What piston position to fire plug at?

[mcjustis]

Okay guys. I have a 1/4 scale perkins hit n miss that I need help with. I am changing over to electronic ignition on this so I need to know where to place the magnet for the hall effect sensor. I'm thinking on the large cam gear. My thought is that it will only fire every other revolution. The magnet is easy enough to position on the brass gear, and there's plenty of room for a little adjustment arm to mount there for advancing / retarding spark.

Here's my question though...

When in the cycle should the plug fire. Is there a rule of thumb? So many degrees before/past or at top dead center?
Thanks,
Martin

 

[Jasonb]

What ignition are you using? The Rxcel ones have an auto advance and need the magnet in a different position to say the ones from S/S. It will also get confused as the timing ring is moving at half the revs of the crankcase where their sensors are designed to be mounted

Normally you want it to fire at about 5degrees before TDC. If possible have the sensor for the magnet on a disc that can be rotated then you can advance and retard the timing to get the best running

 

[gbritnell]

The theory behind spark advance is to have the plug fire so that the fuel/air mixture is burned completely as the piston starts it's downward stroke. This way the maximum power of the burned charge is pushing the piston in the right direction. By that I mean if it fires too early there will be pressure on the piston before it gets over TDC.
On an automobile engine (before electronics) there were basically 3 stages of ignition advance, static or intitial (ususally around 10 degrees before TDC), then once the engine started and produced a vacuum signal more timing was added. The final stage was by centrifugal weights attached to the point cam. These were for high rpm.
Fuel burns at a constant rate so the faster the piston is moving the sooner the charge needs to be ignited.
Now back to your question. For a hit and miss engine I wouldn't go to the trouble of making any type of advance mechanism. I would just mount the magnet on a moveable disc. This could be mounted to the cam gear. As Jason stated I would start at about 5 degrees before TDC. A hit and miss generally runs at one speed (governor controlled) so the timing isn't as critical. When you start the engine see how it runs, then stop the engine and advance the timing a little and see if it runs better if it doesn't then try retarding it a little and see what kind of difference that makes. With a hit and miss the most important thing is for it to run nice and slow so if you have too much advance you won't be able to get the rpm's down.
gbritnell

 

[mcjustis]

To answer your question Jason, it's the TIM 6 from Jerry Howell. This one.
http://www.model-engine-plans.com/partskits/ignitionsystems/transistors.htm

So I'll start about 5 degrees BTDC and I'll have the hell effect sensor on a moveable arm. That's how it's shown on the plans except with points instead of the hall effect sensor. Thanks guys.
Martin