The basically stock 455 H.O. in our ’72 Trans Am had always ran satisfactorily, but when it recently sprung a coolant leak, we yanked it out from under the hood and affixed it to an engine stand for proper assessment.

From its appearance—with the exception of a few corroded frost plugs and maybe a few gaskets—everything appeared issue-free. As you learned in the past two issues of HPP, it was during disassembly to replace those gaskets that we found several areas of concern, and so a full rebuild was underway.

Until that point, the 455 H.O. had given us no indication of any problems. We flogged it on the chassis dyno several months earlier, and it seemingly trotted along like a thoroughbred. After the rebuild, we took the opportunity to measure its performance on the engine dyno, and while we were certain performance had improved considerably, we could only guess at how much since we only had numbers from a chassis dyno.

With the 455 H.O. nestled between the framerails again, we found it a completely different animal. Simply stated, it operated significantly better than ever at every speed. It accelerated smoothly and cruised effortlessly in light-throttle conditions. And like any good-running 455 owner can relate, applying throttle too quickly resulted in clouds of tire smoke.

A question still gnawed at us, however. How much power did we actually gain from the rebuild? The only way to determine this answer was with a follow-up chassis dyno session.

HPP Engine Buildup Worksheet

  • Engine Displacement: 468.8 ci
  • Horsepower: 433
  • Torque: 517 lb-ft
  • Bore/Stroke: 4.195/4.25 inches
  • Engine Built By: Willard Auto Machine

BLOCK

  • Block Description: ’72 code-YE 455 H.O.
  • Preparation: Clean and inspect, soda-blast, bore and hone cylinders to 4.195, align-bore
  • Deck Height: 10.220-inch
  • Crank: Eagle forged 4340-steel, 3.25-inch main journals, 2.2-inch rod journals, 4.25-inch stroke
  • Preparation: ESP Armor finish
  • Balancer: Powerbond stock replacement
  • Connecting Rods: Eagle forged 4340-steel, H-beam, 6.8-inch length
  • Bearings: Sealed Power on mains and rods, Dura-bond for camshaft
  • Pistons: Ross, custom design for Butler Performance spec, 4.191-inch, 8.5cc valve relief volume
  • Wristpin: 0.990 in, full floating
  • Piston Rings: Total Seal
  • Deck Height: Zero
  • Rotating Assembly Balanced: Yes

OILING SYSTEM

  • Windage Tray: Tomahawk Replacement
  • Oil Pump: Pro-Series from Butler Performance, approximately 70 psi
  • Oil Pan: Canton stock replacement

HEADS

  • Casting: ’72 No. 1972 455 H.O. Round-Port
  • Preparation: Clean, install new valveguides, machine
  • Valves: Ferrea stainless-steel, 2.11/1.77-inches
  • Valve Seat Angles: Multi-angle with 30-deg intake and 45-deg exhaust seats
  • Valvesprings: Comp Cams 987-16
  • Valvespring Install Height: 1.8 inches
  • Valvespring Pressure: 125-psi seat, 325-psi open
  • Valve Retainers: Comp Cams 740-16
  • Valve Locks: Comp Cams 614-16
  • Rocker Studs: ARP 7⁄16-inch
  • Rocker Arms: Comp Cams cast-alloy roller, 1.5:1 ratio
  • Combustion Chamber Volume: 100 cc
  • Compression Ratio: 9.2:1
  • Intake Port Volume: 170 cc

Air Flow Numbers at 28 Inches:

Before Rebuild

  • Lift Int -- Exh

  • 0.050 -- 41/24
  • 0.100 -- 74/51
  • 0.200 -- 139/97
  • 0.300 -- 190/133
  • 0.400 -- 209/149
  • 0.500 -- 212/160
  • 0.550 -- 213/165

After Rebuild

  • Lift Int -- Exh

  • 0.050 -- 43/27
  • 0.100 -- 81/57
  • 0.200 -- 153/123
  • 0.300 -- 200/155
  • 0.400 -- 214/168
  • 0.500 -- 222/175
  • 0.550 -- 223/177

