Vacuum SucksIn "All About Advance Curves" (Sept. '03), under vacuum advance, you stated that ported-vacuum decreases under load. I guess that load means wide-open throttle. Well, I know that manifold vacuum decreases with the throttle opening, but ported vacuum should increase with air rushing past the port. Am I confused or are we both confused? I read the article three times and it doesn't make sense to me. Aside from running your sweet black epoxy-primered Falcon into some poor guy's Road Runner, you guys are the best.M. SaulEureka, CA
Let's see if we can make this a little more clear. It's true that running air across a port will create a vacuum in that port, but that's not why ported vacuum is different from manifold vacuum. The source for manifold vacuum is, amazingly, the intake manifold. As the throttle opens and load increases, the vacuum decreases until at wide-open throttle (WOT) you have close to zero manifold vacuum. Ported vacuum still uses manifold vacuum as the source, but this passage only has vacuum present after the throttle blades are opened slightly. The reason for this is to apply vacuum to the advance canister only after the throttle is opened and not at idle. Otherwise, you would have full vacuum advance at idle when it isn't necessary. If you hook a vacuum gauge up to the ported vacuum source, you will read zero vacuum at idle, but then the vacuum will quickly jump up to manifold-vacuum level as soon as the throttle is opened slightly. This is because the ported vacuum source is located just above the leading edge of the throttle blade. Once the throttle is opened, the ported vacuum hole is uncovered and "sees" manifold vacuum. Hope that helps you out.
How Special Is "Special"?I have an '84 Camaro, and on the left side of the rear bumper is a badge that reads "F41". Is this car rare? It seems to be a plain car with a 305ci V-8, manual windows, automatic trans, A/C, and power steering. What's the deal with F41?Les JonesPitman, NJ
The F41 designation is a General Motors assembly code for "Special Performance Suspension." This code has been used for decades on numerous GM models (not just Chevys), and is typically the heavy-duty/handling suspension package. On your Camaro, the package should have included heavier-rate springs, slightly stiffer struts and shocks, a larger front antisway bar, the addition of another bar on the rear, and close-ratio power steering. Although this would be considered a fairly desirable option, it doesn't exactly make your Camaro rare, although most F41 Camaros we've seen from that era were running the 2.8L V-6, so you've got a leg up on them. Not to burst your bubble or anything, but Cavaliers from that same period could also be badged as having F41 suspension.