Well I must say this discussion is getting rather interesting and lively.
The first car I owned was a TR-3A with twin SU’s. I balanced these carbs using the hose/hiss method, which worked ok for 2-carbs with a balance pipe but was really subjective in nature and sensitive to how one positioned the source end of the hose.
Later, I graduated to a Unisynch, which worked better in that it delivered a smoother idle yet was sensitive to orientation. If the glass tube was perfectly positioned vertically and the engine idled reliably, the readings were somewhat repeatable and surely better than the hose/hiss method.
Later, I got my S2 SE Elan with twin Webers. Instead of the near steady state flow of the SU?s, there was the unsteady state or slug of airflow characteristic of Webers, where one throat supplies fuel/air for each cylinder. Suddenly, the hose/hiss and Unisynch methods just didn?t work as well. While still a young pup of 30 ish and studying for my mechanical engineering degree, I decided to try measuring/equalizing manifold vacuum among the throats to see if this would work better than the previous methods. Well, I tapped the bottom of the intake manifold runners and inserted grease fittings, minus the check ball, as my vacuum taps. Although they didn?t work as well as special purpose hose barbs, they did the trick. I used Merium manometer fluid (specific gravity of about 2.95) and had to use about a 12-ft tall column of fluid as I recall, and be very careful not to ingest the fluid after reving the engine and closing the throttle (a high vacuum condition).
Decades passed and I found myself the owner of a 78 Ferrari 308 (8-cylinder). As my TR3A?s were long gone, I purchased a pair a matched pair of BK (bigger brother of the SK) Synchrometers to tune the 4-DCNF, twin choke, down draft carbs. These instruments promised absolute precision. My plan was to idle the engine down as much as possible, select the highest flowing throat and position the ?control? BK to continuously monitor flow on one on the high flowing cylinder and adjust the remaining 7-throats to agree with it. As each throat was dialed in, the rpm would change. Flow to the throat to be adjusted was matched to the flow of the control throat. The process seemed logical to me in that it accounted for changes in rpm due to adjustments. What one couldn?t do well with this method was look at the dynamic behavior of all the cylinders when the throttle was just off the idle stop. To do this, one needs to be able to look at the flow/vacuum of all cylinders simultaneously. Here is where the 2 x 4-bank manometer truly came into its own.
With a bank of mercury (no friction) manometers, one could blip or slowly increase the throttle, and see simultaneously the effects on all cylinders. If the levels are equal, the flows are equal. While the BK/SK Synchrometers work pretty well at idle, it?s also the off idle operation, up to about 2000 rpm, that is important for drivability and low-end torque. Indeed, one should set the off idle synchronization, prior to setting the idle synchronization, to achieve the best driveability.
Finally, synchronization is mostly important during ?every day? driving. If one is racing, I think you only need to be close as you are at open throttle much of the time and a smooth idle and progression are just not important. From an every day, live ability point of view; synchronization is very important and attainable.