Selecting correct carburetor size

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Air-flow capacity: It is important to correctly size the carburetor to the engine's operating air-flow requirements. Too small a carburetor limits horsepower; up to some specific speed, normal torque is obtained. Beyond that point air-flow is limited by the carburetor and power drops off.

A too-large carburetor will cause starting troubles (too low vacuum signal at cranking), idle will be hard or impossible to set, and fuel/air mixture will be incorrect.

However, type of service also has a bearing on carburetor sizing; an engine that is never operated at full-load conditions is sometimes better off with a slightly-small carburetor; it will have improved low-end torque and easier starting. Passenger cars, forklifts, etc are good examples. Engines operated at or near full load, such as stationary engines, generators and the like, may produce better power with a slightly oversized carburetor.

Determining engine air-flow from the charts: The chart below gives air-flow requirements for most engine sizes. Use the next-closest value. Engine RPM should be the highest speed during actual use, such as accelerating under full load at highway speeds. Use the carburetor chart to pick the appropriately sized carburetor.

Calculating exact engine air-flow: You can easily calculate the actual engine air-flow directly. For normally-aspirated engines (ie. not turbo- or super-charged):

CID * RPM / 3456 * 0.85 = CFM required

CID is engine size, in cubic inches (CID = cubic centimeters (cc) * 0.06102), RPM is maximum engine speed. 0.85 is a close approximation of Volumetric Efficiency.

For turbocharged or supercharged engines:

CID * RPM / 3456 * %boost + 1.00 = CFM required

Normal inlet air pressure is 14.7PSI; supercharging merely increases inlet pressure. For example, 6PSI boost means 20.7PSI pressure, or 140% boost.

IMPCO Carburetor models vs. air flow

Vehicle applications


        IMPCO           Max.
        Model           CFM
           50            91
       50-500           108
          100           170
          125           202
          175           210
          200           276
          225           329
  300A-1, -20           348
 300A-50, -70           432
          425           460

Industrial/stationary applications

        IMPCO           Max.
        Model           CFM
           50           118
       50-500           124
          100           197
          125           235
          200           345
          225           380
         200D           468
          425           533
         200T           680
         600D          1600

Engine size and speed vs. air flow

NOTES: These tables assume 85% VE as mentioned in the text. To convert liters to cubic inches, multiply by 61.02. For two-cycle engines, double the CFM value found.

Engine speed, RPM  400 - 2400 RPM
   CID   400  600  800 1000 1200 1400 1600 1800 2000 2200 2400
    50     5    7   10   12   15   17   20   22   25   27   30
   100    10   14   18   23   28   33   37   42   46   51   56
   150    14   21   28   35   42   49   56   63   70   77   84
   200    19   28   37   46   56   65   74   84   93  102  110
   250    23   35   47   58   70   78   93  105  116  128  139
   300    28   42   56   70   84   98  112  126  140  154  168
   350    32   49   65   81   98  114  130  146  162  178  195
   400    37   56   74   93  111  130  148  167  185  204  223
   450    42   63   83  109  129  149  169  189  209  230  251

Engine speed, RPM  2600 - 4000 RPM
   CID  2600 2800 3000 3200 3400 3600 3800 4000
    50    32   34   37   39   42   44   47   49
   100    60   65   70   75   79   84   88   93 
   150    91   98  104  112  118  125  132  139
   200   121  130  139  148  158  167  177  185
   250   151  163  174  186  198  209  220  232
   300   182  196  208  224  236  250  264  278
   350   212  228  244  260  276  293  309  325
   400   242  261  280  298  317  335  254  372
   450   272  293  314  335  357  378  399  419

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