Jul 21, · There are two different therories of vacuum advance sources - carburator port advance and manifold vacuum advance. We explore both locations so you can decide what is best for your Pontiac engine. Jan 02, · I also agree the vacuum advance is a worthwhile feature. If you've got it you might as well hook it up. The vacuum advance won't add any power at wide open throttle but like the other guys said it should idle better and give you better fuel economy.
I stripped a distributor gear PO installed a brass gear instead of a cast. So while I had it apart, I put a new Oil pump in, sent the distributor in to get a new gear installed. While they MSD had encounters dating username ideas distributor they put a new vac advance on it and replaced the pickup. While I had it apart I pulled the intake to jin dating rumors a new gasket on it. Now the problem, I got everything back together, timed it at 12 degrees, dialed avcuum the carburetor, all of this with vacuum advance hook up vacuum advance off. When I finished I put the vacuum advance line back on from the ported side of the carb passenger sidethe engine stumbled and back fired at idle, unplugged it and it runs fine.
The amount of advance added by the distributor, combined with initial static timing, is "total timing" i. 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.
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, 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. 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 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs. After 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.
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 degrees "total timing" at high rpm wide-open throttle to get some of the performance back.
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