Engine Tuning Guide - Speed Tune Your Street Machine

We swapped in a K&N 4-inch-tall, 14-inch-diameter air filter element to replace the paper element filter that was creating a rich mixture because the filter was a restriction. By adding the free-flowing K&N, the A/F became more stable at the top end of the track.

We swapped in a K&N 4-inch-tall, 14-inch-diameter air filter element to replace the paper

The second option for leaning out the idle circuit was to experiment with the idle air-bleeds. One of the advantages of the Holley 950 HP carburetor (or any HP-bodied Holley) is that the idle and high-speed air-bleeds use screw-in jets that make them very easy to change. Since these bleeds meter air, they operate the opposite way that fuel jets do: increasing the air-bleed leans the circuit, while decreasing the size of the bleed richens the circuit. To lean the '55's part-throttle A/F ratio, we elected to first increase the size of the stock 72 idle air-bleed by five numbers to a 77. This also gave us more adjustment in the idle circuit, and we were able to now keep the in-gear, idle A/F ratio around 13:1.

WOT Tuning
Once we had the '55 running crisper on the street, it was time to attack WOT tuning at the track. Luckily, our tuning efforts occurred during the winter months in the high desert when a typical Saturday morning is not nearly as stifling as in the summertime. Our first day at the track netted a 10 a.m. report of 4,374 feet of density altitude (DA) using our PerformAire weather station. Considering the track is at 2,800 feet, this was not bad. Right out of the box, Greg's '55 yanked the left front tire and cranked out a solid 11.53 at 115 mph. This was a half-second better than before with only the fuel-delivery improvements, plus a somewhat better weather day.

From our previous data logging closer to sea level, we assumed the A/F would be even richer at this higher elevation. We also employed the Innovate meter to data-log each pass and again assumed that a rich 11:1 A/F would require leaning the carburetor to pick up power. From the dragstrip-test results chart on page 44, you can see that leaning or richening the A/F from our initial point only slowed the car down. The lesson here for tuning is that we always watch the mph to measure the results of our tuning changes. The e.t. can be affected by traction and other variables, so trap speed is a much better indicator of horsepower. By the end of the day, the DA had increased, and even with the same jetting, the car was significantly slower, so we packed up and went home.

Despite the fact that the A/F appeared rich at the top end, the A/F during the launch appeared lean. When we increased the primary jet size, trap speed improved. Also note the white "whistle" located on the top of the metering block (arrow). This is connected to the vent tube to prevent fuel from sloshing out of the primary bowl and into the carb.

Despite the fact that the A/F appeared rich at the top end, the A/F during the launch appe

The following Saturday, our PerformAire weather station delivered great news. The DA was a shockingly low 3,234 feet at a cool 59 degrees F. We had spent some time the week before studying the A/F trace from the Innovate data logger, and it appeared that the engine ran progressively richer as it moved down the track. Greg was running a 3 1/2-inch-tall paper filter element, so we substituted a 4-inch-tall K&N for the first pass of the day. The '55 ran its best ever 11.42/117 mph, and we felt we were really going to get the car into the corrected 10s, but it didn't happen. We tried a slight lean out of the high-speed air-bleeds, but that only hurt mph. Next, we richened up the primary side, and most of the mph came back, so we tried jets that were one step richer on the primaries, and that helped the mph even more. The next pass killed the sprag in the TH400 trans, and we were done for the day.

Evaluation
We learned some interesting stuff from all the abuse heaped upon Greg's '55. First and foremost, the Innovate A/F meter was an important element in showing us what the A/F ratio was doing throughout each run. But with this information also comes a warning. It's easy to become immersed in the data-log information. During the last half of each run on Greg's '55, the data indicated a very rich mixture-as fat as 11.8:1 A/F ratio at the top end. We changed the air filter and that helped. Changing the high-speed air-bleeds didn't help like we thought it would, and that led us to consider that despite the apparent rich A/F ratio, the trap-speed data were telling us the engine liked to run best at around a 12:1 mixture. Every effort we made to lean out the top end only slowed the car down. The point is that if the car is performing despite the fact that the data don't jibe with your theory, believe the trap-speed numbers and come up with a new theory.

After roughly eight runs down the track on two different days, the combination was very close to ideal. We did improve how the big Chevy runs with better part-throttle response and a cleaner-running engine that is knocking on the 10-second door.