This topic of class size has always been an interesting one. So, many years ago I did some research. What I found after I contacted ASAE, PAMI, the Nebraska Tractor Test lab and SAE is that there were no official industry standards. The original combine classification system was devised and used by Implement and Tractor Magazine for their annual "Red Book" of farm equipment. It was a way for them to provide a quantifiable, non-biased system for their readers to evaluate and compare combines. I and T used NE. Tractor Test lab results for tractor comparisons. This goes back to at least the early 1970's and maybe even further back in history. The person I talked to at I and T back then (about 1992) couldn't remember when they started the system but knew that the people involved were no longer with the publication. Back in the early 1970's the prominent combine design was a conventional rasp bar threshing cylinder followed by straw walkers for separation. More cylinder, straw walker and sieve width and length generally correlated to more capacity if the additional horsepower was available. There were various feeding mechanisms like the MF paddle feeder house and front beater, feeder chains like the White, IH and Deere and the Gleaner system of the front beater feeding directly to the threshing cylinder. All had their respective pros and cons. The only other threshing system in North America that I am familiar with that had gained any degree of acceptance was the New Idea cage sheller for corn. This system probably did the best quality of threshing corn but was limited to the Uni machine parameters of physical size and horsepower. The folks at I and T used basic square inches as a measurement for concave area, straw walker area and sieve area. The premise being more was better. They also used grain tank size and horsepower in a balancing formula to come-up with a classification system. The process, while not exact, was a pretty good way of looking at combines. Obviously, if a combine had a large concave and separation area with very little horsepower, it was limited in total capacity. Very few combines at that time had enough horsepower, let alone an excess of horsepower so that wasn't a big issue. On the benefit side, many models would power-out about the time losses got excessive. Newer machines can often blow grain out the back before the engine is even breathing hard thus created more demand for grain loss monitors. This class system was well accepted for several years until New Holland introduced the rotary combine to North America in about 1977. International Harvester followed and then Gleaner and White. All four were desperate to compete with Massey Ferguson and John Deere for market share and to provide a higher capacity machine vs. their older designs. The marketplace wanted more capacity. They had to come up with something that was different to get any attention in the marketplace. This is when the class system started to fall apart. This is also when manufactures had to increase their advertising and marketing expenditures just to stay in front of the customers. The problem became how to compare rotary separation to straw walker separation. The horsepower issue gained more importance because it simply took more horsepower to run a rotary machine. I and T never developed a comparison system that had any consistency due to several factors. For example... the New Holland design used two smaller rotors than either IH, Gleaner, or White's one rotor design, which took less horsepower to spin and also provided quicker separation due to their higher centrifugal force. This allowed a smaller concave and separation grate area per cylinder but required higher horsepower to power two rotors which were needed for overall capacity. Another reason for two rotors was the ability to feed them the crop mat coming from the header. The IH had more concave area but larger diameter rotors which took more horsepower to spin and had a lower centrifugal force for separation. The single rotor required a transition cone that funneled the crop down to the rotor from a wider feeder house. The IH product offering became difficult to compare within itself by using three rotor diameters with their respective horsepower demands and centrifugal force capabilities. The Gleaner design was different again and further complicated comparisons. It offered 360 degree separation around the rotor cage but used a larger diameter cylinder and therefore higher horsepower requirement with lower centrifugal force. This design didnt require a transition cone so horsepower was less than the IH design. The Gleaner N7 was considered a class 7 machine due to horsepower and grain tank capacity even though it didnt have the concave area or the additional passes of the White 9700. Gleaner made comparisons even more difficult when they introduced the grain acceleration system that threw grain through the air stream thereby adding capacity to the cleaning system without having to add square inches to the sieves. Then Claas came to North America with a smaller diameter conventional threshing cylinder and smaller concave area but offered greater centrifugal force and separation capacity per square inch comparable to the New Holland along with lower horsepower consumption. The Ford combine was here first by was also built by Claas. The Claas intensive separation system increased the capacity of the straw walkers which already had the largest square inches in the business thereby increasing separation capacity to rival the rotary separation systems. They followed this up with the cylinder system using eight separation cylinders over open concave grates. The square inch dimension of separation grates was smaller than the straw walker version but had higher separation capacity. Claas then introduced the acceleration cylinder and open concave in front of the threshing cylinder which rivaled the threshing and separation capacity of the rotary designs without all of the horsepower requirements. Behind the threshing cylinder came either walkers or two tangential separation cylinders. John Deere introduced a conventional threshing cylinder machine with two separation cylinders instead of straw walkers. This design kept the effeciency of the conventional threshing cylinder but added the horsepower rewuired to spin the two separation cylinders. They primarily limited sales to rice country in the early years because the design lent well to the use of spike tooth cylinder threshing. Massey Ferguson introduced the five series conventional machine on a limited basis with a rotary separation cylinder behind the walkers. MF had the cascade sieve system which was shorter but had more cleaning capacity. Square inches still pretty much applied. White introduced the largest rotary design both in diameter and length. This design took more horsepower due to length and the total amount of crop in the cylinder at one time. It had lower centrifugal force but made-up for that with longer rotor length and therefore more passes for grain separation. This was the first true class seven machine in terms of threshing, separating and cleaning area but lacked horsepower