ZAGNUT

My Feedback: (1)

i picked up the little MVVS 2.5cc from Dar last week and even though it hasn't run yet i already love it!

in another thread someone said that the transfer passages were badly mis-matched between the top and bottom case halves. this had me a bit worried until i opened it up checked it for myself.

surprise! surprise!
in the pic you can see that this engine was clearly designed as a rear exhaust. if set up for side exhaust it'll still work but the passages won't be anywhere near matched. set up for RE they will only need a few swipes with a jewelers file for a perfect match.

another surprise is the way that the intake passage of the crank is machined for good flow. i was expecting the standard drilled hole and milled slot. a little polishing near the opening and it'll be perfect.

i also measured the timing durations:

exhaust........160* EDIT: the actual timing was much less, 140 i think....must have read the wheel wrong, 70+70=160??

boost............103*

transfers........98*

intake valve..160* EDIT: this is the timing for the crank's rotary induction valve NOT the transfers in the cylinder


the intake duration of 160* seems a bit low, i'm used to seeing 180-200*. the port in the case under the carb is just a narrow slot that would be easy to open up a bit.

would like input from anyone out there who has experience in making these little guys spin

dave

DualdudE

I know that feeling

I bought a .25 Combat from Pé Reivers... didn't run it yet either..but also love it.. It looks good, it feels good, an it looks well designed. Hope they run as awesome as they look

DarZeelon

Dave,

Smoothing out the flow, especially near the end (between the cylinder casing and the liner), could reduce turbulence, which could in-turn reduce power, more than the increased flow increases it. The turbulence effects a more complete burn.

Exercise good judgment before modifying anything major.

If you do the break-in and then make a modification that would change the "circular" relationship, between the piston and the liner, you are looking into a repeated break-in. And you cannot do too many of these on any engine.

Please note how narrow the exhaust port is. This is also true of other powerful engines; Jett for Instance.
Compare to the McCoy from the PDP thread.

ZAGNUT

My Feedback: (1)

finally did the break-in but not before totally reworking the little beast.

smoothed and polished the crank passage and matched the transfer passages to the new port height.
set exhaust at 175 deg.
transfers and boost at 125 deg.
induction is now 200 deg. starting at 35 ABDC

i didn't shim up the cylinder like i should have so the bottom edges of the transfers are a bit below the piston crown at BDC[]. this is not the smoothest way to have the transfers. there is also some SPI at the exhaust port. next liner/piston will be done right.

fuel is 20% klotz benol and 10% nitro, plug is the supplied nova-rossi 5, prop is APC 7x4

rpm:
open exhaust-----------------------22,000

with tuned silencer-----------------22,500

with tuned silencer,
and a bladder feeding
the hole where the carb
should be[8D]----------------------23,600

idle is a nice 5,000 with very good transition even though i have yet to play with the airbleed

obviously i need to play with the exhaust length, or better yet use a real pipe and i'll gain maybe 2.5K over the open exhaust.
the figure with the bladder is not 100%, the needle i used was too coarse and very touchy, but there is power to be gained with a bigger carb.
i also want to make up some new head buttons and play with the squish band and try the turbo plugs...more to come

dave

DarZeelon

Nice RPM numbers, Dave.

You are currently, with the tuned silencer and the carburettor, getting 0.71 HP. The manufacturer rates this engine at 0.7 at 26,000 RPM, so you are getting more than this engine is supposed to be giving.

Your current carburettor, like most other air-bleed type carburettors (OS FP, for instance) , has a hole in the barrel which is a little smaller than the corresponding hole through the carburettor body. It may be a good idea to enlarge and contour this hole, to optimize what the carburettor can give you.

These high RPM sounds must be heard, not written...

The tuned silencer does give a modest boost over the open exhaust.
It is more for getting at least the same power, with less noise.

In your situation, you are at the very top of this tuned silencer's effective RPM range, so the boost can be expected to be even more modest.
Less than a full length pipe would.

MVVS does not currently carry a full length pipe for this engine.

