I would like to know the difference between carburetors.

[Ben Bruce aka First Born]

  I have recently seen the term small bore and large bore in relation to the Rochester 2GC carburetor.   What is the difference?. Why one or the other on an engine?.  Curiosity!

 Carb King?

 

  Ben

[NTX5467]

Throttle bore size.  Large one should be 1.69" throttle bores, with the other two sizes being 1.56" and 1.44".  Large, medium, small.

 

Seems like the 1.69s were originally used in the later 1950s and into the 1960s on some Pontiacs and Oldsmobiles?  The 1972 Chevy 350s also used them, too.  Most of those Chevy 350 2bbls ran almost as good as a Qjet, from my experiences.  The 1.44" was used on 1970 Buick 350 2bbls and my '77 Camaro 305 2bbl.

 

By observation, that small carb on a Buick 350 has lots of off-idle torque and enough airflow for easy cruising with 2.73 rear axle ratios.  

 

One night, on the way home, my then-new Camaro dropped a ball check out of the direct drum, which meant it also lost governor pressure.  As it was still under warranty, as the service manager noted, I decided to see what a "smog equipment laden catalytic convertered" 305 would do.  Afterall, it would only upshift at WOT anyway.  So when I got to the main road and stopped to check traffic, I put it in "1" and floored it.  The tach did not hesitate to hit 7000rpm, nor was it slowing down, either.  That proved to me that many factory emissions controlls were not nearly as strangling as some seemed to believe.  The engine lived another 524K miles before it was replaced by a 355 short block I had had built in about 1986, as all of the block freeze plugs were seeping.

 

Which size of throttle bores a GM brand would use probably had many decision rules.  One would be emissions compliance, no doubt.  Others would include cost and customer demographics, too.  

 

Enjoy!

NTX5467

[carbking]

Adding to the excellent post by Willis above:

 

This is the first time I have heard the Rochester 2G series referred to as "small bore / large bore".

 

Often, folks, especially hot-rodders or racers, refer to these units as "small base / large base".

 

If one looks in the book "Rochester Carburetors" written by Doug Roe (Rochester engineer) and Bill Fisher, one will find reference to "1 1/4 inch flange" and "1 1/2 inch flange".

 

Each of these has a different mounting bolt pattern. GUESSING there is an S. A. E. standard somewhere for the two-barrel carbs but I have never looked for it.

 

I am aware of the S. A. E. standard on single-barrel carburetors, and referenced it in my article on early carburetor flange sizes.

 

Single barrel flange sizes

 

Note that the "nominal" flange size NEVER agrees with the throttle bore, and this remains true with the two-barrel Rochester 2G series.

 

Back to the "Rochester Carburetors" book: If one refers to page 178 in the 1973 printing, one will find the following chart:

 

ROCHESTER CARBURETOR FLOW RATINGS

 

Model 2G, 1 1/4 inch flange, 1 7/16 throttle bore, 1 3/32 venturi, 278 CFM

Model 2G, 1 1/2 inch flange, 1 11/16 throttle bore

     1 3/16 venturi, 352 CFM

     1 1/4 venturi, 381 CFM

     1 5/16 venturi, 423 CFM

     1 3/8 venturi, 435 CFM

 

Again, this chart was published in 1973. Willis noted an additional bore size, and I am aware of a few 1 7/16 venturi.  There may be other sizes as well.

 

The carburetor, regardless of the mounting base size, was chosen because of the CFM requirements of the engine, and the CFM which could be delivered by the carburetor.

 

If both bases could deliver the desired CFM, I have no clue which would have been chosen. I do know that Pontiac used one on the 389 "E" engine (1960~1961 from memory) where the throttle base was slotted, so the carburetor could be bolted to either a small base manifold, or a large base manifold.

 

Of course, today, all of the above is ignored, and a shiny new one-size-fits-all-works-well-on-none unit is placed on the engine!  but it is shiny  

 

Jon

 

 

     

[NTX5467]

Thanks, Jon.  The cfm figures listed are interesting!

 

I might add that Holley recently introduced Sniper EFI 2bbl self-learning TBI systems stated to replace Rochester 2bbls.  Their website has flange illustrations as to sizing and such.  Modern tech for older engines.  I guess the young 'uns need something they might better understand what it is rather than a crude carburetor that works very well, usually?  To Rochester Carburetor's credit, it always seemed that the 2G carbs were about as bullet-proof in reliability and performance as you could get, at least on a stock engine.  Or at least until they started using some additional circuits to better fine-tune the fuel curve for emisions purposes.

