RE: Modifying a 2 stroke
 

Ducted fan engines that are one third the displacement also turn three times faster. Carburetors are sized more for fuel draw ability than for top performance.

RE: Modifying a 2 stroke
 

I will have to check to see if he has notes on it. Often the info comes from customer beta tests.

I can tell you that the rpm increase is not substantial. Usually what we see with the turbo-jett is more of a torque increase, where you can carry a bit more prop diamter or pitch, at the same rpm you had in the with a stock muffler.

Bob

RE: Modifying a 2 stroke
 

Carb size is proportional to how much air the engine flows. The amount of air the engine flows is proportional to how much power it makes. So any 4HP methanol fueled engine will have similar carb size. Take a look at a carb for a hot car .21 engine that makes 3+HP. It's the same size as the above mentioned engines. Details in the design can have a big impact on flow efficiency for a given throat diamater so some engines make more or less power.

RE: Modifying a 2 stroke
 

After checking again with the calculator of Pe, yes, it's seen that even 300 rpm-increase is worth the effort !
Thus I will pollish all air-paths inside the engine; good exercise for the next winter.

Tunned pipe gives let say even 20% but only at some RPM-range, and takes maybe as well at least 20% on
the other RPMs, compared to open exhaust which "costs" only noise, and that is quite OK for some airfields.
Also open exhaust is the lightest solution. Is all this correct or I am overlooking something else ?

Thanks to Kweasel for the RPMs at open exhaust ! - I will use that to save money and weight from the res.tube
at least at this point.

Thanks to Bob27s for the Power-vs-Carb size explanation; now the only confusion(!) here would be that ducted fan
engines OS.65 and OS.91 with the same carb have pretty big difference in the otput power, nearly 1.5 times i.e.
still there is some "mistery" to scratch out.

Question to all and especially to Dar: - If we forget the special turbo-chargers, still we see couple "inconfortable" facts
like the one I menioned above for Carb-size and still one more - the Barometric Air Pressure, which many of you
professional guys take into considerations always for top-end performance. Then my early speculation and in fact
a hope for cheaper power-increase was to add a small (mechanical- or electric- driven) air-blower, like a small
ducted fan on the top of the carburator and increase the air-pressure as much as possible with that simple
"compressor" and thus add something to the other long list of imrovements, most of them without dramatic
effect anyway. The effort of building that is maybe even less than polishing the engine inside, isn't it ?

How much Air Pressure Increase this would bring, for example if I rotate about 20000 rpm a one-two inch
fan in a tube ? Any numbers you might mention, or all those numberrs are too small, even too small to
bring 200-300 rpm increase in BGX-1 ?

Do you know any one who have done that ? Do you know any WWW-page where I can get those numbers
from, for example the ducted-fan-combustion-engines jet planes pages - which ones to start with ?

Cheers,
Nick

RE: Modifying a 2 stroke
 

A correction please, above I wrote 'Bob27s' instead of 'gkamysz' - Thanks Greg !

RE: Modifying a 2 stroke
 

Nick, 1.5 times the power is hardly enough to worry about, especially when we don't really know if the 91 is sized correctly leaving the 65 with something too large. Maybe the internals of the 91 were refined to give better flow without a larger carb. Carb throat diameter does have an effect on fuel draw. Going oversize on the carb might net a few percent more power at the expense of poor fuel draw. I would think that to some extent the carb size is dependent on intake duration. It limits the percentage of each revolution the carb is flowing. In converting some four stroke engine to diesel I had to reduce the throat to get good fuel draw. I attribute this to the cooling effect methanol has on the intake mixture flowing a greater mass of air during the same operating condition. Kerosene doesn't have this nearly as much cooling effect so less air flows during the intake stroke, and less pressure differential to draw fuel is available. Even after the reduction in throat and airflow the power is roughly the same while burning kerosene. This must be due to differences in how the fuel burns.

Turbochargers in this size are rather impractical. Efficiency of a 31mm compressor I have peaks out at about 65%. This compressor is large enough for a 1 liter engine. At sizes smaller than this efficiency is reduced further and it soon consumes more power than it produces.

A positive displacement supercharger would work in four strokes. With the cost and weight involved it would be far easier to go to a larger engine, as someone mentioned earlier.

Ducted fans for pressurizing air is a complete waste of time. You can get maybe 1 PSI of boost from a properly design fan for such an application. In this size it will be even less. So again you have to fight efficiency to get more power out than you put in.

RE: Modifying a 2 stroke
 

Thanks Greg !

What you wrote, if I could make the link now, is that 173 deg opening of the crankshaft of BGX, nicely explained and
modified by Kweasel, seems to match now the relatively small carburator used for BGX.

Hi Kweasel and Everybody else as well,

You mentioned that you increased the opening of the crankshaft beyond 173 deg. Couple questions here indeed:
1. In which direction did you expand, symetrically or not ?
2. Did you increase the opening of the carb ? - in fact what strikes me here is that BGX has been shipped
with bras-insertion in the carb, which has anything else but smooth edges and many guys avoided the
use of Perry-pump to provide fuel and get rid of the brass-insertion, while trying all kind of resonance
pipes and compression ratios to increase power; is that a bit dummy-approach or there is something
hidden here, like problems with Perry-pump for instance ?

Regards,
Nick

RE: Modifying a 2 stroke
 

As much as I'd like to try some of this , I'm going to size up a 50cc gasser to see if it fits. If so, it's goin on their!. Need to get the first gasser under the belt.

