The Carbs
We've been told that a pair of vacuum-secondary carbs on a tunnel-ram rarely open all the way on the street. We've also heard that this phenomenon is a good thing, because the engine will only use as much air and fuel as it needs. If you can tune the carbs for a smooth transition, a set of small vacuum-secondary carbs is perfect for a mild street machine. To test this theory, we installed a pair of 525-cfm vacuum-secondary Road Demons and strapped the Rambler to the chassis dyno. On the first pass there was a noticeable bog before full throttle as the secondaries snapped open against the secondary spring. After we installed a heavier spring and dropped the secondary jets to 72 to slightly lean out the fuel curve, the Rambler made 365 rwhp. Just to see what it would do, we then tied the secondaries open and ran it again. After a huge lean dip caused by the lack of a secondary accelerator pump, the Rambler made 390 rwhp, a gain of 25 hp. That's when we added a set of 650-cfm Race Demon carbs that were set up specifically for a tunnel-ram application. Using out-of-the-box jetting, the 650s made a peak of 391 hp with a noticeable improvement in the throttle response on the dyno.
Since we didn't order the manifold with bungs installed, we went to Fast Eddie's in Orange, California, for the how-to on the welding 'cause he is a better welder than we are. This is worth the $150 he charges. Here, he marked the location of the holes about 1 inch from the base of the intake and centered them. | 
Drilling the holes can be done by hand, but Eddie prefers to mount the manifold on a press. The idea is to get them as close to 90 degrees from the runner as possible. |
Eddie dropped a dowel with the same ID as the bung into the hole to keep each one centered while he was welding. Neat trick, huh? | |
Out on the street, the 525s felt a little crisper at low rpm but would bog slightly when the throttle was whacked open. The 650s were the opposite, as they sagged a bit at traffic lights and absolutely obliterated the tires when we went WOT.
We asked Steve Brul for some dirt. He's seen vacuum-secondary carbs mounted side by side on tunnel-rams lean out one bank of cylinders at full throttle. His reasoning was that the secondaries were only partially open or not open at all, starving the right bank of cylinders (as seen from the front) and causing the im-balance. And that was the situation here. For an everyday street machine guy who just cruises and blips the throttle once in a while, this likely won't make a difference. On a drag car that sits at full throttle, a lean condition can hurt it. For our nitrous application, it was out of the question.