The Stovebolt Site Forums The Shop Area Engine Shop What do you want to talk about today?

Lets talk about stuff.

Is his a general hypothetical question, or, do you have a engine, carburetor, and intake in mind?

nothing to see here.

Are you drag racing with this engine?
If not, then I would suggest that neither of the choices which you have presented are suitable for a non-racing application, and in all likelihood a 2 barrel carburetor would cause issues with everyday driving. These engines don't breath well due to head port design. More carburation just won't work on the street.
A quadrajet on a weedeater comes to mind. Sure would look cool though. I may just dummy mount a Carter AFB on my weedeater. It may stop my neighbor from thinking he can get away with picking on me.

deleted

The controversy: is the added CFM, reduced pumping loss enough of a benefit to make up for the (possibly) worse mixture distribution caused by moving the insertion point?
I wouldn't.

The only advantage I can imagine would be that a single 2 barrel would be head and shoulders above the same amount of venturi area split between two carbs like the Fenton manifold, etc. A 2-into-1 arrangement like there would be with an adapter on a regular 1 barrel manifold has been proved many times to get worse performance than the 1 barrel it replaces. If someone were to machine or cast a manifold with a properly shaped plenum to accommodate a SMALL 2-barrel like the Stromberg 97 that fit the 239 cubic inch flathead Ford might be worth considering.
Jerry

deleted

From a purely theoretical standpoint, a scientifically designed two-barrel intake COULD provide a measurable (at least on the dyno, probably NOT by the seat-of-the-pants machine) increase in engine efficiency assuming the two-barrel used is something like a Carter WCD with metering rod technology, and the metering rods are custom cut for the application when comparing to a single barrel.

From a practical standpoint: WHY???? Single barrel carburetors exist in a sufficiently large size to feed ANY street 216/235/250/261/292 Chevrolet engine; and if the single barrel is properly selected and correctly tuned, the two-barrel advantage is going to be limited to a very few percent.

The two into one adapter annoys the carburetor, as it defeats the original engineering design, at least for a conventional 2-barrel. Air velocity in the venturii creates a negative pressure in the fuel delivery points allowing atmospheric pressure to push fuel into the negative pressure areas and thence into the venturii area. The air velocity is normally controlled by the throttle plate opening. The two into one adapter upsets this design, and as Jerry mentioned, has been proved many times to LOSE performance when compared to a decent-condition and properly tuned 1-barrel.

However, from a "bench-racing" or "parking-lot-fast" standpoint, it can be any lie you think you can get away with

And, again as Jerry mentioned, due to the port design of the cylinder head, the single two might be better than 2 singles. But if you REALLY want to increase the low RPM volumetric efficiency of the Chevy 6, then design a 3 carb manifold where each carb feeds 2 cylinders; BUT WAIT - didn't Chevrolet do that with the Corvette?

As a good friend on another forum is so fond of posting: "engineers do things for a reason"!

Jon.

If I were considering adapting a 2-BBL carb (with a proper manifold design, not an adapter) I'd look at the Dual-Jet Rochester, that's basically the front half of a Quadra-Jet. That carb has a 3-step venturi, and it transitions pretty seamlessly from one RPM range to another. The throttle bores are also small enough to assure adequate venturi velocity to avoid the dreaded off-idle stumble that big bore carbs of all designs tend to have. There MUST be enough airspeed through the venturi to create enough vacuum for good fuel mixing. That's difficult, if not impossible when the throttle bore and the venturi is too large.

Does this site have spelling auto-correct? It keeps converting "venturi" into "venture!"
Jerry

I ran dual rods, yf's and glass bowl hollys on my 235 motors (and triples on an offy intake) for quite a few years until I swapped over to a pair of Holly/Weber 5200s from a 74 pinto. (progressive 2bbls, I used the 74 version because the inlet is threaded as opposed to a pressed in brass nipple for the earler modles). I used adapters on a vintage Weiand intake. The difference in the way the motor ran was significant. Ran way better on the 5200's. When I installed a 261 in 2015, used that same set up. Ran pretty good. Then I decided to step up to a new Clifford 2X2 intake using a pair of 38 dgev webers (same basic carb, just non-progressive and larger). A big difference, a really big difference. I don't think I could have realized any real gains by just using the 38 mm webers on adapters and the Weiand. Although I loved the Weiand, and had run it for many many years, the clifford was so much better in terms of performance. So yes, I think direct to plenum has its advantages over adapters but I also don't think there is necessarily that much advantage if all you are doing in running a mild built 235 on the street. If that is what you are doing, head to the 5200s on a traditional vintage intake with adapters.

deleted

I have done some testing (not on Chevrolet, but other makes on this subject), and the subject has been beat to death.

