Are there any real differences between Chevy and bop units

[Marx3]

I know the inlet position and the throttle linkage etc are different between these units, but how about internally? I don't mean the calibration, cause that can, obviously be altered.
I sold a 17058276. I know this is one of the desirable units, but why? Other than calibration, herein the small mab's, what can a Pontiac unit do, that any large mab Chevy unit can't do, if smaller mab's are installed?

[Frank400]

I've been wondering the same thing as well.  I've built a few Chevy carbs with .120 main air bleeds and I all converted to .070", both upper and lower, all work very good.
 
   I've noticed on a Pontiac carb 17057274 (same Cliff uses on his Ventura I believe), it has the POE outlets below the secondary air flap.  That might be an advantage, but other wise, can't see why a Pontiac would be better than a Chevy .120 MAB converted to smaller Main Air Bleeds.

   Hope some experts will chime in.

[Marx3]

Well, the POE's being positioned beneath the air flaps, would certainly be a design advantage I guess. I have actually never seen any Qjet that actually had them there... I thought it was as feature, that could only be found on early 70's units on high output engines,
I wonder if there is any performance difference between having them positioned beneath the flaps and slotting the flaps?

[Cliff Ruggles]

Positioned under solid flaps would have a very slight advantage one would think.  When the huge secondary throttle plates open, the low pressure area created under the closed solid flaps will try to immediately pull fuel trapped in the main fuel supply passages to the fuel nozzles and equally from the POE exit holes under the flaps. 

A large hole lined up with solid flaps as used on the later Pontiac carburetors would be right there and probably just as good or a very, very close second......Cliff

[Marx3]

Interesting. So if one grinds a good notch in the flaps, there is no difference between any late model APT Qjet?

[Cliff Ruggles]

Solid flaps and holes that are either under or in line with the flaps would still have a slight advantage, but not enough to spend the time to relocate the holes.  Nothing the flaps is just a lot easier.  If the holes are very close to the leading edge of the flaps you can also open up the entrance like they did on the Pontiac carburetors.  I do this a lot here in lieu of grinding a notch in the flaps......Cliff