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, 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 not as simple as .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. Also good to know is that high octane gas reduces the burn speed, and high squish velocity increases it 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 cab 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 G15, and the best carb size for top RPM racing is shown at H15. For piston port intake engines the best carb size for street/trail is at G24, and the best carb size for top RPM racing is shown at H24.  EXAMPLES: Honda CR500: Honda equipped it with the same size carb as their 250, a 38mm. The calculator recommended 34mm for trail use, and 41mm for racing. Honda chose to use a carb between those two sizes but I think it should have the 42mm from Keihin (PWK) or Mikuni (TM) for racing, or the Keihin PWM (round slide) for trail riding because round slide carbs aren't excessively rich mid throttle for trail riding, whereas flat slide carbs are inherently that way which is good for racing but bad for trail riding. Note the very high peak air velocity thru the 38mm. That prevents as much large fuel droplets from forming in the air/fuel mix. Those are very helpful for racing because they cool the engine down and deliver more oil to the bearings and cylinder.  Honda CR250: The calculator shows a 38mm carb would be best and Honda uses a 38mm. I would personally want to try a 40mm (shown under the 38mm data) if I was racing it.  Honda CR125: The calculator shows a 38mm 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. That is most ideal for racing. 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 is in agreement with if his compression ratio is near 8.1. (He said he didn't know it)  RD400 (2 cylinder): The calculator agrees with 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 a good choice and what my calculator recommends for racing. The top data set (below) is for a 20mm carb which 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 |