Smaller venturi carburettor
 

So many questions to ask and not enough time to post :D

The venturi on my NOVA MEGA 12 5pt turbo came with a 5.4 mm plastic venturi that was in the carburettor, is this standard or is it due to rules and regs etc, if i was to take it out would this increase power but throttle response would be less??

VENTURIS

Internal combustion engines require three things to produce power - fuel, oxygen and heat -- and they need them in specific ratios. Running in situations in which the air is denser (contains more of oxygen) an engine produces power with ease. In situations in which the air is less denser (less available oxygen), an engine will not be able to burn as much fuel as it does at higher density air situations. Less fuel means less power, and that means slower speeds.

What can you do to minimize the effects of less denser air ? Compression creates power, and when the air is less dense, there is less fuel/air mixture available to compress.
To recover some of the lost compression, you can use a fuel containing a higher percentage of nitromethane, or reduce engine-head clearance to increase the compression ratio.

To compensate for less density air, you need to adjust your needle valve to a leaner setting to maintain the proper fuel/air ratio. The opposite is true for higher density situations. The idea is to maintain the optimum ratio of fuel and oxygen by adjusting the fuel volume to compensate for ambient air pressure.

Now, if you have the appropiate compression, and still need some more fine tuning for lower density situations, that is where the different sized Venturis come in handy.

A venturi such as a model engine carburetor's works on the vacuum principle. As air rushes through the venturi, it accelerates and creates a vacuum at the spraybar. This vacuum draws fuel from the spraybar and fills the crankcase with a mixture of air and atomized fuel; at very low density situations, less air accelerates through the venturi and past the spraybar. This reduces the vacuum at the spraybar, so less fuel is drawn through it, and the mixture becomes too lean.

There is a way to compensate for this: Switch to a venturi with a smaller area.

When you have a smaller venturi area, the airflow velocity through the venturi is increased. This also increases the vacuum at the spraybar, which, in turn, draws more fuel.

Obviously, you have to experiment to learn which combination size of Venturi best suits your engine, and ambient situation, and you have to take into account the rules and regulatios of your association or club. IFMAR / ROAR staes 5.5mm for .12 engines, and EFRA states 5.4mm as maximum dia. of venturi at the slide side of carb.

AFM

That is a fantastic explanation :D

Thanks alot!!!

i race at 500 feet above sea level. we use smaller venturi's to broaden the power band and decrease fuel consumption.

So if I understand correctly putting in a smaller venturi increases horsepower?......

Depends on the air density and or altitude at were you race.

The atmospheric pressure, temperature and humidity all affect the density of the air. On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the amount of oxygen available for combustion and therefore reduces the engine’s horsepower and torque. For tweaking the fuel/air mixture and compression ratio, the air density is the most important consideration.

Use the following as a guide to correcting your setting when the weather changes:

Air temperature: When the air temperature increases, the air density becomes lower. This will make the air-fuel mixture richer. You must lean the mixture to compensate for the lower air density, and/ or change to smaller venturi to increase suction and avoid over leaning. When the barometric pressure decreases, the opposite effect occurs.

Humidity: When the percentage of humidity in the air increases, the engine draws in a lower percentage of oxygen during each revolution because the water molecules (humidity) take the place of oxygen molecules in a given volume of air. High humidity will make the air-fuel mixture richer, so you should lean the mixture.

Altitude: In general, the higher the altitude the lower the air density. When driving at racetracks that are at high altitude, you should lean the mixture and change to smaller venturis to increase suction and avoid over leaning, and also increase the engine's compression ratio to compensate for the lower air density.

AFM

WELL said afm! :nod: :nod:
Thanks for the info. :D

Hey AFM. Very nice explanatory for wheater condition. Just couple of things doesn't come together in the pazzle.
1. A venturi such as a model engine carburetor's works on the vacuum principle. As air rushes through the venturi, it accelerates and creates a vacuum at the spraybar. This vacuum draws fuel from the spraybar and fills the crankcase with a mixture of air and atomized fuel; at very low density situations, less air accelerates through the venturi and past the spraybar. This reduces the vacuum at the spraybar, so less fuel is drawn through it, and the mixture becomes too lean-are you running with no pressure from exhaust? You know that pressure from exhaust is about 12-15 PSI (.8-1 bar) and in our carburators if we use pressure line from the pipe it really doesn't metter vacuum effect you are talking about. Put bigger insert, so more air can be suck to the engine.
2 Altitude: In general, the higher the altitude the lower the air density. When driving at racetracks that are at high altitude, you should lean the mixture and change to smaller venturis to increase suction and avoid over leaning-I think I got problem to understend sentnce above-should I lean engine or should I avoid overleaning?
Are you came from airmodeling-on any classes there they don't use pressure and some of your ideas are working there. But when we have pressure line it works slightly different.
One thing I agree with you with no questions-lower air dencity-lower the head volume to bring back compression ratio which lost due the smaller ammount of mixture in combustion chamber. Putting smaller ventury will decrease ammount of air (WE ALREADY HAVE SHORTAGE DUE TO THE LOW AIR DENCITY)- this will bring motor power way down.
Ones again, as long as we use pressure from exhaust-we really don't care of velocity of incoming air through the ventury("which increase vacuum effect")-more will be there is better-never use smaller ventury on high altitude racing condition.
Edward

Hi Edward

Thanks for your opinions:

1.- Reality of racing with IFMAR, ROAR and EFRA rules limits us to 5.5 or 5.4mm venturis, so there is no possibility of increasing size, and yes we use pipe pressure line but very very long line to avoid fuel foaming.

2.- Have you raced at a track 3,500 mts high?? On Road experience gives us the fact that we have to use smaller venturis, not bigger, when using bigger venturi engine chokes and you have to lean it to far and it overheats because to little lubrication is the bottom line result. Engines work better with smaller venturis (5.0 or 5.1mm) and carburation is more stable without overleaning.

Just my experience at such altitudes.

AFM

My home city is Tashkent, Uzbekistan, former Soviet Union. Altitude about 2000 feet over the sea level. I race there from 1976 till 1993. I know little bit about race in those conditions. More then that we use pressure line in Uzbekistan and when we went to see level areas we use no pressure, just centrefugal forces (tether boats and airplanes), on RC boats always use pressure line. On 15 size engines we used 8mm ventury at see level and 9 mm in home. Results were the same.
I checked Lima Peru altutude and it came as 133 meters over the see level. Which track is located @ 3500 mts? It is 11500 feet over the sea level.
I found La Paz capital of Bolivia located @ 3650 (12000 ft)-probably people race there, I was looking everywhere and couldn't find any cities where people race @11500 ft-it doesn't mean they don't exsist-I just can't find them.
And Inever race in that conditions for sure.
Edward

Well Edward, here in Peru appart from Lima, which as you say is at sea level, we have cities like Cuzco which is at 3325, Arequipa which is at 2750, and we do some races over there trying to promote the hobby. Also I have customers in Quito, Ecuador, which is at 2250.
So we had to deal with some weird tuning to make this little buggers work, and also we ourselves have to "tune" for driving there.... jajajajajajajaja

AFM

yes you got to deal with that. I never run at that altitude and probably have to learn one day. Defenetly I will start from 5.4 and will stay there.
May be you should try to squize head clearance to .2mm or .15mm and it should work.
This is extreme situation and might be it will required different tunning-I got make some calculation to tell you more precise.
Edward