Holley 4180 Design & Modifications

I found the following article at a fellow Mustang enthusiasts site and thought it would be great addition to this site. Below I have written out the article exactly as released by Hot Rod. You may also few the original articles by downloading this "Holley4180.zip" file. The article is from an unknown issue of Hot Rod magazine.

Investigating the late Mustang and Capri Holley 4 Barrel Carburetor
by, Marlan Davis

The 1983 through 1985 Mustangs and Capri's came from the factory with Holley 4-barrel carbs, the first production cars so equipped in a decade. The carbs look like the good old Holley's of yesteryear. They run and feel like the Holley's of old. But how can any late model carburetor run so well and yet escape the wrath of the Federals? The answer is that Holley's engineers have retained the best performance characteristics of the old Holley's, while at the same time making the necessary design improvements needed to bring today's product into conformation with currently mandated emissions and mileage standards. Because these changes are extensive enough to limit complete component interchangeability with earlier model carbs, Holley coined a new generic model number. Known as the "Model 4180," today's Ford/Holley production carbs are used on 3/4 and 1 ton pickups and 460 vans, in addition to the 1983-'85 Mustangs and Capri's. Compared to their closest ancestor, the Model 4160, the 4180 features a completely new idle and main metering system designed to improve fuel economy while reducing exhaust emissions. Readily apparent changes include relocation of the fuel/air mixture screws from the metering block to a "tamper proof" location in the throttle body, and annular discharge booster venturis on the primary side. New air/fuel passages in the primary metering block promote more homogenous mixtures while eliminating cross-channel leakage between the metering block and main body.

Because of these changes, earlier Holley primary metering blocks and throttle bodies won't interchange. on the secondary side, minor mods are required to install most standard 4160 metering plates or to convert to a Model 4150 configuration with a metering block and replaceable main jets. Center pivot fuel bowls will bolt on both primary and secondary side; if retaining 4180/4160 metering plates, use nitrophyl float instead of brass float.

HOT ROD wondered how all these changes affect the carburetor's suitability for serious performance applications. Is the carb as tunable as the old Holley's? How does one disable the so called "tamper Proof" systems for off-highway application? What mods are needed (if any) are needed to install traditional Holley Hop-up parts? To get answers, we contacted Fuel Curve Engineering, one of the premier Holley race carburetor builders in the nation. Expert carb pathologists Bob Szabo and Murray Jensen performed an autopsy on a brand new 1984 Mustang Model 4180/List 50265, comparing it to the universal standby 600cfm Holley Model 4160/List 1850 carb. here is the coroner's report....

Naturally, whenever major engine mods are performed, the carburetor usually requires recalibration. We wondered how good the stock Mustang carburetor calibration was on a stock, unmodified vehicle. Would a production carb benefit from some Fuel Curve Engineering fine-tuning? To find out , we borrowed an 84 1/2  Mustang from Fred Colley, owner of Colley Ford, the largest Ford Motorsports parts dealer west of the Mississippi. The rare 5-speed 302 H.O. GT 350 convertible had only 2000 miles on the clock, having been in storage virtually from day one.

The car was driven to 2500 foot high Los Angelas County Raceway, and base lined in stock form with all emissions control equipment hooked up. A best (corrected to sea level) e.t. of 15.75 seconds @ 87.19 mph was recorded. Bob Szabo of Fuel Curve Engineering then installed his reworked duplicate of the stock carburetor, List 50265 (Ford B5ZE-9510-GA). various jet combinations, power valves, and secondary springs were tried, with optimum combination ( on our particular day) listed in the chart below, along with the factories baseline calibration. The relatively light Mustang proved insensitive to secondary spring changes, until the lightest (white) or Heaviest (black) springs were installed. The secondary diaphragm check ball was removed for all modified carb tests. Also keep in mind that at actual sea level drag strip , slightly richer jetting would be needed.

Well, how much did we gain? Slightly over two-tenths and 1 mph, actually: 15.515 e.t. @ 88.70 mph. The factory calibration is pretty good, but you can always gain more power from a little tuning. Still, Ford did their homework-with Holley, and without a computer.

Below are key points and suggestions from the article above.

