What is a 4100 (1.12) Spreadbore?

For the last several years you have been reading about how wonderful the Autolite 2100 and 4100 carburetors are. The reasons that these carburetors perform the best on the planet are:
1. Annular fuel discharge - which atomizes fuel much better than any other metering system ever engineered in any carburetor.
2. A one piece float bowl and main body casting that eliminates leaks and maintenance.
3. A simple appearing design that does not require constant adjustment and attention while providing superior fuel economy and straight line performance!

Focusing in on the 4100, there were 2 size manufactured (really 3, but you will never see a 1958 only 1.19). The most common 1.12" venturi 600 CFM, and the much rarer 1.08" 480 CFM, are the ones you will come across. The large 600 CFM (1.12) was factory installed on Galaxies and T-Birds with 352, 390 and 428 engines. The small 1.08 was used on "A" and "D" code 289's and 1966 only Galaxie and T-Bird's with 390 and 428 applications. (The big blocks used the small carburetor for emissions reasons.) The "K" code 289 HiPo was fed by unique 1.12 600 CFM manual choke performance calibrated 4100. For a normal to even high performance 289 to function the best it must have a carburetor with around 480 CFM, and until now that meant the 1.08 venturi 4100. With the smaller size the driveability is flawless, i.e. no hesitation or flat spots and the fuel economy (with a 3.00 to 1 rear end) can exceed 20 miles per gallon. Add to that, the 1/4 mile performance is actually faster than the 600 CFM version.

There is a big problem with all this. A lot of enthusiasts have "listened" to the rave reviews of this engineering masterpiece and the supply of 1.08 4100's has been gobbled up all over the country. Just try and find one at a swap meet! Until now, the 1.08 was the only correct performance choice for the vast majority of 289/302 owners, and they either paid a hefty core charge ($150-250) or patiently scoured swap meets and salvage yards for a restorable core.

Pony Carburetors has made the "performance" search much easier. They have recently introduced a 1.12 venturi 4100 that works even better than the 1.08 on small blocks. By narrowing the primary venturi diameter to 1.00", it is now possible to get even better fuel economy, sizzling performance and driveability that begins to approach fuel injection. With the small primary (1.00") and the large secondary (1.1.8") the Autolite now has the spread bore characteristics that were so desirable and utilized by several carburetor manufacturers in the late 60's and throughout the 70's. The end result of a spread bore design maximizes fuel efficiency while maintaining neck jarring acceleration when the situation arises. Another wonderful thing out this spreadbore is that it can be used on the regular square bore intakes as all four holes are the same size. It is a spreadbore, but INTERNALLY. It is perfect for use with the original cast iron 4 barrel intakes or with the aftermarket intakes made by Edelbrock and Weiand. If the aftermarket intake is used, a spacer plate also must be used to prevent vacuum leaks due to the narrow mounting flange on these aftermarket items. We sell an ideal one that is 1/8" thick for $21.00

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