RE: Modifying a 2 stroke
 

The closing point of the rotor valve is most important. I cut mine to open 3 degrees sooner and 7 degrees later with no adverse effects, this is the about the same timing as the 140RX. If the closing is too late then you will see some gas reversion at idle and reducing throttle quality at low speeds . I would keep the closing no later than 50 degrees ATDC. If the rpms are relatively high (9000) you can remove the carb sleeve and have good fuel draw. I tried a Perry pump and experienced problems from over pressurization. If you use one reduce its pressure. Remember that the compression is very low and it loves nitro, for no nitro fuel I would cut down the head.

RE: Modifying a 2 stroke
 

Here is a good way to make more power: Mirror finish everything from the carb to the exhaust on the inside of course. polish the entire crankcase, rod and be sure to radius every angle change. You want to make everything as smooth as possible. use lots of polishing compound with the final being the red jewlers rouge. On the intake side of the piston when at top dead center the piston skirt should be above the top of the intake port. If it is not remove everything you can see, following the contour of the top of the intake port. I know what your thinking, these surfaces need to be rough to keep the fuel/air mixture right. An engine is nothing more than an air pump that gets hot. the faster you can get air from point a to b will increase the work that can be done by the engine. Over a long distance the fuel particles will fall out out of suspendsion due to gravity but in small engines this distance is so short that this will never happen. I have used this method on old kawasaki 3 cylinder/2stroke engines with great results, no re-jetting, or changing to different plugs.

RE: Modifying a 2 stroke
 

Thank you Kweasel and Woolfster2 !

Kweasel mentioned the OS140RX and that was very good hint too ! It is surprising to me that OS140 and
also OS160 have lot better Power-to-Weight Ratio than BGX-1. Should I consider that at the OS-company
there are different ideas/teams who do not really share the experience or the BGX-1 came to market
lot earlier than 140 and 160 engines.? Somewhere, I guess it was in the book of Dave Gierke,
'Airplane Engines Guide' is mentioned that BGX-1 was built to counter the development of the
35cc gas engines. Then fuel economy might have been the reason to have BGX-1 performing
poorly compared to 140 and 160 - Is that the Case ?

Hi TrollMaster and Everybody,

I wander why should you go to gas - if you refer to what the competition results were the last couple
years on the big flying copies, Scale-Racing, most of the winers used Zenoa-gas 80cc converted to
Methanol, and some of them even with the help of Zenoa company !?!? - isn't it that a blow to
the Gasoline Line ??? [sm=spinnyeyes.gif][sm=spinnyeyes.gif][sm=spinnyeyes.gif]

In that sense moving to 50cc gas I guess you would achive the perfoamnce of a good, i.e. well
MODIFIED 35cc methanol one and will have the trouble of heavier engine, addign the weight of
a ignition system and battery, spark-intereference to radio, and also the smell of the gasoline
(of course F3A has gone electric and there is no smell there but the $$$$ dollars count is too high).
Of course the gasoline fuel is less amount needed and cheaper too but is it really worthed, after
seeing what those Zenoa-supported guys are doing even at 80cc ??

Well, in a way or another one major Question in this discussion seems to turn to some extend to:
- How much is worthed to Modify 35cc-methanol or better to buy Standard 50cc Gas ???

Cheers,
Nick

RE: Modifying a 2 stroke
 

you guys know that supercharging is useless right?

RE: Modifying a 2 stroke
 

I suspect that with the proper tuned pipe some boost can be attained. However, I suspect it would be best for one speed wide open throttle applications. Unless fuel efficiency at part throttle is not a concern.

RE: Modifying a 2 stroke
 

Hugh,


Boost from a tuned exhaust is achieved in an RPM range. This range can be pretty wide, like with a tuned muffler, or a bit less wide with a cone+disk tuned-pipe, or it can be narrow (with a dual-cone tuned pipe), but it is not effective just at 'one speed'.

It is true that at part throttle settings and below boost RPM, the low-pressure pulse arrives at the exhaust port and does suck some of the mixture from the cylinder into the header, only to be crammed back in when the reflected high-pressure comes too early, pushing this 'potential supercharging' back across the cylinder, through the still open bypass and boost ports, so no supercharging takes place.

But no actual fuel waste takes place either.

If the engine is allowed to over-rev, beyond the boost RPM range, the potential supercharging mixture is stranded in the header, by the rising piston masking the exhaust port shut, before the reflected high-pressure wave manages to push the charge back into the cylinder.

Only then you get wasted fuel that can amount up to 20-25%, but you would need a much smaller prop for the engine to reach this RPM. This is why too long a pipe is a very effective 'RPM limiter'.

At really low throttle settings and RPM, the reflected wave can theoretically increase sub-piston pressures to super-atmospheric, resulting in some of the intake charge being spat out through the carburettor. But the lower magnitude of the pressure waves at part throttle and the lower initial pressures in the crankcase (at part throttle the average crankcase pressure is sub-atmospheric), this will rarely happen, it at all.

RE: Modifying a 2 stroke
 

This was in referance to using superchargers with two stroke engines. As such I suggested they would work with a tuned pipe, but only at one speed or perhaps more precisely with a very narrow range. Though a tuned pipe works at a wide range, the supercharger would only result in wasted fuel except when the tuned pipe is working at its best RPM.

RE: Modifying a 2 stroke
 

Hi Guys,

Do you have or have you seen for any MVVS and OS, and maybe Webra or ST, the curves which tell the
Power Boost/Damage versus the RMP, of course plotted in the same graphic field where the open exhaust
performance is given ? I would imagine that with Open Exhaust that curve is most flat i.e. "monotonic" one,
while with the pipes and muflers there are certain rpm-areas of higher and lower Power, isn't it.

Regards,
Nick