Timing Advance

[Mister 4x4]

Much cleaner and less confusing - Thanks! ::thumb::

 

[TommyK]

The difference in ported vs manifold vacuum is only apparent at idle. Certain engine combinations require and/or benefit from additional initial advance at idle and using full manifold vacuum is one way of achieving this while avoiding the hard start issues often caused by increased initial timing. Whatever vacuum source is utilized the distributor advance must be tailored to suit.

In essence the carb butterfly acts as an on/off switch when a ported vacuum source is used. When the switch is on (butterfly open to expose the vacuum port in the venturi) the vacuum advance behaves just as it would if hooked up to a full manifold vacuum source.

IMO if your engine is not one of those that requires additional vacuum advance at idle then there is no significant benefit to using full manifold vacuum. A properly curved distributor with vacuum advance hooked to a ported vacuum source will work just fine.

As stated in the article the vacuum advance can "tune" is critical in the proper management of timing when using full manifold vacuum. GM made hundreds of different cans each tailored to a specific engine/drivetrain combination (and vacuum source), much like carbs were. This is not so easily done using the original Ford vacuum cans on our cars or the available aftermarket versions.

As an alternative to using full manifold vacuum, an ignition timing start retard can be used to run more initial timing while avoiding hard starting issues when running ported vacuum advance. Some versions of the Duraspark ignition had this feature. The GM 5 pin HEI module also has start retard. MSD makes an add on start retard.

 

[barnett468]

Much cleaner and less confusing - Thanks! ::thumb::

ok great, thanks for letting me know it was a bit confusing so i could try and clear it up.

 

[goodnigh]

Just to get me straight.

I have a 351C 4V quench with some kind of cam.

Disconnect both vacuum lines and plug, adjust to 600

RPM. Timing should be where? These are Ford shop manual instructions.

mike

 

[barnett468]

Just to get me straight.

I have a 351C 4V quench with some kind of cam.

Disconnect both vacuum lines and plug, adjust to 600

RPM. Timing should be wherw? These are Ford shop manual instructions.

mike

sorry but i for one don't completely understand your question.

do you want to keep the dual vacuum advance?

do you want to keep the retard side of the "timing vacuum module" functional?

do you want to set your timing to the optimum level for your particular engine?

 

[Don65Stang]

Just to get me straight.

I have a 351C 4V quench with some kind of cam.

Disconnect both vacuum lines and plug, adjust to 600

RPM. Timing should be where? These are Ford shop manual instructions.

mike

For smog and emissions timing follow the factory manual otherwise don't ever open it for timing instructions. Set your initial timing to 16 - 18 degrees. Total timing when the engine is at 3000 rpm should be at 36 - 38 degrees. See my lengthy post above that show the pic of the distributor guts. Follow those steps and your car will run like a scalded dog. Your mechanic should or may have set this for you already. Ask him to set it at 16 and 36 degrees if not.

 

[barnett468]

.

and dont use your vacuum advance at all . . this way you wont need to anguish over which one is "right".

.

As mister4x4 pointed out in a round about way, timing will vary from one engine to another.

yes, timing is affected by several things . . just a few are the fuel being used . . some fuels have a faster burn rate than others although this is purportedly not as much of a problem with todays fuel as it was with fuel in the 70's.

it is also affected by dynamic compress, not static compression although static compression affects dynamic compression.

it is also affected by cylinder head design . . for example, a heart shaped high swirl head can not run as much timing as some others.

the best timing for power is to have the fuel develop maximum pressure at 14 degrees after top dead center.

one size does not necessarily fit all.

.

Plugged the vacuum advance into the driver side port, and the advance shot up to 40.... still running nicely.

although it was "still running nicely", this is way too much initial timing unless you have an 8,000 hp engine.

also, this is typically a sign of an engine that has low compression . . you can not do this on a street engine that has maybe 7.0 dynamic compression or more because it will try to run BACKWARDS.

.

 

[jbojo]

This timing stuff is so dependent on each motor. Mine is 10.5 CR with 62cc heads and medium performance cam. I use 14* BTDC initially with 10* ported vacuum (motor does not like manifold vacuum). Timing starts to advance at 950 RPM and has 34* by 2500 RPMs. Motor idles at about 825 RPM.

 

[Don65Stang]

This is a good discussion on timing tonight.

John, your numbers correspond and are in line with my (Chuck's) recommendation of 16/36 just backed off 2 degrees. These numbers are what "most" 351c engines like and are a good starting point to dial in the correct timing for "anyone's" particular engine.

Barnett is spot on with his diagnosis of the 40 degree comments. Good stuff.

