Does your stock or mildly modified Pontiac engine idle higher than it should because each time you try to dial it down, it runs rough? If so, this article is for you. We’ll tell you why it’s happening and how to fix it.
No Pontiac mill was ever designed to leave the factory with an excessive idle speed. Rather to the contrary, it was meant to idle very slowly. The hallmark of a refined and well-tuned engine is a low idle speed. The information supplied will be just as effective with a stock Pontiac as a modified one.
What is Slow?
Our subject engine was a '79 Trans Am with a 400 and four-speed manual tranmission.
It’s easy to state that the engine should idle slowly, but a value needs to be attached. The component that has the most impact on the potential idle speed is the camshaft profile. The carburetor is the second determining factor.
If your Pontiac is stock or nearly so, then the factory specification for idle speed should be your first target. Once that is achieved, it’s good practice to try and get the engine to idle about five to ten percent slower than that. For example, if the idle specification is 650 rpm, then the goal may be between 590 rpm and 620 rpm. The caveat is that the engine needs to be as smooth at the lower speed and run as well.
You may ask, “Why should we try to have our engine idle slower than the specification listed by Pontiac?” All car manufacturers suggest an idle speed specification that will mask or make up for a poor state of tune as the vehicle experiences a lack of care during its life. Thus, the specification is slightly on the high side to guarantee a smooth idle regardless of the abuse the owner has given the Pontiac or the lack of maintenance.
A modified Pontiac with any style of performance camshaft will traditionally have an increased amount of overlap. This describes keeping both intake and exhaust valves open simultaneously for a longer period of time, measured in rotational degrees. When overlap is increased, there is a reversion of exhaust gas back into the cylinder bore, which dilutes the incoming charge. The effect mandates both a richer air/fuel mixture (due to the dilution) and a higher idle speed. The propagation of the flame in the bore is not as smooth due to the reversion (the inert exhaust gas disturbs the burn rate). Thus, a Pontiac engine with a modified cam profile may require more fuel at idle but still should be able to run at a slow speed. In these applications it becomes more of a balancing act of finding the proper mixture strength and then decreasing the idle speed. An aside to increased overlap and other factors that are part of a performance camshaft, is that engine vacuum will be lower at idle and light load.
A modern EFI Pontiac, like the LS2 GTO shown here, has no mixture or traditional idle spee
An all-out race engine with a very aggressive cam profile and large ports in the cylinder heads and intake manifold runners will want a higher idle speed (maybe 100 to 150 rpm more) but the goal should still be the same. The mindset needs to be, “How slow can I get this combination to idle?”
A street/strip Pontiac engine should be able to idle at a speed no more than about 100 rpm above its stock specification. As always this is a guideline and will be application specific. But if your engine needs to run at 850 to 900 rpm when the stock specification is 650 rpm, you are out of the ballpark.
Under analysis the required idle speed of the engine is impacted by many factors. The throttle plate needs to be opened sufficiently to produce the required power to overcome all internal engine friction. This is a design area and is one of the three loses in any engine when converting chemical to mechanical energy. Approximately 25 percent of the potential energy in the gasoline (Btu) is used to turn the flywheel. The other 75 percent is assigned to three areas of loss with each taking almost the same amount of energy. These losses are pumping, thermal, and frictional.
The carburetor should always be
kept clean inside and out. Pay
attention to the air-bleed
Tighten up all fasteners that hold the carburetor together and to the intake
manifold to e
The PCV valve needs to be of the proper flow rate. A PCV is a controlled vacuum leak and a