aspeed

Same 7-5" Master Airscrew as my other .15 tests. 10% nitro, has a rotatable exhaust with the side exhaust being used as the muffler is set up for that. I opened it up and noticed the ports don't line up properly for side exh. but the rpm's were the same with rear exh. and side exh. anyway. I am kind of dissapointed it barely made 14,000 rpm. with or without muffler.My MVVS.21 was sogood, I thought this would be too.The muffler is pretty quiet though. It has an rc carb around .196" dia. with an airbleed that doesn't idle too well either. Swap meet item, good compression, looks good.

Mr Cox

The half-pipe gives a little better performance than the muffler. I think they stopped making the mufflers near the end, and supplied all with the half-pipe.

aspeed

I think it is the half pipe.  It doesn't lose any rpm with it. It is made of tubing not a casting.  It just seems slow to me,  An OS LA is faster on that prop at least, really everything I have that size is faster except the Thunder Tiger.  Maybe I will check the crank timing, the exh timing is around 160 degrees which should be ok.

Mr Cox

It sounds like there is something wrong, it should do a lot more than 14000rpm on a 7x5 prop.

aspeed

I checked the crank timing, it is likely that.  I think it should usually be 40 deg. after bottom dead centre and about 60 degrees after top dead centre.  It is opening "about" 48 deg. after BDC and 40 deg ATDC closing.  I think the closing should be ground a bit (a lot) and the opening timing is too far to grind.  The headspace worked ok at .015" to .030" with little measureable difference.

Mr Cox

Which engine do you have, is it a very early model?
It might be designed for moderate rpms and then I don't really see anything wrong in those timing numbers. I think the opening is not as critical as the closing is, and the closing depends on the rpm region you want to operate in. Opening at around 50° ABDC is quite normal. Closing at 40° ATDC indicates that it is set up for low revs i.e. <15000rpm. A closing at 60° would then be setting it up for >20000rpm which would be a totally different engine. What are the exhaust and transfer timings?

aspeed

120 deg. transfer and 160 exh. I am guessing 15 years old, but have no idea. So I guess just run an 8-4 or so on it.

aspeed

I changed the closing timing to 55 degrees after, and it goes 16,200 rpm, peak without the muffler, but doesn't stay there.  It sags back to about 15,500.  The bearings are a tight fit too, it needed another shim.  It seems to want to go 14,500 with the muffler.  I guess it isn't worth making a plane for.  It sure looks like a fast motor, maybe someday I will try some different props or something.  Blew a plug too, Probably my fault for squeezing it too lean for some numbers.

Rudeboy

Modified engine (sleeve, case, piston, crank), MVVS venturi (enlared to 6mm), Rossi .15 one piece pipe (shortened slightly), 80/20 castor fuel, APC 6.5x5.5 slightly cut down.

26.500 rpm

Haven't run the engine in years, but there's still more to be had IMO with some nitro and custom head buttons...but that was beyond my capabilities at that time.

Mr Cox

I ran my MVVS .15 engine today, but once I got it diled in the rain began so I haven't had it in the air yet. On the ground it gets onto the pipe on an stock APC 6.5x5 and 10% nitro fuel. Rock steady at 22200rpm so it might be able to take a 7x5 prop and still get onto the pipe in the air.

According to the "prop_power_calculator" this corresponds to about 0.5hp, not to shabby from an old-school engine...

aspeed

OK, I tried the 6.5 - 5" APC. I got 19,900 to maybe 20,000 once in a while. (burned the plug too) Put on an Ucktampipe, and it got to 21,500. It still wasn't really steady enough to run in a plane. Maybe with some pipe pressure and some fiddling. Then I took an OS .15 LA to compare because all my other tests were with the 7 - 5" prop. It does 19,900 to 20,000. What is my conclusion? The MVVS likes higher revs, but is back to the disappointment shelf, The LA is lighter and easier to use/set. I guess a new Turbo head may help things out, but it would on the LA too. Edit- your plane looks real good!