Thursday, April 6, 2017

The Story of an Old Firebird, Part 2....

The base engine that came with the car when I ordered it was a 400CID 4 barrel lump rated 230HP @ 4400RPM and 325lb-ft @ 3200. It had an advertised 8:1 compression, “Big Valve” (2.11 Intake, 1.66 Exhaust) heads, a 750cfm QuadraJet, and dual exhaust.


Heads and Cam -

Pontiac used many different heads over the years, but after about 1967, and up through the end of Pontiac engine production, the biggest changes were in the combustion chamber volume, and whether they were “D Port” or “Round Port” heads, referring to the shape of the exhaust ports. The “D Port” heads were the standard heads, redesigned for better flow in late 1967, while the “Round Port” heads were high performance only, and were first seen on the 1968 “Ram Air II” engine. The Round Port heads would go through several designs, culminating with the 1973/74 Duper Duty 455 heads, the best heads Pontiac ever released to production. There were some other differences, like whether they had screw-in rocker arm studs, or pressed-in rocker arm studs, with the 4-bbl engines almost universally getting the screw-in studs. The porting was pretty good on the “4X” heads that came on the car, and except for the large combustion chamber to lower the compression, they were very good street heads for 1973. One change Pontiac made in 1973 was to reduce the exhaust vale size from 1.77” to 1.66”, primarily to cut down on exhaust flow out of the combustion chamber, allowing them to use less recirculated exhaust gas in the EGR system.

The cam used was the “067” camshaft. This was almost a “performance” cam, as the next one up was the fabled “068” cam that was originally used in the “TriPower” GTO motors. The 068 cam definitely had a “rumpity-rump” idle, and the 067 had just a trace of it. I helped a buddy put an 068 cam in his ‘74 T/A, and he called it his “Mini Super Duty”.

The 068 cam had an advertised duration of 288* Intake, 302* Exhaust, and a “Duration at .050” of 212/225. Valve lift with 1.5:1 rocker arms was .408”/.407”, and it was rated as being good for “Idle to 5,000RPM”.

The 067 cam had an advertised duration of 273*/289*, a duration at .050” of 200*/213*, with a valve lift of .408/.407. It was rated as being good for “Idle to 4500RPM”.

One thing to note is that all Pontiac “performance” camshafts had about 10* more duration on the exhaust side. This was used to overcome the inefficiency in the stock Pontiac exhaust ports. If you look at a cross sectional view of a Pontiac cylinder head, you’ll notice the exhaust gas has to travel quite a distance inside the cylinder head from the combustion chamber to the exhaust manifold flange, almost 180*. The extra duration on the exhaust side helps to alleviate this restriction by allowing the exhaust gas more time to exit the chamber.

The first time I ran the car with open exhaust I was amazed at how freely the stock engine pulled to 6,000RPM, which is getting a bit scary with the stock Pontiac cast “ArmaSteel” connecting rods.


Intake and Exhaust -

The stock engine had a 750cfm Rochester QuadraJet carburettor on a cast-iron intake manifold. Except for the facts that it was a) cast iron, and heavy, and b) it had an EGR system, this was a “good” intake manifold, as “good” as any earlier Pontiac stock 4-bbl intake manifold with the exception of the aluminum “455 H.O” intake manifold.

The exhaust manifolds were terrible. A simple cast-iron “pipe” (a “log style” manifold) bolted to the cylinder heads. They were heavy, and didn’t flow very well. The OEM cross-flow muffler at the rear of the car was generally considered to be an effective “cork”, even though it had 2.25” pipes to and from it. A fairly quiet muffler, with a decent sound, but pretty restrictive.


Block and Rotating Assembly -

In 1973, all Pontiac 400 CID engines had two-bolt main bearing caps. Pontiac blocks were pretty beefy, and even though the main bearing caps “only” had two bolts holding them down, they also had large steel dowel pins pressed into the block that the caps mated to. This kept the caps from moving around under high RPM/high load situations, and made the blocks with “two-bolt” mains nearly as good as the ones with “four bolt” mains.

The stock crankshaft was nodular cast-iron and very durable.

The stock pistons were cast aluminum and very durable.

The stock connecting rods were cast steel (“ArmaSteel”) and not so durable. They were generally considered “safe” for ~6,400RPM in a 400, but were pretty much guaranteed to fail at that speed in a 455. The extra HALF INCH (actually .461”) of stroke in a 455 pushed the stock rods well beyond their design limits, and they’d snap.

The stock flywheel was cast-iron, and weighed about 40 pounds. The stock clutch disc and pressure plate were 10.5” in diameter, and of a diaphragm design, pretty standard GM stuff.


Drivetrain -

Muncie M-20 wide-ratio 4-speed transmission (2.52 first gear) with an OEM “Hurst” shifter. The OEM versions of the Hurst shifter were quite a bit different than the aftermarket ones you could buy from your local speed shop. The shift rods were smaller diameter (more flex), and where they connected to the transmission shift arms, and the shift mechanism at the base of the stick, had larger, softer bushings to keep them quieter (more slop). The stick was also attached by two injection molded plastic pins, rather than being a bolt on stick. I’d heard stories of the stick coming off in the driver’s hand after repeated slam-shifting, which caused the plastic pins to fracture.

The rear axle was a GM “10 Bolt” with a 3.42 ratio, equipped with Pontiac’s version of GM’s Positraction, called Safe-T-Track. Combined with the M-20 transmission, this combination gave excellent gearing for acceleration in 1st, 2nd, and 3rd, with 4th gear being a great cruising gear on the highway. You could think of it as a “3 speed with Overdrive”.

That pretty much covers the engine and drivetrain the car came with. Next section will cover the engine I built for it.

2 comments:

  1. I had a 1978 firebird. I bought it and then got orders to Okinawa. It sat in my brother's barn under a tarp for 14 months. Then I got orders to Italy. I sold the Firebird and bought a VW rabbit convertible. The Firebird was a nice car, I just hardly got to drive it.

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  2. Reminds me of my 66 Goat... sigh

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