downdraft carb

German guy

German guy

Senior Member
Any consensus out there on the best down draft carb and manifold that does NOT require a fuel pump? Looking to increase power after having added a Zipper Dist. My '30 Tudor has a recently rebuilt engine with stock head and no leaks. I plan to add a Snyder's cast iron header. Lots of hills where I live and I need more power without turning the under-hood view into a hot rod wannabe.
 
For power, get a 6 or 6.5:1 head! The freight cost to Germany may be bad, but it is bolt-on and might be cheaper overall.
 
I second what Bruce says, If an original look is important, the higher compression head will not be obvious. Neither will a B carburettor to the casual observer and it will give you even more power than just the HC head, especially if you bore out the manifold's vertical tunnel.
 
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I like my Weber downdraft carburetor but I have a fuel pump. It is an inline pump that is mounted inside the passenger compartment between the fuel valve under the tank and the firewall. Not noticeable.

I was told that the Weber would operate without a fuel pump if the tank is kept full but I have not tried that.

The higher compression heads will really make a lot of more torque but the engine will be starved for air at the higher RPM with the normal updraft carburetors. I have often wondered if a high air flow updraft carburetor from a tractor and a high volume manifold would work. I have seen photos of a special manifold with two standard updraft carburetors that may be an option for you.
 
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Gutentag German guy! I tried a Weber side-draft with no pump, it didn't work, the gas wouldn't flow into the carb. Your '30 has a higher gas tank than earlier A's, that helps. If your manifold is not very tall, and you keep your tank reasonably full, a Stromberg 97, or similar, will work OK as long as the hills are not too steep or too long. I did that on my '31 pickup for several years, and it worked. The B carb will work great, but isn't much better, power wise, than stock.
 
Good suggestions here and I agree with nkaminar's point that an electric fuel pump would not substantially change the appearance of the engine bay. The downdraft carb itself will be the thing people see, plus the air cleaner.
 
I just got a rebuilt engine, and chose the 6.5:1 head. Observations from the first few hundred miles:
1. Louder. This was the first and most immediate impression. The standard exhaust system was not designed for the greater gas energy. The Wretched Roadster has no sound insulation, no windows, and a cloth top. I would expect an insulated closed car would be quieter.
2. More power. For example, pulls up hills in 3rd gear that required 2nd gear with the standard head. This roadster was pretty peppy with the old engine but it’s noticeably stronger now. This would be of greater benefit with a heavier body style.
3. The higher power suggests that a higher (lower numerically) rear end ratio, or an overdrive would make it more pleasant. The engine revs more quickly to your intended speed and feels eager to go faster but the engine does not sound happy at these revs. I presume it would still have enough horsepower if turning a bit slower, and would sound and feel less frantic.
4. More sensitive to small throttle adjustments. It reminds me of a terrier eager for another inch of leash. I readjusted my foot position and the car jumped suddenly. With the original head it just didn’t have the ready torque to change velocity so quickly.
5. Much stronger compression braking. I used to remove my foot from the accelerator in third gear when I saw a traffic light a block away, and be at easy speed for braking when I neared the intersection. Now I must slowly reduce the accelerator or the car slows very abruptly.
6. Speed of rotation of the starter. It is still using the 6 volt system and the starter spins the engine more slowly. Of course, the compression is higher so more resistant. I’ve seen no need for 12-volt conversion so far. We’ll see if the 6 volt system can cope.
7. Forget hand cranking to start. See number 6. No big deal, a rolling start will suffice if needed.
8. It is not as sweet, not as smooth, not as leisurely. The original power restricted me to drive more gently. Again, the puppy pulling on the leash. Philosophically it is less satisfying because I value experiencing the same sensations as people who drove the car for nine decades before me. This applies to me, maybe not you. It’s not measurable on a dynamometer or stopwatch.
I am a relative newcomer to the Model A hobby and I only represent these thoughts based on my personal experience, not suggesting they are rules.
 
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I just installed an FSI Zipper distributor so spark advance should not be an issue. The 6.5:1 would probably require a modification to the Zipper. That is why I asked if the 5.5 would make enough of a difference. Also...maybe a fairy tale...but I am led to understand that the 6.5:1 puts a lot more wear and tear on the bearings and I really do not want to do an engine rebuild in the future. Thanks for you reply.
 
6.5:1 will not put more wear on the bearings, unless you drive like a mad man. Think about this, it takes a certain amount of horsepower to move a Model A down the road at a given speed. Therefore you are putting out only that much power at that speed, no matter what the compression is. The power output of the engine is governed by throttle position, so is the wear on the bearings. In theory, the oil cushion in the bearings prevent bearing wear. You will likely need to use less spark advance to prevent knocking.
 
German guy said:
I just installed an FSI Zipper distributor so spark advance should not be an issue. The 6.5:1 would probably require a modification to the Zipper. That is why I asked if the 5.5 would make enough of a difference. Also...maybe a fairy tale...but I am led to understand that the 6.5:1 puts a lot more wear and tear on the bearings and I really do not want to do an engine rebuild in the future. Thanks for you reply.
Click to expand...


