British Vacuum Unit Special Supercharger Distributor

Rob Medynski of British Vacuum Unit manufactures original, no longer supplied, Lucas distributor vacuum units, other ignition parts and is also a rebuilder of Lucas distributors. One of his latest projects has been to develop a distributor to give efficient ignition timing on British cars using the supercharger.

In the past, supercharged cars such as the MGB required limited total timing usually no more than 28 degrees under full boost load to prevent pre-ignition. Setting a distributor up this way you lost efficiency the other 99% of the driving time when you’re not under boost load.

This has been a age old problem when installing a supercharger on our classic British cars. I’ve also discovered this has been a unsolvable problem with most all make and model cars. Timing has always been a problem because of the wide range of engine performance conditions.

First and most important is maximum timing under full boost. As boost pressure increases so does flame speed. The amount of timing needed for the most efficient performance under any and all conditions is calculated by the fuel mixture flame speed. For maximum power and performance, the fuel mixture needs to start to expand and build pressure when the piston is at its most power potential of 20 degrees ATDC.

The fuel mixture has a delay from spark to expansion. The 14-to-1 air fuel mixture usually takes around 14.5 ms from spark to full pressure with a normally aspirated engine. This is why spark is BTDC so the air/fuel expansion starts at the 20 degrees piston power potential, not before or after.

This is the tricky part. A normally aspirated engine relies on load and RPMs to control timing. With a supercharger we now have enhanced load conditions.

With a naturally aspirated engine “use of a carburetor”, the air intake is limited by atmospheric pressure—it can only draw a limited volume of air/fuel mixture. The air /fuel mixture has a flame speed from when spark occurs to when the mixture starts to expand and apply pressure on the piston.

Most supercharged engines are low compression. Not under boost can tolerate up to 36 degrees total timing. The supercharger under boost now supply’s air/fuel by lbs of pressure psi greatly increasing the flame speed requiring a reduction in timing.

Carbureted or naturally aspired engines relied on two conditions. Mechanical advance that controls timing by RPMs and vacuum advance that controls timing by load and RPMs. Now with a supercharger, we need a third condition, boost retard, to handle and reduce the over all timing when boost pressure and faster flame speed is present.

Ideally, when there is no boost, we can run with 36 degrees mechanical advance with another 12-to-14 degrees vacuum advance under light load “cruising speeds” for a total of 48-to-50 degrees. With slight load, the timing falls back to the mechanical advance 36 degrees.

Once the supercharger starts to build pressure, the timing needs to retard with the increase in flame speed regulated by boost pressure from 36 degrees to around 28 degrees.

Our simple solution was to design a distributor curve and vacuum unit to give the desired timing under all conditions.

1- We set our distributor for a low compression 36 degrees total at 4000 RPMs.

2- Vacuum advance is regulated by load and RPMs, mainly at cruising speeds when the fuel mixture becomes leaner and slower, to burn to maintain pressure at the maximum piston power potential. A slower flame speed requires a increase in timing, usually 48-to-50 degrees.

3- Boost retard regulated by lbs of boost pressure with a retard range of 1-to-5 lbs boost for 1-to-8 degrees retard.

I also designed the vacuum advance/boost unit to handle the extreme vacuum and boost pressures and incorporated a adjustable advance limiter.

Our dual unit design parks the point plate at the same location when there is no vacuum or boost. It is not a floating unit. We usually supply our distributors with points and condenser. Electronic ignition
can easily be installed.

With the British Vacuum Unit complete supercharger distributor, special advance, boost retard unit along with our unique distributor advance curve, allows full efficient ignition timing when not under load, vacuum advance when under light load, and boost retard by lbs of boost pressure. This allows over all efficiency, the “best of all worlds” throughout all RPMs, loads and driving conditions, and greatly enhances the performance and life of the engine. We also designed our distributor to be a drop in, connect one vacuum line, set 10 degrees static BTDC, and you’re done!

Some of the disadvantages with other supercharger systems are low compression engines can safely run 36 degrees total mechanical advance when not under load. Most other systems have limited total timing. Most others do not have the proper vacuum advance rate and do not have a retarded timing option under boost.

Some come close, but often at a very very high cost, need a separate boost retard, and you often need to know how to program the package.

If you need any additional information contact British Vacuum Unit at 603-731-1788 or email us at bvu@britishvacuumunit.com.

Rob Medynski
British Vacuum Unit
112 Briar Bush Road
Canterbury, NH 03224