The most visual external difference is the relocated idle mixture screws. Instead of the previous primary metering block location. (A, Arrow), they're now found in the main body, hidden behind "Tamper Proof" block-off plugs (B, Arrows). Corresponding internal fuel delivery passages redesign precludes use of easier-to-modify old-style metering blocks-- could be a hassle with rad cams in all-out race applications.

Hidden screws are accessed by sawing or chiseling away casting surrounding the plugs, then pulling plugs with pliers. Recessed screws than adjust conventionally (turn out to richen) with Allen wrench.

As delivered, 4180 metering block (left) comes with close-limit main jets (A). Compared to conventional jets (B), close-limit design has less percentage flow variation between jets of like number size, yet are completely interchangeable with standard design. Note how 4180's jets are installed at an angle-making some internal race passage mods harder. Block dowels are different sizes, too. Accelerator pump bleed hole (C) reduce hot-soak and peculation problems. If running big cam and/or 50cc accelerator pump, this hole should be plugged to avoid lean condition-but hot start characteristics will suffer. The 4180 requires unique float bowl-to-block gasket, part No. 108-56 (Lower Left).

Factory-pre-set choke also has anti-tamper provisions, with choke cover retained by breakaway-head screws. To remove the special screws, carefully file heads to permit gripping them with conventional pliers: replace with standard screws.

Secondaries were converted to Model 4150-style metering block with replaceable main jets. Side pivot bowl was retained to clear factory heat isolator plate and EGR valve. Center hung bowls aid cornering, and will fit if EGR is deleted.

The 4180 comes with a two-stage power valve (Left). High-capacity "window-style" single-stage valves are recommended for performance use. Be sure to use plain round power valve gasket 8r-1579 with window valves having internal shoulder.

Main body side of primary metering block graphically illustrates internal design changes. Model 4180 (left) uses separate accelerator pump transfer tube with O-ring (A). Most non-emissions Holley's don't have a separate tube, which was designed to correct previous internal leaking problems. Non-transfer-tube metering blocks tended to heat-warp under long-term usage, causing loss of gasket tension and consequent leakage. Use gasket 108-55 (lower). Fix cures the problem with no performance drawbacks.

Reason for different metering plate design: 4180 (left) transfers fuel directly through the main body, compared to the 4160 (right), which has transfer passage as part of the metering plate. The 4180 design prevents internal leaks.

Like earlier Model 4160's, 4180 carbs use metering plate in the secondary side in lieu of main jets. To change secondary calibration requires either different metering plates or conversion to Model 4150 secondary metering block with replaceable jets. Model 41809 secondary plate (left) doesn't interchange directly with most 4160 plates (right)- the only exceptions are metering plates used on Holley Model 2300 2-barrel Chrysler 3x2 Six-Pak end carbs, and certain Model 4160 Chrysler OEM 4-barrel installations. All other 4160 plates can be used on the 4180 only if their fuel delivery passages are blocked off as shown by arrows. Metering plate gasket must match metering plate design-use 108-13 for 4180 plates:108-27 for 4160 plates.

Metering plate hassles can be avoided by changing secondary metering to 4150 configuration. Kit 34-6 includes metering block, gaskets, screws, and all other necessary parts. Main jets must be purchased separately. On Mustang, use No. 70 secondary jets as sea-level baseline.

Because the 4180 main body meters secondary fuel internally when converting rear to 4150 style, use the two lead balls contained in kit 34-6 to block off new metering blocks fuel passages, as shown above.

Like the latest Holley race carbs, Model 4180 features annular discharge booster venturis (B compared to conventional design, A). Annular discharge boosters improve overall fuel metering efficiency and broaden low-end response because of a stronger signal felt at the booster throughout the operating range, as well as improve fuel atomization.

The 4180 carbs use vacuum secondaries, whose rate of opening is varied by changing to different tension springs, as contained in Kit 20-13. The lighter the spring tension, the faster the secondaries will open. Use the lightest spring that does not cause the carb to bog. removing check ball also quickens opening rate. On light cars like the Mustang, Fuel Curve Engineering recommends first removing this ball before changing springs.

Replace restrictive, clog prone bronze in-bowl fuel filter (arrow) with inline fuel filters for high-performance work.