Don Terrill’s Speed-Talk

The stuff is corrosive.So how about this. Put a vacuum port on the dist./ dist cap and use metered engine vacuum to pull out Ozone? I'm going to do it. I my self have noticed corrosion in my dizzy.MSD......

Be careful about drawing in excess water vapor. One step forward, two steps back.

BigBlocksOnTop wrote:The stuff is corrosive.So how about this. Put a vacuum port on the dist./ dist cap and use metered engine vacuum to pull out Ozone? I'm going to do it. I my self have noticed corrosion in my dizzy.MSD......

Known and common problem with MSD distributors. Less a fault of the ozone production, more a fault of MSD manufacturing's failure to properly coat/protect the parts.

Other distributors don't rust like MSDs.

You put a vacuum port on the distributor, what do you do if the carb backfires? How about fuel vapor entering the cap when the engine is off? I can see blowing the cap right through the hood upon re-start.

MSD suggests venting the cap, but they don't say anything about adding a vacuum source.

i have seen some old chrysler marine inboard engines had a vacuum line from intake to distributor to remove moisture and humidity from cap .

danta wrote:i have seen some old chrysler marine inboard engines had a vacuum line from intake to distributor to remove moisture and humidity from cap .

Do you know if the caps had a different type of seal? I don't think the vacuum capsule advance/retard plate would seal very well either.

GM LT1 engines used a vacuum port to draw air from a fitting in the intake duct through the front-mounted distributor to vent the cap. A local police force converted some to propane. Bad move; a gas accumulation in the intake was drawn through the distributor in one vehicle, resulting in an explosion and the bomb squad being called in...

Some later engines had a fan on the distributor shaft to dissipate ozone.

MadBill wrote:GM LT1 engines used a vacuum port to draw air from a fitting in the intake duct through the front-mounted distributor to vent the cap. A local police force converted some to propane. Bad move; a gas accumulation in the intake was drawn through the distributor in one vehicle, resulting in an explosion and the bomb squad being called in...

Some later engines had a fan on the distributor shaft to dissipate ozone.

I like this story. A good one for an engineering prof to have at hand as well.

Robert Heinlein wrote:It is impossible to make anything foolproof, because fools are so ingenious.

I run a MSD in my dragboat (it sees limited lake duty due to high compression and expense of fuel) and have no problem at all with corrosion. Maybe I'm just lucky.

No need to overthink this; take a look at a big Ford Duraspark cap. Stock pushrod Ford distributor caps are all vented. Ozone is a volatile gas and the rotor moves quite a bit of air under the cap.

I have a sbc in a 62 lowrider that when I built it back in 94 would continualy corrode under the cap, Accel dual point, one set removed, triggering crane Hi 6 ignition, no matter how much inox or wd40 sprayed, after a few hundered klm's the points would wear and close up, and car would stop,after putting up with this on a round trip to the summernats, when I returned home I proceeded to drill several horitontal holes in the dissy cap between posts to vent things, Car still cruises to this day without ever looking under the cap since, and it does see a hose of water under the bonnet when washing the car, all good.
Cheers Steve

Kevin Johnson wrote:

danta wrote:i have seen some old chrysler marine inboard engines had a vacuum line from intake to distributor to remove moisture and humidity from cap .

Do you know if the caps had a different type of seal? I don't think the vacuum capsule advance/retard plate would seal very well either.

there was vacuum advance on marine ignition and i dont remember if the cap had a seal or a gasket .

the OEM marine engines i worked with had vented caps with fine screen

I'm pretty sure a lot of NHRA cars inject pressurized nitrogen into the distributor cap to prevent corrosion and cross fire.

I wouldn't connect engine vacuum to it. I have heard of people who did that, the fresh air line plugs, and then no spark because the cap was under enough of a vacuum.

Someone here recommended a nice 1/2" hole in the cap, lower than the terminals. I did that and my distributor cap and rotor life went up dramatically. Before that, I would have corrosion issues in as little as 5 minutes of high RPM driving. Have to pull off the road halfway home and scrape the junk off of the terminals. (Aluminum terminals, because nobody makes brass for my distributor anymore)

There is the same issue with the new marine electronic distributors. The ionized air affects how the build up on the terminals reduces spark to the plugs. They use a vacuum port on the distributor that goes to the flame arrester which only pulls enough vacuum to keep the ozone out of the cap but they also have a brass screen in the bottom of the distributor as well .

I have done this mod to several MSD set up's and it cures the issue of the build up on the inside of the cap terminals that creates misfires or rough running issues. It also has shown to dramatically extend the life of the dist. cap to more then a single season of boating.
The drop in distributors don't suffer the same as the systems using a ignition box and those that do show similar improvements in cap durability/longevity. That said in an over the road application there is no reason why it would not improve the situation just that with a MSD box in the system you will need to maintain the vacuum hose every once in a while and not consider it a permanent deal as it may clog after a while from drawing the crap from the distributor.