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Turning a 1955 Chevy Into a Modern Hotrod - No Jive For Your '55

1955 Chevy Modern Hotrod Superflow Flow Bench
On SLP's SF-1020 SuperFlow... 
   
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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.
1955 Chevy Modern Hotrod Pushrod
We used Lunati's pushrod-length... 
   
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1955 Chevy Modern Hotrod Pushrod
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.
1955 Chevy Modern Hotrod Performance Engine Components
The new heads were torqued... 
   
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1955 Chevy Modern Hotrod Performance Engine Components
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.
1955 Chevy Modern Hotrod Rocker Wheel Position
The position of the rocker's... 
   
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1955 Chevy Modern Hotrod Rocker Wheel Position
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.
1955 Chevy Modern Hotrod Aluminum Heads
Tri-Fives with the popular... 
   
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1955 Chevy Modern Hotrod Aluminum Heads
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.
1955 Chevy Modern Hotrod Headers
Ironically, it all went back... 
   
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1955 Chevy Modern Hotrod Headers
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.
1955 Chevy Modern Hotrod Afr Headers
We were stoked to see the... 
   
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1955 Chevy Modern Hotrod Afr Headers
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.
1955 Chevy Modern Hotrod Engine Parts
Next, we replaced the four... 
   
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1955 Chevy Modern Hotrod Engine Parts
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
1955 Chevy Modern Hotrod Rear Wheel Horsepower
The as-cast AFR 305s were... 
   
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1955 Chevy Modern Hotrod Rear Wheel Horsepower
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.

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