 

Obviously, the intent was to have the systems for GM applications, BUT those same Rochester flanges also fit Carter BBDs and Strolmberg WWC 2bbls, too.  Which widens the possible applications into later-1950s to 1980s Chrysler territory, too.

 

Perhaps the terminology First Born mentioned should have been "Small Flange" and "Large Flange"?

 

Enjoy!

NTX5467

Edited July 31, 2022 by NTX5467 (see edit history)

[carbking]

Willis - other carbs fitting the 1 1/4 flange size: Carter WCD (some), Carter WGD, Carter BBD (some), Stromberg WW, Zenith WWZ, 28ADA10. Some of the O.E. Holleys, but not my area of expertise.

 

Other carbs fitting the 1 1/2 flange size: Carter BBD (some), Stromberg WWC (some).

 

There are also some proprietary flange sizes, a couple of which are the Holley/Autolite flange for Ford, and the Stromberg WWC for G.M.C.

 

Jon.

[Ben Bruce aka First Born]

Thank you, sirs.  That is what I suspected.  So, 1.44 [ small]  should be sufficient for a 263 starlight eight, then.  Even in an   EFI.  

 

   I ran across the terms while reading about the Holly 2GC EFI.  

 

  Ben

 

 

[carbking]

Ben - Pontiac used the small base (1 7/16 inch throttle bore) on the 1959 Pontiac 389 for taxi use; so if it were a carburetor, yes, definitely sufficiently large.

 

Sorry - no comment on efi sizes, I do not know.

 

I would really have thought Holley, selling carburetors since 1903, would do better with the terms.

 

Many of us talk about the "new math"; is this the "new English"?  

 

Jon.

[NTX5467]

The small Carter BBD would be what I term "318 size", although it was also used (one model year) on the 1980 360 2bbl V-8.  The large Carter BBD would be for the 383 2bbls with 1.56 throttle bores, which has a larger and rounder float bowl area in the main body casting.  The larger BBD was replaced by the Holley 2210-family 2bbl, with the same flange and throttle bore sizing.

 

By observation, the Carter BBDs tended to side-step the somewhat common issue with longer-term high-torque on the carb stud wingnut deforming the air horn and causing various problems.  How?  When Chrysler made the transition from the carb air horn-tightened air cleaners in 1966, they used a thick wire to snap into two holes in the same area of the air horn, with the air cleaner stud attached to that.  Which put all of the "pull down" force to that outer area of the air horn, away from the center area.  To me, it looked flaky and Mickey Mouse back then, but it had its advantages, too.

 

Enjoy!

NTX5467

[MRJBUICK]

Not to hijack the thread

but i know there is at least 2 different venturi sizes on the quadrajet

 

and a quick story

 

a couple of years ago the young guys across the street who are very involved in modern performance cars

were trying to start a 289 65/66 mustang. could not get started cranking and cranking, so seeing an opportunity i wandered over and after watching for a few min realizing they had flooded it, i pushed the throttle wide open and opened the choke plate (which they had no idea what its function was, and thought it was the throttle plate) and vroom it started, and told them what it everything was and wandered back home with a smile. i don't know if they even understand the basics to this day lol

the old fart strikes again

Edited August 1, 2022 by MRJBUICK (see edit history)

[JohnD1956]

On 8/1/2022 at 2:30 PM, MRJBUICK said:

i don't know if they even understand the basics to this day lol

Yeah. And likely at this day that Mustang is sporting an aftermarket Fuel Injection unit with headers and flame thrower exhaust tips. Lol

[Ben Bruce aka First Born]

5 minutes ago, JohnD1956 said:

Yeah. And likely at this day that Mustang is sporting an aftermarket Fuel Injection unit with headers and flame thrower exhaust tips. Lol

 

 Well, the aftermarket FI WOULD be an improvement. As long as the right 2GC is used😁.

 

  Ben

[JohnD1956]

2 hours ago, Ben Bruce aka First Born said:

 

 Well, the aftermarket FI WOULD be an improvement. As long as the right 2GC is used😁.

 

  Ben

👍I know you're right about that Ben. You've proved it with your car.  I am impressed with what you have set up and use.  😃