Total advance

[Gary S.]

Today my 72 LT1 came out of storage and I want to check out the ignition system as well as give it a typical tune-up. Can anyone explain in simple terms how to measure total advance using either a standard or dial back timing light (I have both).

I have read the archives about issues that fellow owners have had with either weak distributor springs, weak vacuum advance units or bad distributor gears. My car is totally stock down to what I can only assume is the orignal vacuum advance can. My car only has 36000 miles on it and unaltered, to the best of my knowledge.

Last fall I looked for a suspected vacuum leak but didn't find anything wrong with the system on my car. I have a very slightly rough idle and at steady state throttle, I have extremely minor surges or mini-hesitations. My wife swears she can't feel it but I know I do. Under full throttle acceleration, the car accelerates very nicely. This description is what I had always assumed to be vacuum relate but the vacuum is a steady 15" Hg at idle. I am starting to wonder if the issue isn't ignition related, which is the reason for the post.

Any and all help is appreciated.

Thank you,
Gary Schisler
72 LT1

[John H.]

Re: Total advance

Gary -

Total advance is the sum of base (initial) timing and the amount of advance provided by the centrifugal advance mechanism in the distributor (vacuum advance doesn't enter into total advance). Set the dwell first (assuming you have a dwell meter), as dwell affects timing. Disconnect and plug the vacuum advance line, idle it down as low as you can, and check base timing with your light; you idle it down as low as possible to avoid the influence of an early-starting centrifugal advance curve confusing the reading/setting. Write down the observed timing (for example, 8 degrees BTDC).

With your dial-back light hooked up, increase revs in 1000-rpm increments and observe the timing (turn the dial until the index mark on the balancer aligns with the "0" mark on the timing tab, and the dial will tell you the advance at that rpm). Note the readings at each step, and the rpm at which the advance no longer increases - this is the "end" of the centrifugal advance curve, and the reading at this point is your "total advance". That total advance number, minus the initial base timing reading, is the amount of centrifugal advance being added by the distributor.

Re-connect the vacuum advance line and check the timing again - if your vacuum advance can is working (and is connected to full manifold vacuum, not to "ported vacuum" as many smog engines are), this timing reading will tell you how much advance the can is adding - should be about 15 degrees added at idle. For example, if your initial base timing (with the vacuum advance disconnected and plugged) was 8 degrees, you should show 23 degrees after re-connecting the vacuum advance.

As a general rule, most small-blocks like about 36 degrees total advance, with the centrifugal "all in" by 2600-3000 rpm.

[Clem Z.]

Re: Total advance

measure the circumference of your damper,divide by 10 and this will give you where to mark the damper for the 36 degrees. put this mark on the advance side of the TDC mark on the damper. remove one advance spring so you do not have to rev the engine so high in RPMs,with the vacuum advance plugged rev the engine till the advance stops moving and align the 36 degree mark you put on the damper with the "0" mark on the timing tab and then lock down the distributor. replace the spring you removed.

[Gary S.]

Re: Total advance

John and Clem,

Thank you both for your excellent responses.
Gary

[Duke W.]

Re: Total advance

Not sure, but a '71 LT-1 may have ported vacuum advance and TCS. Also the entire ignition map is biased for emissions, not performance and ecomony.

You could achieve better all around operation including better idle quality and stability with full time vacuum advance and bypassing the TCS. Also, since your engine was originally designed to operate on regular unleaded you can set up a very aggressive centrifugal curve and run premium.

I would rework the map staring with changing the vacuum advance and centrifugal springs to approximate the 64/65 SHP map and set initial so that total initial plus full centrifugal is 38 degrees. The total idle timing - the sum of initial, full vacuum, and maybe a few degrees of centrifugal if it starts below idle speed should be in the range of 36-32 degrees, and and revving the engine to the point where the centrifugal is all in with no load should be initial, plus full vacuum, plus full centrifugal and should typically be in the range of 50-54 degrees.

Duke

[terry plata]

Re: Total advance

Duke and Clem and all, Good advice but I have a question: My impression was that MOST vacuum advance diaphragms were hooked to VENTURI VACUUM ports so that as RPM increased, advance increased. Supplying full MANIFOLD VACUUM would cause FULL vacuum advance at idle when throttle plates are closed and then drop the advance under full throttle when plates are open wide.. The only advantage I can see to maniofold vacuum at the distributor is that it may prevent some spark knock upon acceleration. Thanks for clarifying this for me.. Terry

[Duke W.]

Re: Total advance

Vacuum advance is NEVER signaled from venturi vacuum. Prior to emission regulation the norm was manifold vacuum. As mixture density decreases, flame propagation speed slows, so more advance is required, and manifold vacuum is a good analog of mixture density. Also, exhaust gas dilution slows flame speed and whenever there is manifold vacuum exhaust gas dilution is significant, particularly at low speed/idle. This is why Corvette engines need 20-30 degrees of total idle advance to idle most efficiently.

