British Vacuum Unit
Briar Bush Rd.
Canterbury N.H. 03224
We manufacture new, rebuild and restore Lucas vacuum units to original with
the highest quality parts and set to factory operating advance specs.
We don't supply a one unit fits all!
Starting in the early 1970s many British cars came with vacuum retard or limited vacuum advance.
We manufacture vacuum advance units to enhance maximum performance.
Zenith carburetors without ported vacuum, we machine the correct ported vacuum on Zenith carburetors
that had never been completed.
E mail for details.
We also rebuild and restore distributors, adjusting mechanical advance through out the RPMs for efficient full burn of the fuel enhancing power, mileage and cooling. Install a new vacuum advance for maximum performance when not under load. Many British cars from 1969 on, had vacuum retard and mechanical advance was increased from 9 to 13 degrees to as much as 18 to 22 degrees. Many distributor over advance at low RPMs because of weak springs and over advance at high RPMs because of high mechanical advance limits.
See our distributor re-build page.
We can also help you set up ported vacuum and vacuum advance if your car is 1969 or later.
Email for advice firstname.lastname@example.org
First understand how the vacuum operates and the importance.
Lucas Distributor Vacuum Advance Units
One unit fits all? Not likely!
There are hundreds of thousands of older antique and classic British vehicles under restoration, restored and/or still driven on the road to date. Over the years essential parts such as the distributor vacuum advance unit have dried up, becoming No Longer Supplied, NLS. This is the case with the Lucas distributor vacuum advance unit. Most vacuum advance units have been superseded to non-original and at best, close in operation to the original equipment. A needed piece added to the ignition system to enhance performance, mileage, and operation temperature. Most of the early so called hex units manufactured through the 1940s to the early 1960s have completely dried up or have been superseded to a limited number of at best “close match in operation”, hose connection, cup chamber vacuum advance units. Units produced from the early 60s through 1967 with the cup chamber, threaded vacuum hose connection have been superseded to units, again limited to close to their original operation spec’s supplied with only the hose type connection. Suppliers have tried to fill the needs of customers by superseding the limited number of later style units close in operational code to fill the demand. In many cases as the years have gone by, this range has widened. Vacuum unit part listings have been scrambled with superseded numbers to where the original units and codes are virtually lost.
Between the Jaguars, MG’s, Minis, Triumphs and Healey’s there were over 130 different units manufactured from 1950 to 1970 by Lucas. The three-digit code indicates starting advance, full advance and the total degrees of advance. Between the many different unit codes, units can have a starting range of mercury vacuum from 2 to 10, full advance mercury vacuum from 4 to 18 and total ignition advance as little as 3 to as much as 12 degrees. “At the crank", this is 6 to 24 degrees ignition advance. A unit with a code of 8-18-7 will start the advance at 8 mercury vacuum. Fully advance at 18 mercury vacuum and has a total ignition advance of 7 distributor degrees, that is 14 degrees at the crank. Installing a 3-13-12 would advance too soon and be 10 degrees over advanced if your vehicle called for an 8-18-7.
Most people are unaware that the total degrees of distributor advance, including the vacuum unit advance, is twice the ignition timing degrees at the crank. It has become a lost art to properly tune and check the distributor for proper operation. Many mechanics and “do it your selfer’s” never removed a distributor to service or check the mechanical advance springs or vacuum advance unit. The mechanical advance in many older distributors advance too quickly. Springs that control the rate of mechanical advance through the RPM range, fatigue and allow the mechanical weight to advance way to early causing pre-ignition. This leads to retarding the distributor for a quick fix and now reducing the needed advance at higher RPM. Too often the vacuum unit is changed or disconnected believing the vacuum unit is the over advancing problem, when the advance springs are in need of replacing. Keep in mind; the vacuum unit operates to enhance performance when not under load. Therefore pre-ignition under load is over all ignition settings or mechanical advance problems.
Ported vacuum is engineered at the throttle valve where vacuum varies under slight load, mainly at high RPM and cruising speeds. Units designed for manifold vacuum, have some degree of advance almost all the time unless manifold vacuum drops depending on RPM to throttle opening.
Over the years I have personally serviced thousands of English cars and rebuilt hundreds of performance British engines. Ignition timing is to optimize proper fuel burning pressures at a particular crankshaft speeds under a particular load and RPM condition. The more fuel, the faster the burn, the more pressure. The higher the RPM, the earlier the fuel needs to start burning to pressurize when the piston is on the down stroke. Cursing speeds ‘not under load” are usually leaner “slower” fuel mixtures at high RPM’s and need to fire earlier to completely pressurize the down stroke. This is where the vacuum advance unit operates. Not only did just about every vehicle manufacture had a unique unit to their make and model, most had several different coded units for the year, make, even model depending on the many different engine options. An example is the 1961 Jaguar XK150. Depending on the many engine options, the XK150 was available with a compression ratio of 7:1, 8:1, 9:1, 8:1 3.8 litre 9:1 3.8 litre, XK 150 S type, 8:1 3.4 litre, 9:1 3.4 litre and a 9:1 3.8 litre. In 1961 the many different options called for 6 different coded distributor vacuum units alone.
British Vacuum Unit manufactures all early original Lucas distributor vacuum advance units, detailed in appearance, operation, with the proper earlier heavier aluminum casting and larger print or later lighter style casting with smaller print, stamped part number and code. We not only produce units as they were originally supplied to the British Auto industry as vehicles ran off the assembly line. British Vacuum Unit manufactures units in the many different limited versions such as the first of a newly introduced early hex unit supplied by Lucas with brass vacuum cap, stamped code “or” part number only. These units were not available over the counter when an original replacement unit was needed back when the vehicle was still new. The majority of hex chamber units were manufactured with stamped part number “and” code on an aluminum vacuum cap.
British Vacuum Unit is a manufacturer of the most authentic vintage Lucas replacement vacuum unit available and the only manufacturer of early hex units in the world. It is an ongoing task re-searching the many different Lucas catalogs. Many cases the catalog printed the “year” of the vehicle must be referenced to accurately find the correct unit part number or the following year; the unit may have already been superseded. In some cases the original unit listing cannot be found! An example is the early Jaguar XKE 3.8 liter, Engine # R1001 to R9999 & RA1001 to RA1381 with distributor 40617. Early cars rolled out of the factory with a 54410415 vacuum unit. Unusual because this is an 8 digit later number used on an early style hex unit. The 1961 Jaguar Lucas parts catalog list unit number 54412348. This is a cup chamber unit with a threaded line fitting! Unless you reference the original Lucas books for the year of your vehicle, you most likely have the wrong information that has been superseded to a later unit close to what was originally called for. Installing the proper unit enhances cooler water temperature, over-all performance and mileage. A good tuning practice would be replacing a 30 year or older vacuum unit when you consider by now, most are dried out, frozen, leaky or have warn out moving parts. The original vacuum advance units manufactured specifically for the distributor number is a very important part of fine tuning, many times increasing over-all performance by 10-20%. British Vacuum Unit manufactures all the original units.
Our units are sold bagged without adjuster kit or in the vintage Lucas box with the full 4-piece micro-adjuster mounting kit. If we do not list a particular unit you need, send it in and we can reverse engineer the unit at no extra charge. All our units have a 2-year warranty. We have a 30-day no reason return policy if our products do not exceed you expectations. We stock most units listed on our web site and usually ship within 24 hours.