And My Excel Calculator Now the Carb Calculator comes with a reed valve calculator on its sheet 2. With it you can compare reed valves, virtually try different reed materials (carbon fiber, fiberglass, steel), and try out the modification of removing the valves ribs along with making your own reed petals. See my Youtube video about it. There are many contributing factors to the combustion burn rate which should match the target RPM (which is lower than top RPM for street and trail bikes). The idea is to have the combustion mostly complete before a certain crank degrees at the target RPM. The contributing factors are carb size, jetting, compression ratio, cylinder bore, gas octane, and squish velocity.  I found out that the old formula people have used to figure out the correct carb diameter is .8 x square root(cc x RPM/1000). That formula and any carb sizing chart are just over-simplifications for the non-technical public.  More velocity atomizes the fuel more which increases the burn speed. More compression increases the burn speed. High squish velocity increases burn speed but I doubt manufacturers use that trick much because it also contributes to detonation. If you are wanting the best carb for racing then consider how much time you spend at the last 1000RPM of the powerband. A racing sized carb would be good for that but for anything else then consider a smaller carb which will help out with mid range power. Sometimes the crispness of the power exiting corners is just as important as the top RPM power. Surprisingly, carb air velocity at the needle is not a linear increase with RPM. Since the only two variable factors both contribute to a faster burn as they increase, then it's easy to average them together (see green graph below) to get an idea of their combined effect. The graph of their combined average shows a steady increase up to max RPM.  Below is a screenshot of the Excel spreadsheet. To use it just enter the needed data into the light blue cells and the calculated results will display in the other cells. For reed valved engines the best carb size for street/trail is at G11, and the best carb size for top RPM racing is shown at H11. For piston port intake engines the best carb size for street/trail is at G16, and the best carb size for top RPM racing is shown at H16. Large carbs produce more large fuel droplets which cool and lubricate better which is important to race bikes. And smaller carbs give better economy although they tend to let the engine run hotter.  EXAMPLES: Honda CR500: Honda equipped it with the same size carb as their 250, a 38mm. The calculator recommended 37mm for trail use, and 39mm for racing. 38mm is a good all-purpose compromise. Also consider if the carb is a round or flat slide. Round slide is better for trail riding because round slide carbs aren't excessively rich mid throttle, whereas flat slide carbs are inherently that way which is good for racing but bad for trail riding.  Honda CR250: The calculator shows a 38mm carb would be best and Honda uses a 38mm.  Honda CR125: The calculator shows a 37mm carb would be best and Honda uses a 38mm. Notice with a 36mm (top data below) how the air velocity only just barely is beginning to get in the red zone. Stank Dog, racing a Husqvarna TC125, says he prefers the 36mm carb.  I went on some forums and asked for full data on peoples rides if they had experimented with different carb sizes for the highest power. Here are three examples: 2 cylinder 250cc for karting: I'm not sure which specific engine he has but karting engines are set to rev high and need big carbs. He selected the 42mm which my calculator says ideally should be 39mm or 40mm (39.4mm). It shows only half of the engine (125cc) which is how it's done.  RD400 (2 cylinder): The calculators recommended 39mm is pretty close to his selected 38mm. This bike was modified with pipe, big carb, and higher compression. The owner said it had been bumped up to a compression ratio more than 9.1:1. He said top RPM was 10K so I'm assuming peak power at 9K RPM.  Honda NSR50: The stock carb was a 20mm but someone put a 28mm carb on it, which you can see here was OK but what my calculator recommends is 24.9mm for racing, so a 24mm is closest to that. The top data set (below) is for a 20mm carb which shows it is OK for moderate street use.  This graphic illustrates the realtionships between carb sizing, atomization, and ideal top RPM for carb size. 
This real life dyno gives creedence to my concept. See how the middle of the dyno hump for the 125cc with 28mm carb is the correct peak torque RPM according to my carb size calculator? For a 34mm it's around 11,000 RPM which is higher than the graph which falls off probably due to a combo of peak RPM allowed by the pipe and because of insuficient blowdown in the porting. 
Click here to watch my Youtube video on this subject. Click here for my 2 stroke calculators list (including this one) and how to buy. HOME |