Timing curves

[canyncarvr]

Consider a situation where initial timing is advanced a pretty good bit due to cammage and the setup of mechanical advance to take that into account.

With a 'normal' setup as example, you might have 10ºBTDC set for initial timing with mechanical starting in the low-teens-RPM somewhere...call it 1200rpm. If initial timing is set to 20ºBTDC should the centrifugal be sprung to start at a higher RPM, or should the mechanical curve START be at the same place it was? I don't see how the latter would work.

Granted, 10º will have to be taken out of the distributor to get to the same total timing you started with given the 10º initial change, but that is not my question.

Seems like a reasonable situation to get rid of the secondary 'long loop' spring, to put a secondary spring in with a holding-action early in the RPM sweep. Yes, I understand that would do away with the 'two stage' advance curve....but you're already starting at 20ºBTDC in this example.

 

[GTX JOHN]

I start mechanical at same place.
I remove strong spring and bend tab out
as far as possible on weak spring in Mopar Dist.

 

[canyncarvr]

GTX John: What is your initial timing and how much advance is in your distributor (state dizzy º or crank º please) for at total (no vacuum numbers) of...how many degrees and at what RPM?

Those measurements not particularly useful without build and fuel information..but I am NOT asking that.

 

[Bewy]

The reason for a LOT of initial [ idle ] timing is the same reason you want to start the centri curve early. The big[ger] cam produces a less dense mixture at lower rpms, which means more ign advance is reqd.

 

[canyncarvr]

The reason for a LOT of initial [ idle ] timing is the same reason you want to start the centri curve early. The big[ger] cam produces a less dense mixture at lower rpms, which means more ign advance is reqd.

The big(er) cam part, yes. The rest of it....are you saying that with the added initial timing that the centrifugal should be left alone except for limiting the total? As per my example, it would be 20º initial, keep the primary and secondary distributor springs the same...just change the limit (probably to 12º or so crank degrees)? 32º works in my engine just fine.

I'm wishing I had adjustable air bleeds in my 4779. At 20º BTDC initial, I don't think the engine is going to idle below 1000rpm. But...that's another matter.

 

[GTX JOHN]

Initial Timing around 15 depending on fuel octane
Plate 9 x 2 =18 in distributor.
All in by 2200 Engine.
This is in my 340/360/440 street cars when running pump 91
@ Sea Level (slightly more as elevation increases)
They all run in the 10s thru the Mufflers with Air Cleaners in place.

 

[canyncarvr]

Initial Timing around 15 depending on fuel octane
Plate 9 x 2 =18 in distributor.
All in by 2200 Engine.
This is in my 340/360/440 street cars when running pump 91
@ Sea Level (slightly more as elevation increases)
They all run in the 10s thru the Mufflers with Air Cleaners in place.

Thanks for sayin'.

 

[GTX JOHN]

If you bend the tab out on the spring = be sure it
does not hit or run on anything especially the screws
that hold to clips that retain the cap. If need be, you can
grind or replace them with shorter ones. This is a trick that
i have been using since early 1960s.

 

[Bewy]

Some engines require 50* to idle properly [ see below ]. My engine idles with 48* @ 800 in gear & has a smooth idle with a big cam.
Unless you know how to do this electronically, using an adj vac adv connected to manifold vac is the way to have a lot of idle timing........& have the timing drop back to the static amount under load. See the links below.
So if your engine idles best with 35* [ turn dist while idling to get highest rpm ], you can use the factory 12* initial + 23* added with MVA. If your engine wants 36* at WOT, then modify the centri curve accordingly. I would NOT have the centri curve start too early; that requires light springs that may over time cause the weights to stick.

 

[Bewy]

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