You mentioned that the off-idle sneeze when the engine is cold was a problem even before you swapped carburetors. First, you might try adjusting the electric choke to come on stronger and stay on longer. Next, we'll assume the sneeze occurs after the Avenger's electric choke is off but the engine is still somewhat cold. This leads us to think you may need more timing at part-throttle. Your question about vacuum advance is actually right on target. Adding a vacuum-advance distributor, like the new Crane distributor, offers additional timing at part-throttle when the engine needs more timing but only under light load. As you open the throttle, this additional timing gives the engine a better chance at acceleration. A little more initial or mechanical advance at these speeds is almost always a good idea. If you generate too much part-throttle timing, the engine will start to surge at light-throttle. A sneeze through the carb is generally caused by a too-lean condition, which would point to the carburetor, but because both carbs suffered the same problem (and the 650's off-idle circuit could be slightly richer than the 770's), our money's on a more aggressive timing curve with vacuum advance to solve the problem.
What's interesting is that 12 hp should not cost a half second in the quarter-mile. Our guess is that since the secondaries are not opening completely throughout the pass, it equates to a substantially greater torque loss. That would explain why your car is so much slower.
Your question of how much more power you could gain with the AFR 180 heads is a bit academic, since we would work on making your existing combination work to its fullest extent before considering swapping heads. The AFRs appear superior on the flow bench, so you could expect a little more power, but frankly, the Edelbrock heads are so good there is power still waiting to be tapped from your existing combination.
The latest-generation ATFs are generally synthetic-based, which means improved high-temperature lubricity.
Changes in ATF
Steve Anderson, Car Craft reader: I've heard about the problems with engine oil and how they are taking zinc levels out, but I was wondering about automatic transmission fluid. Has that changed and is there anything I need to know for my TH400 trans?
Jeff Smith: We called Quaker State oil engineer Mark Ferner to go over what changes have occurred in automatic transmission fluid (ATF) and what things you should know that would help your transmission survive. According to Ferner, there are certain types of antiwear-additive packages that are added to both petroleum-based and synthetic-based ATFs to protect against gear and pump wear in the transmission. The amount of zinc in engine oil has been reduced because the ash deposits from zinc can find their way into catalytic converters, which tends to reduce the efficiency of the catalytic materials. There's no easy path for ATF into the exhaust, so the antiwear agents have tended to remain the same.
However, ATF formulations have changed over the years. The old Ford Type F and early Dexron spec fluids have been superseded now by much higher-quality lubricants. These new fluids employ slightly lower viscosity levels to improve cold-weather operating performance but can still withstand the much higher operating temperatures prevalent in a performance application.
Ferner offered an example of the kind of high temperatures possible in an automatic from a Raybestos test where surface temperatures of clutches and the adjacent intermediate plates exceeded 800 degrees F. At these localized high temperatures, a lower-quality fluid will tend to evaporate, which contributes to excessive wear. The current Ford fluid is called Mercon V and is compatible with all Ford transmissions built since 1983. GM Dexron III and III-E are for all GM automatics, while Chrysler-specific transmissions that don't require a Dexron or Mercon fluid can be filled with a fluid called ATF+3.
Ferner also mentioned the older Ford Type F fluid, originally spec'd in 1967, does not contain friction modifiers that are present in fluids like Dexron III or Ford's Mercon fluid. While it is true that using an older Type F spec fluid will tend to allow the clutch materials to grab more aggressively, most automatic transmission clutch manufacturers design their clutch materials to accommodate the friction modifiers, and these newer fluids are highly recommended. The latest GM fluid is a Dexron IV version that should be on the market very soon. These latest fluids will be lower viscosity yet with improved additive packages that can handle higher temperatures without suffering significant thermal breakdown.
Despite the newer fluid's ability to withstand higher temperatures, transmission sump temperatures of more than 200 degrees F will still contribute to increased transmission wear and damage, even with these improved fluids. According to Ferner, an ideal temperature range for a typical street-driven automatic is between 175 and 200 degrees F. If your trans is subjected to higher operating temperatures, a more efficient cooler is a wise investment, as would be shortened drain intervals that can perhaps avoid a costly transmission failure. Quaker State has recently introduced a high-performance ATF lineup called Q Power automatic transmission fluid that is a full synthetic offering additional thermal stability as well as lower volatility. This means it won't vaporize as easily when hit with 500hp big-block high-gear shifts.