> LSLB RXb wrote:
> I just really am lobbying for people to report timing on setups in
> degrees so the information can be compared across platforms. we can
> start to make gains on timing approximation from setup to setup if we
> use like terminology.
degrees work for me! but to do any cross platform comparison, you still need to know stroke and rod length. in which case, piston position is just as useful.
> so to say:
>
> If an adequately aircooled 10,000 rpm 40mm cylinder/pipe setup on a 40mm
> stroke need to be timed at 15º BTDC, does an adequately aircooled
> 10,000rpm 38mm cylinder/pipe setup on a 43mm stroke need to be timed the
> same?
OK now we are talking about actually changing the stroke, not the rod length. so, yes, changing the stroke would change the relative piston position. but by how much?
according to that calculator, with a constant 75mm rod:
40mm stroke @ 15° BTDC = .860mm
43mm stroke @ 15° BTDC = .939mm
difference in piston position is .079mm or .003"
> and what about
> if an adequately aircooled 10,000 rpm 47mm cylinder/pipe setup on a 40mm
> stroke need to be timed at 13º BTDC, does an adequately aircooled 10,000
> rpm 45mm cylinder/pipe setup on a 43mm stroke need to be timed the same?
40mm @ 13°BTDC = .648mm
43mm @ 13°BTDC = .707mm
difference is .059mm or .002"
in these examples, a degree of crank rotation is approximately .1mm ~ .004" piston travel.
so even when splitting a tenth mm is possible with a dial indicator, or a half a degree on the wheel, you still face visual aid timing mark alignment, and - with points- firing inconsistency. and even so, spec timing is often a range of a couple tenths mm or degrees.
with a 10" wheel and a sharp pointer, I can confidently degree cam and relative ignition timing in within .5° . It only really matters if you have a way to measure the effect of your changes, which is where having a dyno is a big help.
so i guess a question i have is - how do you know when something is timed where it "needs" to be? like how can you tell the difference between 13° and 13.5° ignition advance?