Monday, November 20, 2006

VW - CDI Modules

My articles about capacitive discharge ignition modules, archived by Mr. Richard Kurtz on his web site (the address of which I’ve lost again due to another system crash) generates a fairly constant stream of messages. The questions most commonly asked have to do with the following:

I do not work for Universal Corporation nor do I have any arrangement with them. I recommend their product because it has worked reliably for me in more than a hundred installations.

My recommendation is not based on any formal, quantified product-testing program. I’ve used a number of other electronic ignition systems but MSD is the only brand-name I can recall. The CDI module from Universal proved more reliable than the three MSD modules I tried. I have not used a Pertronics (sp?) system. (Revision Note: I has been about eight years since I wrote this article. Since then I’ve bought, tested and used the points-replacement modules offered by Pertronics and Compu-Fire. Both worked, and equally well.)

I do not know the absolute maximum rpm at which the Universal CDI module will operate with a 4-cylinder, 4-cycle engine but I’ve personally run them for relatively short periods of time, typically less than thirty minutes, above 7,000 rpm with no problem. (For sustained high-speed operation I think it would be wise to direct a blast of ram air at the CDI module’s cooling fins.)

I use commonly available silicon-insulated ignition harness (carbon-core) in conjunction with a stock (black, blue or what-have-you) Bosch ignition coil and Bosch platinum-electrode spark plugs. Using these wires, I’ve not seen any evidence of insulation failure on vehicles fitted with CDI modules. My baja-bug, in which the ignition harness is exposed to the weather, has used the same wires . . . El Cheepo stuff from J.C.Whitney . . . for the last ten years. Indeed, with silicon/graphite ignition harness the only cause for replacement has been failure of the air seals or distributor-tower boots.

The longest service I’ve gotten from a set of stock spark plugs was about 26,000 miles. I had a platinum plug fail after 56,000 miles and changed-out the whole set but I know of a V6 that has accumulated more than 80,000 on a set of platinum plugs and is still going strong.

I use commonly available ignition points, whatever is hanging on the rack at the local FLAPS. I observe the usual precautions when installing the points and lubricate the rubbing block with high-temperature silicon grease about twice a year. The rubbing block lasts . . . however long it lasts, usually about 25,000 miles but I’ve had some last more than 40,000. The points must be adjusted as the rubbing block wears, usually once a year, depending on the miles driven. Aside from some minor flattening, when triggering a CDI module the contact points show no signs of wear.

When it comes time to change the points, I just plug in the spare distributor, replacing the points at my convenience. I do not see the use of a points-triggered ignition module as a liability. The VW distributor normally requires service at about the same interval as the rubbing block wears. It is no problem to include point replacement with adjusting the distributor’s thrust shims. Indeed, the use of points-triggering provides an excellent back-up since it allows me to return to the standard Kettering ignition should the CDI unit fail.

I know of a case where a vehicle got a 25% boost in its mileage after a CDI module was installed... (20 mpg vs. 16) but I suspect there were other factors involved. Typically, I see a 5% to 10% improvement in highway mileage (i.e., constant high speed) after installation of a CDI module, based on a test-run of approximately 102 miles (that is, from my shop to Indian Truck Trail & return). These appear to be valid figures, borne out by longer runs on equally good roads but under less controlled conditions (San Diego to Kansas City & return). I was disappointed by my mileage on the Inuvik Run . . . about 22.5 mpg for about 8,500 miles . . . but the vehicle was carrying a fairly heavy load and the route included about 2,000 miles of bad roads.

CDI modules are a practical way to retrofit modern-day technology to a 1930's-era engine. The engine remains in tune much longer than those fitted with the stock Kettering-based ignition, which works best at low engine speeds. The spark voltage delivered by the CDI module is fairly constant across the operating range of the Volkswagen engine, providing a cleaner burn and better economy at higher rpms.

The strongest endorsement for this type of ignition modification comes not from me but from the automotive industry, which abandoned the Kettering ignition as soon as electronic systems of equal reliability became available.

Several messages asked how it was possible for a CDI module costing less than a hundred dollars to perform as well as a unit costing more than $600. In my opinion, the real question should be ‘How good a system do I need?’ Most of the messages which raise that question are comparing the Universal CDI module to units which claim they can power an eight-cylinder engine at twelve thousand rpm. In theory, when such a unit is used on a four-cylinder engine, it should provide a good spark all the way up to twenty-four thousand rpm. If you commonly run that fast with your Volkswagen, then by all means, buy something other than the Universal CDI module.

No bug or bus uses an engine turning 24,000 rpm. Nor 12,000, for that matter. For the owner of the typical air-cooled VW, the Universal CDI module is more than good enough.

-Bob Hoover

No comments: