Everything2Stroke Forum banner
1 - 7 of 7 Posts

·
Registered
Joined
·
55 Posts
Discussion Starter · #1 ·
"Bigger is Better" is the common theme when it comes to carb size but in reality there is a sweet spot for size, and other than trial and error the only way to determine it is by calculations. Changing the carb size is changing the air velocity and the air velocity past the needle is what determines the fuel droplet size which is what affects performance. Different droplet sizes (and the % that is vaporized) affects the rate of combustion, how fast the flame front expands. When you select a size you are basically just selecting the air velocity and how atomized the fuel becomes. There is a perfect size for each engine/RPM and you can go too big or too small and miss out.
Factors mostly affecting burn rate are carb air velocity, engine compression ratio, gas octane, and squish velocity. A formula to help determine the optimal carb size would also require inputting the cylinder bore mm since time for the flame to reach the cylinder wall should be around the same degrees of crank rotation as other successful engines. Of course also needed is the RPM that you want to target for power by the carb size whether it is peak power RPM or lower.
Don't forget the ignition timing to match everything which should be adjusted also. (go by the clean band on the end tip of the plugs center electrode).
So the gist of this all is that it can be computed without having to guess and be out a lot of money buying different carbs. Here’s my video on the subject:
 

·
Registered
Joined
·
56 Posts
I like your quest to find the correct info, however there are a few mistakes in your reasoning. Fuel/air burns at around 1 m/s approx. This would mean that engines would not run above 1000 RPM. Turbulence and tumble from squish and angled intake ports is the process used to speed up combustion.

=> A formula to help determine the optimal carb size would also require inputting the cylinder bore

Nope, carb size is related to cc, not bore (and to max RPM) . Bigger cc, bigger carb.

For atomization, the carb size does not matter to some extent. If you close the slide, the air velocity increases at the restriction raising the speed and this helps vaporize the fuel in conjunction with the air jet, idle jet, needle jet etc. All these work together over a wide range to help get the air fuel ratio and droplets be correct. PS The holes in the needle jet help with making the fuel froth so it vaporizes easily.

Carbs are pretty amazing things as they work over a 1000 to 12000+ plus and range from 0 to 100 % power.
 

·
Registered
Joined
·
33 Posts
OK so I guess it's my time to return corrections to you.
Many engines don't even have a squish band, my Suzuki AX100 included. Turbulence increases with RPM which makes the mixture able to burn fast even without a squish band. And even without a squish band the burn speed is much higher than what you mistakenly listed. Gordon Jennings said engines reving under 9000 don't need a squish band although there are engines reving much higher without them (but probably better power can be had with squish).
Yes it is related to cc but bore is part of the equation to determine that. The vacuum in the crankcase that pulls in charge from the carb starts when the rising piston closes the transfer ports, so an accurate estimate of cc the carb provides depends on bore and stroke from transfers closing to TDC.
Matching a carb diameter to an engine depends on max RPM so your words about atomization are just off the mark since they are about gradually opening the slide as the RPM's increase.
 

·
Registered
Joined
·
56 Posts
Yes you don't need squish as there is always some turbulence. Ideally you need the turbulence in the order of 22 m/s. Squish helps.

As far is crankcase suction from the piston goes, that is old school. The exhaust these days creates the vacuum to suck the new charge in. Once a modern motor is running, you can virtually open the crankcase to the atmosphere permanently. I now visualise the 2 stoke as a pulse jet with a piston in the middle. It makes thinking about it much clearer. It all works on pressure waves.
 

·
Registered
Joined
·
33 Posts
When making a spreadsheet calculator you just depend on the volume of the piston stroke above the transfers because the amplitude of the vacuum wave from the pipe varies greatly and more than anything it works by matching the crankcase vacuum so the intake charge that entered the cylinder by BDC will not be sucked back down into the crankcase as the piston rises. But by doing this the flow thru the reed valve starts earlier than when the transfers close. But like I said it is a great unknown value so it's safer to just stick to the base value due to the piston rising past the transfers. An adjustment factor is added to shift the calculations into the range of what is commonly experienced. This works out really well without a super complex program that requires a mind bending amount of data.
 

·
Registered
Joined
·
56 Posts
Which can do more work, -14 PSI (Maximum vacuum) or several hundred PSI pressure when the exhaust opens? The exhaust has much more energy. This energy is used to suck the new charge into the engine and it sucks in more than the actual cc of the engine. That is why 2 strokes can make such incredible power for their size. It is like supercharger. If you don't believe me, remove your exhaust and see what happens to your power.
 

·
Registered
Joined
·
33 Posts
You obviously have never looked at a pressure graph from a probe inside the header. It's only a peak near 22 psi and the return suction wave around -10psi
Slope Rectangle Organism Font Line
 
1 - 7 of 7 Posts
Top