MOPED ARMY

Pretty sure I have a spare

I'll trade you this ugly Mondial I got sitting in my garage for it

Ohhhh my baby is so close!

still makes a difference :)

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.

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?

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?

if you aren't reporting in degrees, it just sort of makes it harder to ask those semi-difficult questions.

thanks for that calculator tho, that thing rocks.

Boom, the math to prove my caveman esc test and try theory, thank you for the help!

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.

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"

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?

errrr i knew i forgot the part where the conrod is 90mm on the 43mm stroke setup...just like a puch/tomos.

Which honestly works towards your argument because 13 degrees for 43mm stroke with a 90mm conrod is .681mm, or only .033mm different. The dial gauge micrometer I use for setting timing without a woodruff key totally measures up to .01mm, so I can dial in that close, but you are correct, timing is all just a loose construct based on how many RPMs you are spinning, what type of cooling you have, how much over-rev you'd like and other factors.

but OK, for me, timing something means starting EXTRA low for timing (closer to TDC) and gearing. Once I verify I'm making all of the RPMs I should be (thank you exhaust nomograph, tuned length calculating and port timing maps) then I can start to add the gearing back. Once I'm geared where i wanted to be, I verify that my set up can pull that gearing and still hit all of my RPMs with temps being in check. Then I can start add timing back (farther away from TDC) while rejetting and watching my head temps. It can be kinda hard to do long WOT pulls here in Boston, but there are a few big main drags where I can hold WOT for like 5 to 10 minutes and usually get some true readings.

that all being said, I honestly don't know what my tomos is timed at because of the parmakit box, but I can tell you I never saw temps over 270ºF - 280ºF when I had my temp sensor on and my take off is more than adequate, so I haven't really had any need to bumped up my timing yet.

but yea, I sorta have wondered if extremely oversquare setups need less timing then closer to square setups. anyways, I'll stop jacking this thread with my timing talk :)

Yooo you're the man!

I found this thread but no conclusion, just Adam nerding it up worrying about .0012" of piston travel.

Lee was the one who found the info from Mars I tested it, so thank you bud