OT: How do you size a carburetor?

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Model T Ford Forum: Forum 2018: OT: How do you size a carburetor?
Top of pagePrevious messageNext messageBottom of page Link to this message  By William Hoffer on Sunday, October 07, 2018 - 07:22 am:

Perhaps the speedster guys can help with this? I have a 1912 model 34 Buick that could use some more pep. The engine was rebuilt not many miles ago. Two winters ago I lapped the valves into their cages and lapped the cages into the jugs. Compression is 50 lbs across the board. The carb is a simple Zenith 68 series, rebuilt and tuned to run responsive and smooth. Ignition is by an Atwater Kent distributor, which seems to be just fine - no misses, stumbles, or other problems. It is all very nice, but it would be better if it had more hill pulling power.

So I am wondering if a different carb would be the answer. How do you determine the proper size of carburetor for a particular engine? Any other suggestions?

Thanks, Bill


Top of pagePrevious messageNext messageBottom of page Link to this message  By Stephen, South Texas on Sunday, October 07, 2018 - 08:29 am:

There's libraries full of books on the topic and thousands of calculators on the interweb. You don't give enough info to even start a suggestion. Your question is like asking "what size shoes should I get".


Top of pagePrevious messageNext messageBottom of page Link to this message  By kenneth w delong on Sunday, October 07, 2018 - 09:24 am:

I do not know anything except i would ask Stan Howe what he thinks either here or by e-mail?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Scott Conger - Wyoming on Sunday, October 07, 2018 - 09:30 am:

Neat car!

Did you get the cam reground to stock specs?
Do you have the correct rear axle ratio?

At 23 HP, your car isn't any better prepped to go up a hill than a Model T, which when stock, doesn't have much guts for long hill pulling.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Steven Meixner on Sunday, October 07, 2018 - 12:57 pm:

Years ago I drove a 11 Buick that had lots of power. The rockers were switched to 216 Chev I think, this gained a rocker arm ratio.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dave Hanlon N.E.Ohio on Sunday, October 07, 2018 - 01:52 pm:

The carburetor can't be any bigger than the intake manifold opening, or it's just wasted.


Top of pagePrevious messageNext messageBottom of page Link to this message  By charles fanucci on Sunday, October 07, 2018 - 03:48 pm:

carter updraft BB 1D used on most old engines with great results


Top of pagePrevious messageNext messageBottom of page Link to this message  By Gilbert V. I. Fitzhugh on Sunday, October 07, 2018 - 05:14 pm:

I have a 1912 Model 35 Buick. It has the same engine as the Bill Hoffer's 34, but a longer chassis, bigger wheels (33x4 tires) and a full touring body. It's the entry level Buick for 1912; there were two larger sizes. I'm running on the original Schebler carburetor. This engine is a tad smaller than a Model T- same bore, but 1/4 inch less stroke. It has overheard valves. It has a three-speed sliding gear transmission; the ratios are beautifully chosen but the linkage is an abomination. Fortunately, the engine pulls like a freight train, so I rarely have to downshift, even though the car weighs half again what my Model T weighs. One day on the BBC tour last May, I had Steve Hammatt from and Jan Kendrick as passengers. Steve must be well over 250 pounds. Jan has a 33-horse Hudson, and she asked whether my car was a 30 or 35-horse. When I said it was rated at 22-1/2, she said: "NO WAY!!"


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Gregush Portland Oregon on Sunday, October 07, 2018 - 06:22 pm:

If you get a hold of a copy of the Dikes Carburetor book, using this as an example, in the Stromberg section it list size of venturi for CI size of engine. While the base carb might be an OE-1, the venturi and jets would be sized to the engine it was used on.

funny spell check does not like that word; venturi


Top of pagePrevious messageNext messageBottom of page Link to this message  By Terry Woods, Cibolo, Texas on Sunday, October 07, 2018 - 11:09 pm:

I would try a Stromberg OE-1 like used on Studebaker sixes and other large engines. The can even be used on a T, and Stan will tell you; they make a T go like stink.


Top of pagePrevious messageNext messageBottom of page Link to this message  By William Hoffer on Monday, October 08, 2018 - 07:36 am:

Sounds like you have the magic combo, Gil. Clay Green's car is a sister to mine and has more zip as well. I have to believe it is my carb that is holding it back, so I will start digging using the suggestions posted here. Many thanks for all the help! Cheers, Bill


Top of pagePrevious messageNext messageBottom of page Link to this message  By George John Drobnock on Monday, October 08, 2018 - 09:43 am:

Any one consider this post:

Need clearification on low CFM ratings of carburetors used on Model T Fords (from 2009)

http://www.mtfca.com/discus/messages/80257/112906.html?1257130720

"In my book a Ford Model T has 177 cubic inches of volume and can be made to turn 3000 revolutions or so. A Ford T engine will displace 265500 cubic inches of air in one minute at 3000 revolutions because it is a four cycle engine and only sweeps half of the piston-cylinder volume each revolution. A single cubic foot of air contains 1728 cubic inches. So when you divide 265500 by 1728 you get an answer proving that a Model T ford requires a carburetor that will flow 153 cubic feet per minute at 3000 revolutions if it has 177 cubic inches of displacement. (Harris 2009)"


Top of pagePrevious messageNext messageBottom of page Link to this message  By Stan Howe Helena, Montana on Monday, October 08, 2018 - 01:48 pm:

1. The size of the throat and the size of the venturi are only two of the things that control the performance of any carburetor.

2. ALL carburetors are a compromise between performance and economy and simplicity or complication.

3. Most early carburetors are simple two circuit systems. There were many attempts to regulate the air/fuel flow with automatic air valves, compensation valves, adjustable venturi etc.

4. Many carburetors have a throat size larger or smaller than the intake throat size. It has little to do with performance.

5. "Flow," which so many people are concerned about is only one factor in carburetor design and performance.

6. All carburetors operate on the theory of pressure differential, the difference between the pressure inside the engine and the atmospheric pressure outside the engine.

7. Liquid gasoline will not explode. Liquid fuel must be changed to a gaseous mixture of gasoline and air in order to be burned in an internal combustion engine. Riser length, intake manifold design, etc., are all part of the equation.

8. 1910, 1915 etc., carburetor designs do not run like 2018 fuel injection systems.

9. 90% of carburetor problems are ignition.

10. 90% of ignition problems are carburetion.


Top of pagePrevious messageNext messageBottom of page Link to this message  By William Hoffer on Monday, October 08, 2018 - 05:57 pm:

OK. So to pack more fuel/air mixture into the cylinders to make a bigger bang, we need a bigger pressure differential and the means to deliver a bigger charge, right? I have not brought up the exhaust system, but if I rebuild the exhaust to original specs, (it is a glass pack now with rigged up piping), perhaps the system will move more fuel and exhaust per cycle? Would time be well spent opening up the intake manifold, bolting up a Stromberg OE-1, and cleaning up the exhaust path? Is there a way to predict this stuff?

Thanks to all for your input. Bill


Top of pagePrevious messageNext messageBottom of page Link to this message  By Scott Conger - Wyoming on Monday, October 08, 2018 - 07:18 pm:

Gilbert and William

I would really suggest that you both install degree wheels on your cars and compare opening/closing degrees on both intake and exhaust valves and compare notes.

This honestly does not sound like something that is going to be remedied by "opening up the intake manifold...", etc.

If William's engine rarely needs to be downshifted and "pulls like a freight train" and yours "could use more pep" that really sounds like a torque issue.

Put a vacuum gauge on the thing, make sure you don't have a vacuum leak, and then check your valve events.

Diagnose your issue. Don't shotgun it.


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