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Velocity Stack on the 35 gasser ??

Old 02-18-2008, 04:02 PM
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the Wasp
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Default Velocity Stack on the 35 gasser ??

so I was wondering if any of you MVVS tech guys have tested a Velocity Stack on the MVVS 35cc gasser (or even the 35cc glow)

I was wondering what the results were, more power, more RPM maybe ??, it would be nice to pick-up another 200 RPMs cheaply

Jim
Old 02-18-2008, 04:31 PM
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DarZeelon
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Default RE: Velocity Stack on the 35 gasser ??

Jim,


What most people think is a 'velocity stack', isn't...

To actually increase the performance of a two-stroke engine, at its second harmonic in its operating RPM range (7,500-9,000), it would typically have to be ~7.9" inches long...

The nice 'skirt' intake may smoothen the flow a bit, but adding power requires the length for optimum flow, for a pulse strength of ~10%.


Can you imagine an 'intake chimney' nearly 6" long (2" are in the carburettor and the crankshaft), on your MVVS 35 cc engine's carburettor?...
Old 02-19-2008, 01:10 AM
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Default RE: Velocity Stack on the 35 gasser ??

OK, LOL Carburetor Trumpets as RC Showcase calls them (see links),, Velocity Stack is what the Auto Parts Industry call (ed) them for years,,

anyway, I was thinking it might help with that big tapered mouth right behind that big prop..

http://www.rcshowcase.com/html/acces...strumpets.html

Jim
Old 02-19-2008, 03:37 AM
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Default RE: Velocity Stack on the 35 gasser ??

Tried this first on the 35 gasser


Result, only a very small (100) increase rpm, but unstable running. Removed it.

With the MVVS stack

Same rpm but a more stable running, so it is a keeper.
Old 02-19-2008, 10:03 AM
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Default RE: Velocity Stack on the 35 gasser ??

@hans:
That more stable running probably was due to the more recessed location of the choke valve.

@dar/wasp
in my 50cc racer, we found a best total intake duct length (by cut 'n try) of 40cm, with the carb located 1/3rd that distance from the piston skirt.
Torque started above 9000 rpm. Anything below that rpm caused the engine to flood instantly if the throttle was more than just slightly open, due to the multiple blowback gas passage past the jets.
Old 02-19-2008, 10:44 AM
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Default RE: Velocity Stack on the 35 gasser ??


ORIGINAL: pe reivers

...Anything below that rpm caused the engine to flood instantly if the throttle was more than just slightly open, due to the multiple blowback gas passage past the jets.
PĂ©,


Tuned length runners work great with port fuel injection, like in many cars.

This is because those multiple reverberations don't affect the mixture strength, with the fixed amount of fuel injected at the port.


Any power boost will only happen at the various harmonic RPM values, but the mixture will not be affected.

The [link=http://www.bgsoflex.com/intakeln.html]software for the calculation of the intake runner length[/link] assumes the runners precede the point of injection. It is also designed for four-stroke engines, so the RPM numbers must be halved...
Old 02-19-2008, 05:06 PM
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Default RE: Velocity Stack on the 35 gasser ??

Dar, the link is for 4- strokes, and lacks valve cam timing input possibilities..
With two strokes, timing is more complicated, and must take into account whether you hace rotary, piston, or reed induction.
Also, the Bowling & Grippo guys have their facts wrong (in the text). The result is OK though.
A suction wave is reflected as a pressure wave when it meets an open ended tube. So in very short tracks, the wave is bounced back to and fro, until it eddies out. This is the least critical, but causes blowback at low rpm if the piston or disk closes late.
Full length, or 3/4 length tubes work better, but are critical at rpm below resonance.
Injectors (active fuel injection) should sit as close to the crankcase volume as possible, carbs (passive injection) should be as far away as possible.
Old 02-19-2008, 08:46 PM
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Default RE: Velocity Stack on the 35 gasser ??

I think MVVS should look to the future and start with fuel injection. Have an EEPROM fuel management unit(FMU) with a couple of basic tunes and a USB cable for tuneability. Make a simple program for changine fuel and injector timing, RPM, etc. A small piezo electric injector(already developed) and a throttle position sensor. Map sensor installed in the FMU. Just your basic speed density EFI. Just like new age hot rods. I can tune my cars all day long with my laptop in the front seat. Would be fun to have the ability to do it to an airplane. Rid ourselves of carbs and tuning issues!
Old 02-20-2008, 09:00 AM
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Default RE: Velocity Stack on the 35 gasser ??

you want me to tell them?
Could be fun if prices soar due to small production quantities.
Old 02-20-2008, 05:53 PM
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Default RE: Velocity Stack on the 35 gasser ??

thanks for the info guys !!

but you know what we really need ? an in-cowl muffler that will increase power a bit over the today's available mufflers, something like the Youngblood MP2/MP5 heli mufflers,,

http://www.heli-world.com/detail.aspx?ID=5679

a 300 or 400 RPM increase would be quite noticeable on my GBY !!

personally I wouldn't mind a long tuned pipe mounted to the side on my GBY, I think it would look cool, but the guys at the field would really give me a lot of grief about it !!

Jim
Old 02-20-2008, 10:00 PM
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Default RE: Velocity Stack on the 35 gasser ??

