capaci said:
Thanks for the post Ron
Right on the mixture.. well, please do get a 02 sensor or EGT exhaust gas temp sensor and adjust for best mixture maybe a bit rich on your engine.
This is the drag about carbs and tuning for races and best performance there is no manual mixture adjustment it is fixed for the automotive stuff.
heck get Stan to place a main mixture control on the carb that can be adjusted from the drivers seat, you sell a few of these without doubt.
The chemically optimal point at which this happens is the stoichiometric ratio (sometimes referred to as stoich), where all the fuel and all the oxygen content in the air of the combustion chamber will perfectly balance each other out during combustion.
This is what is good about EFI and digital controls you can tweak the mixture if you have the controls
On my aircraft I adjust lean to get max exhaust temp then go a bit rich on the stoich side. Best power and runs cooler then lean. This is at cruise.
best
I developed the throttle bore modifications that you see in the photos about 7 years ago during the heyday of SCCA Trans-Am road racing after Mike Lozano of Lozano Brothers Racing Engines contacted me about some part-throttle fuel distribution/drivability problems he was seeing in his customer’s engines. The “intermediate circuit” 830cfm carbs that some other racing carburetor suppliers were building in response to this issue succeeded in masking (drowning?) that original problem, but were creating others. After a bit of dyno testing it became apparent that this problem was being caused by poor-to-practically-nonexistent atomization of fuel being discharged into the air stream via the idle circuit. The radial grooves cured the fuel distribution problem practically straight away, and also allowed us to significantly reduce the total volume of fuel discharged through the curb idle and transfer-slots (and subsequently, the accelerator pump circuits as well), resulting in better mileage while maintaining the functional in-cylinder A/F ratio that Mike wanted to see. Later refinements to the concept (directed, multi-point discharge from these circuits and modifications in the metering blocks themselves to allow the idle circuits to respond more quickly to changes in the engine’s fuel demand) actually provided a nice gain in WOT power, which is hard for many people to grasp until they consider what is actually happening in the combustion chamber and why an engine needs an absolute A/F ratio
slightly richer than stoichiometric in order to produce maximum power. It’s important to note here that for the most part, fuel arriving in-cylinder as a liquid (either in the form of droplets or wet film) might as well not have even gotten there at all…
in fact, for several reasons the overall situation would be better if they hadn’t. Hydrocarbon molecules beneath a droplet’s surface have no access to oxygen molecules until something vaporizes the droplet…and if that “something” is compression heat -- or worse yet --
combustion heat, then the game is essentially over before those hydrocarbon molecules have a chance to participate in any beneficial way. All they can really do at that point is react (burn) while on their way out of the cylinder during the exhaust cycle…very much to the detriment of exhaust valve and header life.
All sorts of things can and do cause vaporized fuel to condense back from vapor to liquid while on its way to the cylinder. Localized pressure increases – usually due to turbulence in the intake tract – are the chief troublemakers here, but some of this is unavoidable; simply opening the throttle (and who among us is willing to give
that up?) raises pressure in the intake manifold plenum sufficiently to cause a significant portion of the fuel suspended at that moment to coalesce back into liquid form…which is one of the main reasons that carburetors need accelerator pumps. All of these potential problems are compounded when any part of the carburetor fails to introduce fuel into the air stream in a way that readily allows it to be vaporized. Small numbers of large droplets not only evaporate more slowly than do large numbers of small ones, but since a droplet weighs much more than the air it is displacing in the port, it has a tendency to centrifuge each and every time it has to change direction. Obviously then, the less efficiently the carburetor atomizes the fuel it discharges into the incoming air stream, the higher the percentage of that fuel arriving at the cylinder in a liquid state…and the richer (in absolute terms) we have to start out in order to end up with a mixture that
functionally approaches stoich in the combustion chamber.
(As an aside, there is a "peak to the vaporization mountain”...and it is possible to go past it and start hurting yourself power-wise. Explaining how this can happen requires a bit of longhair physics that I won't bore everyone with here; although the phrase
latent heat of vaporization should be enough to get the other physicists and engineering types headed down the correct path. Anyone else interested can contact me via PM and I’ll type something up)
Regarding in-car A/F mixture adjustment capability, Holley actually offered a retrofit kit to do this back in the middle 80’s. Unfortunately, it wasn’t very well engineered and reliability issues kept it from ever becoming very popular with racers and was discontinued after a couple of years. Come to think of it I haven't seen one in decades; even on ebay. Even though the reliability problems would be easily solved today, I’m not sure just how much of a market there would be for such a system (the street muscle car guys not withstanding). Given the current movement afoot in most motorsports sanctioning bodies toward reduce the cost and complexity of the cars themselves, I don’t see any of those sanctions which currently require the use of carburetors allowing in-car mixture control these days.
capaci said:
Flow vs Delta Pressure across the throttle plate at % throttle open, say 10, 25, 50, 75, 100 % would be good for a chart. If you could get 02 or exhaust temp this would tell us how linear the mixture is or what the curve is on a dyno so we could add engine load.
Total venturi air flow and fluid flow through the various fuel discharge circuits are both measured at 8.2 degree intervals of throttle shaft rotation as part of the final calibration process for each carburetor we build. It’s the only way I’m aware of to assure proper transition between the circuits as the engine’s demand for fuel changes. In the event that our baseline calculations are off however, the range of fuel curve adjustment on our pro-level carburetors is extremely wide and I’m happy to work with the customer to get every aspect of the carburetor’s performance as close to perfect as it can be for his application.
