Vacuum advance and timing adjustment

[Timothy B.]

I have been reading past threads about timing and vacuum advance. Several posts discuss ported versus non-ported vacuum, and how non-ported is better for various alleged reasons. For the experts: Why did GM design them with ported vacuum advance in the first place?

Also, I assume at proper idle speed with the vacuum advance hooked up to the proper ported fitting, that there should be minimal or no actual advance being applied. Since this is the case, why do the manuals say to plug the vacuum advance line when adjusting engine timing?

[Duke W.]

Re: Vacuum advance and timing adjustment

Ported vacuum advance is an emission control strategy. By eliminating vacuum advance at idle the total idle timing is reduced to just the initial timing setting, which increases EGT and aids in HC and CO oxidation with injected air.
It also increases idle fuel flow and transfers more heat to the cooling system.

As far as why the manuals say to disconnect and plug the vacuum advance signal line when setting initial timing, I expect it is just a vestige from the procedure used during the pre-emission era when most engines had full time vacuum advance.

Duke

[Timothy B.]

Re: Vacuum advance and timing adjustment

So, with full time vacuum advance, when you plug the line and adjust timing to specs (10 deg. BTDC or so) as soon as you reconnect the line again the timing lurches forward? During deceleration would be the only time that the vacuum advance mechanism would actually release, right?

[John H.]

Re: Vacuum advance and timing adjustment

Tim -

You set initial timing with the vacuum advance disconnected, and when you re-connect it, the idle speed will increase; then you adjust your idle mixture screws (with a vacuum gauge, to highest steady vacuum) and idle speed, and you're done.

The vacuum advance is fully-deployed under high-vacuum conditions (at idle, cruise, and when you take your foot off the gas), and reduces under load (which reduces manifold vacuum), disappearing entirely at WOT.

[Joe C.]

Re: Vacuum advance and timing adjustment

Tim:

I see that you have a '67 small block. Everything else being equal, and assuming that the engine is healthy, and/or rebuilt meticulously, then the 300HP engine will handle more spark advance than will the 350 HP engine. This applies to burning pump gas @ 93-94 octane, with no additives.
The best rule of thumb that I can provide: For best fuel economy and power output, run as much spark advance that the engine will tolerate before detonation occurs.

Joe

[Duke W.]

Re: Vacuum advance and timing adjustment

If your '67 did not originally have CA emissions, it will have full time vacuum advance. If equipped with K-19 AIR for CA emissions it has ported vacuum
advance.

If full time advance the line must be disconnected and plugged for the initial timing setting. When the vacuum can is disconected the revs will drop. Set the timing with the engine just barely chugging and then bring up the revs and note when the advance begins to increase. This is the start of centrifugal, and you want to make sure it is at least a couple of hundred revs higher than where you set the initial. Otherwise your the initial timing you read will not be true.

When you reconnect the vacuum can the advance will jump to full value because idle vacuum should be higher than the vacuum at which the vacuum can adds full advance.

For ported vacuumm advance used on emission control models, it is usually not necessary to disconnect and plug the vacuum can because there should be not vacuum advance at idle, but the service manuals usually still say to do so.

When you lift off the throttle following acceleration the vacuum advance will go to full value because trailing throttle vacuum is greater than idle vacuum.

Duke

[Jack Corrao]

Re: Vacuum advance and timing adjustment

To further confuse this issue is my 71 LS5 that has TCS or transmission controlled spark ( I think ). I read a lot about this in all the performance mags but just dont get it. It seems as you accelerate vacuum advance reduces as vacuum goes away but at the same time mechanical advance is advancing the timing on another "curve". I think my TCS keeps vacuum from going to the distributor at all based on an electrical signal from the tranny or shifter somewhere. I cant figure it out, and I'm not sure if I'm getting full performance from my engine without vacuum advance. Say you were doing a 1/4 mile run, does vacuum advance come into play at all or just mechanical advance based on RPM alone? I think vacuum advance is to improve driveability under partial engine load under light throttle, hence high vacuum signal to the advance canister to compensate for the lack of mechanical advance at that point , then mechanical takes over as rpm increases. Any comments? I welcome someone setting me straight. I tried connecting the distributor directly to ported vacuum on my quadrajet, but the car came with it connected to non ported and connected throught the TCS. I dont feel a difference. What a dilema. Anyone familiar with this TCS in addition to basic advance theory.

Thanks,

Jack

[Randy Adams]

Re: Vacuum advance and timing adjustment

Try a power tune. Remove your weight springs in your distributer, get an adjust timing light, set it at 36# and rev moter to 3000rpm or when timing marks stops climbing. Adjust so timing mark is at 0 degrees and lock down the dist. Set your light back to 0 and read your timing--this is your static reading. reconnect your springs and vacuum hose adj. carb. idle and air.

[Jim T.]

Re: Vacuum advance and timing adjustment

Jack I have a 70 350/300 that came from the factory with the TCS system. My 70 is automatic and the TCS withheld vacuum until it shifted into third. I did not like this and by passed the TCS vacuum distribution solenoid with a direct full vacuum direct line to the vacuum advance. Did this when the car was about a month old and have never changed. The city mileage went from about 11 to 14 MPG with the change. Never overheated in peak of summer in Dallas using the air. The highway mileage of course stayed about the same since full vacuum was applied in third gear.

