The Baseline: Iron D-ports
To establish a baseline for a typical street 455, we located a set of D-ports that would create a mid-9.0:1 compression ratio on our mule motor. This can be a bit tricky, as Pontiac never actually offered a 455 with 9.5:1, having started with 10.25:1 in 1970 and skipping all the way down to 8.5:1, then to 8.2:1, and finally to an insulting 7.6:1 for the last couple years. However, since Pontiac used flat-top pistons in most of its engines and manipulated compression ratio with combustion-chamber volume, dialing in the right ratio is a matter of finding the right heads. In this case, we needed something in the neighborhood of 96 cc's instead of the 124 cc of our original heads. Fortunately, some early '70s 400s have exactly what we needed. Pontiac guys have sought out the casting number 96 heads from a '71 400 two-barrel motor for years, since they have the potential to make about 9.5:1 on a 455. We happened to find a set of 7K3 heads from a '72 400 that also have 96cc chambers. We thought we'd scored, since the newer heads even have heat-treated valve seats for unleaded gas, but then Ken Crocie of H-O Enterprises pointed out that these castings are among several that don't have the outer exhaust bolt holes, or the bosses to drill them, making header use a problem.
But, for the sake of our dyno test the 7K3 castings would still do the job. We took the hideous junkyard score to JMS Racing Engines in Monrovia, California, for a quick cleanup and rebuild. Once reassembled, the 7K3 heads were ready to establish a baseline for a typical iron-headed street 455.
 The 7K3 heads we scored in...  The 7K3 heads we scored in the junkyard had the desirable 96cc chambers we needed for our 455, but they were a mess. After dismantling them, the first step at JMS Racing Engines was the shot-blast cabinet to strip off 30-plus years of baked-on filth, and then Nick Prohaska of JMS handled the task. |  After the shot-blasted heads...  After the shot-blasted heads turned up crack-free, Pete Hillemeyer mounted them in the Serdi machine to cut new seats. While he was there he made some deeper cuts to the bowls to clear and smooth some of the errant iron. The Serdi cutting bits used create a three-angle valve seat in one pass. |  While Pete was cutting the...  While Pete was cutting the seats, Nick was cutting the stock 2.11/1.77-inch valves on a Kwik-Way machine. Then, after checking the relationship between the valves and seats, Nick assembled the heads with a fresh set of mild performance springs and checked the seat pressure, finding 120 pounds on the seat. |
The 455 was returned to the...
The 455 was returned to the dyno, sans rebuild, but with yet another set of heads. The iron 400 heads made a bit over 9.0:1 and gave us no trouble, even with nonadjustable rockers and an aftermarket cam. Still, this many inches should make over 400 hp but didn't . . . yet.
On the Dyno Iron D-ports
The 455 mule was assembled with the 7K3 heads using Corteco gaskets and stock Pontiac head bolts. With the original pistons 0.015 inch in the hole, the 96cc combustion chambers, and the 0.036-inch compressed thickness of the head gasket, compression was right around 9.2:1. The Comp Cams XE274H hydraulic cam remained from our last Pontiac dyno session. The grind features 230/236 degrees duration at 0.050-inch lift and 0.488/0.491-inch lift on a 110-degree lobe separation angle. Stock-type pushrods were used with Pontiac 1.5:1 stamped-steel, nonadjustable rocker arms, mainly because our rollers wouldn't fit the stock studs of the 7K3 heads. Despite the lack of adjustment, the stock rockers seemed to attain the proper preload when tightened to factory specs.
Once the engine was mounted on the dyno at Westech, we topped the Edelbrock Performer RPM intake with a Speed Demon 825-cfm carb. We were about to use a set of "three-tube" cheapie headers since that's all we could get our hands on in time when Ken Crocie of H-O Enterprises came to the rescue with a set of D-port 151/48-inch long-tube headers. As it turns out, Ken's no-frills Summit headers were pretty economical at $99.50 and worked perfectly.
With the MSD Ready-to-Run distributor dialed in, the 455 fired up and was soon making power. After a series of pulls and some tuning, we found the sweet spot with 37 degrees total timing and 83/87 jetting in the Demon. With this combo, we averaged our best runs to obtain a peak power figure of 390 hp at 4,700 rpm and 494 lb-ft at 3,500. Note that the engine was grunting out 450 lb-ft at the start of the pull at 2,500 rpm.