Turning a 1955 Chevy Into a Modern Hotrod - Air Fuel Mixture Ratio

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1955 Chevy Modern Hotrod Superflow Flow Bench

On SLP's SF-1020 SuperFlow flow bench, the AFRs, stock 990s, and ported 291s went head-to-head in a flow comparison test. The iron and aluminum heads were flowed with a realistic bore size fixture of 4.280-inch (typical 427 or 454, .030 over). Most BBC heads are flowed using a 4.600 bore. A larger bore unshrouds the valves to help show better flow numbers. Surprisingly, our flow bench results (with the smaller bore) were similar at the lower (.200-.400-inch) lifts but not as much at high lifts (.500-.700-inch). Still, the AFRs flowed so much more cfm at all lift values (see the charts) than the old castings that we couldn't wait to see the difference on the dyno.

On SLP's SF-1020 SuperFlow flow bench, the AFRs, stock 990s, and ported 291s went head-to-

Dyno Time
In Part 1, the five upgrades upped the ante to 402 rear wheel horsepower at 5,400 rpm and 415 lb-ft of torque at 4,800 rpm (this month's baseline). Once again at SLP Performance Parts we met up with Ken Estell (engineering fabricator) and Will Seppelt (product design engineer) and strapped down the Shoebox. Hank Daneicki (director of engineering) handled the dyno-driving duties.

Hank put the hammer down and we witnessed 452 rear wheel horsepower when he let off the gas at 6,200 rpm (this was the shift light setting). On all the previous pulls Hank was letting up at 6,200 since the previous peak power was at 5,400 rpm. Now at 5,400 rpm, the AFRs helped produce 35 more horsepower, and at 6,200 rpm the power was up by 79 hp! With more pulls to come, we didn't want to risk revving the old Rat too high.

During initial back-to-back pulls we noticed the air/fuel mixture ratio was too lean at 13.5. We reached into our Holley jet box for size #82 jets to replace the Holley 950 HP's stock #79 jets on all four corners. The bigger jets added response and three rear wheel horsepower. Still, the A/F was too lean at 13.3 to 13.4. Next, we swapped the high-speed air bleeds (#32s) for a richer set (28s). This brought the A/F to a safe but slightly rich 12.6 along with consistent power output. If we'd tested on a cool day at 65 degrees Fahrenheit, the A/F would have been close to an ideal 12.9. It was not conducive for us to further try more tuning on this extra hot and humid day. Besides, after realizing 82 rear wheel horsepower at 6,200 rpm from a head swap, we were content to leave well enough alone.

Everyone's heard the saying, "It's all in the combination." Selecting the right parts is usually a phone call to the company's tech line. Be thorough with the specifications of the motor, driveline, vehicle weight, and intended purpose. This big-block responded favorably to its new upgrades, and Joey D enjoys driving his No-Jive-Five-Five more than ever. The head swap certainly helped the whole combination come together to become a serious street/strip contender.

We used Lunati's pushrod-length checker (PN 80121) to determine the proper-length pushrods. The AFRs will usually require pushrods anywhere from .080-.200-inch longer than stock. This must be checked for correct rocker arm geometry. Carefully follow the instructions.
We used Lunati's pushrod-length checker (PN 80121) to determine the proper-length pushrods
The new heads were torqued in three steps to 35, 55, then 75 ft-lb using ARP head bolts. Notice the adjustable-length pushrod and rocker in place and ready to help us determine the correct pushrod length. Again, be certain to check proper pushrod length due to varied deck heights, cam base circle, head milling, etc.
The new heads were torqued in three steps to 35, 55, then 75 ft-lb using ARP head bolts. N
The position of the rocker's wheel is important. The wheel needs to roll across the top of the valve tip. At midlift the bottom of the roller wheel should be at the middle of the valve tip. Seen here, the wheel is at the first third of the valve tip at zero lift. Also, be sure there's at least .030-inch rocker to retainer clearance. At full lift, check that coil bind clearance is more than .060-inch.
The position of the rocker's wheel is important. The wheel needs to roll across the top of

Tri-Fives with the popular "605" power steering box will encounter header clearance issues using the latest aluminum heads with raised exhaust ports. We tried 2-inch headers made to fit with stock heads, but they would not fit with our new castings. The 2-inch tubes also rested against the bellhousing area of the block and trans, creating a N-inch gap between the header flange and the head. AFR's Tony Mamo recommended we use 2-inch (minimum) or 2 V-inch tubes for the higher-flowing heads to produce optimum power.
Tri-Fives with the popular "605" power steering box will encounter header clearance issues
Ironically, it all went back together with the '80s Blackjack 1Y-inch headers (no longer produced). Even though we couldn't use the right size headers, at least this would be a true head swap-only test-for now. Perhaps in a future installment we'll try larger tubes.
Ironically, it all went back together with the '80s Blackjack 1Y-inch headers (no longer p
We were stoked to see the AFR heads deliver peak-to-peak gains of more than 50 rear wheel horsepower. The stock heads produced max hp at 5,400 rpm, while the AFRs were still climbing at 6,200 rpm when the tach light lit, causing us to automatically lift. At 6,100-6,200 rpm the AFRs were making nearly 80 more rear wheel horsepower. On the first few pulls we noticed the A/F mixture was too lean at 13.5 (12.9 to 13.0 is usually ideal). We came armed and ready for tuning with boxes of Holley tuning parts.
We were stoked to see the AFR heads deliver peak-to-peak gains of more than 50 rear wheel

Next, we replaced the four high-speed air bleeds (they're the four inner screws; the four outer screws are for the idle/cruise circuit). The stock #32s were replaced with #28s, which made the A/F mixture safer but a little too rich at 12.6. Throttle response was better and power was maintained, but we felt it was counterproductive to try further tuning on this hot and humid (90-degree) August day. At that we called it a day and went home for supper
Next, we replaced the four high-speed air bleeds (they're the four inner screws; the four
The as-cast AFR 305s were definitely worth the effort and expense after realizing a peak-to-peak power jump of more than 53 rear wheel horsepower. At 6,200 (we let up at 6,200 for the baseline) there was an additional 82 rear wheel horsepower. Starting with the Part 1 baseline (329 rear wheel horsepower) to after testing the six upgrades (cam, intake, carb, spacer, 18 volts, and heads), the 454 picked up 126 horsepower to the back tires (455 total). The total combination (especially after the head swap) produced a powerband that kept pulling up the rpm scale after the stock heads had power dropping.
The as-cast AFR 305s were definitely worth the effort and expense after realizing a peak-t