Boring out "A" manifold to "B"size ?

G

Greg out west

Senior Member


Hi Guy's,


Been working on grinding out the A intake manifold to the B size..I'am running a B block and carb & h/c head so why not take advantage of the bigger carb, ect.



I have a B manifold,, but just don't like the look's of it !!! I remember on the old FordBarn that you could pick up a few H/P doing this. Does anybody know how much H/P it add's ?




I had the B carb sitting in a box for year's. cleaned it up and tried it and it seem's to run alot better than my old A carb..I don't think it was used every much as there is no play in the throttle shalf..



I understand the B carb has a high speed circuit ?.



Greg out "West"
 
I bore all of mine out to 1.300" ....but the biggest thing I am starting to notice is my combination is starting to run out of fuel. With a modified high-compression head, to oversized intake valves, to good ignition, etc. , what I have found is that when I installed a better exhaust system, I now can flow more air than what the stock carb. can flow fuel. I have been working with Borla Exhaust to make a stock appearing exhaust system that flows even better for expanded performance. Since Borla has a factory in my area, --and since Alex Borla enjoys coming to my shop, he has allowed his engineers to work on development of an A and T exhaust system. Before we were even able to get it on the chassis dyno, we have noticed that most Zeniths won't flow enough to "keep up". I really hate the look of a downdraft carb/manifold on an aestetically authentic restored Model A with hidden performance mods, so I am exploring options in that area.
 
I use adjustable reamers to bring the model A intake manifolds out to model B size. I carefully set the manifold up in a bench vise and ream by hand. It takes a while but if you like doing things yourself it works. I got a set of adjustable reamers at Harbor Freight and they have also really came in handy to make some repro parts fit. It works for me. Good luck.
 
In reply to the "B carb. high speed circuit": What you are probably thinking of is the power jet. This enriches the fuel mixture at about 3/4 throttle opening and higher. It is similar to the "power valve" on newer carburetors.
 
As the owner of a Model B Ford I ask what is wrong with the way the intake manifold looks? Having gotten that off of my chest I would like to ask if you know there are two types of Model B intake manifolds. One has the vacuum intake at the front (this is probably the one you don't like) and the other has the vacuum intake at the rear.

Charlie Stephens
 
I also run a "B" Manifold & it has the vacuum port in the rear & I think it actually looks better than the "A" intake. I have never seen the one with the front port.
 
Hi charlie,


I have a B manifold . I just like the look's of the A intake better. It goes with the car.. If I had a Model B, I'ed probably use the correct intake.. Maybe by using the A intake,,I'am in a way trying to hide the B engine ? Go figure ??

Greg out west,,


I see your in L.A.,, Me too I'am in the so. bay.
 
Beyond Bore, Could The B Be Better?

Opening up the A intake to equal the B is a popular modification, but has there been any dyno test that compares the two. There must have been some reason for Ford to change the intake to that flat topped look that identifies the Model B from A rather than just opening up the left over A intakes as might otherwise have been done. Could there be some flow advantage in the B design? Trouble is that the seat of the pants is too subjective. I have often used down draft carb setups and found them to be quite satisfactory most of the time as long as there is enough fuel in the tank to keep it flowing, also some have been of a low enough profile to improve the gravity feed, even on hills. The lowest downdraft I've found is the Cragar which may be a result of the added height of the overhead valve cylinder head. Downdraft usually doesn't look at all Model A. Good Luck: Fred A
 
I have a B carb on the A manifold. However using the B manifold was not an option for me. As my car is right hand drive the inlet manifold has a built in crank on it to pivot the advance and retard rods. Of course the B had a centrifugal advance on the dizzy so didn't need the advance and retard hence no pivot on a B manifold even in RHD country.Why I might like the increased performance of the B carb I still want the car to look like a Model A and hence I wanted to stick with the original ignition set up
 
Those of you that are running a Zenith B2 carb, what do you do for an air filter? The filter for the Zenith A carb is a bit too small.
 
"I really hate the look of a downdraft carb/manifold on an aestetically authentic restored Model A with hidden performance mods, so I am exploring options in that area."

There are a lot of old marine engines that used up draft carbs that are
small to Huge.

For enlarging the insides of a manifold, try blocking off all openings and filling
it ALMOST full of steel shot. Then bolt it to a paint shaker...Different materials will take different times to get an enlargment so check it often so you don't go too far and break through.
 
Although boring the up tube of an A manifold helps, one big bottleneck remains- the more-than 90 degree sharp turn as it splits front and back. The B manifold is easier in this transition and also larger in cross section on the two arms, something that cannot easily be bored out on an A manifold. Unfortunately, a B manifold looks, well, like a B manifold.
 
This was cut on a milling machine.
I used a long shank carbide ball up inside in an attempt to releave the sharp 90 edges.
 

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If you are going to bore the intake out for the first time, be sure that you align the bore perpendicular to the bolt holes of the manifold, NOT the carburetor mounting flange. They are not the same.
 
Brent, Could you detail how boring out a manifold is done? The 1.300 is at which end? The carb end or the block end? How about the block? How high a finish should be on it after the boring? How do you locate center of the ports on the block, a gasket?
TIA Terry



BRENT in 10-uh-C said:
I bore all of mine out to 1.300" ....but the biggest thing I am starting to notice is my combination is starting to run out of fuel. With a modified high-compression head, to oversized intake valves, to good ignition, etc. , what I have found is that when I installed a better exhaust system, I now can flow more air than what the stock carb. can flow fuel. I have been working with Borla Exhaust to make a stock appearing exhaust system that flows even better for expanded performance. Since Borla has a factory in my area, --and since Alex Borla enjoys coming to my shop, he has allowed his engineers to work on development of an A and T exhaust system. Before we were even able to get it on the chassis dyno, we have noticed that most Zeniths won't flow enough to "keep up". I really hate the look of a downdraft carb/manifold on an aestetically authentic restored Model A with hidden performance mods, so I am exploring options in that area.
Click to expand...
 
Terry said:
Brent, Could you detail how boring out a manifold is done? The 1.300 is at which end? The carb end or the block end? How about the block? How high a finish should be on it after the boring? How do you locate center of the ports on the block, a gasket?
TIA Terry
Click to expand...

See this link
Boring out the A Manifold
for a detailed explanation of the job.

Joe
 
Joe & David have pretty well covered your first 3 questions. The only difference is I do not detail (radius) the end of the draw tube because I am wanting the reversion effect. As for centering, IMO you need a lathe and a mill to make a plug. If you notice, Dave has a fixture to hold the manifold however it takes someone with machine experience to make that. I do not have a fixture for modifying manifolds but I just make a plug.

As far as how to port match, mount the manifold to the block without the intake valves and stick your finger down the pocket area and feel with your finger.
 
BRENT in 10-uh-C said:
...As far as how to port match, mount the manifold to the block without the intake valves and stick your finger down the pocket area and feel with your finger.
Click to expand...

If the manifold is fairly clean and dry, a quick shot of spray paint should give a good outline of the block against the manifold surface. Same for exhaust.

Joe
 
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