But:

(A) If I were really interested in this subject
(B) Independently wealthy
(C) Had an infinite amount of time on my hands

I would proceed in this manner.:

(A) Rebuild the original Rochester (need a baseline) and yes, I CAN rebuild these things, just don't like to rebuild them, and rebuild them, and rebuild them and...........
(B) Acquire and rebuild either a Zenith or Stromberg carb of the correct venturi size for the engine (better efficiency).
(C) Acquire an original Chevrolet single barrel intake and spare cylinder head.
(D) Do some homework on FACTORY inline 6 cylinder two-barrel applications, at first blush, the 232 A.M.C. looks promising.
(E) Once a suitable donor (eg. the AMC) was determined, acquire an intake for this engine.
(F) Bolt the original Chevrolet intake to the Chevrolet head, and cut out the entire plenum assembly
(G) Remove the plenum assembly from the donor, cut the Chevrolet runners to fit the donor plenum.
(H) Take the whole thing to a welder (my welding looks like the underside of my desk in the 3rd grade!!! )
(I) Rebuild the Carter WCD from the AMC (another reason the AMC looks promising) with a calibration 2 sizes richer than stock.
(J) Fabricate a second set of rods which are only one size rich.
(K) Make an appointment with a dyno operator and test it all.

GUESSES, based on testing on other engines:

Original Rochester - 100 percent
Stromberg or Zenith - 110 percent
Two barrel Carter ON MODIFIED TWO BARREL MANIFOLD (after final calibration on dyno) - 115 percent
Two barrel Carter (or any other two barrel of your choice) on original single barrel manifold with two into one adapter - 65 percent.

The only way you will know for sure is to test it.

Jon.

Good stuff, if you were to guess, where would the appropriately sized Carter YF fall in relation to the Rochester/ Stromberg/ Zenith?

I do not know jack regarding the answer to your question, but here is some Zenith YF information.

JW - the reason I did not include Carter, was that the original poster did not specify engine size. As you are aware, Carter DID offer Chevrolet replacements for the 216 and 235, and a very difficult to find replacement for the 250. No direct replacement was offered for the 261 or 292. I was trying to make the post as generic as possible. I would rate the Carter YF right at, or just a percentage or two below the Zenith or Stromberg.

Back on the adapters:

We were trying to come up with an inexpensive fix for some VERY expensive cars, whose original carburetors, when available at all, were well into 4 figures! We spent quite a lot of time, fruitlessly, on testing; finally arriving at the conclusion is was simply much better to fabricate a new intake. We fabricated several different adapters, and did find adapters 4 inches tall or taller, with shaped ports, would add a couple of percentage points to the adapted system. Still way below what was possible without the adapter.

Tim - took a look at your reference link, and quite curious as to the original source for this data, as Carter DID NOT specify a solid needle for the Chevrolet YF replacements. Did not check any other information on the link.

Jon.

You may have missed some of the point. I ran rods, yf and glass bowel 1bbl on a series of Weiand, offy and a cyclone 2x1 intake so direct to plenum. Ran decent. Ran Holley/Weber 5200 2bbls On same Weiand manifold with adaptors, ran better. If the intake was a 2x2 may have run better. Ran bigger carbs direct to plenum on a Clifford, ran way better. I doubt that the bigger carbs would run well on adapters. It’s all about what the motor needs and wants as to whether direct to plenum makes a difference. A mild built motor might not respond better or worse if carbs are fed through an adapter or direct to plenum. On the other had, a highly modified motor would run better direct to plenum. I think that is what I was trying to say in my inarticulate way, lol.

Carter YFs, pretty good carb. I like them better then running rods. Simpler, easier to rebuild, the early models (think 216 versions, manual choke and the later automaticc choke) are smaller and run better in my opinion on a dual set up. Main jets are the same as wcfb jets or maybe AFB I forget, will have to look at my jet box to confirm, and are still available. The yf uses a rod arrangement to meter idle fuel, and the rods can be swapped around. Never played with the zenith but I have heard they are a terrific carb

I like the way Jon's facts, figures, and common sense approach to things rains all over the bench racers' parade and bursts their bubble of fantasy! It seems all those claims of ultra performance, huge gas mileage gains, and riding unicorns sidesaddle over rainbows never seem to have any dyno figures attached to them!
LOL!
Jerry

Chrysler offered a 2 bbl carb on the 225, called it the Super Six.
It did run a wee bit stronger but was a pig on gas compared to the single.

