Fuel Atomization by a Carb

[MadBill]

I've seen it written that it is possible for a carb to atomize fuel too well. I'm not so sure this is true. My experience with some billet body carbs with annular discharge venturis, leads me to believe that a carb that has better fuel atomization just needs closer control of the A/F ratios at the top end of the curve. I found that the engines that these carbs were run on, definitely did not tolerate a rich A/F ratio after the rpm's got a little past peak torque.

A carb that atomizes fuel well,is basically increasing the amount of burnable fuel that gets to cylinders, compared to a carb that has less atomization. The "delivery A/F ratio", calculated from air flow and fuel flow measurements, may be the same for 2 carbs. But with the carb that atomizes fuel more completely, the effective A/F ratio in the cylinders, which I call the "burning A/F ratio", will be richer because there is more fuel ready to burn.

I found that a richer top end mixture, with one of those annular venturi carbs, resulted in a rather dramatic loss of hp in the top area of the power curve. Leaning the top end A/F ratios brought back the power, and ended up making slightly more power on less fuel, compared to a similar size carb with more conventional venturi boosters. The more conventional carb was more forgiving about A/F ratio control, made a little less power compared to the annular venturi carb, and required more fuel to make its best power.

What has been your experience with this fuel atomization issue?

In one of his books, DV talks about a trick super-atomizing carb he built for a NASCAR team. In early season testing it showed significant power gains, but come Daytona and higher temperatures, it fully vaporized too much of the fuel, displacing more air and thus reducing the power. Also, he relates that the BMC Minis he raced in his early years made more power with fuel-blobbing Weber carbs than with the similar but finer-atomizing Dellortos.

[cjperformance]

They were sold in the early '70's, I'd used one on a dirt track car at the time
the rule stated any two barrel carburetor and that was the largest available
at the time, they must not of sold well or were trouble some cause mid to
late 70's you didn't see anymore and I don't believe Holley ever used that
venturi design again.

I had read that the 650 2bl was designed initially for some boat racing class where a 2bl was the rule? No idea wether thats actually the case though.
That tube on the carb pic above,, does that just have an open hole at the end or a series of holes along its underside like a T-Quad secondary tube? Also what is the numner on that carb?

[cjperformance]

Whats the part number on the metering block on that one.
I have one thats missing the metering block, for ages I've meant to try out an 850 or 1050 block on it.

Here's what mine has, when I pulled the block I was surprised it would
be metering block limited being it has the accel. transfer tube in it,
usually see that on the emission type carbs.

0-101.jpg0-102.jpg
[/quote]

Thankyou

[bigfoot584]

list is 6425, earlier post with the two metering block pics. has number of the block used.

[cjperformance]

The Quadrajet has no boosters in the secondaries, Yes? How does that affect atomization?

They may not have had a booster we know it, but it was a tube exiting to
the center of the venturi acting similarly to a booster, something similar to
below.

031.jpg

Part number of this carb i was meaning? Not the boosterless carb the one with extended main venturi and tubes protruding where the booster would usually be. Thanks

[bigfoot584]

The Quadrajet has no boosters in the secondaries, Yes? How does that affect atomization?

They may not have had a booster we know it, but it was a tube exiting to
the center of the venturi acting similarly to a booster, something similar to
below.

031.jpg

Part number of this carb i was meaning? Not the boosterless carb the one with extended main venturi and tubes protruding where the booster would usually be. Thanks

Sorry about that, it's off this site.

http://www.candsspecialties.com/store/h ... llets.html

[BLSTIC]

That kind of venturi is common in dirt track racing over here in Australia. The rulebook for one class says that carburettor equipped vehicles must run a 350 holley casting if they want to run methanol, so standard fare is to fit larger throttles and annular discharge venturis with no booster to get enough airflow for the roughly 300 wheel horsepower they can make.

Just a bit of background info, not answering the topic at all