More Performing With Efficiency

With the 1.1 mile/quart reading repeated three times in a row, we knew it was accurate and a good baseline. In that state of tune, the Pontiac produced 199.01 hp at the rear wheels. The mixture was leaner (13.0:1) at wide-open throttle than at cruise. A fresh set of spark plugs were installed since the old ones were exposed to a rich fuel mixture for a long period. They did not look bad, but we wanted accurate results. We were not concerned about the engine oil being diluted with gasoline since it was changed the week before and had only about 10 miles on it. If this were not the case, the oil would need to be dumped since it would be polluted with raw fuel from the rich condition. The AC-Delco plugs were gapped at 0.035 inch.

At this time, we knew the carburetor was the main cause of the high fuel consumption rate, so it was removed in preparation of a teardown and look inside.

Wise went around the intake manifold and snugged-up all of the bolts to assure a tight seal. This is good mechanical procedure that should always be followed.

Bob ordered an AC-Delco tune-up kit, but the distributor cap, rotor, and breaker points were all new. The advance weights were clean and lubricated. An advance kit had already been installed by the owner. We would confirm the ignition lead and modify that aspect of the engine after the carburetor problem was eliminated.

With the carburetor apart, Wise noticed a number of problems. The air horn was warped (not shown). This is common with Q-Jets since most people overtighten the two front carburetor hold-down bolts and twist the casting. Once this happens, it is not repairable. If a new gasket does not work, the only choice is to double up the gasket to try and make up for the warpage. A warped air horn will allow an air leak into the transfer passages and create a rich mixture at cruise while also causing the carburetor to not react to tuning via the mixture screws. In addition, the well plugs, (both primary and secondary) were leaking at one time and were not repaired properly.

The caps epoxied onto the secondary well plugs were so thick the throttle body was not sealing evenly over the main body. Wise employed a file to remove material to allow the two parts of the carburetor to bolt together as they should. He also re-epoxied both the primary and secondary plugs since they were still leaking and adding to our problem. This condition also caused a hard-starting issue after the car sat for a few days.

The primary metering rods were measured, and it was found they were too thin. These rods were 0.041 inch (gauge readings depict a thinner step) and would be replaced with 0.043-inch rods. The main jets were already the smallest the engine could use at 0.072 inch. There was no need to be concerned about the secondary side of the carburetor since the mixture was correct at full throttle.

Bob used an Edelbrock Q-Jet calibration kit for tuning parts. He also changed the power piston spring to a gold one (4 in. Hg.). This delayed the power-enrichment circuit of the carburetor until the engine is under more load. It will also help lean out the mixture at cruise and hopefully provide some fuel mileage gain. The new gasket and internal changes produced a cruise air/fuel ratio of 14.8:1, just where we wanted it, lean of stoichiometric.

With the engine running once more, Wise checked the breaker point dwell, and it was right on at 30 degrees.

Cruising at 3,000 rpm at 65 mph, the ignition timing at that speed was quantified. The Pontiac had 40 degrees total advance (with the vacuum advance hooked up) at 65 mph. With the carburetor tuned up, we were able to go just under 1.5 miles/quart of gasoline. We then tried advancing the timing five degrees to 45 and the fuel consumption went back up to 1.4 miles/quart. The engine ending up liking a total of 38.5 degrees at 3,000 rpm (with the vacuum advance hooked up) and rewarded us with a little more than 1.5 miles/quart. This was a whopping 37 percent decrease in fuel consumption. The gain was without employing any of the other tricks represented in Part I, such as synthetic engine, transmission, and differential lubricants; reduced aerodynamic drag; wheel alignment; and brake drag; along with playing with fuel brand and octane.

The Trans Am's owner was happy with the results. Given all of the data, a fuel-mileage estimate on a highway trip in the baseline tune would be around 12 mpg. The 4.4 mpg measurement was due to the short test cycle that was under acceleration for almost one-half the time. A 37 percent increase would yield 16.44 mpg on the highway. By employing the other tricks HPP spoke about, we are confident with some careful driving technique this 400 can return more than 19 miles for every gallon of gas pumped into the tank. That would be a 58 percent improvement over its previous consumption rate. With premium fuel selling for around $2.75/gallon at the time of testing, the cost per mile to drive the "tuned" F-body would equate to $1.73/gallon gas over the way it was before. With an 18-gallon fuel tank, it would be an increase in range of 126 miles on every fill-up. We cannot make the gasoline any cheaper, but we can make it last longer.

Engine Speed (RPMx1000)
(Green) Baseline: Run Conditions: 61.83*F, 30.27 in-Hg, Humidity: 17%, SAE: 0.95, Max Power 199.01
(Red) After Tuning: Run Conditions: 64.75*F, 30.20 in-Hg., Humidity: 7%, SAE: 0.95, Max Power 211.08
This dyno graph shows horsepower and A/F ratio before and after the tune. Not only was fuel efficiency drastically increased, but horsepower went up 12 at the peak--even more importantly, it increased by 26 ponies at lower engine speeds. Peak engine torque increased 3 lb-ft (from 292.82 to 295.81). Fuel economy and improved power can coexist.