Most owners pushed them hard then complained to us service guys about poor mileage.

But where would we be without innovation and experimentation.
Better to adapt injection than fool with carbs. Much easier to tune, just read the downstream O2. Don.t guess, measure it!
Have fun. Howie

Howie - the 225 is another example of doing one's homework. I mentioned the A.M.C. simply because it uses a Carter WCD, which in my opinion is one of the finest, if not the finest 2 barrel ever made. It is infinitely tunable, as it utilizes Carter metering rod technology. If I remember correctly, the 225 used the Carter BBD, which also used metering rods, but only as an on/off switch similar to a power valve. They did not use stepped diameters.

The point to the post was to suggest to attempt to utilize a plenum that was actually designed by engineers, and adapt the entire plenum to the Chevrolet runners; and the added benefit of getting the carb along with the plenum.

There are probably other 6 cylinder factory 2 barrel installations (I know G.M.C. used one on larger 6's), but I am not sufficiently interested to do the research. Another possibility MIGHT (or might not be a straight 8).

And innovation and experimentation is a wonderful thing. I truly believe this, which is why I posted a possible solution with the A.M.C.

Have done my share of experimentation, generally with a less than the desired result, as in the weeks I spent 30 years ago trying to make the two into one actually work!

We actually put a Jaguar double overhead cam 6 cylinder in an MGA. Did you ever try to shift gears with your right hand two feet behind your back? Handling wasn't much either. Kind of nose-heavy.

Jon.

In the 70's Chevy put a std 2bbl (Rochester GC) on truck spec'd 250 I6's and went to a progressive 2bbl (Rochester 2SE Varajet) in 79. They further modified the 2SE with electronic controls to further meet emissions. Those 250's also came with factory split dual exhaust manifolds.
The 2SE came w/a small 35mm primary bore and a much larger 46mm secondary bore.

My research classifies the 2SE similar to the Rochester B, you either like it or you don't lol....

That carb had a nasty habit of wearing out the connection between the secondary air valve and the metering rod, and sucking that brass metering rod into the engine. It makes some interesting noises when it gets imbedded in the top of a piston.
Jerry

Jon, this will be a little off topic but I am curious...in your opinion, what is the best carb for a 261? Especially since Carter didn't make the YF for that engine?

I'm currently running a 261 version of the Rochester B. My 261 is bored .080 over, so the displacement is actually about 272 cubic inches. Because of the increased displacement, I installed a .067 jet and that seems fine based on the spark plugs.

I'm happy with the Rochester B, but I've been curious about your opinion about the best carb for that engine.

I had a couple Zenith 228's off of late 50's GMC 270's that I wanted to experiment with, but the mounting holes were a little too close together to fit the Chevy intake. I have read that the Zenith 228's were also available(but very rare) with the wider mounting holes to fit a 261 intake and am always on the prowl for one when I scour junkyards.

Thanks,

Matt

Jon gives us good technically-based answers, so starting with addressing his advice/suggestions is appropriate.

I have used a variety of carburetor(s) on my 1960 261 (bored 0.060 over - it is a little under 270ci).

After reading some of Jon's posts years ago, I opted for my "lame logic" and looked for carburetor/carburetors that matched the cubic inches (CFM might have been a better measure, but that is not easily measured after a over-bore, I guessed).

I now have settled on one Holley AA1 two-barrel carburetor from a GMC 302, with two adjustable jets. Maybe a GMC 270's Zenith (?) one barrel carburetor (with an adjustable) would be as good, or better. A vacuum port had to be drilled into it, and I had to find an intake manifold adapter plate (Larrowe & Sons, I think).

My 261 has never driven more smoothly and with as much power (all empirical "measurements" of performance). Come to eastern NC, and I'll take you for a ride.

Tim, CFM (theoretical, not actual) is easy to figure. Divide the cubic inch displacement by 2, multiply it by RPM, and divide that number by 1728. The resulting number is cubic feet per minute, assuming 100% volumetric efficiency. That number is virtually impossible to achieve on anything but a radically modified race engine, so 90% VE is a much more realistic number.

