Advancing Opportunity - Effects of Vacuum Advance on Pontiac Engines

The HP-series carburetors... 

   

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The HP-series carburetors from Holley don't contain a ported-vacuum source, so we sent ours to Cliff's High Performance in Mount Vernon, Ohio, where one was added. Located just above the throttle blades, our vacuum gauge reads the same vacuum level as a manifold-vacuum source once uncovered.

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.

Other than the camshaft, the... 

   

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Other than the camshaft, the only non-original component on our '72 Trans Am is this rare '71 455 H.O. Quadrajet. For increased airflow, these unique units lack the traditional booster rings found in other Q-jets. Like many carburetors of this era, its idle circuit was already lean to combat emissions, but once we added a slightly larger camshaft, the mixture screws had little effect. So it was sent to Q-jet guru Cliff Ruggles for recalibration.

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 tapped into the ECM of... 

   

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We tapped into the ECM of our '01 Trans Am to see if tuning practices had changed with current technology and emissions regulations. Using an OBD-II-compatible diagnostic-scan tool, we found that at normal operating conditions, our LS1 idled with 22 degrees of advance, and it peaked at 38 degrees at part throttle. Consider that the cylinder heads of these engines are designed for the quickest possible burn, as you can see in this LS1 chamber shot with its heart-shape and spark plug biased toward the exhaust valve, among other features. As a result, maximum full-throttle performance requires just 28-30 degrees of advance.

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.