I'm thinking about making a custom exhaust header, when I move from the current single downdraft carb to a pair of sidedraft carbs.
On the one hand, since the intake/exhaust gasket has cylinders 2 and 3 siamesed with just one big rectangular hole, my initial assumption is there's no point trying to do long individual header tubes that join down lower.
On the other, when I look through pictures on the site I see stuff like ddgonzal's header that do in fact have two separate header tubes dropping down quite a ways before joining.

So what's the scoop? Is it worth putting a bit of extra metal in the header top to fill in between the two exhaust ports where the gasket is open? Are there gaskets that have a divider there to try to keep the two separate?

Good question. Yes, I have heard of this for some engines, but it is very rare. Never seen one. Basically the divider is on the exhaust manifold and fits into the port.

Does it really matter?

Mini engines have siamesed exhaust port, and use a 3-branch exhaust. They are circular and the way performance is improved is to use tubular exhaust with a long, larger diameter, center branch, called a LCB (long center branch) exhaust.

On some divided port heads, hot-rodders grind away the divider to make a siamesed port. For these heads the flow increase is more beneficial than the scavenging effects.

So it's not so cut and dried. The exhaust port design on the A-series is fairly good and receives very little attention.

Small-block chevy has separate center ports, but a siamesed exhaust manifold. Replacing this with tubular headers makes a huge improvement. Not so much with the A-series, as the cast manifold is already a 4-into-2 design.

The GX manifold, although cast, is a long Tri-Y design, nearly as good as a header, and certainly more durable and reliable. Racers still use tubular headers for tuneability (changing length of collector, tube diameter, etc). For a street engine ran in a variety of RPMS, and where fine tuning will NOT happen, the GX manifold is the answer.

Dividers have been sold


But never dyno tested, so no one really knows. See Do I really need a port divider? Will I get better performance or an increase in mileage?

If I understand right you're talking about building your own headers/ extractors? A little leakage between the ports will have little or no effect. Don't worry about sealing 2 from 3. You're talking 0.5 to 1mm.

If you merge the pipes efficiently you can get good results with short primaries and long secondaries (much like the GX exhaust). This is because there's no overlap between 1&4, and , 2&3 so merging them is ok, only 1 cylinder is using the port at a time. The trick is minimising losses on the merge.

Wow, a wealth of information! Thank you both.
This is for a street car, and I'm trying the best I can to raise low to mid rpm torque, so I'm going to try for long headers (given the pretty serious clearance constraints I have.) So as long as I'm making them, why not make them the best I can?
At the same time, I have an H72 or 77 head, so there's an air injection hole drilled through the head that connects all the exhaust ports anyway. It doesn't make sense to go totally overboard.
Now, I get to figure out how to imitate the GX header given my space constraints.
Thanks!