For instance: A 350 Chevy bored .030" oversize is 358 cubic inches. Half of 358 is 179. (It takes 2 turns of the engine to pump the total displacement through it.) 179 X 5000 RPM = 895,000 cubic inches per minute. There are 1728 cubic inches in a cubic foot. 895000 divided by 1728 = 518 cubic feet per minute (theoretical). 518 X .9 = 466 CFM. Now- - - - -how much too big is that 650 CFM Edelbrock 4 barrel everybody recommends?
Jerry

Matt - both Stromberg and Zenith offered aftermarket carbs that would work exceptionally well, also Zenith offered a type 228 for the G.M.C. 270 that works exceptionally well.

Tim: here are two links on CFM/carburetor selection:

http://www.thecarburetorshop.com/Carburetorsizeselection.htm

http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm

Your method of picking one from an engine of similar displacement also works MOST of the time. It does NOT work if the engines are extremely dissimilar. Engines with vastly different maximum RPM, engines with different configuration (oversquare/undersquare), and generally sufficiently dissimilar to muddy the waters. Thus a carburetor designed for a 350 Pontiac (high torque, low RPM) doesn't generally work well for a 350 Chevy (low torque, high RPM).

As far as CFM is concerned (read the article), one of the most knowledgeable comments came from Mark Twain "Figures don't lie, but liars figure!".

Jerry - take a look at the last paragraph on the first link

Jon.

Timely stuff, I was going to post a thread with a related question.

I can certainly get my head around the CFM calculation for engines. Reading Jon’s links, looks like 75% VE is the right number for our old 6 cyl engines? What should one use for the rpm piece?

Here’s my bigger question, can one back their way into a CFM rating for a carb with throttle bore and Venturi size? For instance, the Stromberg “BX” replacement carbs. I see them come up for sale. I can find charts that show Venturi and throttle bore. But haven’t yet figured out how to convert that into something useful. I think I can still do algebra, just can’t find the right formula.

JW - I don't think you will ever see such a formula; there are entirely too many variables.

Jon.

I built race engines for a track where we were required to run a Holley #7448 2 barrel carburetor, commonly called a "350 CFM" carb. Now, it was possible to flow a LOT more air than that through it. A 350 Chevy engine would run 6200 RPM on that carb, which was WAY above the advertised volume, but the manifold vacuum went up to the point that the engine simply couldn't suck enough air, air/fuel mix, or peanut butter, I suppose, to wind any tighter. The carb served the same purpose as a restrictor plate on NASCAR superspeedways. There was no point in doing a lot of mega-dollar engine building with big valves, big ports, and ultra-hot camshafts and trying to feed a beast like that with an undersized carburetor. It was the racing association's way of controlling engine building costs and making the guys on a limited budget competitive. Did we find ways to cheat?- - - - -Of course we did! The same carb on a 250 cubic inch engine with a super short stroke would turn at least a thousand RPM faster than the 350's. Once my guy managed to get out front and keep the engine wound up, nobody could get within half a lap of him. My engines wouldn't pass a full teardown, but nobody wanted to put up the big bucks the track required for a protest, and their tech inspectors were too dumb to figure out what I was doing.
Jerry

That’s what I was afraid of, but good to know that I can quit trying.

Have a 55 Hudson Hornet in the shop right now that has the V8 option which is the 320ci Packard engine. It runs like scat and has the factory ONE BARREL carb. On the GM I6 engines doing anything speed related is a waste of time in my opinion. Now for rock solid dependability, great low end torque, and long range durability they are among the best but not suited to hot rodding. Show me any Chevy/GMC six cylinder of any size or configuration, multi carbs, super/turbo charged, ignition, exhaust, etc and apply the same thing to an engine with seven main bearings and a 12 port head and it will show how much time and money was wasted on the GM engine. All is not lost as after just 65 years GM has given us the 4.2 (or Atlas) I6 which should be able to run with the Aussie equipped Ford I6's. Jerry "Hot Rod Lincoln" is now working on a "Dumb" Atlas engine equipped with carb(s) and conventional ignition to make it a doable bolt in replacement and finally giving a GM six with balls.

LOL at edited title and first post.

I would concur that the thread went off the rails. Lots of interesting stuff, however.