This is in response to two recent situation on the forum where engines were running hotter than expected. This thread will support the argument that having vacuum advance set right can make the difference between a cool running engine and one that creeps up in heat. I hope it helps. bro. d The Effect of Vacuum Advance on Cooling http://ampperformance.com.au/technical/overheating-modified-engines/ Vacuum advance can help cool a hot running engine by giving the engine a little extra ignition timing advance under light load conditions that goes away under higher loads. In most cases when you are using a "modified" mechanical advance curve along with a vacuum advance, the amount of additional advance from the vacuum advance should not exceed 10 degrees and not be in before 10 inches of vacuum. http://www.rootesparts.com/id201.htm There is a lot of misinformation about ignition timing and cooling. Retarded timing contributes to overheating. Advanced timing helps cooling. Bump up your initial timing a few degrees and see if it helps the car run cooler. It's an easy and practical fix. Of course, if you advance enough to enter pre-ignition or detonation you will start to overheat. Detonation contributes to overheating. If you start to detonate back off the timing. Overheating cars should always run vacuum advance. Vacuum advance helps cooling. http://www.superchevy.com/how-to/ad...know-about-vacuum-advance-and-ignition-timing There’s a tiny silver can on the side of most distributors that is easily the most misunderstood component of any distributor-based ignition system. Feared by many, and ignored by many more, the vacuum advance can is an important component of your ignition platform that offers both performance and economy. Leaving it unplugged is akin to throwing free engine efficiency straight down the drain. To fully understand why the vacuum advance can is a necessity in any street-going car, we need to dive into spark timing as a whole and cover some ignition basics. http://www.v8buick.com/index.php?threads/overheating-comments.184544/ Many of us, especially those of us with drag racing experience have opted for a mechanical advance only distributor. For drag racing, they work great, but on the street? Another matter entirely. There's a ton of cars out there running these distributors on the street, their cars overheat at idle and in traffic, but they figure that's the price to pay when you have a hot rod. You need to run a vacuum advance distributor on the street. Extra advance is required at idle and low speed to burn the lean idle/low speed mixture. Along with this, the vacuum advance can needs to be sourced to full time vacuum. Granted, factory engines ran ported vacuum, but what most don't realize is that GM developed the ported advance system to force the engine to run hot on the low end. Done for better emissions. They did set the engines up with a temperature controlled vacuum switch that swapped the vacuum source to full time vacuum when the engine got overly hot. Once it cooled down, it was back to ported vacuum. An ok system I suppose, but when the engine runs that hot there's no cushion when running hot days with a heavy load and starting at the bottom of a long grade. *** Part of the confusion about vacuum sources may be the Carter/Edelbrock carburetor instruction sheet that points to the ported vacuum bib on the front of the carb as the vacuum advance line. (Ported bib is on the right and full time bib on the left as viewed from the drivers seat.) Simply selecting the vacuum source to full time vacuum can take an engine that overheats in 5-10 minutes on a summer day to one that will idle through the In and Out Burger line at now more than 192* on a 100* F + ambient temp day. A simple swap that yields big results. http://www.vetteclub.org/warehouse/tech/Ignition/distributor.pdf Background The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response due, in part, to the slower flame travel in the combustion chamber under these conditions. Once the engine load increases, this “overadvanced” condition must be eliminated to produce peak power and to eliminate the possibility of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well. Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed. The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec: How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches of Mercury (expressed as inches Hg), a vacuum advance control unit that needs 18 inches to peg out would be a poor selection. How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit at cruise. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance. Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times. Ported vacuum was used as an emissions method control to retard timing at idle (by eliminated vacuum advance) in order to reduce hydrocarbon emissions. Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles. Running straight manifold vacuum to the vacuum advance control unit is recommended for most applications where emissions are not an immediate concern. Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we use it to just “tweak” the car a little bit to provide that last little bit of optimization for drivability and economy. The vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide open throttle.