[77 TRASHCAN]

I've read here that 2G's (the big and small) were 350 cfm for the large 2 barrel and 250 cfm for the small carb. Is this true, or exact for all the large and small 2G's ???

I've heard that there's actually different size venturi's for each type of 2G...I know nothing of these claims.

Just wondering???

Thanks!!!

[ponyakr]

Here's what a guy posted on Speed-Talk. Can't verify.

"...the large 2G carb came in four sizes352 381 423 and 435 cfm, venturi sizes were 1 3/16, 1 1/4, 1 5/16, 1 3/8 respectively..."

Most everything I've read about 2-barrel carbs say something about how the 2-barrels are tested differently in some way, so that the cfm for the 2-barrel does not equal the cfm for the 4-barrel.

https://www.thecarburetorshop.com/Ca...izesandCFM.htm

Has never made any sense to me. Probably just over my head.

Anyhow, you can see some of the different vent sizes listed on the Class Racer Info site.

When I was asking about some 2-barrel Pontiac combos for Stock Elim racing, a well known, long time racer, Billy Nees, told me that most Pontiacs had the smaller vent version. He has been running a small block Nova, which uses the big 2-barrel, for years. He also runs a '79 301 4-barrel Lemans.

After lookin up some of the Pontiac 2-barrel combos, I found that the vent size is NOT listed on many of 'em. Guessing that not many 2-barrel Pontiacs have been run in Stock Elim racing. At the factory hp ratings, the Pontiac combos would not be competitive, at all.

Here's a combo that is listed as using a 2-barrel with 1.186" venturis.

http://www.classracerinfo.com/Engine...3&MAKE=Pontiac

This '70 sbc 350 uses a 2-barrel with 1.375" venturis. The carb size should make a big difference in performance potential.

http://www.classracerinfo.com/Engine...MAKE=Chevrolet

Some of the old dirt track classes required a Roch 2-barrel for all GM engines. So everybody wanted the biggest there was. I ran several of these, myself, on 400, 428, & 455 engines.

https://www.motortrend.com/how-to/ct...g-carburetors/

https://jetchip.com/shop/performance...er-2g-500-cfm/

https://biggcarbs.com/shop?olsPage=p...doo-2-by-big-g

[Stan Weiss]

Originally Posted by ponyakr

Here's what a guy posted on Speed-Talk. Can't verify.

"...the large 2G carb came in four sizes352 381 423 and 435 cfm, venturi sizes were 1 3/16, 1 1/4, 1 5/16, 1 3/8 respectively..."

Most everything I've read about 2-barrel carbs say something about how the 2-barrels are tested differently in some way, so that the cfm for the 2-barrel does not equal the cfm for the 4-barrel.

https://www.thecarburetorshop.com/Ca...izesandCFM.htm

Has never made any sense to me. Probably just over my head.

Anyhow, you can see some of the different vent sizes listed on the Class Racer Info site.

When I was asking about some 2-barrel Pontiac combos for Stock Elim racing, a well known, long time racer, Billy Nees, told me that most Pontiacs had the smaller vent version. He has been running a small block Nova, which uses the big 2-barrel, for years. He also runs a '79 301 4-barrel Lemans.

After lookin up some of the Pontiac 2-barrel combos, I found that the vent size is NOT listed on many of 'em. Guessing that not many 2-barrel Pontiacs have been run in Stock Elim racing. At the factory hp ratings, the Pontiac combos would not be competitive, at all.

Here's a combo that is listed as using a 2-barrel with 1.186" venturis.

http://www.classracerinfo.com/Engine...3&MAKE=Pontiac

This '70 sbc 350 uses a 2-barrel with 1.375" venturis. The carb size should make a big difference in performance potential.

http://www.classracerinfo.com/Engine...MAKE=Chevrolet

Some of the old dirt track classes required a Roch 2-barrel for all GM engines. So everybody wanted the biggest there was. I ran several of these, myself, on 400, 428, & 455 engines.

https://www.motortrend.com/how-to/ct...g-carburetors/

https://jetchip.com/shop/performance...er-2g-500-cfm/

https://biggcarbs.com/shop?olsPage=p...doo-2-by-big-g

2 barrel carbs are rated at 3.0" Hg

4 barrel carbs are rated at 1.5" Hg

A cfm rating for a carb rated @ 3" Hg will about 1.414 times higher then the same carb rated @ 1.5" Hg.

Stan

[carbking]

Doug Roe published a PARTIAL chart in his book on Rochester carburetors.

In 1971 and 1972, Pontiac sold a low compression 455 with a 2-barrel. These carbs are virtually impossible to find because of the dirt circle track racers.

In the day, we built quite a few circle track carbs using 400 Pontiac 2 barrel cores. Ours were popular locally (except for the "claim" class, where they were too expensive) because we left the power circuit in place and left the choke plate in place. The engines using our carbs would pick up quicker just exiting the corners.

As far as CFM ratings are concerned, the very best comment I have seen on the subject would have been spoken by Mark Twain.

And my memory is not as good as it once was, but I think I remember a 1 7/16 venturi Rochester used on a truck engine with a governor.

Jon.

[carbking]

Originally Posted by ponyakr

https://www.thecarburetorshop.com/Ca...izesandCFM.htm

Has never made any sense to me. Probably just over my head.

