saraholt61

Can someone tell me what is the correct initial timing for a 67 427/435?

aaronz28

Initial really won't matter.

you want to set Wide Open Throttle timing to 36-38 degrees (with the vaccuum advance unplugged)

most likely your intitial will fall somewhere between 8-16 depending on who has setup the distributor...

On all of my cars, I set up WOT timing to 36-38 and try to get as much initial advance as possible without having rough starting...


this not only lets the motor run a tad cooler, but seems to keep the motor running clean, and is really smooth off idle.

i also use full manifold vaccuum running to the Vaccuum advance can (if so equipped)

this winds up putting timing at idle (with the vac advance hooked up) around 30-35 degrees.

On all of my cars this has proven to provide the best street manners and on all the cars I've had on the Chassiss dyno, changing the timing never improved anything... only hurt low end when we tryed to fool with it.


GOod luck

Aaron

67L36Driver

Spec in the chassis service manual gives 30* centrifugal advance in the dist by 3800 rpm. That coupled with the 5* BTDC at idle gives you a total of 35* BTDC all in (vac advance disconnected).

Vetterodder

Keep in mind that a 40 year old distributor may have a curve very different than it's original specs. These are performance cars and performance car guys often modify their cars (including their distributors). Start with total timing like Aaron suggested just to be safe.

aaronz28

my point exactly... and I'm not sure that even the factory timing curve is as good as you can get with those motors...

assuming someone modified the distrubutor... you could be way off by trying to set initial... especially if someone put a quick curve kit...

on my Z28... i have it setup to have 20 degrees centrifugal... so to get my 38 on that car, i have to have 18 initial.. (which is about 14 more than the OE spec calls for)

if I set it at 4 degrees initial.... it runs horrible becasue it is only seeing 24 degees at WOT.

so, unless you know exactly what the weights and springs are in the distributor, you have to set the WOT timing and let the initial fall where it may....

Aaron

Joel 67

sorry, wrong thread

JohnZ

Your 435 will also run a lot better (and cooler) with full manifold vacuum to the distributor - the stock arrangement was "ported vacuum", which was an emissions aberration to eliminate advance at idle.

saraholt61

I tried 5 Degrees BTDC bit the engine ran terribly so I am now running it at about 15 degrees initial.I don t understand the previous comment.Ported vacuum(above throttle plate) increases with engine rpm and manifold vacuum(below throttle plate)decreases with engine rpm.Vacuum moves the diaphram on the vacuum assembly which advances the timing.If it was connected to vacuum on the manifold it wouldn t work on acceleration????Did I miss something here?Pls. explain.

Vetterodder

Once the throttles are open ported and manifold are the same. Hooked to manifold, the extra advance at idle is benefitial. You don't want vacuum advance under hard accelleration as the increased load will result in pinging.

BarryK

Neither ported nor manifold vacuum operate to advance spark timing via motor rpm, that's how the distributor's mechanical advance operates. Vacuum advance operates via motor load.

now, lets look at how motor vacuum is produced. This is a somewhat simplified explanation..... vacuum is created by the piston's downstroke and the throttle blades in the carb. The blades create a restriction in the air flow. If the blades are closed they create more of a restriction in the "leak" of air so more vacuum is created and when they are opened they create less of a restriction so less vacuum is created.

ported vacuum and manifold vacuum operate the same except at idle.
Manifold vacuum will give full vacuum advance at idle which is beneficial for better idle characteristics, better throttle response, lower operating temps and possibly a little better fuel economy. Full manifold vacuum operates by allowing full vacuum advance at idle than when you give the car more throttle you are opening up the throttle blades reducing vacuum and decreasing the amount of vacuum advance. The more load the car is under, such as acceleration, going up a hill, etc to less vacuum the motor produces and the less advanced the timing is.
with ported vacuum there is no vacuum being seen by the vacuum advance can so it gives no advance. As soon as the throttle blades are cracked open slightly it will give full advance and than continue to decrease the amount of vacuum and therefore vacuum advance as the load on the motor increases and the throttle blades are opened more.

both ways work the same with the difference being at idle.

JohnZ suggested changing your vacuum advance from the ported configuration the factory set up to full manifold vacuum for the advantages listed above.
Keep in mind that the ONLY reason for the factory switching from full manifold vacuum as they had it set up in the past to ported vacuum on your car was for emissions reasons.
One way to help lower the HC emissions to start to comply with the new federal requirements was to retard initial timing and to increase operating temps to help burn off excess HC. Retarding initial timing and having no vacuum advance at idle both help contribute to higher operating temps which was their goal at the time to help lower emissions but it took away from performance.

you can get a better understanding of the vacuum advance system and how and why it works by reading JohnZ's tech article and also one from Lars Grimsrud.

Here are links to them:

http://www.lbfun.com/warehouse/tech_...101Article.pdf

http://www.lbfun.com/warehouse/tech_..._explained.pdf

http://www.lbfun.com/warehouse/tech_...ance_Specs.pdf

Viet Nam Vett

Barry K... The Link Master..

aaronz28

deleated

buns

For future referance, here are the distributor specs from the '66 Service Manual.

JohnZ

Original factory specs for the '67 427/435 (L-71):

Base Timing: 5* BTDC
Centrifugal Advance: 0*@900, 30*@3800
Vacuum Advance: 0*@800, 15*@15.5"Hg. (none at idle - "ported")

solidaxel

I have had good luck setting my inital timing with a vacuun gauge, then check it with a light and see what the engine likes at idle.
my .02