ignition timing

[keith.hodgenia]

Does anybody out there know the formula for converting distance BTDC into degrees BTDC. Thanks. Keith.

[charles.bulloch]

Hi, Keith,

since x = rcos A + sqrt{l^2 - r^2sin^2 A} you can't get the angle in a simple manner. I once wrote a program in basic which got the angle by iteration, but probably I could not do it anymore. The program did it in ten steps. l is the con-rod length and r is half the stroke.

http://en.wikipedia.org/wiki/Piston_mot ... ght_angled

Charlie

[keith.hodgenia]

Thanks for that Charles, I think! I'm working on an old blind head two stroke with easy access to the crank shaft end. I had hoped doing it in degrees would be easier and more accurate than a pencil down the plug hole Perhaps not! Keith.

[John.Cheetham]

Keith, You could try this website

http://www.torqsoft.net/piston-position.html

As in any calculation you will require the length of the con rod between centres.

John C

[PeterW]

My father used to irritate me. I always liked to set ignition timing by the book, he'd set it by trial & error. He'd set it, take it for a spin, advance it, see how it felt, was it pinking etc., repeat a few times and settle on whatever he felt ran best.
His argument was that although the manufacturers settings were best when new, by the time he bought whatever it was it was old and worn. tolerances had changed, compression might be lower, adv/retard springs might be weaker and petrol might burn faster/slower/hotter/cooler. I don't think I'll ever have his feel for mechanics but I still try setting it a degree or two either side of correct just to see where it runs best.

So here's two questions.
1. what matters with timing - the travel of the piston or the time it takes to do it?
2. does modern petrol burn faster or slower?

[keith.hodgenia]

Thanks John, that's just what I need. Peter, your Dad was right on the money, this bike is new to me so I am not too sure what" feels about right" is just yet. I took it for a spin today and it was running hot and pinking on the hills. So retard the ignition or richen the mixture or look at the none standard exhaust. Banbury is in 2 weeks so I need to sort it out. Many thanks Keith.

[charles.bulloch]

There was a question on this very subject in this years Advanced Higher Physics paper. Unfortunately nobody at the exam board has an Ariel because the question ignored the effect of the con-rod length. Because the crank and piston is actually a linked system. So the x = rcosA equation is not valid.

Getting out more in future

Charlie

[leon.mitchell]

"I'm working on an old blind head two stroke with easy access to the crank shaft end."

35 degrees BTDC at full advance is probably fine.

Leon

[keith.hodgenia]

Thanks to everyone for their input. I seem to be making progress. As I mentioned I had some doubts about the exhaust system and whether it produced enough back pressure. It has the "coffee tin" expansion chamber often seen in the twenties, with a very short tail pipe. Period drawings show a much longer tail pipe back to around the rear wheel spindle. I have lengthened it a bit and reduced its diameter. The engine now revs well, Hopefully a bit more tweaking to the timing and mixture will see it running in an acceptable manner. My obsession with the ignition timing has been more about repeatability than ultimate accuracy, with very little room above the engine, measuring the advance by degrees could be a lot easier.

[Bob.Murphy]

This bike is new to me so I am not too sure what" feels about right" is just yet. I took it for a spin today and it was running hot and pinking on the hills. So retard the ignition or richen the mixture or look at the none standard exhaust. Banbury is in 2 weeks so I need to sort it out. Many thanks Keith.

Aaaagggh . . I can't stand the suspense . . . WHAT IS IT

Show us some pictures and put me out of my misery .



Bob.