By Rick Gonser, editing by Larry Gorden
See Pontiac Service Craftsman News, March 1957 for description of the tri-power engine.
The 1957 tri-power set-up was similar but not identical to the 1958 Pontiac. Differences are as follows:
1) vacuum diaphragm on the rear carb for 1957, front carb for 1958,
different vacuum slider switch,
3) compression fittings (instead of double-flared) on fuel lines,
4) fuel lines "tees" with in and out openings,
5) passenger-side front and rear return springs tied to airhorn instead of concentric,
6) front and rear passenger side throttle arms and front-to-rear rod different than 1958,
7) last but not least, the $5,000+ (increases weekly) sunburst air Cleaner.
1957 Tri-Power Engine
Related differences include passenger side valve cover, breather, breather stack, generator bracket, generator pulley, water neck, valley cover, valley road draft tube, and whatever else I've forgotten...
1957 was the first year of tri-power, and advanced projects was doing their best. Even so, they made lots of mistakes, a majority of which were corrected in 1958, which leaves 1957 as a unique year...contrary to popular opinion...
The bottom line is that it's very easy to deduct 25-35 points for all the stuff that one needs to be "correct" on a '57. In Greenville last year, I offered a flat -20 points during tech for "non-authentic" 1957 tri-powers. Or, one could take their chances with the judges. 2 out of 3 accepted the deal...
The absolute worst coat-hanger style mechanical linkage is far more reliable than any vacuum set-up. Pontiac didn't start making mechanical linkage in 1958 because they thought it was a good idea. They were responding to litigation demanding immediate corrective action to their vacuum plumbing nightmare...
In a points-judged environment, mechanical linkage in a 1957 is deemed "non-authentic", and will get you a 5 point deduction in a heartbeat...
The following pictures are of a 1957 tri-power setup. This is not a "correct" setup because it has mechanical linkage that engages the secondary carburetors rather than vacuum linkage.
Passenger's side (Click image for an enlarged view)
Driver's side (Click image for an enlarged view)
Center Carburetor (Click image for an enlarged view)
Rear Carburetor (Click image for an enlarged view)
Below is a 1958 tri-power with stock vacuum linkage. The major diff between it and a '57 Is that the vacuum diaphragm is on the rear carb instead of the front and the fuel lines are different.
1958 Tri-power (Click image for an enlarged view)
One advantage a '58 tri-power car has is that mechanical linkage was an option. Not so on the '57s, and the vacuum pieces are virtually unobtainable. Thus stock original 57 tri-power setups are rare.