- John

and a lot of other worthwhile information.  I have the material together to try match porting a set of heads and intake myself, but suggest practicing on some expendible parts first.  To match port the intake alone, and not the heads, wouldnt do any good.

This is what you need to do.  Slot the holes together.  Then look into the right side hole no matter which end you look from and you will see an obvious restriction where the passage on the right side dives under the cross runner.  Grind on this to open it out. The left side is not so restricted and doesn't need much.  Radius the holes where they start at the flange so that the flow does not have to go over the sharp edges left over when the holes were machined.

This pic shows a manifold that has the grinding finished but it has not cleaned up by blasting.  After blasting fill the surface of the flange including the space between the slots with an epoxy glue mixed with the iron filings that you have saved from the grinding.  Grinding is not the right word because I don't use abrasive stones for any of this, I do it all with carbide tools and a high speed, electric die grinder.  Level the hardened glue/iron mix with a file. I also plug the hot air tube with a 5/16 bolt on both sides and I don't run the heat in the exhaust plenum,  However this means things are slooooow to warm up! 

I like a 4 hole spacer for slightly crisper throttle responce.  I used a manifold modified like this before the BlueThunder was available and had my engine dyno tested.  This manifold modification supported over 325 hp on pump gas normally asperated, so it does work.

         They don't always. Combination of plenum volume and mixture velocity issues decide that. In many cases people add HP with 4 hole spacers under the carb. My Windsor powered Ranger being one. It liked the 4 hole better on the original Wieand manifold. With the Edelbrock that's now on it, the reverse is true. (?)

Are you trying to create a smooth surface on the top of the flange

Besides checking the Throttle opening as Steve suggests you should also be sure that the spacer you use will seal to the carb and manifold.  I am using an aftermarket 1" high 4 hole on my truck (which might be too high for a '57 Pass car) that has some recesses in the bottom. These recesses nearly overlap the manifold and if they did there would be a severe vacuum leak so check that also.  If you are using the stock Ford 1/2" 4 hole you will be OK.

I think the rectangular crossover slot is an attempt to provide "balanced" manifold vacuum - but I don't think you have to have it for the system to perform well.

I have thought about tapering the primary holes of a 4 hole towards the rear giving a smoother transistion to the rear of the engine. This should allow the air go to the back of the engine easier at part throttle.   I have never seen such a modification described or tested but maybe someone else has tried it.   Remember that as difficult as it is for air to turn corners the droplets of fuel have a far harder time. So this modification of a 4 hole might give more uniform distribution of fuel from front to rear thus giving better milage than is otherwise obtained.  The effect might not be very large so it probably would take an engine dyno test to demonstrate if the idea works.

Also as Frank notes these effects are very combination specific, what helps in one case may hurt in another.

- John 

Initial Thoughts w/ no carb tuning:  starts great, crisp throttle responce.  I don't know if I'm just expecting gains, but the car feels like it has more power across the board.  I can definately tell the difference if I hit the gas at about 40...  Firm downshift and it pulls nicely. 

It does bog down a bit if I'm at about 35 mph and I give it a little throttle, but I think some tuning will clean it up.  But I'm very happy over all!  Worth the time / money / effort.

- John

I think what he's referring to are the sharp edges on the bottom of the carb deck. Where the machining has broken through the "ceiling" of the plenum. If you maintain the four holes -and don't cut slots - to try and keep the crossection fairly constant below the primary... say bore them oversize to match the spacer and carb if you will - then this would be "flaring" the exit on the bottom side.

When slots are used, the trailing edge of the carb bore would be abrupt with an open spacer - could be eased with a four hole spacer flared to the rear on the bottom. But forward - the flare would be on the underside of the plenum ceiling.....I can draw this better than explain it - but need more coffee.....

Did your finished job look like which picture? (two full slots or two holes joined) and what spacer do you intend to use?

My outfit has the OEM holes -so I made the tapered spacer I sent the diagram of. If you are using the OEM spacer - does it have holes larger than the carb base?

Is there some meat in the spacer to allow creating a bit of a transition taper? I used a phenolic spacer and tapered/blended beneath the throttle plate to get that smooth transition to the manifold hole.

This picture is an old one - but - I really "funneled" the spacer openings under the carb to make all of the edges match up as smooth as possible. Between the 1/2 inch of phenolic material - and the air gap under it (this is an unmodified intake) - the carb stays pretty cool. 

I would encurage you to play with it and see what feels the best. We had one motor that actually preferred the Motorcraft 3/8" tall heat insulating spacer and that motor gained almost 20HP every time we went back to it after trying something else.

Just a few thoughts,

SPark