Carburetor Flow Rating

[Bob Bidonde]

What is the flow rating of the Zenith 1, 2 or 3 carburetor?

I am not asking for the jet flow rates. Using the CFM Calculator at Summit Racing, and entering 201 Cubic Inches engine displacement and 2781 max RPM @ 60 MPH, the necessary CFM = 138. I doubt that the Zenith 1, 2 or 3 have 138 CFM capacity.

[Flathead]

I think Ford was a little conservative on carburetor sizing, but I bet they had charts and formulas too.

[BRENT in 10-uh-C]

Bob, my calculations for what a stock Model-A engine's requirements are do kinda differ from theirs (-Summits). I suspect theirs is calculated more for racing engines where the Volumetric Efficiency is greater. The ability for the fuel mixture to fully load into a stock Model-A engine cylinder is generally around 75% -or less if the camshaft is worn or if an economy-styled Tillotsen carburetor is used.

The formula that is typically used for carburetor CFM sizing is Engine Displacement X RPM X Vol Efficiency ÷ 3456. So in the case of a stock engine you will need approximately 122CFM at your RPM range however we all know that a stock carburetor is maxxed-out at 2700-2800 RPMs. Zenith advertised their 1" bore carburetor with a 27/32" venturi will flow about 88 CFM which if you use the calculation formula I posted above, their CFM range works well in the 2,000-2050 RPM range which is almost 50 mph. Also realize that fuel economy was a huge concern back then and so engineers did rate on a conservative side when it came to sizing a carburetor for performance vs. economy. The Model-B carburetor with the 1¼" bore was advertised to flow a little over 100 CFMs, which allows the engine to quit flowing at around 2300 RPMs which with a stock gear ratio should come out to about 55 MPH.

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[Marshall V. Daut]

"Also realize that fuel economy was a huge concern back then and so engineers did rate on a conservative side when it came to sizing a carburetor for performance vs. economy."

With gasoline prices around 17 to 18 cents per gallon in 1931, who WOULDN'T be concerned about fuel economy back then?

Marshall

[Bob Bidonde]

Brent,

Thanks for tour response and especially for sharing the FOMOCO venturi drawing.

[Y-Blockhead]

Sponsored Links (Register now to hide all advertisements)

Renner's Corner https://www.rennerscorner.com/slide-show--info.html has a flow chart on various Model A carburetors that I believe will interest you.

[Richard in Anaheim CA]

I wouldn't attempt to contribute to this technical discussion but Henry Ford essentially "guaranteed" that every Model A Ford would attain a top speed to 65 mph.

None of these results would seem to support his claim but I think most of us, with a heavy foot, would agree with Henry.

Richard
Anaheim CA

[johnneilson]

carburetor flow ratings are just a point of reference to compare against one another.
the testing criteria (test pressure) is critical.
Dave Renners work here is showing a comparison of carbs is very helpful, but not a definitive CFM rating. If you compare to Brents comment about Ford rating of 100 cfm. (approx 20% difference on the "B"carb)
Both figures can be correct.
For example, a Holley 2bbl rated at 500 cfm has the exact same venturi/TP and base as the 650 4bbl setup per bbl. The difference in the ratings is the amount of test pressure applied to the different carbs. The 2bbl rating is different than that of the 4bbl. (3" hg vs 1 1/2" hg for example)
Yes, this is not necessarily a 2:1 ratio as there are some other factors that affect the total airflow such as the proximity of available free air to the 4 individual venturies vs the 2 bbl.

Driveability is another subject completely, as is the potential for economy.

John

[jack backer]

Formulas are important but can’t accurately pinpoint airflow due to a variety of variables. Suffice to say the Zenith flows at half of the capacity of the engine. To utilize the full performance potential ( not racing, Street) matching the two flow rates yields a better result than Ford intended. Carburetor design is a factor as well ‘two updrafts better than one updraft, one downdraft better than two updrafts’. The Stromberg 81 closely matches the stock A engine..
Volumetric efficiency changes the equation as well, how much can the engine do with the air flowing through it.

[Bob Bidonde]

Thanks again to all who replied above. Considering all of the info I have collected, it appears that the Zenith 1, 2 & 3 flow rate is in the neighborhood of 88 to 122 CFM as the throttle goes wide open.

As for Henry advertising his new Model "A" should be capable of 65 MPH, I have verified that speed and more, but it eventually cost me a destroyed engine. Fatigue life is a factor if you are running original reciprocating & rotating parts. In my case, a rod cap bolt had a fatigue failure, and the rod went through the block at 63 MPH.

[johnneilson]

Originally Posted by Bob Bidonde

Thanks again to all who replied above. Considering all of the info I have collected, it appears that the Zenith 1, 2 & 3 flow rate is in the neighborhood of 88 to 122 CFM as the throttle goes wide open.

As for Henry advertising his new Model "A" should be capable of 65 MPH, I have verified that speed and more, but it eventually cost me a destroyed engine. Fatigue life is a factor if you are running original reciprocating & rotating parts. In my case, a rod cap bolt had a fatigue failure, and the rod went through the block at 63 MPH.

Hmmm,

If that is the untouched photo of the damage, it was not a rod bolt failure.

Sorry, done too much forensics on failures.

John

[JayJay]

Originally Posted by Richard in Anaheim CA

I wouldn't attempt to contribute to this technical discussion but Henry Ford essentially "guaranteed" that every Model A Ford would attain a top speed to 65 mph.

Downhill, with a tailwind...


JayJay

[AzBob]

Ford ad excerpt from Literary Digest Aug. 1928.

[rotorwrench]

Henry's team was having a hard time getting 40 hp out of the new 200 cid engine so they brought Harold Hicks over from the aircraft plant to have a look at it. He recommended a change to the block to improve cooling around the exhaust valves which was an easy sell. He also recommended a change to the Zenith carburetor. This kind of left Henry skeptical that it would change their readings. When it did, Henry was surprised. Harold had a told you so attitude and that didn't sit well with Henry. Henry was cantankerous that way but cut Harold some slack after he wrecked a model A he was testing and was injured pretty bad. He stayed on with Ford till Henry stopped aircraft production a few years later. Harold Hicks has an oral history transcript over on The Henry Ford site. It's interesting reading for sure.

[Bob Bidonde]

John, look at these photos of the #4 Connecting Rod. One of the cap attaching studs failed from fatigue. The stud failure was not tension or shear.

[jack backer]

Rod bolt failure down stream of root cause.. the journal tells the story.

[johnneilson]

Bob
Did you take the remaining stud nut off for the picture?
Yes the bearing failed quite some time before the rod broke
It should have been banging loud enough to be heard and stopped

Cannot see any bearing tang cuts so assumption is Babbitt and splash oil
If so is there shim debris in the pan with the Babbitt?

Sorry to see this but it is a lesson, expensive not to be repeated

John

[BRENT in 10-uh-C]

Originally Posted by JayJay

Downhill, with a tailwind...


JayJay

Originally Posted by AzBob

Ford ad excerpt from Literary Digest Aug. 1928.

From another Ford Dealer booklet...

[AzBob]

Originally Posted by JayJay

Downhill, with a tailwind...


JayJay

Here's another one right from the horse's mouth, none other than Henry himself.


Ford Motor Company ad from 1927.


"The new Ford will ride comfortably at fifty to sixty miles an hour. It has actually done sixty five miles an hour in road tests." Henry Ford, 1927

[Bob Bidonde]

The bearing babbitt failed when the rod cap went loose.