Interpreting Dyno reading when it comes to different final drive ratios provide a difficult obstacle. To begin, there is a potential for disparity because the various gears each have their distinct values for inertia, produce a greater degree of friction, and modify the amount of tire sliding. All these factors contribute to the fact that there is a possibility of disparity. In most cases, and especially when using an inertia dyno, power levels will drop noticeably whenever the numerical value of the gear ratios is increased. Higher numerical loads have a greater propensity to be less efficient than lower numerical gears. The torque is multiplied whenever the gear ratio is increased, which tends to enhance the amount of tire slippage that occurs.
Because rpm and torque have an inverse relationship to horsepower calculation, changing the rear axle ratio or conducting the test in a gear other than one with a balance of one to one on the transmission should not, on the surface, have an influence on the horsepower results.
This is a point that many people fail to take into consideration. On the other hand, this does not consider the fact that the engine’s acceleration rate can change based on the gear ratios that are being utilized in the vehicle. Take, for instance, the following: On an engine dyno, the flywheel power figure (in bhp), for example, will drop if the acceleration rate of a sweep test is increased from, say, 300 revolutions per second to 600 revolutions per second.
This is something that we are aware of and expect to happen. This is due to the higher rate of acceleration that is experienced. When an engine accelerates quicker, the power required to rev the engine increases; however, a more significant percentage of that power is used up before it reaches the flywheel. Increasing the rate of acceleration during a sweep test is equivalent to increasing the number of gears in the transmission (while performing the test in a lower bag) or the number of packs in the rear end. Whether or not this results in a difference in the findings presented by a particular chassis dyno is determined by how a specific dynamo maker performs its calculations. Always perform the test in the same gear on the gearbox, which is typically 1:1, and then re-baseline the vehicle after adjusting the gear ratio on the rear axle. If you do so, you will get the most reliable results possible.