Ported vacuum advance was an emission control strategy. By eliminated vacuum advance at idle, the idle timing was retarded well below the ideal. This increased EGT which oxidized HC and CO in the exhaust with injected air. It also rejects a lot more heat into the cooling system.

The last non-emission control engines from the mid-sixties have very good ignition maps given the limitation of the relatively crude centrifugal and vacuum advance, but they work quite well.

Duke

[Gary S.]

Re: Total advance

Duke, the TCS does control the vacuum in the 72. Many people run an extra length of vacuum tubing to a straight uncontrolled vacuum. I have a source for this off of the front of the Holley. I should have mentioned that I knew of the problem and had a way around it. I have seen a couple of TCS equipped cars that bypass the system during normal operations and only hook up the correct vacuum for judging.
Gary

[chris burbage]

Re: Total advance

I have followed a number of these threads over the last year or two. Someone always slips in that small blocks like 36 degress total advance all in by 3000 rpms. What about the BB cars with TI. Should we follow the same guidelines?

[Duke W.]

Re: Total advance

The type of ignition makes no difference. I think BBs with closed chamber heads like a little more total WOT advance - in the range of 38-40.

Open chamber heads can probably use a little more, but I don't know what is ideal.

Optimum WOT ignition advance is a function of combustion chamber geometry and spark plug location. In general, the smaller the chamber and the closer to the center the spark plug is located, the less total WOT advance is required for peak power.

As a rule it's also best to get the total WOT timing in as soon as possible, but detonation often sets a limit to the rate of centrifugal advance.

Duke

[John H.]

Re: Total advance

Terry -

The only application for venturi vacuum (created by air velocity) is the opening signal for the diaphragms that open the secondaries on Holley vacuum-secondary 4-barrels and the end carbs on 3x2 setups. Vacuum advance is controlled by intake manifold vacuum - no load, high vacuum, lean mixture, max vacuum advance; heavy load (like WOT), essentially zero vacuum, rich mixture, no vacuum advance.

[terry plata]

Venturi vacuum yes!

Duke,

Thanks for the response, one comment though. Among other toys I have a 1953 Oldsmobile Super 88 with the original rocket engine. The distributor advance mech gets its signal from the venturi vacuum port on the Carter WCFB four barrell. That is why my original question about vacuum source, some older "performance" engines used the venturi rather than manifold vacuum signal.

Following your logic, I assume the 1964 Corvette fuel injected engine used manifold vacuum for the advance as well?

Perhaps I should switch my 1978 Ford F-150 400 ci engine to manifold vacuum? This should cut advance during hard acceleration thereby eliminating ping! I am running the original distributor but have changed the old Motorcraft two barrrell out for an Edelbrock manifold and Edelbrock 600 CFM four barrell.
What do you think? Terry Plata

[Duke W.]

Re: Venturi vacuum yes!

I have to ask how you ascertained that the vacuum advance signal line is venturi vacuum on these engines. If venturi vacuum was used vacuum advance would increase with air flow. This is just the opposite of of the requirement. All Corvette engines that I am aware of use full manifold vacuum for the vacuum advance until 1966 when ported vacuum advance came into use for emission control.

Duke

[Doug Flaten]

Re: Total advance

Terry, could it be that you meant ported vacuum source rather than venturi vacuum source? The ported source is in the throttle bore above the throttle plates. So at idle, the plates are closed and ported vacuum is low. Someone correct me if I am wrong, but with the exception of the venturi itself, the manifold vacuum is going to be greater than the ported vacuum. The cylinders are the source of the lower pressure, and as you go upstream in the flow path, the pressure is going to be higher. At least that is the way it has always worked in the pipeline fluid flow systems that I have built and modeled. I could see a slight vacuum increase at ported sources as the edge of the plate lifts and creates a higher velocity region around the edge of the plate over the orifice port. This would be a temporary "venturi effect" that essentially goes away as the plate opens more fully. It also would not be very stable. As the plate opens more, there will be little difference between manifold and ported vacuum.

[terry plata]

Re: Total advance

Doug,
Interesting observation! I will have to check the WCFB to see if the spark port is "ported" or "venturi". Vacuum signal at that port increased coming off idle so I assumed it was venturi. I never did rev it up without load to see how the higher rpm affected vacuum readings.

Think for a moment about your last statement :"As the plate opens more, there will be little difference between manifold and ported vacuum". Well following your earlier logic, if the port is above the throttle plates, it would make a difference between manifold and port vacuum. Unless the throttle plates were fully open, manifold vacuum would always be stronger.