I've got so much initial timing on my motor I'd knock on race gas if I hooked up the vacuum advance. I've had several people comment that ported vacuum is not the same as manifold vacuum because at low throttle positions the throttle blades act as venturis which makes ported vacuum stronger than manifold vacuum. Ported vacuum has been used on pre-emission motors but not too often; it's become common on emission motors as it helps with reducing emissions, but nearly all I've seen used a temperature-controlled vacuum source to switch between manifold and ported vacuum. If I had Jason's car I probably wouldn't run a vacuum advance either. If you've got it you might as well hook it up.
And even though 455ci engines power both, the two combinations are very different. This is called ported vacuum compared to manifold vacuum which comes straight from the intake manifold. Initial timing refers to where the timing is set at an idle. I'm just going to buy one and play with it. What is my idle vacuum? So what advantages does vacuum advance offer? Mechanical advance timing offers better engine performance in high speed applications such as race car driving. I think it came from a blow through ea81.
Test away remember, you can change things back if you don't like them , but you need to test to learn. The first thing you should do is find a shop manual for your particular car. Oh yeah, all the vacuum advance pods I've ever seen were adjustable. Or the advance plate itself? There is a diaphragm inside the canister which is connected to a linkage connected to the pickup plate in the distributor. At part throttle, high manifold vacuum moves the diaphragm in the vacuum advance canister on the distributor to add more timing. You see, lean fuel mixtures burn very slowly and, at cruise, the engine should be approaching a stoichiometric ratio of right around 14.
This means advancing the ignition timing. What we did discover, though, was a smoother transition when reapplying the throttle after coasting at speed, likely caused by the spark lead that manifold vacuum advance maintains while coasting and decelerating. As the throttle opens during higher engine loads, the vacuum diminishes, enabling the timing advance to move back to the base, or original advance. It idles nice though heavy chop from the camshaft , cruises nice, starts right up hot, no dieseling when shut off hot etc. With the engine off, remove the distributor cap and use a vacuum hand pump to operate the advance mechanism. How do I know if I have too much vacuum advance? No, it would not, but in the name of simplicity, most billet, race-oriented distributors leave them out of the mix. Not to start a manifold v.
There are two ways to change the total, adjust the initial advance by turning the distributor or adjust the mechanical advance mechanism. I have long assumed that it's actually nothing to do with the dizzy, and more to do with some strange carb problem which leans it off to the point where the extra advance of the vac unit is too much advance and poor running results, but I have never solved it, just left the vac pipe off and run like that perfectly well. I ordered the Crane adjustable vacuum advance can, but in their directions there are no mention of actual numbers. Total timing is the term given to the highest amount of advance. These statistics all need to be considered when you're looking for the optimum timing setting for your application. Remember that electricity takes the path of least resistance to a ground so if the gap is too large the spark may short to another point with less resistance.
Ago I've continued tweaking my tune to see if I can get better performance or mileage out of this. It comes off the venturi above the throttle plates and is effectively isolated by the throttle plates from manifold vacuum, more specifically the throttle shaft as the defining point much like an expansion valve in a refrigeration circuit. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Bowties are for Pee-wee Herman. The Holley must have one there someplace. 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. Hard to have a universal answer.
You probably won't have enough timing marks to test total advance. This can be accomplished by choosing from different fixed vacuum advance canisters, as most of the adjustable canisters add very little advance at the low settings. You do have to be careful on a street car at high vacuum part throttle not getting too much vacuum advance to cause detonation. The mechanical advance feature then takes control of the ignition advance under those conditions. We just set the timing at start up.
Now he serves as a regular contributor to OnAllCylinders. I've posted similar questions before and it was said that the vacuum advance may be adjustable, but mine does not seem to be adjustable. Guys like that are trapped in thier box and cant see outside it. What kind of timing should I be running and should I disconnect the vacuum advance? How much vacuum are you getting on the advance at part throttle? Short duration, low overlap cammed engines may idle fine on less timing. Unplug the advance and I get about 32 degrees so I interpreted that as the can gives me about 9.
Internet forums are rife with opinions on both sides of the argument. This lets you set the exact amount of vacuum advance that your engine requires. Unlike points, this pickup is maintenance-free and is capable of accurate trigger signals throughout 10,000 rpm. The advance only differs between the two when the throttle is closed. If you search the internet, there are a couple of forum discussions where all these canisters and their values are listed. In this case, you should consider hooking the vacuum advance line to straight manifold vacuum.