 

[barnett468]

.

My understanding is if a car is at optimal initial timing (around 16 - 18 degrees) instead of factory initial (6 degrees or so) then using manifold vac will advance the timing past optimal and take it to 26 degrees or so initial.

It actually depends on the vacuum can setting as tommy k implied. Please see more info below.

As stated in the article the vacuum advance can "tune" is critical in the proper management of timing when using full manifold vacuum. GM made hundreds of different cans each tailored to a specific engine/drivetrain combination (and vacuum source)' date=' much like carbs were. This is not so easily done using the original Ford vacuum cans on our cars or the available aftermarket versions.[/quote']
You can buy adjustable vac cans . . for small blocks the part numbers are vc31 and v311 . . there is an allen screw inside the hose fitting . . when you turn it counter clockwise to reduce the amount of advance it provides, it also raises the amount of vacuum required for it to begin advancing.

As an alternative to using full manifold vacuum' date=' an ignition timing start retard can be used to run more initial timing while avoiding hard starting issues when running ported vacuum advance. Some versions of the Duraspark ignition had this feature. The GM 5 pin HEI module also has start retard. MSD makes an add on start retard.[/quote']
Yes, ice ignition and some msd boxes also have this as well as some others.

.

 

[goodnigh]

For smog and emissions timing follow the factory manual otherwise don't ever open it for timing instructions. Set your initial timing to 16 - 18 degrees. Total timing when the engine is at 3000 rpm should be at 36 - 38 degrees. See my lengthy post above that show the pic of the distributor guts. Follow those steps and your car will run like a scalded dog. Your mechanic should or may have set this for you already. Ask him to set it at 16 and 36 degrees if not.

So 16 and 36. Never did a 3000 rpm check.

16 with the vacuum connected or not and at what

rpm. Then there is the issue with the cam, don't

know what cam except valve lift was measured at 0.58

Thanks for all your advice.

mike

 

[c9zx]

For smog and emissions timing follow the factory manual otherwise don't ever open it for timing instructions. Set your initial timing to 16 - 18 degrees. Total timing when the engine is at 3000 rpm should be at 36 - 38 degrees. See my lengthy post above that show the pic of the distributor guts. Follow those steps and your car will run like a scalded dog. Your mechanic should or may have set this for you already. Ask him to set it at 16 and 36 degrees if not.

So 16 and 36. Never did a 3000 rpm check.

16 with the vacuum connected or not and at what

rpm. Then there is the issue with the cam, don't

know what cam except valve lift was measured at 0.58

Thanks for all your advice.

mike

Mike, Since the lift is .580 I have to assume it is a pretty aggressive cam. So, both vacuum line plugged, lowest idle speed possible, set initial to 16 BTDC. Slowly raise rpm while watching the timing to see at what RPM maximum mechanical advance occurs. Maximum mechanical advance should be 20 degrees by about 2600-2800 RPM. So total advance should be 36 degrees (without vacuum advance). Now connect the vacuum lines to the distributor and check timing again It should be no more than about 10 degrees vacuum advance for a gross total of 46 degrees. These are ballpark number for any 351C with a performance cam. There are a LOT of variables here so optimal setting may vary by a couple of degrees or a couple hundred RPM. It is very likely that distributor calibration will be required to achieve this timing curve. Changing the springs (rate of advance) is easy. Setting the length of the advance slot (see picture) takes more effort. Limiting vacuum advance can also be a challenge. If anyone needs details, email me and I'll give you my phone number so we can talk. My typing skills are limited. Chuck

 

[Joe Stang]

Hello Don , Mike and Chuck . It's a sign from the Mustang gods I needed to fix my password and get back with your guys and gals . I have been playing with the timing on my 71 and I can tell it's not happy yet . I am running a Crower cam , 530/540 ish lift and and dura spark dizzy with MSD cap . Also Hyfire 4 cd box . I have the plug gap set @ .35 on plugs now and will try .40 when I get the timing debacle handled.

In the past with a bigger cam (solid 588) I can remember running darn near 40 deg. total . It was easy with the Mallory dizzy I had . I had 28 mech built in it with no vac . Seemed like it was all in @ 2800 rpm too . I was hoping the dura with Vac would be a better deal but so far I have been lost with it . Also I noticed the base of the Dura dizzy can be rotated a bit because the base clips are a bit loose . I am tempted to epoxy or screw the base down so things a not not so loosy-goosy below the cap .

Sorry I have been gone so long . I did miss you guys and gals . I will update you all with the new stuff do to my 71.

Thanks for the great info and support . Joe Stang