CAMSHAFT

  • Brand: Comp Cams custom hydraulic roller
  • Duration at 0.050: 224/236-deg
  • Duration Advertised: 275/287-deg
  • Lift with Specified Rocker Arms: 0.502/0.520-inch
  • Lobe Separation Angle: 113-deg
  • Intake Centerline: 109-deg
  • Lifters: Comp Cams hydraulic roller
  • Pushrods: Smith Brothers 8.65-inch length
  • Timing Chain: Federal Mogul CTS-3112R

INDUCTION SYSTEM

  • Carburetor: ’71 455 H.O. Quadrajet, #7041268, 828-cfm
  • Primary Jets/Rods: 74/43
  • Secondary Rods: CV 0.0527-in
  • Secondary Rod Hanger: Stamped “R”
  • Fuel Pump: Carter M6405
  • Intake Manifold: ’72 455 H.O., No. 488945 cast-aluminum

IGNITION

  • Distributor: Pontiac points-type No. 1112126
  • Points: NAPA CS-89
  • Distributor Gear: BOP Engineering composite
  • Coil: Stock ACDelco
  • Wires: Stock-replacement ACDelco
  • Spark Plugs: ACDelco No. R45TS
  • Total Timing: 34-deg
  • RPM Total Timing Is Reached: 3,200

EXHAUST

  • Exhaust Manifolds: Reproduction round-port by Ram Air Restorations with 2.5-inch outlets
  • Exhaust Pipes: 2.5-inch
  • Muffler: Reproduction crossflow-type from Waldron Exhaust

GASKETS

  • Brand: Fel-Pro

VEHICLE SPECIFICATIONS

  • Year/Model: ’72 Firebird Trans Am
  • Weight With Driver: Approx 3,850 lbs

We regularly tune our Pontiacs using the Mustang Dyno chassis dyno at H.S. Automotive and Dyno in Omaha, Nebraska. The model MD-1750 unit it uses contains a 2,500-pound roller drum and an eddy-current brake that applies a varying amount of resistance that replicates the vehicle weight and load the engine sees accelerating down the street or race track. The power at the tires that spins the roller drum is quite usable when tuning for peak output, but trying to convert that number to actual output at the crank for the sake of discussion can be tricky.

There are a number of power-robbing accessories, as well as driveline components like the transmission and differential, and a full exhaust system, that negatively affect gross engine output. As such, numbers recorded at the tires are generally much lower than those recorded at the crankshaft. Since those components and combinations can vary greatly from one vehicle to the next, it’s difficult to accurately state how much power is lost from equipment that would apply to all vehicles. The owner/operator of this particular Mustang Dyno unit told us that he has previously estimated this parasitic loss at about 25 percent for our particular Firebird, and we would later find at least that and a little more.

Pontiac offered its Round-Port 455 H.O. for ’71 and ’72. While there were a few subtle differences between the model years, Pontiac published a gross power rating of 335 hp and 480 lb-ft for ’71, and it’s safe to say that number is reasonably close for ’72. (From ’72 forward, Pontiac published brake horsepower, not gross horsepower!)

The 455 H.O. in our ’72 Trans Am was essentially nothing more than a stock rebuilt engine fitted with a larger-than-stock hydraulic flat-tappet camshaft. Our initial dyno session yielded a maximum of 262 hp at 4,600 rpm and 327 lb-ft at 3,700 rpm at the rear tires. Doing the math, our relatively stock 455 H.O. was likely generating around 350 hp and 440 lb-ft at the crankshaft, numbers that seemed reasonable for the combination.

The rebuild gave us a chance to freshen up many areas, and add components such as a forged rotating assembly with a stroker crankshaft and a hydraulic roller camshaft. The details of the build were thoroughly covered in the two preceding issues of HPP; the current combination with complete specs is outlined elsewhere in this story.