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Low initial timing but still some pinging


astrochimp

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So that New HEI disty is in and i set the timing to about 4 degrees before TDC but under load, I still get some slight pinging. Vacuum advance is attached to the constant vacuum port on edelbrock 600 performer. 

 

I have been using regular gas so is that just to low octane or is there another issue?

1970, 350ci, 4bbl performer carb and instake

 

 

THANKS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  :shifter:

 

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My stock 1971 350 4BBL also has some pinging so I now use a slightly higher 89 octane fuel.

Try unhooking the vacuum advance from a constant vacuum source and attach it to a ported source. Take it for a ride and see.

Bruce

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NEVER!

vacuum advance should be on manifold vacuum

 

Right on, Sam! Here's a great article on Timing and Vacuum Advance that was originally shared with our club by Bruce (I believe). The topic of "ported" vs. "manifold" vacuum source is specifically covered in paragraphs 7 & 8.

 

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As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

 

TIMING AND VACUUM ADVANCE 101

 

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

 

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

 

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

 

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

 

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

 

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

 

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

 

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

 

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

 

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

 
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Glen, I think I know what you mean but to be clear, many carbs have two vacuum advance line connectors - a manifold vacuum connector, as well as a "ported" vacuum connector.  The "manifold" vacuum connector leads to an opening in the carb body that is below the throttle plates while the ported vacuum connector leads to an opening in the carb throat that is above the throttle plate.  So, if you have a carb like that (Edlebrock, for example), your vacuum advance line is "hooked up to the carb" when using either connector, but the performance is vastly different (as explained in the article).  It's easy to tell which connector is which because the manifold vacuum connector will register a significant vacuum when the engine at idle.

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Great article. Keeps it simple and easy to understand. I am a lot more knowledgeable now than before. Thanks. You are right Summit and other "performance car" sellers are in business to strictly sell parts each one claiming they have the fix for your problem.

John S

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Glen, I think I know what you mean but to be clear, many carbs have two vacuum advance line connectors - a manifold vacuum connector, as well as a "ported" vacuum connector.  The "manifold" vacuum connector leads to an opening in the carb body that is below the throttle plates while the ported vacuum connector leads to an opening in the carb throat that is above the throttle plate.  So, if you have a carb like that (Edlebrock, for example), your vacuum advance line is "hooked up to the carb" when using either connector, but the performance is vastly different (as explained in the article).  It's easy to tell which connector is which because the manifold vacuum connector will register a significant vacuum when the engine at idle.

I use the port at the bottom of the Edelbrock carb on the passenger side of the carb. Should I not use this?
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I use the port at the bottom of the Edelbrock carb on the passenger side of the carb. Should I not use this?

 

Glen, the passenger side vacuum port on both the Edlebrock Performer and Thunderer AVS series is designated as the "Timed Vacuum Port" and is NOT the one to use for our motors.  The vacuum port on the driver's side of the carb is designated as the "Manifold Vacuum Port" and is the correct one to connect your vacuum advance line to for proper operation.  If there is any question pull the cap/plug off the driver's side port and check for vacuum there when your engine is idling.  Put your finger over the open port and you can feel the suction, in fact, it may idle rough when you first uncap the port because you have created a manifold "vacuum leak".   

 

Another easy way to confirm the correct port is to look at the front base of your Edlebrock carb and notice that the driver's side vacuum port sits a bit LOWER than the passenger side port.  That's because it comes out BELOW the throttle plate where the passenger side comes out ABOVE the throttle plate.   Any point below the throttle plate is reflecting manifold vacuum which is why the manifold vacuum drops to nearly zero at wide open throttle.

 

You can see this clearly in the photos of the various Edlebrock carburetors at the following links: Performer Series and Thunderer AVS Series

 

BTW, you will probably notice an improvement in idling, throttle response, mileage and engine temperature when you switch your vacuum line from the passenger side port to the driver's side port.  You should check your initial (static) timing at idle (with both vacuum ports plugged) as it may be too high once you connect to the manifold port because the vacuum advance will be adding up to 20 degrees of advance at idle (high manifold vacuum).  It should be around 4 degres BTDC static but might be set as high as 16 degrees BTDC now.  Let us know if you see/feel any difference.

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  • 3 years later...
47 minutes ago, sendo said:

sorry for posting on a 4yr old post, but is my vac hoses crossed? should I connect the dis hose to the port with the plug in front?

Sendo,

It appears from your photos that you are running a Holley spread-bore on a stock manifold.  Your distributor's vacuum advance is currently connected to the "timed" (or ported) vacuum port on the metering block of your Holley carb (as shown in your first photo).  If you want to switch to "full" (or manifold) vacuum for your distributor advance the answer to your question is "Yes". 

Remove the black cap from the port at the front base of your carb (as shown in your second photo) and connect your distributor's vacuum line to it.  Then place the black cap on the port where you just removed the vacuum line on the side of the metering block.

As discussed in the thread above, you may or may not have to tweak your initial timing when you switch between timed and full vacuum advance.  I'd just try it first and mess with initial timing only if I got some pinging.  Good luck.

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