I have been driving Model As with The Police head (5.2 to 1) (i.e. the head with large "B") since 1963.

For your money the higher compression head is the best deal and least expensive. The increase in low-end torque and power going up hills is outstanding.

You WILL be happy with the results!

I did notice in 1963 that spark needs to be retarded even with the 5.2 Police Head!

If you are adding a higher compression head (i.e. above 5.5 to 1) to a Zipper equipped car you need to contact First Street Ignitions and get the correct "spark curve Kit" for you application!

Running a stock spark curve distributor with a Head higher than 5.5 to 1 WILL damage engine.

Benson says that this info is a quote from FSI website:

https://www.fsignitions.com/product...nd-With-or-Without-Coil/35?cs=true&cst=custom


Advanced Curve Options: Four options available.

1. (Stock Head) 24-26 degrees of centrifugal advance @ 2,000rpms. (for stock heads and 5.5:1 compression ratios)

2. (High Compression Head) 16-18 degrees of centrifugal advance @ 2,000rpms. (for High compression heads with 6:1 compression or higher)

3. (Miller OHV) 32-34 degrees of centrifugal advance @ 3,200rpms.

4. Custom advance. (available anywhere from 12 to 40 crankshaft degrees of advance). $30 Up-charge
 
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Gutentag German Guy, I have a Zipper on my Burtz 6.5 head and this is what Shawn Buller @ FSI said about the Zipper advance setting for this head. "For 6.5 compression ratio you only need 20-22 total degrees advance. I would not add more than 24 degrees of total advance, so with our 16-18 degree advance distributor you would run 4-6 degrees initial advance at idle for a total of 20-22 degrees." When I ordered my Zipper Shawn ask me if I had 6.5 head.
I have a stock updraft zenith on my Burtz engine that has about 1.6KM on it and it is very much different in performance especially when climbing hills. My previous stock engine fitted with the Zipper would hold a speed on a hill. The new setup allows me to accelerate on the same incline.
 
Another possibility is that they make a manifold for two side draft Zeniths. Originally they were made by Trojan Manufacturing in San Francisco (if I remember correctly). A friend of mine had one he liked. Interesting to note he only had the choke hooked up to the rear manifold and it worked fine.

Charlie Stephens
 
Charlie Stephens said:
Another possibility is that they make a manifold for two side draft Zeniths. Originally they were made by Trojan Manufacturing in San Francisco (if I remember correctly). A friend of mine had one he liked. Interesting to note he only had the choke hooked up to the rear manifold and it worked fine.
Charlie Stephens
Click to expand...

Looks like I need a name change. Maybe an operation?
 
Bruce of MN said:
Yes, per this chart https://www.snydersantiqueauto.com/content/Snyders/Product_PDFs/High%20Compression%20Heads%20Dyno%20test%20news%20release.pdf

The 6:1 is better, but not on the Snyder website now. The Burtz 6.5:1 head is the same price as the 5.5:1. The higher compression head requires a bit more manipulation of the spark lever, but to me, the extra power is worth it.
Click to expand...
He has the Zipper distributor which has centrifugal advance. The newer one have springs where you can change the curve.
I need to send my early Zipper in to get it recurved to work with my 7:1 Brierley head.
 
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As others in this thread have stated, the first order of business for increasing power is a higher compression head....6:1 to 6.5:1 is ideal and with attention to spark advance will not pound your bearings.

Next would be carburetion...a Model B carburetor with the vertical runner of the intake opened up will be fine and provide a noticable increase...The B carburetor has an advanced accelleration circuit that will help provide additional fuel/air to feed the higher compression head when accellerating...I may have the terminalogy incorrect, but this is close...And yes, the B carburetor will work fine with your gravity feed on a Model A. There is written documentation from the early dry lakes days showing guys running a pair of Model B carbs with gravity feed running more than 75 mph...no fuel pump needed...(You won't need a pair of carbs and in fact running two will make it difficult to set up a road friendly engine.)

I use a Brumfield higher compression head (6:1) along with a Winfield AA updraft carburetor (which I consider the ideal carb for this application) and an intake that has had the vertical runner opened up...this gives me a noticeable increase in power without drawing attention at events when I open the hood...most don't even notice the Winfield carb...those that do are "in the know".

FWIW, the zipper ignition will make your car easier to drive and provide some assistance with matching spark advance to driving conditions, but ignition on its own does not actually add power to an engine.
 
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The easiest way to increase the power output of your engine is with a "Polizeikopf". Here is an era advertisement, from BV (Aral), for the Ford Polizeikopf:

554268


554269


Hope that helps, and good luck finding an original Ford Polizeikopf in Germany.

Brad in Maryland


PS: Note also the German Ford OEM Bosch Z45 sparkplugs show in the above advertisement.



 

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