Originally Posted by RC NEW KID
Hi, I just got a RB v12,,,it had a white insert in the carb,,I thought it was not supposed to be there, so i removed it. I come to find out it is something called a venturi. Will someone please explain to me what this is and what effect does it have,,both with it in and with it out of the carb. Thank you.
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 whivh the air is less denser (less available oxygen), an engine will not be able to burn as much fuel as it does at higher dnsity 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 dendity situations, that is whwere the different sixed Venturis como 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 size of Venturi best suits your ambient situation.