, the outlet on the stock G90 is a loose 5/16" and the outlet on the Tower muffler I have here is a tight 5/16". The hole in the stinger I soldered into the can is 21/64" which is one drill size up from the 5/16".

At one time the mousse can came loose and blew off leaving just the header pipe. The rpms dropped by a couple hundred. Boy was it loud.

Raine, it is quieter with the mousse can than the stock (baffle removed) muffler. There's a lot more volume inside of the header and can, so the crackle is a lot softer, not as raspy or harsh. Our field is noise sensitive (ask Dave when he was here last summer) and I think it will be fine.

Yes, good eye. That Tower engine in the background has the ST carb on it and the G90 has the Tower carb on it at the moment. They have the same mount and ventui size and seem to give the same results. With the stock muffler I thought the Tower carb might have been a little less rich in the midrange, but with the mousse can it's a moot point, they're both good.

Ernie

Not24

My Feedback: (1)

Your test makes me think that perhaps the stock muffler is still too restrictive. Are the outlets on both mufflers the same size? If it's a plain exhaust tuning issue, you're making me want to try that on the 2300. I can't find a header for it anywhere, so I may have to make one.

Duh, just reread the post explaining exit size. My bad. I'd like to see an 800 rpm gain. That would be sweet!

Ernie Misner

Not 24, it is not necessarily the amount of restriction, but it is the tuned length from exhaust port to the end of the can or muffler. As the exhaust charge enters the expansion chamber it will suck the remainder of the exhaust gasses out of the engine, and as the charge hits the back of the can or muffler it will send a high pressure wave back to the (still open) exhaust port that will tend to block the loss of the fresh charge of fuel that has entered the combustion chamber. The length of the system will tune it to a frequency or harmonic that controls the timing of when these things occur. In a system that is way too long or short (especially) the timing of these return waves are off and not in sync with what the engine and exhaust port is doing. I've probably slaughtered the engineering badly here. Maybe Dar or someone can explain it better.

Ernie

Harry Lagman

Ernie,

Nice piece of testing. Thanks for documenting the results for us.

It looks like you may have isolated one of the causes of the ST rich midrange issue.

They look like pretty competitive figures for a .90 two-stroke. Do you have an APC 12x8 handy? It would be interesting to see what she pulls with that prop. For comparison, my Jett .90L peaks at 13,200.

Ernie Misner

I have the test setup torn down for now and I'm actually getting ready to get it in the air next week on an Ultra Stick 60 that will be easy to mount the can onto. With the engine mounted horizontally, the pipe and can will come down inside of the gear.

Not likely that a ST will outpull your Jett. I'll take a look at my props for next time though. Yes, the carb(s)? still probably go a bit rich in the midrange. I wonder why they don't give is 3 needles like on the helis?

Ernie

Dr Nitro

That was some great reading Earnie, good job!
I love to read posts like this.

wcmorrison

My Feedback: (2)

Amen!

Cheers,

Chip

bob27s

My Feedback: (19)

Nice report Ernie.... and some good results too.

The ST90 with the Jett-Stream muffler performs similar. Keeping in that 11K to 13K rpm band is a good approach. So it looks like you are in about the right ball park. As you have seen, with a good exhaust system with good pressure balance, the engine does everything very well.

What you might want to do, when time permits, is see how the engine runs with a 15x6. I've found the torque curve slide is pretty respectable. Dont change the length of the system. Just add prop until it doesn't run right or show any gain. You can probably get good results pulling down closer to 10K rpm.

Keep in mind, it may seem like just a little at 200-700 rpm gain, but at those prop sizes, those increases corresponds to a large increase in HP.

Bob

m.gramling

My Feedback: (8)

I build a moosecan for Paul Swanson, he's here on RCU, and the www.the******************.com a few years ago for his ST G.90, I used a 2"x10" can. I don't remember what the stinger O.D. was. But it made his throttle response perfect. I think it picked up his RPM by around 900-1000RPM