flyrotary@lancaironline.net Mailing List Archive

In a message dated 11/22/2006 10:27:56 P.M. Eastern Standard Time, eanderson@carolina.rr.com writes:

Never measured the trailing because its hole into the chamber is
too small to worry about any part of the electrode poking into the chamber..

Ed

This has to do with automotive racing use of the rotary engine. Some information my not apply to aircraft use.

A question from the “No pistons list” RE: Nonfoulers and the trailing plug holes. The recess looks and acts like the ages old cure for fouled plugs in oil using motors.

I had a complete set of those nonfoulers on my old Chevy van. They worked so well that it would loose some porcelain from a tip now and then. (very high nose temps) But they were working on a slightly different mechanism than the same look in the rotary. The plug temps are somewhat controlled by the degree of exposure to the flame of the combustion chamber, and somewhat by the distance the heat must flow from the plug shell to get to the cooling jacket and be carried away.

The longer the path the hotter the plug runs. The nonfoulers had a broken path at the plug threads into the nonfouler, and another at the plug threads in the head, that was twice the thermal breaks of a stock plug. Nothing to do with the heat range of the plug. Most of the path is designed into the head or rotor housing. How thick is the aluminum where the threads are cut through the housing. How much coolant flows past this location and so on. The rest of the path is built into the plug, and that path length determines the plug's heat range.

This is very difficult to see on those plugs with the Mazda looking 360 degree ground electrode. Note here that Mazda plugs look just like aircraft plugs, and for the same reason. But on a more conventional plug you can see how long the center electrode insulator is before it joins the shell. In low performance equipment the path is long so as to maintain high enough nose temps to keep the plug clean and not foul up with carbon. Carbon is a conductor. It is what is used inside of resistors in electronic equipment. So a long heat path means a hotter tip. A short path, as in the porcelain joins the shell right near the tip, means a colder plug and low nose temps.

Using too cold a plug (low nose temps) for the performance situation, just means too low a nose temp and this may lead to a carbon fouled plug. This is a pain in the tailfeathers, but is cheap to cure. Using too hot a plug range means high nose temps, and when the engine is used hard, may lead to preignition from the white hot center electrode igniting the mixture before the plug fires (preignition). This can rapidly lead to detonation, and loss of the apex seals. So, if you plan on missing the heat range, miss it to the low side. Put in a colder plug than you think you need for the performance you plan to us.

In the olden times with the Kettering system, (where I came from), we used to warm up the engine with real high heat range street plugs. Once the coolant was up to temperature, we would shut down and switch to the ice cold racing plugs for the race. Everybody did this and it worked just fine. If there was a problem on the false grid that required a long shut down, there would be a good chance that one or two cars would foul plugs from having cooled off to far and not be able to restart. If one of the street plugs was not changed for the race, the engine would be destroyed. Both, very unhappy situations.

We need not do that anymore because we have the Mallory Hifire or MSD ignition systems where we can insert over one amp of electrical energy into the plugs per thousand RPM. The rise times are so short and voltages are so high, that even a carbon fouled plug fires before the lower voltage of the old Kettering system, (points or a points eliminator switch) can leak down across the carbon path and misfire. Note also that these systems provide enough energy per strike to kill you. The MSD also fires once when you turn it on and once when you turn it off. If you don't want a Spiedel (watch band) shaped scar on your wrist, and a really bad memory, do not manipulate the plug wires with the battery connected.

We use the NGK R6725-115 plugs, gapped at .010". They have a retracted gap and fine wire electrodes. The heat range is ice cold. I have never fouled one. We use a MSD for leading and trailing. Even driving slowly to the false grid on a cold morning, no problems.

Autolite and Champion both make plugs with a cold enough heat range to race in a NA rotary.
We used Champion N57Gs for years. That is 3/4" reach medium tip gold palladium center electrode. Whatever they say is the replacement for that plug should work fine. A very cold plug.

I forgot the Autolite number but I saw a box while rooting in the trailer the other day. I will try to find them and publish that number. Auto Zone will order them under $7.00 a box of 4.

Always screw a new brand plug into a spare housing in both holes to be sure that there is no contact of the tip or threads running out in the hole. Usually it is the extended tips that cause the problems. Also there is a slim chance that the tip can extend into the chamber and destroy the apex seals. You wouldn't be the first to do it either. Look at everything.

OK I couldn't wait and went out to look. Frost on everything. the Autolite number is AR2592. A little off ice cold. I rebuilt a rotary race engine that had 4 of these in it. No problems. And the price is right. I also noticed several boxes of Champion N86. So they may have replaced the N57Gs from years ago. Check with Champion on that.

Somebody is always asking for plugs at the track, so I gave away the Champions (we got for free) or the Autolites we got for $7.00 and I kept the NGKs (we got for free for earning another invitation to the national championships).

Rounded electrodes and missing porcelain?

Yes, obviously too high a heat range, and, or, too lean a mixture or advance for the RPM used. The trailing running a split timing gets more mixture in its little chamber before it fires. The bigger the split, the more it gets. So it is exposed directly to the combustion process. The mixture when lit then reenters the chamber as a jet of fire, instead of just a spark, and that works wonders for lighting the bigger charge in the chamber.

This trickery is used to help reduce the unburned hydrocarbons pollution problem the rotary is famous for. It does not help power much, and it doesn't hurt it much. Some builders drill out the little hole to about the same size as the leading hole. I never did it and I never saw one with that mod on the dyno, so I cannot comment on the power it might or might not generate.

A housing that has been damaged by too hot a heat range plug, or long time running close to over heated, will get a tiny crack across the little hole. So long as it cannot be felt, or no surface appearance changes around the crack shows up like the apex seals are stressing the area, there is no need to repair anything.

However if there is evidence of a step or chrome changing color, the surface may be reduced with a stone and the housing reused barring any other problem. The apex seal will tolerate a depression but not a bump.

A housing with radial cracks (in all directions) from the little hole indicates a bigger overheating incident and greater care must be used and extensive inspection of the housing for distortion and so on must be conducted before reuse.

Lynn E. Hanover