[Patrick H.]

Re: Vacuum advance and timing adjustment

Jack,

As Jim noted, the TCS allows vacuum to your distributor in 3rd and 4th gear (assuming you have a manual trans, which you don't state). To see if it is functional, while sitting in your driveway and the car running put your foot on the clutch and shift into 4th gear. You should eventually hear the idle speed rise (on my 72 it's about 20 sec, on my 71 it is, ah, disconnected so I can't say It may be right away). In fact, you have to show this test during your Ops check for judging, assuming your judge knows the subtleties of your year. You may have to "tip" the throttle slightly to check this in 4th, as you have to allow the solenoid on your carb to work (i.e. extend). I don't think so, based on what I recall from when my system was still connected 3 years ago.

That nice solenoid on the left front of your carb? When it receives a signal from your trans, it extends the plunger. This also uncovers the vacuum port at the bottom of the solenoid, and vacuum is allowed to complete its circuit from carb to TCS solenoid to your distributor.

Note that if this solenoid is working, you will also have an increased rpm for about 20 sec when you first start your car, even if warm. That is triggered through a different part of the system, however.

I hope this helps.

Patrick

[Duke W.]

Re: Vacuum advance and timing adjustment

Vacuum advance is added as manifold vacuum increases. At WOT there is very little manifold vacuum, so there is no vacuum advance. Vacuum advance has no effect on WOT power. Vacuum advance is there to improve idle and part throttle fuel economy. The increased fuel consumption from disabling the vacuum advance under certain operating conditions raises EGT for emission control and also rejects more heat to the cooling system.

TCS with ported vacuum advance was there purely for emission control.

This is why pre-emission control engines just had a signal line to transfer full manifold vacuum to the vacuum advance at all times. The requirement for emission control considerably complicated the basic engine control systems.

Before emission control (and neglecting cold start and warmup control) the engine control system consisted of the throttle, centrifugal, and vacuum advance. In the early emission era there were all these add on systems, like TCS, AIR, EFE, etc. Nowadays engines are controlled by a microprocessor using input from various sensors to determine fuel flow, and advance, and it's updated every engine revolution or two.

Duke

[Jack Corrao]

Re: Vacuum advance and timing adjustment

That explains why it's a good idea to bypass TCS, I guess Chevy had to do it for emmisions compliance. It seems when I connect to full vacuum, or non ported my advance jumps 8 degrees at idle, whereas if I connect to ported the advance comes in when you tip in the throttle. Why would you want vacuum advance at idle, wouldn't that be the same as advancing your static timing? I thought you need vacuum part of the advance curve for partial throttle driving. What is a good way to check and configure vacuum and mechanical advance for best driveability? It is a 4 speed with 3.08's. I under stand that 36 to 38 degrees total is best for a BBC, but that pesky curve...

[Joe R.]

Re: Vacuum advance and timing adjustment

Jack,

As Duke states above, the vacuum is lost when the throttle is opened thus retarding the timing and reducing the knock under acceleration. That's why a vacuum advance works so well with carbs.

Regards,

JR

[Duke W.]

Re: Vacuum advance and timing adjustment

Low density and exhaust gas diluted mixtures - such as what you have at idle need more timing than dense rich mixtures due to slow flame propagation speed.

Medium performance engines need 20-25 degrees of TOTAL idle timing to achieve minimum fuel flow and minimum EGT. SHP engines should have 25-30 degrees total idle timing. The combination of full vacuum advance and initial advance usually achieves these ranges on pre-emission engine.

TCS and ported vacuum advance are there purely to meet emission standards. By eliminating vacuum advance under idle and light load conditions, EGT goes up (as does fuel consumption) and the higher EGT promotes oxidation in the exhaust system to reduce HC and CO at the tailpipe.

Emission controlled engines typically have more total vacuum advance and less total centrifugal advance than their pre-emission antecedents. If you want to disable the TCS system, you should select a vacuum advance from an antecedant non-emission control engine and modify or replace the distributor autocam to achieve the same total centrifugal advance as the antecedent engine.

Once the total vacuum and centrifugal advance is corrected, you can try lighter springs to get the centrifugal in as soon as possible. On low compression seventies vintage engines, you can probably get all the centrifugal in by 2000-2500 if you are willing to use mid-grade or premium fuel. The faster you can get the centrifugal in - detonation being the limiting factor - the more low end torque the engine will generate.

SBs seem to work best with about 36-38 degees total WOT timing - the sum of initial plus total centrifugal. BBs about 38-40.

Maximum vacuum advance of 16 degrees is usually a good starting point, and it must be all in at least 2" less than idle vacuum to achieve the smoothest and most stable idle. If the vacuum can is not pulled to the stop at idle the advance with "dither" which will lead to an inconsistent idle speed and/or idle instablity. To achieve the best idle quality the total idle advance should be "locked" at a fixed value.

Duke

[Jack Corrao]

Re: Vacuum advance and timing adjustment

Thats why it pays to ask people who know. I never thought of vacuum advance as a "vacuum retard" under acceleration. All the articles you read talk about how much vacuum and mechanical for "total timing" which can be confusing. I will configure as duke states with full vacuum to see what happens. I have never been able to get the thing to idle well, I'll give it a try...when it's not -20 degrees outside.
Thanks for sharing your knowledge.

Jack