what the vernier adjustment is for

[JHertzman@xxxxxxx]

In AD7-117 Andrew Watry asks 

"From what I have seen, there were three different horsepower levels of 
the twin-carb 1600 engine in 1967-era 105 models.  The Giulia Super 
had 98 HP, the GTV 108 (I think) and the Duetto 112 (I think).  How 
were these different power levels achieved?  Was it all in the carbs, 
or were there compression ratio and porting differences?  The Alfa 
Ricambi and Centerline catalogs show that the same cams were used in 
all these engines, with the same cam timing.  So that wouldn't account 
for any difference.  Did the Super have, for example, 9.0 pistons and 
the other two 9.5?  Did the Super have smaller ports?  I'm mainly 
wondering why the carbs on each are set up differently.  Did different 
venturi and jet sizes create the HP difference, or rather serve an 
engine better that mechanically produced more HP?

Andrew may find it interesting to check p. 572 in his copy of Fusi, which has
a chart "curve di potenza motore vetture tipo Giulia 1600 4 cilindri
allesaggio e corsa 78 x 82 mm Cilindrata 1570 cc". Chart has five curves, for 
(A) Giulia TI, Sprint, Spider, Spider 4R Zagato: 92 (SAE 106) 
(B) 1600 S: 95 (SAE 109
(C) Giulia TI Super, Sprint Speciale, TZ1: 112 (SAE 129)
(D) Giulia Sprint GT, GTC, GT Veloce, Spider Duetto: 109 (SAE 125)
(E) Giulia GTA:115 (SAE 132)

P. 598 a further chart, mislabeled (I think) as GTA & TZ2 but apparently GTA
Corsa and/or TZI Corsa, TZ2 gives 170 (SAE 194) and 175 (SAE 200) at 7500 rpm,
11.4:1 cr.

Curve (B) is much fatter but runs out of steam 500 rpm sooner than curve (A);
curve (D) is appreciably fatter than curve (C) up to a crossover point at
about 5500 but runs out of steam about 500 rpm sooner, while curve (C) is
essentially flat from 6000 to 6500. My understanding is that with the most
apparently directly comparable engines the difference is in fiddling the cam
timings and overlap by using those lovely vernier adjustments of the cams, not
only to get desired power but usable torque curves according to the projected
use of the car type. I haven't checked but if the Alfa Ricambi and Centerline
catalogs show that the same cams were used in all these engines, WITH THE SAME
CAM TIMING, I would look for a second opinion on the cam timing in factory
literature.