asmund

Hi, does anyone know the venturi size of the stock carb on the MVVS 26?? I think it is called WT-613, but I can`t find anything useful on the web about it. I`m wondering if a WT-391 at 11.1 mm would give more power over the stock one?

Charley

My Feedback: (2)

One of the hop-ups that Pe Reivers suggested was to drill the 613 out to 11 mm, so I suppose the 391 ought to work the same.

CR

asmund

Thanks, I just found out that Walbro stamps the size inside the venturi in 64` of an inch. The 391 is stamped 28 so it is 11.1mm and my stocker is stamped 24, making it about 9.5mm. I think I will have to try a bigger carb or drill up the stock one and see what happends.

Thanks for your reply

asmund

I just tried my MVVS 26 with the 391 (11.1mm) carb and got 9000 rpm on MenzS 18-6. This is an increase of about 200 rpm so far but I can propably lean it some more as I have not leaned it till it bogs yet (then of course richen it up again), so I`m hoping for 100-200 rpm more

Oh, I forgot. I`ll have to move on to a bigger prop now, maybe a 18-8 or 19-6 or something like that

pe reivers

The 2007 redheads were fitted with a 9.5mm carb.
The beefed up 2008 engines have a 11.1mm carb to take advantage of the increased engine load capacity.
Menz 18x8 at 8250 rpm, not completely leaned out yet on Standard MVVS tuned pipe. This is serious pulling power.

asmund

Are the new ones really beefed up? I thought the only "new" was the tilted plug and the lipo ready cdi. Other than the bigger carb on the new ones is there other power releasing differences? So, my goal now must be to achieve 8250 rpm on MenzS 18-8, do you think that is within reach based on my 9000 rpm reading on MenzS 18-6?

I`m thinking about doing the pipe mod to get even more power, but I`m not sertain that the increase in noise will be worth it. The article says 97 db on 1 meter distance after the pipe kutting, so it isn`t too bad if that holds true

Here`s the article about tuning the 26cc engine: http://tech.flygsw.org/evolution26review.htm

pe reivers

Yes, the crankpin went from 8mm to 9mm dia, the pin length from 10mm to 12mm, providing 40% more load bearing capacity. The crank balance weight had to be altered to follow suit. To accomodate the heavier crank, the housing needed rework as well.
The engine has had a thorough make-over using all lessons learnt.

asmund

Thanks pe.
hmmm... now I want a new one (I have the red head from a few years back)

Do you think it is a bad idea to try to squeese more power out of mine by doing the pipe mod?
My guess is that there`s a reason for the beefing up of the crankpin, what do you think?

pe reivers

The reason for the upgrade was the advent of this little powerful gasser to 3D use. With the high big end loading, oil needed to be of high viscosity, that is why only 50 grade racing oil would do. This oil has in excess of 10 centistokes viscosity as the bearing needs it.
with 3D use and sloppy cowl designs, operating temperatures crept up. The oil got thinner, and could not keep up with the load. So a redesign was mandatory to make up for the abuse.

To do in older engines:
Add a gasket under the head to reduce compression ratio, and keep static full power rpm above 8000.
Use only SAE 50 grade fully synthetic racing oils.
Use a good cowl design so the engine gets the cooling it needs. Glow engine practice just won't do. (a need for ALL gas engines)

asmund

Thanks alot for your input. 3-D is the name of the game for me now, my engine is doing duty in my Harrier 90 and I have a peak of 9000 rpm on the MenzS 18-6 (don`t think it is fully leaned out yet) I do not use a cowl and it is not overly warm up here in Norway so I think I will be fine then.
I`m running 98 octane petrol with 3% Putoline TT Sport oil (same oil as works great for thousands of kilometres in my supertuned Yamaha Aerox 50cc, now 80cc with everything else that goes with it, about 20 hp)

pe reivers

18x6 seems the prop to go with good acceleration, and sufficient pull-out power. 18x8 pulls harder, but spools up slower.
I do not know the putoline oil you mention. I did have intensive contacts with the Putoline tech rep, but this oil never came up. Do you have spec's ?
Putoline oils I know of, and stock:
MHX: top of the line SAE 50 grade all out racing oil, hot case engines like in watercooled, advised for the MVVS 1.60
RS 959: SAE 30 grade racing oil, for air cooled engines
cart oil, castor based premix. Can be used in glow, and diesel engines.

Putoline (now subdevision of Kroon oils) is made in Holland, and the TT sports oil is not known here?

viflyer

My Feedback: (13)

Pe,

So what is considered "running hot " with one of these motors, and where is the best place to measure operating temperature?

pe reivers

The best place to measure is under the plug, using a sensor integrated in the plug gasket.
The second best is to place the sensor, protected from cooling air flow, snug to the cylinder wall at the highest point possible. In most cases this is between the first and second horizontal cooling fin.

As to temperatures:
A well tuned engine will ultimately reach 200°C head temperature, which also should be considered the limit, as documented by Rotax in their ultralight engine manuals. Roy (tired old man) has made a connection between 160C and major failure limits, using sophisticated systems during long term use. I ran my test engine up to (short term) 220C in tests for lubricity of oils and testing cooling systems. (second rib system)
Mark Fues ran his engines quite a bit hotter at 230C, as evidence by the liner colour of his disassembled engine. Mark did run his engine not slightly, but way too hot! see
http://tech.flygsw.org/evolution26bearings2.htm
The engine internals show conditions much better than any measuring device, but you got to know how to read all of them. I am learning a little bit more every day I am allowed to.

viflyer

My Feedback: (13)

Hi Pe,

So when the pre-2008 motors are run too hot, is there usually an order of failure in the internals?

It would seem that "hope ups" like Marks might require frequent rebuilds??

pe reivers

Not only older motors (engines)
The cantilever crank design in general (MVVS, DA etc.) , like much in use these days, have a crank bending moment on the bearings without any support at the rear of the engine. This puts a higher demand on the oil to keep the moving parts in the bearings from galling and pitting. The oil must therefore have a higher viscosity. If the engine gets too hiot, the oil viscosity can drop below the treshold that is required, and bearing damage will result.

I do not know if Mark needs frequent overhauls. A lot depends on the way he cares for and runs his engines. Why not ask him?