A combine harvester works the opposite way to a truck. It crawls slowly across the field, but the engine drives the whole threshing system at once: the header, the drum, the cleaning and the unloading auger. The load is high and constant, not now and then. The cereal season in Spain is three or four weeks in June and July, ten or twelve hours a day while the grain is dry. If the engine runs short when it hits a patch of dense wheat, the revs drop, the threshing suffers and you lose either capacity or grain quality. When someone brings their combine in to be remapped, it is not to go faster, it is to hold the engine speed steady under load and burn less diesel per hectare in the weeks the machine cannot stop.
A combine engine works at a fixed speed, around 2,000 or 2,100 rpm, because that is what sets the threshing and cleaning speed. The trouble shows up in dense or slightly green crop: the load jumps, the engine bogs down, the revs fall and the drum speed drops with them. The operator has to ease off the forward speed to avoid blocking the machine, and that is where output is lost. With a custom remap we work the torque right at that working speed, so the engine holds its revs in the loaded patch without fading. We are not after more top speed, we want the threshing to stay steady where it used to sink.
A combine burns between 20 and 35 litres an hour depending on the model and the crop, but what counts to the farmer is diesel per hectare, not per hour. With more torque available at the working speed, the engine does not have to be thrashed to hold the threshing, and the operator does not need to drop the forward speed so much in the dense patches. In continuous work that cuts fuel use by around 5 to 10% per hectare, and each field gets done in a little less time. Over a season of several hundred hectares that percentage is quite a few drums of diesel, and a few hours less of machine and labour.
The harvest window is short and the weather rules it: when the grain is ready you have to cut, and a three-day breakdown can spoil part of the crop. That is why with farm machinery we are even more conservative than with a car. We set the torque to what the engine, the transmission and the cooling take with margin in continuous work, in dust and heat, not at bench maximum. We do not touch the AdBlue, the EGR or the DPF: disabling them is illegal and it also causes regeneration problems right in the dust of threshing. We always save the original map to put it back before an official service, or if the machine is still under warranty or on finance.
If it works hard in dense crop and covers a lot of hectares in the season, you notice it. The standard engine runs short when the load climbs, and that is where a custom remap helps: it holds the working speed under load and stops the operator having to ease off the forward speed so much. We always set it to what the mechanicals take with margin in continuous work.
In continuous work fuel use per hectare drops by around 5 to 10%, because the engine holds the threshing with less effort and you do not have to drop the forward speed so much in the loaded patches. Over a season of several hundred hectares that is quite a few drums less. The saving comes from not thrashing the engine, not from going faster across the field.
Done sensibly, no. Precisely because the season cannot stop, we stay conservative: we leave margin in the torque and respect the cooling and the transmission, which work at their limit in dust and heat. We save the original map in case it needs putting back. An aggressive remap on a machine working twelve hours a day is not worth the risk.
We do not touch the emissions system or the AdBlue, so the part that type approval looks at is not altered. We save the original map to put it back before an official service, and if the machine is under warranty or on finance we talk it through before touching anything. It is worth recording any change so it is on file.