How do you calculate piston movement versus crank rotation?

[tony27]

I've read it mentioned that there is a formula for working out how much a piston moves in relation to crankshaft rotation.
I need to work out where to set the timing on my CZ so that at full advance it sparks in the same place it would if I was still using points, forgot to take that into consideration with the TS cdi conversion & ended up holing the piston

Measurements I have are
Conrod length 130mm-possibly 131mm
Stroke 72mm so throw is 36mm
Advance is either 4 or 6deg, I figure work on 6deg as if it's 4deg I'm on the safe side & plan on strobing at max advance once running
Desired max advance figue of 3.5mm

[Montynut]

Calculation of piston travel versus crank rotation is a very complex maths problem as the vertical piston travel is not linear with crank rotation. For example at BDC and TDC the piston basically does not move for several deg. of crank rotation while at mid stroke piston travel (speed) is at its maximum for each degree of rotation. The easiest way is to measure it.

The easiest process to set the timing is to find exact TDC and mark it on the flywheel against a fixed point on the cases. Measure the circumference of the flywheel and divide by 360 and then multiply by the amount of advance desired and mark the fire point on the flywheel.

four deg would be the safe option to avoid problems as 6 is more likely to cause detonation although it may kill the performance.

[YZ250H]

How about this 

http://www.dansmc.com/mc_software2.htm

[pancho]

 Two great replies, thats what makes this forum so good.
cheers

[Mike52]

Degree wheel [ Repco ], even a cardboard one will do, bit of wire bolted to something/ somewhere to act as a pointer for the wheel  and a dial gauge or a piece of something round to measure TDC.
I've chopped the guts out of an old spark plug and stuck a dial gauge in it.

Can even stick the wheel on with blutack.

Find TDC , mark the flywheel or take note of the existing marks , mark the case somewhere at the required advance off the degree wheel.
Then a battery strobe light and a mate to kick the bike over while you check the marks. [ Plug out of course ]
Works good.

[Nathan S]

If you know the rod length and stroke, it's high-school maths.
Start with the crank at 90* if you need help visualising the trig.

[tony27]

How about this 

http://www.dansmc.com/mc_software2.htm

That works great, threw the numbers in & found that 3.5mm BTDC is around 22.5deg.
Scary part is that 6deg less(16.5deg) is 1.9mm & 4deg less(18.5deg) is 2.4mm so I'm not surprised I holed the piston :'(
Going off the possiblities of adding 4 & 6deg to the 3.5mm firing point I had the bike set at came up with 4.8mm & 5.5mm 

[Lozza]

I've read it mentioned that there is a formula for working out how much a piston moves in relation to crankshaft rotation.
I need to work out where to set the timing on my CZ so that at full advance it sparks in the same place it would if I was still using points, forgot to take that into consideration with the TS cdi conversion & ended up holing the piston

Measurements I have are
Conrod length 130mm-possibly 131mm
Stroke 72mm so throw is 36mm
Advance is either 4 or 6deg, I figure work on 6deg as if it's 4deg I'm on the safe side & plan on strobing at max advance once running
Desired max advance figue of 3.5mm

The basic question would be if you were doing a CDI conversion why use such a shit ignition?
The calculation for 3.5mm is 22.6deg
Theris only way your going to find out the advance curve is to graph it, you can get advance built into some stators. 23 deg is not a lot of advance, a good cylinder head and good ducts can run 30deg.

[tony27]

I'm using the TS ignition because it's pretty close to the same weight & easily gotten, pretty hard to justify about $800 for an ignition after customs sting you 15%gst on a bike that cost quite a bit less than that when originally bought
I wouldn't call a CZ head a great design with the elliptical pocket to the rear of the cylinder, I've got a damaged 74 model head at home that may get welded up remachined to a more normal design if I can ever get motivated

[Lozza]

There are a million other ignitions that would be just as easy to get a hold of like a YZ 100 or 125 models with the ignition on the right. A modern DC/DC ignition would be < $500 or so all up and you could buy most things from NZ.
I didn't say the CZ head was good either but just doing that will unlock a lot of usable HP and it's pretty much for free.