and grain tank capacity. They later designed but didn't release the smaller diameter rotor machine which MF did release when they bought-out the White combine line. They then went through (as did IH, Gleaner, and New Holland) several horsepower increases. Now manufacturers had introduced to the marketplace several designs for farmers to choose from with no definitive comparison system in place except maybe for PAMI. PAMI tried to compare machine designs and capacities and found that the only meaningful way was to compare the test machine to a base standard machine which were older straw walker designs. This yielded results for bushels of grain and MOG vs. a standard level of acceptable grain loss. Comparison tests conducted over multiple years had to be done verses the base machine due to crop conditions being different year to year. The system worked pretty well when all the manufacturers had a conventional combine design. This got more difficult as new larger machines were introduced. The base machine simply couldn't keep up. So prospective buyers were always forced to compare the machine they were considering against the PAMI base machine. The whole PAMI test process took a lot of time and money and was only done officially in Canada when manufacturers were willing to invest the money. So very few were conducted on an entire line of combine models. PAMI also had limited capacity to conduct tests during a given harvest season. Combine tests and evaluations were conducted primarily in small grains which left the corn and soybean farmers wanting. PAMI became the accepted standard for Combines just like Nebraska Tractor Tests became the standard for tractors. Nebraska Tests for tractors at that time were mandated by Nebraska state law if a tractor was to be offered for sale in the state. Combine tests were never mandated anywhere as far as I know so were not conducted on a regular or widespread basis. This was one of the reasons why I and T came up with their formula. In recent years the game of marketing has in my opinion gotten out of hand and the fall guy has been the class system and quality comparisons. The reason is horsepower and grain tank size mean very little to throughput of MOG and grain saving capacity. Yes they mean a lot when overall productivity is considered. For example...if grain tank capacity is not large enough to get the combine through the field opening pass, then other measures need to happen to empty the grain tank so harvesting can continue. This maneuvering takes time and therefore decreases machine efficiency (productivity). If conditions are soft or muddy and grain is wet then more horsepower is handy. Theoretically the operator can slow down to borrow horsepower from the threshing process to plow mud or unload. Few operators wanted to do that. That's why power bulges became available with the introduction of electronic fuel injection during the unloading cycle, especially with the market preference moving toward the turrent style unloading auger. The turrent style consumed more horsepower vs. the older design that started at the bottom of the grain tank and angled up and out. The turrent style caused a little more grain damage but had better clearance for grain carts and trucks. Convenience won-out over quality. IH and Gleaner started the horsepower game by using the same rotor size and design and then adding more horsepower and grain tank capacity. The net result proved to be higher field efficiencies due to faster ground speed capability and fewer unloading cycles. As market demand increased for larger machines and fuel remained relatively cheap, horsepower increased and has spiraled upward as more horsepower is also required to carry heavier machines with larger grain tanks and headers. Combine engineering has faced many challenges to increase capacity. There are three basic criteria that need to be addressed with either conventional or rotary designs. In the conventional threshing cylinder_straw walker design, capacity is limited to width, depth or density of the crop mat entering the machine and the throughput time. Increase width and capacity goes up under the assumption there is enough horsepower to process it. This works until you can no longer feed the crop evenly across the width of the cylinder. Increasing the depth or density of the crop mat works until threshing and separating are compromised. Throughput time can be decreased to increase capacity down to the point of not saving all of the grain. The same criteria work in rotary designs. Increasing rotor diameter and length allows the rotor to hold more crop. Spinning the rotor faster augers the crop through faster but may start damaging the grain. All of these factors have to be considered in relationship to combine transportation and grain quality. Combines are built on an assembly line in a factory and have to be transported to the farms. The farmer or custom harvester has to be able to transport from field to field. Roads, bridges, poles, fences, ditches, laws etc. all present constraints. So the outside dimensions of the "BOX" are limited. This requires designs that get the job of cutting, feeding, threshing, separating, cleaning and grain handling done within the available box dimensions. These were the reasons for looking at tangential threshing and separating vs. transverse threshing and separating, i.e. rotary vs. conventional designs. Otherwise we could be looking at 12 foot wide conventional threshing cylinder and walker systems. Rotary designs had the additional advantage of decreased through-put time. They could screw the crop through the combine faster than walkers could carry it through. The Claas CS system had the same advantage. The ClAAS straw walker machine had a throughput time of about 17 seconds from feeder house to straw chopper. The CS machine cut that to about eleven seconds. Rotary machine throughput times vary depending upon length of rotor, crop density and rotor speed. So how is a combine owner supposed to sort out all of this capacity and classification messIJ First of all use common sense and listen to your heart. Secondly, don't believe all that the marketing people are advertising. Much of it is just hype. To the best of my knowledge there are no criteria established for class 8. It used to be that class 7 was a catch-all category for machines above class 6. Maybe class 7 has since been defined. Third start demanding from the manufacturers the criteria that they are using to stake their claims. Then see if all of the manufacturers are using the same criteria. Forth, ask the farm publications to ignore the advertising dollar sources and find non-biased engineering based criteria for comparing combines. The above information is derived from years of servicing, selling, demonstrating, attending trade shows, owning and running combines in many crops, in many conditions in many states. It is only one persons' "OPINION". If it serves your needs or thirst for information, you are welcome to use it. If it does not serve you, just discount it or discard it. But maybe it can serve as a catalyst to define a meaningful, nonbiased combine comparison system discussion.