ZAGNUT

My Feedback: (1)

i cant really do much to the carb because the barrel is only 8mm in diameter...at the most i could drill it out to 5.5mm and i'm thinking that i'll need at least 6mm to let the engine breath. i could make the throat rectangular and gain some area...what you say?

not worried about the availability of a pipe, i'll be making my own from carbon. i've had some trouble with finding the proper resins for the bigger pipes that i've made but this thing runs so cool that i shouldn't have a problem with pipe life.

even though the engine already screams i'm hoping to get it up to 26,000 static rpm with the 7x4 once every thing is sorted out.

dave

Rudeboy

I tried to do the same thing... made a spraybar that bolts into the pinch bolt hole and uses the original needle valve... but my tuned pipe doesn't give enough pressure and fuel draw is zero with this setup...

Does your mini pipe give enough pressure or do you pressurise the fuel some other way?

downunder

My graph for a calibrated APC 7x4 shows .88HP @ 22,500 revs...and needs 1.36HP @ 26K. If the same prop is trimmed down to 6.02" then it absorbs .64HP @ 22.5K and .98HP # 26K.

DarZeelon

Brian,

I get the HP estimation from a Pé Reivers prop calculator.
You can download it free from his web site [link=http://home.wanadoo.nl/pereivers/prop-power.xls]here[/link].

Or you can PM me your E-mail and I will send it to you.
You will need Microsoft Excel, or another electronic spreadsheet program (Lotus, Quattro Pro) to use it.

I believe the Reivers prop calculator is also calibrated on APC.
If so (Is it, Pé?) these numbers cannot both be right.

Unless the one you are using is calibrated on a toothpick pylon type APC prop....

Dave,

To go to higher power at higher RPM, your current tuned silencer configuration will help very little.
You will need to shorten the system, but I cannot tell you how, or by how much.

der_steuermann

My Feedback: (2)

Hi Dave,

for good rpm results I'd go with the MACS tuned pipe # 1110 and set it at 8" from the glow plug to the pipe's biggest part. Also I'd use the MVVS .15 venturi carb. Drill this carb out to 5.8 mm dia carefully. To get the engine on the pipe properly stick with the APC 6.5X6.5 pylon prop and cut it down to 6". If torque is not the main issue the exhaust timing should be 180°

downunder

Dar,
I get my figures from an article written for the April '94 issue of Aeromodeller magazine where a Norwegian guy built a power absorbtion rig to test various prop sizes. APC were chosen simply because they had the least variation between similar props, not because they were necessarily the most efficient. Where too large a gap was found between APC props, Graupner nylons were used.

I've converted the graphs into a spreadsheet so that typing in a rev against a prop size gives both HP and torque. These figures should still hold today unless APC have drastically changed their moulds. But seeing that your (Pe's) HP figures are lower then possibly his were calibrated with a pylon prop.

ZAGNUT

My Feedback: (1)

der_steuermann,
how much rpm is your pipe adding to the open exhaust figure??
also, i want to stick with the 7x4 or maybe a cut down 7x5 as this is intended for a sport plane (most likely a delta) and i also want a little thrust and not just balls-to-wall speed. till now i've flown the 7x4 at around 16-17k and on a small model this reasonably quick. the reason for the MVVS is to get more speed from the same size planes without sacrificing vertical.

Rudeboy,
to get pressure for the big open intake i used a pressure bladder made from surgical latex, look at the attached image(stolen from combatpigg over in the 1/2A forum)

to run like this you need a needle with a fine thread/long taper. for starting you prime the engine, hit it with the starter and once it fires unclamp the fuel line and then adjust the needle. i used a needle from an enya and the starting position was less than 1/3 turn open so it was VERY touchy.

you can also use crankcase pressure to the tank. this also allows the use of an exhaust throttle. used to be popular with boaters and was called the "california mod" as it was mostly used out west.


dave

der_steuermann

My Feedback: (2)

Well, actually I never tested a 7" diameter prop in combination with the MVVS .15 since this is too large for speed application. Using the shortened APC 6.5X6.5 pylon prop the rpm peaked at 26.300[:'(]. This number has been recorded with a fully modded engine without R/C-carb...