Discussion and questions related to the course Practical Dyno Tuning
Since I am using KTM 500exc the TDC mark is not visible outside, it's inside, though it becomes visible after removing head. But if I crank with the head off the oil will spill and even if I do it on cranking how will I be able to check it at high RPM?
Here is the Picture of TDC mark with the Head Removed.
Read the service manual procedures.
I had to find the TDC of a KTM SuperDuke V-Twin, where I was replacing the OEM ECU with a standalone. I rigged up a pressure sensor to the spark plug hole, and was able to capture the crank pattern and the pressure pulses to determine the offset for TDC. I might have been off a bit, but any small error is dialed into my tune as I maximized the ignition timing with a dyno.
I suspect you will need to do something similar.
That's a really great way of doing it David! I'll keep that in mind.
Thanks, Here is the Picoscope plot showing the pressure signal and the ignition coil signal -- I set the test timing to 0 deg, removed the other spark plug, and just cranked it on the starter motor (176 RPM you can see in the lower right).
I'm not sure if this was my final timing, but it is the only scope trace I saved from that experiment.
What the project, what is the goal, what modifications have you done to the engine?
Are you trying to check for spark timing drift on an aftermarket ECU, but you don't have a timing mark? How is timing set in the service procedure with the stock ECU? How is valve timing set? You can make your own mark and all that but at least understand how the engine is supposed to be serviced first. Normally there's a mark under the timing cover to set the valve timing, and a mark outside the timing cover to check the spark timing or otherwise service the engine. Look at the maintenance procedure for the engine; there should be a part about checking and setting timing. If you are reflashing a stock ECU and using stock sensors you shouldn't need to deviate from the factory procedure, but if this is a full custom setup it's a different situation.
Hey Raymond, I think the angle that Guru is going with here is that this engine (as is common with most motorcycle engines) has no external pulley and hence there's no way of referencing TDC with a timing light like we would with a car engine. Since these motorcycle engines normally use a wet clutch assembly that's also enclosed, there's literally no external location to get a TDC reference from.
Just thinking things through further, another option that 'might' be worth testing would be to use the drive sprocket from the gearbox. If (and it's a big 'if') one of the gears is 1:1 then you could theoretically use the sprocket as a reference but I fear the backlash in the gearbox would probably make that less precise than David's idea.