Advancing Opportunity - Effects of Vacuum Advance on Pontiac Engines

Modifying The Carburetor
Whenever an engine is modified to increase power, the carburetor needs to supply additional fuel. Although most are familiar with the metering jet and/or rod changes that affect the primary and secondary circuits, many often overlook the idle circuit. In instances where engine modifications are limited, the original carburetor may supply just enough idle fuel to get by. But in today's world where hobbyists commonly install smog-era carburetors onto high-performance engines that deviate greatly from any original application, idle mixture is oftentimes severely lean. This may be why excessive amounts of manifold-vacuum advance seem effective in certain applications.

Knowing that the idle circuit of the Quadrajet on our '72 Trans Am had never been modified, we sent it to Cliff's High Performance in Mount Vernon, Ohio, for proper calibration. Upon its return and reinstallation, we found that the mixture screws had a greater effect on overall idle quality, throttle response noticeably increased, and the transition from idle circuit to primary was less noticeable. But we found something else while verifying the static initial setting that was totally unexpected.

When the vacuum-advance unit was disconnected, idle speed decreased slightly, but actual idle quality changed very little. It appeared that the additional idle fuel allowed the mixture to burn more efficiently with less spark lead, and that manifold-vacuum advance was not totally necessary. This prompted us to readjust the carburetor settings and reconnect the vacuum-advance unit to ported vacuum for further testing. The result included a strong, stable idle and seamless transitions without any effect on economy.

What We Learned
After logging several hundred miles with both Firebirds in their final states of tune, we are completely satisfied with the results. We were able to tame a radical camshaft and dramatically improve temperament with manifold-vacuum advance in one combination. In these instances, the long-duration camshafts required for high-rpm operation have excessive valve overlap, which tends to dilute the incoming mixture with residual exhaust gas. Additional spark lead provides extra time for a more complete burn of the mixture in each cylinder.

We also found that relatively stock combinations with limited modifications do not always benefit from manifold-vacuum advance, especially if the carburetor's idle circuit has been modified. It appears that in these instances manifold-vacuum advance will not always improve street manners but can smooth idle. Ported-vacuum advance can then be used to improve part-throttle response and overall fuel economy. If the original carburetor is unmodified or a different carburetor is used entirely, manifold-vacuum advance will likely provide noticeable improvements in all operating ranges.

Conclusion
There is one constant generality about vacuum advance that likely applies to every combination: A suitable amount (whether manifold or ported) can noticeably increase part-throttle response without any negative aspects. But because most of our engines are modified in some manner, neither form is necessarily "correct" for any vehicle. Since vacuum advance offers such distinct advantages for any car driven on the street, we suggest experimenting with both forms to determine which works best for your specific application. Once you find the right combination, you'll wonder why you waited so long to perform your own vacuum-advance experiment!

Special thanks to Jim Hand for his assistance.