Not all is what it seems...
There seems to be a lot of misunderstanding regarding the shift times that are achievable when using a semi-automatic shift system. Most of this misunderstanding comes about because of pit-lane exaggeration and unrealistic, sometimes ludicrous, claims made by some suppliers.
Claims of incredibly fast shift times must be treated with suspicion. We've all seen the undoubtedly impressive claims, and even seen 'evidence' to substantiate them, yet when you dig a little deeper, all is not quite what it seems.
If the gearbox is not under any load then it's entirely possible to make a shift in 20mS or thereabouts. However, when the gearbox is transmitting torque, it becomes a very different story. So, when a supplier claims an industry-leading shift time, and supports the claim with dyno plots, then we would advise you to carefully examine the data to make sure the test has not been conducted under non-typical off-load conditions. In our opinion, any test conducted under such conditions is highly misleading, and any data is unlikely to be representative of the typical shift times that you can expect on the track when the vehicle is under full load.
Even if it was possible to change gear under full load in as little as, say, 20mS, we would not consider it to be desirable given that there is an increased risk of transmission or engine failures for an almost immeasurable gain in performance. When we make an upshift, the engine speed must be reduced to match the higher gear. Depending upon the gear ratios and the point at which we shift, the drop in engine speed may be in the order of 2000rpm or more. Taking things to extremes, let's imagine a scenario where a shift was performed in no time at all (zero milliseconds). When the dog-ring engages with the higher gear, the engine would still be spinning 2000rpm too fast to match the current road speed. The inertia of the car will then mechanically force the engine speed to reduce, and a shock loading will go through the entire transmission - and the engine crankshaft. Without effective management (in the form of compliant drive couplings), this shock loading could result in transmission or engine failures. To give you an example of the sort of forces involved, the torque required to change the speed of a typical 4-cylinder engine by 2000rpm over the period of 20mS is in excess of 1000 lb/ft (1350Nm).
The ideal shift time is very closely linked to engine inertia. Too fast, and the engine will not have chance to slow down. Too slow, and the engine speed will fall to much. Both scenarios result in increased shock loadings and aggressive gear shifts. We have found that pneumatic operation, along with precisely controlled engine torque reduction results in the optimum shift time. This is borne out by our data log evidence. You can see from the sample log that when we make an up-shift, the engine speed falls at the correct rate to exactly match the next gear. If it didn't then you would see excessive 'ringing' and spikes on the RPM trace as the next gear engaged.
Finally, a lot of weight is placed on shift times in terms of improved vehicle performance. Grossly exaggerated claims of savings per gear shift are common. If a semi-auto shift was performed in, say, 20mS compared with a manual shift taking perhaps 100mS, then it's easy to make the assumption that each shift saves 80mS of track time. Extrapolating this, you may be lead to believe that during a typical lap with a dozen or so up-shifts, you will save 1 second per lap. This is not the case, for it would assume that the vehicle came to a standstill during each shift, which of course it doesn't. People making such claims are either deliberately trying to mis-lead, or do not have even a basic understanding of vehicle dynamics or indeed schoolboy mathematics! Any increased straight-line performance of a semi-auto shift system comes about due to an increase in the time that the vehicle is accelerating rather than coasting. In this example, if the vehicle was accelerated from a standing start up to 6th gear, it would be accelerating for 0.4s longer than the same vehicle with a manual shift. This does not equate to a track time saving of 0.4 seconds, therefore claims of performance gains in excess of 1 second are simply laughable. In reality, unless you have a vehicle that is capable of extreme acceleration, the improvement in straight-line acceleration is so small that it is difficult to measure.
We don't sell our system on the basis of reduced shift times and improved straight-line performance. We prefer to be realistic and tell you the truth regarding performance increases that are achievable, even if it's not what you are expecting to hear or have been led to believe by others...