Probably the key document for purchasing a toilet is the Maximum Performance (MaP)
Testing of Popular Toilet Models This chart displays the efficiency of various toilets according to the amount of standardized material that is moved out of the bowl per flush. The range is from 1000 grains per flush (best) to under 300 grains per flush (worst). It should be noted that one of the key ingredients that correlates with good flusing action is the size of the flush-valve flapper.
Generally, toilets with 3" flush valves do the best and toilets with 2" flush valves do the worst. Type of flushing mechanism, such as gravity or power-assisted, are not the key ingredients. Generally, gravity flush toilets may then have the advantage since they are simpler. Price also does not seem to correlate with efficiency. Some toilets under $200 seem to function as effectively as toilets costing several times more.
An example of an efficient toilet for under $200 is the American Standard Cadet.
Awesome Library does not endorse this product but provides it as an example.
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and Dr. R. Jerry Adams