I am not one to be a price shopper. Infact I spend more money to have the best product for the best power. I spend tens of thousands on cars and hp, yearly. Just for fun. I do see your point though.
I laid money down once to have a supercharger custom built for an engine but once the builder stated that he was going to charge me more money to make it than it would cost to produce the system for other people, I pulled my 10k+ money back. I dont want to pay to be the guinea pig when others will benefit. Thats the companies job.
Just tell MVVS to make great and powerfull engines please. Thank you Pe.
Old 02-21-2008, 01:27 PM
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Default RE: Velocity Stack on the 35 gasser ??

I think they are doing just that. If there is a market for fuel injection, and a sustaining profit can be made, I am convinced they will do it.
But just look at OS. Some aerobatic crowd people use the engine, and pay through the nose for it, to be beaten by the humble MVVS glow engine in matters like reliability and userfriendliness. OS is a huge company that can afford to have up front devellopment, even if they loose bigtime on it. The spinoff is advertising value.
Judging by the teething problems, fuel injection has to come a long way before it is ready for our engines.
Old 02-23-2008, 02:45 AM
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Default RE: Velocity Stack on the 35 gasser ??

Which MVVS Engine are you referring to Pe?
Old 02-23-2008, 03:56 PM
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Default RE: Velocity Stack on the 35 gasser ??

None in particular.
With RC aerobatics, the 1.60 glow competes alongside with OS glow engine types, and holds its own. The gas engine was at a very slight disadvanage because of it's lower specific output. Not much, just slight. For champs that is enough to notice.
Old 02-28-2008, 03:36 PM
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John Serjeant
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Default RE: Velocity Stack on the 35 gasser ??

Hello from the U.K.
Velocity stack or trumpets?.
Anyway, I,ve two MVVS 1.60 engines and both run well.
However, I do notice a certain amount of unused fuel, or more precisely, the oil part of the gas, seems to "get out" of the carbie and sprays the engine and interior of cowling.
Not to a large degree, but enough to ask, why does this happen?.

Would a "stack" minimise this fuel being blown out, thrown out of the carbie?.

I do like the look of the MVVS stack fitted, as in the picture in a previous post to this thread. Where do you get one!!.
Comments welcome please.

John Serjeant.
Old 02-28-2008, 04:16 PM
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Default RE: Velocity Stack on the 35 gasser ??

John,


As you noticed already, only a small amount of fuel is thrown out from the carburettor.

This tends to be more noticeable at part throttle settings...

There is a certain number of crankshaft degrees, that while the crankshaft rotary valve is open, the piston is descending, raising the crankcase pressure beyond atmospheric.

When this pressure change happens at high engine speed, the inertia of the air coming into the carburettor and running through the crankshaft, will keep the flow in the correct direction, i.e. into the engine.
But when this happens at mid-low engine speeds, the flow is actually reversed and mixture is expelled through the carburettor, taking some more fuel with it, as it flows out.

This creates a small 'cloud' of fuel-vapor close to the carburettor entrance.


For this, an extended intake 'trumpet' will offer a full solution, as that cloud is contained within the trumpet and not thrown, by the prop-wash, all over your plane. It is then sucked back into the engine by the engine's next induction.


You will have to close the low-speed needle a noticeable amount, to compensate for the lost fuel that no longer is.

Old 07-14-2008, 11:52 PM
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Default RE: Velocity Stack on the 35 gasser ??

Question for you all...

I have a hangar 9 sundowner(second one by the way) with a Evolution 35 and I'm having a hard time with dead sticks...


The first flight on my first sundowner ended in a deadstick that I believe was caused by a lose spark plug wire. I didn't seat the clip correctly. I'm beginning to question that reasoning as I had another deadstick today and at this point I'm losing alot of trust in this engine...

Reading sundowner fourms people are recommnding closing off the half of inlet hole in the cowl and adding big holes in the bottom to maintain airflow. They are recommending this as they believe that there is an airflow distrubance caused by the choke lever. Well I did all that and I'm still having this engine quit on me. This is my first gaser and I'm assuming the mixture is good. Good transitions and it four strokes at idle and no where else in the throttle range. Other Evo 35 owners at the field says that it sounds fine. Mixture is 32:1

So my question is, will a velocity stack help? If so, where do I get one. The wider one shown in the picture above looks interesting. This looks like one, but I'm concerned this will not allow me to reach in and choke the carb by using the actual plate versus the lever. http://www.horizonhobby.com/Products...ID=EVO30941321 Because of the fit in the cowl, I don't have access to the lever.

Any suggestings?
Old 07-15-2008, 12:20 AM
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Default RE: Velocity Stack on the 35 gasser ??

personally, I had 2 dead sticks on my MVVS 35 and I believe it was do to too lean on the low-end, so richen up your low-end needle and fly again, but keep flying around the landing strip LOL

I will comment on props,, my PA (ARF 80 inch span) Gee Bee Y is rather fast at full throttle, and this plane sure is not as fast as your Sundowner,, I use a 3W 20x8 and I can get 7100,7200 tuned for flight, peak RPM is 7370..
Old 07-15-2008, 07:48 AM
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Default RE: Velocity Stack on the 35 gasser ??

That is not a velocity stack, but the carb mounting base.

Your dead sticks are probably due to lean mixtures. The engine should four stroke slightly up to half throttle. Above that, tune for clean running.
Set idle speed at 1600 - 1800

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