Technical Progressive Multi-Carb Question

This is a hypothetical question. I don't have a manifold with several carbs on it on the table in front of me, nor am I even particularly thinking of a specific application. Most importantly, I'm not interested in what to do instead of a progressive multi-carb setup. It's something I've been reading up on and I can't find a specific answer to my question anywhere.

My question is: every guide to setting up a conventional progressive multi-carb setup emphasizes that all the carbs should reach WOT at the same time. Is this purely a function of the type of linkage commonly used – which I can completely see – or is there an engine-dynamics reason for this? In particular, would there be any advantage (or disadvantage) in the primary carb reaching WOT significantly earlier than the secondary carbs, assuming that a linkage could be devised to do this? (perhaps some kind of cam arrangement?) i.e. the secondary carbs begin to open when the primary is nearly or actually at WOT?

(It is after all not impossible for the primaries on a vacuum-secondary progressive multi-barrel carb to be wide open while the secondaries are completely shut.)

 

I don't have an answer for you, but I'll bump this back up to the top for you. I think that if you could design the linkage so that the secondary carb(s) could continue to open after the primary is already fully open there might actually be an advantage to this, allowing for fine tuning of the rate of opening of the secondary carb(s). I can't see a reason why all carbs have to reach full open position at the same time, other than design limitations of typical linkage, but I'm not an expert on this so be interested too in hearing from someone who is.

 

I always thought that would be a good thing. And have figured out how to do it. Even though I haven’t put it in action yet. You would need a linkage with slides and springs. The spring would open the center carb and the linkage would slide on the out side carbs. At the same time the center carb is at wot the outside carbs start opening, compressing the spring on the center carb.
I was going to do this on my 462 cubic inch FE, with a 500 cfm carb in the center. But I found all three original carbs and linkage..... so this idea went to the back burner!








Bones

 

It is only a matter of geometry on the on the linkage.

 

I have a tripower setup that uses the center carb till half throttle and then the linkage engages the other two. They all reach wot at the same time.

 

I would think it would make a difference as to the number of carbs and the manifold layout. The only places I see progressive linkage working really well would be an inline 3 or 4 carb setup where you have 1 or 2 'center' carbs and tip in the outboard carbs progressively. And I don't see the need to have all of them reach WOT at once either. Controlling the tip-in rate would be a very useful way to prevent any flat spots from appearing. IIRC the Mopar 'six pack' used vacuum outboard carbs, no mechanical connection, as well as some 2-4V Holley set ups where the primaries are ganged but the secondarys come in on vacuum.

Non-inline intakes, synchronized WOT would be critical though to avoid mixture distribution issues.

Just my .02....

 

I wrote a long answer, then decided maybe I should just sit back and listen.

 

An interesting question.

Guessing that if one polled H.A.M.B. members with multiple carburetion WHY they have multiple carburetion, one would get:

75 percent maximum performance
20 percent eye candy
5 percent other

I fit the other category, as mine was done for advertising; although both power and fuel economy increased (slightly) over a single 4-barrel.

Of the 75 percent that voted maximum performance, probably less than 20~25 percent are correct, based on what they are running.

So to the OP's question:

For those doing eye candy, the question is moot.

For those that are truly into multiples for performance; the theory behind multiple carburetors on a plenum style manifold (progressive linkage is not an option on any IR manifold of which I am aware), is to improve the average fill density of all cylinders by having a more equal runner length to each cylinder.

Looking at factory designed multiple carb set-ups, virtually all would have all carbs have all throttle plates at W.O.T. simultaneously, with the exception of the ones using Holley vacuum cannisters to open the end carbs or Rochesters using the single vacuum cannister for the same purpose. These could have the center of a tripower wide open, and the ends closed until the vacuum told the ends to go wide open. Pontiac prevented W.O.T. on the end carbs on many of their vacuum set-ups.

Ford did have a sequencing on their tripower for the early 60's where one end began to open 5 degrees prior to the other end, but still all went W.O.T. simultaneously.

Guessing here that linkage to maintain the highest average cylinder fill density where less than all plates going W.O.T. together would be difficult at best, and probably impossible.

Jon.

 

I’ve run manual progressive linkage for 5 years. I see no reason not to complicate a system that’s worked for years as factory whether sliders or vacuum. I like sliders and they are dependable and easily adjustable when you need it..

 

Ok, so think of why OEM firms in the USA originally used multiple carburetion. I can only assume it was because in the 1950s they couldn’t get the CFMs they needed with a single carburetor. If so, then the multiple carb options, say 3x2s, should operate with one carb as a ‘primary’ and then others as ‘secondaries’ akin to a 4 bbl. It then makes sense to open the ‘secondary’ carbs together.

I’m sticking my neck out a bit here, because that’s simply the logical answer to me. If true, I also assume by the mid-60s the OEM folks were using 3x2s more for sex appeal than for strictly engineering purposes. Who didn’t want a 440 six pack or a 427 tri power in those times!

Perhaps more info on this from others. Carbking?

 

Almost - the comment about insufficient CFM from a single carb is a popular myth. Both Carter and Holley had 600 CFM (or more) carbs by 1957. Carter had a 950 CFM carburetor in 1963. If you get into military, Stromberg had a two-barrel rated 600 CFM on the 4 barrel scale (850 CFM on the 2-barrel scale) in 1939. Larger carbs were certainly available in the '50's and '60's. And while it was a low RPM engine, Zenith produced a single barrel carburetor that fed a 5,063 CID engine in 1930 (that is correct, over 5000 cubic inches).

Contrast these figures to a 1957 J-2 or 1957 Pontiac where the 3x2 barely made 500 CFM (if that).

Even in the 1930's and 1940's when grass roots hot rodding was beginning to become popular, there were single 2-barrel carbs that would easily outflow 3 Stromberg 97's. They just were not cheap. Lots of V-8 Fords in the salvage yards, thus lots of Stromberg 97's (correct name Stromberg EE-1), and they WERE CHEAP! While there were folks in the '30's that could afford to play with blown Duesenbergs, there were a lot more that could almost afford to play with flatheads.

And I answered your question (barely) but this is a tangent on the original thread. If someone wants to start a carburetor history thread, I will be happy to post in it.

Jon.

 

Most likely it's a function of the type of linkage commonly used, because we like to build the easiest linkage that will get the job done.

 

.....and I would sure as heck read it!

 

Interesting thread, keep it coming

 

I thought of that exact approach, too. The other way would be to operate the primary throttles via a snail cam, and the secondary throttles via a sliding linkage off the snail cam.

 

Thanks Jon. That does answer a lot.

Another thing I haven't been able to find online is just how airflow varies with barrel cross-sectional area. I suspect that it isn't a direct correlation? Increase in cross-sectional area is sharply progressive relative to throttle angle: 50% area is only achieved at around 60° throttle angle. But does this correspond to 50% of the flow at WOT?

How responsive were the vacuum-operated multi-carb setups? I have no direct experience of them.

 

If you design a spring loaded linkage system that will open the center carb to wot before the outside carbs start opening. Then open the outside carbs, it will/ can act just like the vacuum operated tri power set us, even better! .... if you have a smart foot!
On the vacuum operated systems you can open the center carb wot while the outside carbs are closed! Just my thoughts.








Bones

 

I used the setup from Hot Rod Carbs on the roadster and works great.