Spark mod

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kell
Posts: 369
Joined: Sat Jun 19, 2004 2:58 am

Spark mod

#1

Post by kell » Sat Mar 13, 2004 4:13 pm

Description: Came across a new product called Direct Hits.

Came across a new product called Direct Hits. It's a capacitor about 80 pF that goes in parallel with the spark plug. Just slips over the plug (you have to use a non-resistor plug). So I thought I'd try a bench test. I made a variable capacitor with aluminum from soda cans and 11 mil polyethylene (or some such plastic) from another drink container. I set up a coil and spark gap, and battery on the bench. Got a spark. Then I connected the capacitor in parallel with the spark gap so I could compare results with and without the cap. It made a very big difference. Without the cap I got an okay spark with about a cm gap. I slid the capacitor plates together (without changing the spark gap) and the spark got much heavier brighter and louder. So I think this product would work. To get a set of 6 or 8 plug caps for a car is over a hundred bucks, I saw on the internet, don't know yet what the deal is for a Harley. If they even list bikes fifty years old. Am thinking of rolling my own.



Jack_Hester
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Re: Spark mod

#2

Post by Jack_Hester » Sun Mar 14, 2004 12:27 pm

Kell -

Let me know the results of your prototypes. Very interesting, as I have an application for an alcohol engine. Definite need for a hotter spark.

Jack

kell
Posts: 369
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Re: Spark mod

#3

Post by kell » Tue Mar 16, 2004 2:14 am

Rounding up some better materials. Experiencing dielectric breakdown with the pop bottle plastic, which is only good for a few thousand volts.
I expect the first version will run on a lead coming out of the spark plug boot. May take the form of a cylinder rolled around the plug cable, just for mechanical integrity, not for electrical reasons. The other lead would have to go to ground somewhere on the engine, with a short lead length if possible.
A .05" mylar dielectric and 6 square inches of surface area on the plates would make about a 80 pF capacitor that could withstand 50 kV.

57pan
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Re: Spark mod

#4

Post by 57pan » Tue Mar 16, 2004 6:16 pm

Very interesting. Keep us posted.
If dielectric breakdown is a concern maybe there is a way to just have it come into play while you are starting the bike.
Once it gets running it may not be needed, but it sure would be great to have a hotter spark when you're kicking it over.

kell
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Re: Spark mod

#5

Post by kell » Wed Mar 17, 2004 12:01 pm

Got it!
Just wrap some aluminum foil around your spark plug cable and ground it!
Cover about a foot of the cable with foil, lay a few inches of the stripped end of a wire on the foil and tape it down, and ground the other end of the wire.
I had to cover six inches of cable before I got a noticeable effect. A foot might work better but I wouldn't go overboard and cover several feet of cable, too much capacitance could simply turn into a damper.
This will subject the cable insulation to a strong electrical field, and will likely shorten its life. Also, if you have old cables that are worn out get good ones so they don't spark through. You could also take a preventive measure by putting a few layers of polyethylene garbage bag underneath the foil.
If you cut aluminum from a soda can round the corners and burnish or file the edges so they aren't sharp, sparks love sharp things.
Doesn't matter if the foil touches metal parts on the bike, it's getting grounded anyway.
Another choice is the "slug and snail copper barrier tape" hardware stores and garden centers sell. And get this: the brand name is SURE FIRE.
Are we having fun yet?

Billy
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Re: Spark mod

#6

Post by Billy » Fri Mar 19, 2004 3:52 pm

A company called "Nology" Engineering has been around for a while now & has plug wire called "Hotwires" that have a capacitor built in the wire. The spark is up to 300 times more powerful than stock. They have a very short spark duration of about 4 nanoseconds.
They can be found at http://www.jpcycles.com If you need that much fire.

kell
Posts: 369
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Re: Spark mod

#7

Post by kell » Mon Mar 22, 2004 2:01 am

Note about wrapping plug cables...

Suppression cables are probably not very good for this purpose. Capacitance requires surface area, and the conductor in a suppression cable is just a piece of thread. With copper plug wires you actually have a dimension you can measure with a caliper and plug into a formula for capacitance of a cylinder. That and the dielectric constant of the insulator.
Andrew -- if your alcohol engine has a distributor with a center cable coming from the coil you can save yourself some trouble. Instead of doing the spark plug cables, replace just the central distributor cable with a good quality cable (8 mm?) at least one foot long that has a copper conductor and wrapping it ( with aluminum or copper foil/tape) and grounding the wrapping. I can do the numbers for how much of it you have to wrap if you let me know what the insulation is and measure the diameter of the center conductor. Twist the wire a little before you put the jaws of the caliper on it so the filaments don't smush out flat.

kell
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Re: Spark mod

#8

Post by kell » Sat Apr 10, 2004 3:43 pm

Okay I wrapped the plug wires on the right side of my bike's double-plug heads. Screwed the ground leads to a couple of the 10-24 pan cover screws. It starts a little easier.
Here's what I did:
I got the economy spark plug copper core wire kit from J&P (part number 381-283). Since I was going for a specific capacitance I had to measure the diameter of the copper conductor and the outer diameter of the cable. For the constant that determines capacitance of the insulation I had to estimate a little bit. These wires use hypalon insulation which has a high dielectric constant of about 8 or 10 (good), but only makes up about half the insulation (not as good). 2 or 3 is typical of most other plastics so I averaged the numbers for a guesstimation of about 5 or 6. Then I was able to calculate: covering about 18 inches of cable with aluminum foil puts me in the ballpark for capacitance. (Expensive silicon wires, with the same size conductor, would have required about a yard of foil covering, so these $5 wires are ideal for the purpose.) I wrapped electrical tape around the aluminum foil. The tape also holds the grounding wire down on the foil.
What kind of spark plug you use will determine the duration of the spark. With short ground leads and non-resistor plugs you get super-short sparks. Direct Hits or Nology, one of those web sites, mentions a figure of several nanoseconds (billionths of a second) for this kind of setup.
Resistor plugs should cause an 80 pF capacitor to discharge between about one microsecond to a hundred microseconds (millionths of a second), or more, simply because a resistor plug can be anywhere from about 10 kilohms to approaching a megohm. (I measured some plugs and found a very wide range. Even among plugs with the same manufacturer and part number the variations in resistance approaches a factor of twenty.) With resistor plugs you don't get the super-duper peak amperage but if theory is correct the impedance-matching effect of the capacitor still improves energy transfer efficiency greatly -- no matter what kind of plug you use. Also, spark duration is a good thing if you have a lean mixture, which is hard to ignite.
If you want to connect this setup during starting only, you could use an alligator clip on the ground lead.

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