Ignition timing technical logic

[KurtJL]

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Please point me to a technical resource that explains why the AeroVee spark plugs fire at different BTDC timing.

Thanks, Kurt

[gammaxy]

I assume you're talking about the secondary ignition timing being different than the primary timing on the Turbo Aerovee? I've asked and seen similar questions asked here before with no answer. Someone should ask Sonex.

Another difference on the turbo is the valves are adjusted with a lot more clearance.

[Sonerai13]

The for the timing differential on the turbo AeroVee is that, during our initial testing we found that if the timing was run the same as on the non-turbo AeroVee the CHT ran higher than we wanted. Retarding the secondary ignition a few degrees brought the temperatures in line.

There are several examples of "split" timing on type-certificated engines as well. Some engines are timed with both mags at the same BTDC setting, and other engines run one mag a few degrees ahead of the other one. I'm sure the reasons are the same for them as it is for the AeroVee.

[gammaxy]

Joe, thanks for that answer. I'm assuming this is the relevant quote from the manual that Kurt is asking about:

The electronic (secondary) ignition on a turbo-equipped AeroVee is timed to 10 degrees BTDC, unlike the Magnatron (primary) ignition, which is fixed at 28 degrees BTDC.

I interpret that to say there's an 18 degree difference in timing. There have been recent posts that indicate these statements in the manual might be typos and the true setting is 10 degrees after the primary ignition or 18 degrees BTDC.

viewtopic.php?f=7&t=3265&p=24895

In either case, (18 degree or 10 degree difference) it does seem to be a little more than "a few degrees". Mike Farley reported 24 degrees BTDC worked for his secondary--this sounds more like the "few degrees" you mention and might even be in the same ballpark as the normally-aspirated AeroVee.

[daleandee]

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[Brett]

A long time ago nissan made 4cyl engine that had dual plugs. They fired at different times for a more complete burn to reduce emissions..... they said that if they fired at the same time the flash fronts would meet each other and cause issues...

[KurtJL]

Thanks for the replies.

Sounds like it's purely a turbo driven issue (?)

Regards, Kurt

[Sonerai13]

This was covered in a different thread I believe, but I'll repeat it here. There is a mistake in the AeroVee manual regarding timing of the ignitions for the turbo. The secondary should be timed approximately 10 degrees AFTER THE PRIMARY. We will correct the manual ASAP.

Some builders have experimented with the timing of the secondary, so the 10 degrees after the primary is not set in stone. All we know is, when we timed the two ignitions together like on the non-turbo engine, the CHT on the turbo would spike when at higher boost settings. Retarding the secondary approximately 10 degrees solved the issue.

[KurtJL]

Thanks for the replies.

Per Dale Williams' comment, I'm interested in the "science" (or rather the engineering) behind the timing issues.

If there's good empirical data about what's happening inside the combustion chamber then it'd be nice to know about that; e.g. detonation issues related to 1st and 2nd plug timing, fixed-timing vs. variable-throttle (manifold pressure) trade-offs for managing head temperature, other?

I appreciate that there are ongoing turbo issues, but my original question was hoping to also understand the non-turbo timing technology.

Thanks again, Kurt

[KurtJL]

Example relative to "... the CHT on the turbo would spike when at higher boost settings" --- as I remember it, a "spike" in CHT is a sign for having entered detonation conditions. Was there detonation detection during the testing process?

Regards, Kurt