Well, not exactly, but what he says contradicts to some extent what I've said repeatedly here.

In the December 2006 Popular Hot Rodding issue, he had an article about intake manifolds. While a wealth of good info regarding why certain manifolds work better than others, especially in combination with other parts, he had a section on optimizing two-plane manifolds (we call them dual plane around here, typically; they are also referred to occasionally as 180 degree manifolds). He had conducted dyno tests with various carb sizes - 740, 920, and 1030 verified CFM carbs - on a healthy 383, and noted that the bigger carbs did produce slightly more peak HP & torque than the smaller, but only charted 3500 RPMs and up. He then said this: "Below 3,500, they did not produce as much as a stock 750. Having said that, there is a fix. Our test carbs had mechanical secondaries, so the tests were done wide-open on all barrels. Running on just the primaries under 3,500 produced far better results with the big carbs. The lesson here is that had we used vacuum secondaries, the poorer performance of the bigger carbs under the 3,500 mark would have been entirely cancelled out. So if you think that a vacuum secondary is only for street sissies, think again. The fact that you choose a vacuum-secondary carb means you now have the option to run with a carb having greater overall flow, as those secondaries will only come in when the engine needs them. In short, a high-tech two-plane intake with vacuum secondary carburetion can really deliver the goods on both ends of the rpm band."

Some things should be noted here: 1) He spent a week with each carb size on the dyno and flow bench optimizing each on that engine (used an RPM Air Gap for the dyno tests); 2) he didn't back up his statements with dyno pulls (but who am I to question him?); 3) the discussion was for two-plane intakes, not single plane.

In my own case, my VS to DP gains were observed with a relatively poorer two-plane, an Action +, and with inadequate stall for the cam being used. I haven't put the VS back on with the RPM Air Gap and higher stall converter, so I can't say the VS would not be better than the DP at this point. When I go WOT, I'm at 3500 instantly, so it might not make any difference, although a 950 CFM carb might be a different story.

So, I guess you could still say the VS carbs protect against over-carbing an engine, he just put a different light on it with regard to low-RPM power vs. peak.

 

Interesting.
Well, vacuum secondary carbs working to help prevent overcarbing is a pretty common thought right?

Didn't mention a not-so-loose convertor, ie, a standard trans, and the thought of throttle response. Throttle responsiveness doesn't show up on a dyno eh?
Are you talking about your VS->DP on your 396? I didn't think you had tried a DP on your camaro due to the computer controls and whatnot?

 

DP at the track run faster than a VS in every case i've ever seen. Even with tuning and jetting changes, etc etc. Every racer basically agrees with this for WOT conditions.

 

Yes, I was talking about my carb experiences on the 396.

Vizard is particularly well-known for practical vs. bench racing results, but this does smack a little of the latter. Of course, below 3500 RPMs is more street than racing territory. But, when you're talking big cams, you're probably talking over 3500 RPMs for the start of the "real" powerband, anyway.

I still don't like VS for manual applications, because the secondaries don't close immediately after letting off the throttle. Come to think of it, I don't like that aspect with an automatic, either.

After starting this thread, I read another carb article in Chevy High Performance November 2006 issue, and they were trying 750 to 1000 CFM carbs on a very stout 454, 460-510 RWHP range. The lower HP # was with the 750 carb, the highest with the larger. Another article put 830 to 950 CFM carbs on a 415 SBC in a '71 Nova, 4 tenths and 4 mph difference between those models. All DP's. But, both of those engines had single plane intakes.

Most of us don't have a week to spend on flow benches and dyno to tune a carb. So, perhaps it just comes down to what you can expect to work best out of the box with minimal tweaking.

 

Maybe in a situation where you don't have enough stall or too high a gear and have to lug the motor up from a low RPM the VS would work better. But I agree that a DP has an advantage in a drag race setup- you get it all instantly. If you've got a loose converter and proper gearing you're not going to spend any time in the lower RPMs anyway and all your time is going to be in the 3000+ RPM range right from the tree.

No matter how you slice it, it takes time for the secondary throttles to open up on a VS carb. And that applies even if you have a 4000 stall converter. The secondary throttles can only open so fast without causing a bog becuase they don't have their own accelerator pump. That's time that isn't wasted with a DP.

VS carbs are a lot more tolerant of low RPM lugging like often happens on the street when you're in too high a gear or whatever and give it a quick stab.

 

Again, this discussion was with regard to dual-plane intakes, which essentially only draw from one side of the carb. When you have a carb big enough to feed the engine all it wants at the peaks, it may be so big that at lower RPMs (perhaps even the magical 3500-4000, into the cam's powerband), the velocity might be so low that having the carb completely open won't pull through the boosters adequately.

Daily driving throttle response is a toss-up for the same sized VS or DP carb. You're only using the primaries in either case. Smaller carbs are going to win that battle - or spreadbores.

The other "issue" I have with vacuum secondaries is consistency - somehow they just don't seem to open exactly the same from run-to-run.

Bottom line - you don't race or drive on a dyno. . .