Apologies!

I tried to make it as clear as possible.

Let us try a different method:

Think about filling a water bucket with a water hose connected to an outside faucet: it takes longer to fill the bucket if the faucet is turned on maybe half way as it does if the faucet is turned all the way on.

The hose orifice (carb equivilent - venturi) is the same; but the water pressure is different.

In measuring air flow, we are using negative pressure (call it vacuum, call it suction, whatever makes sense, but it is negative pressure).

The higher the negative pressure, the more air will be pulled through the orifice.

Here is a picture of an experiment that was taught in the Carter Carburetor school, which may help.

https://www.thecarburetorshop.com/Ca...le_venturi.jpg

There are several glasses of water open to the atmosphere, with a tube connected to varying points in a venturi. The purpose of the experiment was to demonstrate the differences in negative pressures from different parts of a multiple venturi, but if one considers that each tube has a different negative pressure, one can see the results of how much water will be lifted from each glass in the tube.

Jon.

[ponyakr]

"Doug Roe published a PARTIAL chart in his book on Rochester carburetors..."

Since I have that book, I looked 'em up.

The larger ones have a 1 11/16" throttle bore.

Of those, Doug shows there to be 4 different venturi sizes. They begin @ 1 3/16" & go up from there in 1/16" increments.

So, the sizes listed are 1 3/16", 1 1/4", 1 5/16", & 1 3/8"

The listed cfm ratings are 352, 381, 423, & 435 respectively.

[carbking]

The carb I was thinking of would have been late '70's; maybe after the chart was printed.

Jon.

[padgett]

My. Difference is ratings is essentally same reason 1/8 mile drag racing replaced 1/4 mile, too much. Originally flow benches measured carbs at a 3" drop. Then 4 bbl carbs flowed too much to get down to 3" so they started rating them at 1.5".
OK 2G - no choke, 2GC - integral choke, 2GV manifold choke.
Biggest 2Gs I know of were the end carbs on a late tripower. Late 60s big Chevs (think trailer towing) had the biggest 2GC. In general a big 2GC will have more grunt off the line than a QJ particular with an automagic.

[Tom Vaught]

[QUOTE=padgett;6304435]My. Difference is ratings is essentally same reason 1/8 mile drag racing replaced 1/4 mile, too much. Originally flow benches measured carbs at a 3" drop. Then 4 bbl carbs flowed too much to get down to 3" so they started rating them at 1.5".Quote]

Mind posting up where you got that false info. Whose Flow benches?
The Holley Flow Benches in WW-II could flow 3,000 cfm at 1.5" test pressure.


Tom V.

[carbking]

Of course, if you really want a fairly large 2-barrel :P



Stromberg 2-barrel on left, Stromberg 97 on right.

Jon.

[Tom Vaught]

Thank You Jon for an example of the aircraft units.

Tom V.

[carbking]

Originally Posted by Tom Vaught

Thank You Jon for an example of the aircraft units.

Tom V.

Close, but no cigar.

Tank.

[shaker455]

Jeff,
Here's a nice feature if you like to increase CFM.

[chrisp]

Have 1 3/8 if interested.

[chrisp]

Originally Posted by chrisp

Have 1 3/8 if interested.

Unavailable , may have another .

[Formulabruce]

We called the "435" carbs "Canadian 2bbl"
Also found on 2bbl 305 and 350 marine engines .
Both these scarfed up for racing.

[Sirrotica]

When I couldn't get the Rochester 2 bbl carbs with the largest bores, I took one of the smaller models, and bored it myself. I finished the inside up with a wheel cylinder hone. I shaved everything I could off of the venturi cluster to get the maximum venturi area possible. I shaped everything as close to possible like and airplane wing with the wide rounded side towards the upstream airflow and the tapered areas downstream. That included the venturi cluster, and throttle shaft.

This was for the novice class at the local dirt track that required a 2bbl and cast iron exhaust, and intake manifolds. We used one of the 4bbl adapters to a 2bbl intake and just reversed it, blended all the edges and corners and ran it on a factory 4bbl intake. With less than $300 dollars in the engine rebuild it would run with any SBC that was using the 500 CFM holleys. The top running chevys (383s and 400s SBC) had thousands of dollars in their engines, but they couldn't pull away from a almost stock 400 from a 75 T/A............ It had the stock 5C cylinder heads, and we ran regular in it, not race fuel like the competition did.....

The guy that owned, and drove the car won a bunch of races over the 3 years he raced it. It was low buck compared to the other competition, we just worked hard to make it very competitive. Chevy guys really hate to lose to a Pontiac........

[77 TRASHCAN]

So... are there different cfm ratings for the large 2G (obviously as stated)...AND the small 2G???

[ponyakr]

"... Chevy guys really hate to lose to a Pontiac.."

So, you noticed that too, huh ?

And if you really want see 'em mad, let 'em see a 100lb female driver step out of that Pontiac that just beat 'em ! I've seen 'em, cussing & slinging tools & makin a scene, while loading their stuff up as fast as possible. Made me enjoy her wins, even more !

[carbking]

This is the "street" forum.

If one is reasonable in the selection of a carburetor, in general CFM numbers on street vehicles are more important for bench racing than actual performance of the engine.

How well you tune what you have IS important.

Jon.