C-85 Timing

[flyguy]

It is the bottom cylinders that get 28 degrees and tops are 30, correct? TCM Bulletin just says right and left mags.
Darryl

[Dan Michaels]

Re: C-85 Timing

The right mag is the upper plug.

Dan

[flyguy]

Re: C-85 Timing

If so then the timing is the other way around with the 28 on top and 30 on the bottom plugs. Anyone care to confirm that for me?
DC

Edit: I realize it probably doesn't make a lot difference but if TCM specifies it, it might add a couple of HP. Flame front travel and developing max presssure, stuff like that. Anyone ever hear why not just 30-30 for both?

[Dan Michaels]

Re: C-85 Timing

I think it has to do with the duration of the cam. I got the info out of the Overhaul manual for the C85.

Dan

[Dano"T"]

Re: C-85 Timing

http://www.bowersflybaby.com/tech/fenton.htm

Good source of info for small Continentals

[flyguy]

Re: C-85 Timing

Thanks Dano, looks like a good source.

Now that I think about it Continental tends to set the advance at a value that they figure any engine will tolerate. Twenty eight on top probably means the fuel ignition/burn is more efficient from the top plugs and requires limiting to 28, not 30. Similarly the bottom will tolerate more advance as the ignition/burn from there is poorer. Tracks rather well with the way the engine runs from my experience.
DC

[mulwyk]

Re: C-85 Timing

will running a 28/30 ignition timing on an A-65 result in more power? Or does that just work on the C85?
(Disclaimer: not that I would ever modify the ignition timing on my A-65 to get more power. No, I'm just wondering is all.)

[flybikefarm]

Re: C-85 Timing

The A65 is timed at 30 and30. Timing one mag slower would probably result in less power. George

[Robert Lees]

Re: C-85 Timing

What timing is used for a C-85 modified as per Don's Dream Machine?

[flyguy]

Re: C-85 Timing

I expect anything over 30 on a production aircraft engine would be pushing it, and likely not get you anything. I understand the F1 racers use 32 or so. My modified Camaro engine seems to optimize out at 32 and it gets wound to 6000 RPM regularly.
DC

[NY86]

Re: C-85 Timing

The lower plugs are closer to the exhaust ports and run in a leaner mixture than the upper ones. Leaner burns slower so you fire the plug 2 degrees earlier to give it a little more time to burn. Keeps the mag drops closer...

[flyguy]

Re: C-85 Timing

Now that makes sense. Scavenging is not as good down there, right? Thanks for the info. Learn something every day.
DC

[NY86]

Re: C-85 Timing

Scavenging is not as good down there, right?

Exactly!