Quote:
Originally posted by Big-Foot
StanJ - While I disagree about the primary and secondary throttles needing to be actuated in a linear fashion..
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Hmmm... I’m not exactly sure what my previous post might have implied regarding secondary throttle bore actuation other than the fact that street carbs with “1-to-1” ratio of primary/secondary opening tend to be difficult to manage in traffic, and likewise problematic on long-cammed, low/no flywheel inertia racing vehicles when negotiating the infield and pit lanes. The question does serve well as a springboard for the subject though… and actually, a 1-to-1 primary/secondary opening ratio does have a couple of potential advantages; it just requires a bit of engineering to utilize those advantages without liability.
Looking at the fuel metering circuitry of practically all Holley 4150 series (double-pumper 4brl) and 2300 series (2brl) carburetors, you will see two separate fuel delivery circuits for each venturi: the “main” circuit which discharges fuel from the venturi discharge nozzle (or “booster”), and the “idle” circuit, which discharges fuel alternately through the curb-idle orifice below the throttle plates (the rate of discharge here is controlled by the position of the mixture screws on the sides of the metering blocks) or through the transfer slot depending on throttle opening. Proper function and optimization of the idle circuits is a book unto itself and in the interest of verbiage probably a subject better saved for another post, but for now the point is that while fuel entering the air stream by way of the main circuit has the advantage of having been pre-atomized to an extent, fuel entering the air stream through the idle circuits does not. Certainly, both circuits utilize air bleeds…but - given its design and routing – the idle circuit’s air bleed does little more than act as a signal corrector with regard to circuit timing. Where allowed by class rules, we have some throttle bore modifications that help this situation, but you can’t completely cure it.
From one of my posts on another board:
The real issue is the actual Air/vaporized Fuel ratio present in the cylinders at the time of ignition. For a moment, let’s assume that a theoretically “perfect” stoichiometric air/fuel ratio of 14.7 to 1 (on gasoline) is optimum for an engine under all conditions (this isn’t the case under all conditions, but bear with me here). Fuel that arrives in the cylinder as a liquid (either film or droplet) cannot participate in the combustion process until it is vaporized…usually by the heat of compression (which is too late to have any bearing on detonation), or even later by the heat released by combustion itself (which it too late to do much of anything other than raise exhaust temperatures). It’s a matter of hydrocarbon molecules having physical access to oxygen molecules; and hydrocarbon molecules beneath the surface of a droplet or film have no such access. So, to achieve an effective 14.7 to 1 A/F ratio in the vaporized charge, we’ve got to start out with something a bit richer than that at the carburetor in order to allow for the fuel that puddles on the manifold floor, gets centrifuged out of the air stream at all of the bends in the intake runner, falls out of suspension due to local pressure increases in the intake runner, etc. Just how much richer depends on how well -- or not -- we can keep each of the previously mentioned things from happening. Therefore, it stands to reason that a carburetor which atomizes a given amount of fuel into the greatest number of droplets of the smallest size – thereby minimizing the centrifuge effect while simultaneously providing a greater amount of droplet surface area from which those droplets can evaporate on their way to the cylinder – is the most desirable. The main circuit discharge droplet size of an “out of the box” Holley 750HP 4br. averages about 60 angstroms in diameter at 1.5” Hg, while our fully prepped Trans-Am pieces produce droplet sizes that average around 20 angstroms and can safely run a little over 1 full point leaner in A/F due to a greater percentage of it’s total fuel discharge arriving in the cylinder in a combustible state. This works out to 15-18 lbs of fuel per hour at 750 horsepower, or about 1.5 mpg advantage at Elkhart Lake.
Now to tie this all together, let’s consider tooling down the highway at part throttle (assuming that any of us here actually
do that). With a conventional “progressive” secondary linkage on our carburetor, the primary throttles are considerable farther open than the secondary throttles under these conditions, with considerably more air passing through them. The main circuits of those venturi are online, and the airspeed down the primary throttle bore walls is sufficient to begin “curtaining off” the idle circuit discharge points. But over on the secondary side, the throttle blades are still almost closed; likely open just enough to expose the curb-idle discharge and transfer slots to full manifold vacuum. This results in a
lot of wasted fuel as well as well as a host of other potential problems (reduced spark plug life, cylinder wall washing, high header temps, etc.) toward the rear of the engine; single plane intake manifold-equipped Fords being particularly susceptible due to their runner departure angles.
How do we best address the problem? By going back to the 1-to-1 linkage that I damned in the beginning…but with a twist: progression built into the throttle actuation itself. This is fairly easy to do with “cable” systems, easier still with “throttle-by-wire” servo actuation, and one of our customers (Lozano Bros. Racing Engines of Cibolo, TX) builds a very nice “adjustable” mechanical set up for Trans-Am that could be adapted to road cars. 1-to-1 linkage does sometimes require some circuit timing modifications to the carburetor in order to optimize things, but it’s all “win/win” stuff…no real downside performance-wise.
A lot of our customers don’t want to go to the effort and expense of re-engineering their throttle systems to take advantage of it, but yes…1-to-1 primary/secondary throttle actuation at the carburetor can be made to work well.