Exhaust Manifold Bolt Holes: Why So Few?
I have an original '78 WS6/W72 T/A that has been stored for a number of years. While getting it roadworthy, I found an exhaust leak between the driver-side exhaust manifold and the cylinder head. This was due to a broken bolt on the last threaded hole of the manifold/cylinder head near the firewall. I removed the head and manifold (unfortunately breaking a few more bolts), which brings me to my issue.
The two end bolts are different from the center four. Each of them is hollow at the tip (threaded end) to a depth of approximately 8-10 mm. This appears to be where the first bolt broke. The bolt at the other end was removed intact.
What is the reason for this design? Is it a thermal issue? It threads into an open passage. Do I need to replace it with a similar bolt, as it seems to affect bolt strength and durability?
I'm the original owner of this Bird, so I know there haven't been any previous repairs.
Rocky Rotella Responds:
It's not uncommon to find Pontiac engines with exhaust manifolds retained by less than six bolts. It seems that as early as the late-'60s and early-'70s the division assembled some of its engines with four or five manifold bolts per side. By 1972 some of the cylinder heads were cast without provisions for outer exhaust-manifold bolts entirely. It's unclear if this was an attempt to save material costs, but it seems that engineering found satisfactory sealing quality with fewer than six bolts for a short time.
In 1973, the division began casting its cylinder heads with provisions for six holes once again, all of which were drilled and tapped. I can't recall seeing any D-port engines from that point forward assembled with less than six exhaust manifold bolts per side. Pontiac literature contradicts this, however.
According to Service Information Bulletin No. 74-I-34, dated April 5, 1974, the end exhaust-manifold retaining bolts and locks were eliminated from all engines (except the SD-455) with Engine Unit Number 191552, claiming it improved manifold durability. It's unclear whether this practice carried over into the '75 model year, but the '75 Pontiac Engine Assembly Manual shows 12 identical exhaust-manifold bolts (PN 943537) used on all engines that year. The same information appears through 1977, and this seems to correspond with what I've seen over the years.
Hearing that your original '78 Trans Am has different outer bolts is interesting but not shocking. I've never noticed unique fasteners in those positions when disassembling engines, but that doesn't suggest my observation is correct or that your Firebird's engine was assembled incorrectly. I can't imagine a hollow bolt having as much integrity as a solid bolt, however, and suggest replacing the outer bolts with conventional 3/8-16-inch units in the appropriate length. It's a simple fix that will get your Pontiac back on the road in no time.
I read Jim Taylor's Tech Q&A response to Jeff Knautz concerning his No. 48 heads. A few years back, I was given a R/A-III H.O. 350 out of a '69 Firebird, with an XC block and No. 48 heads. Due to my inability to glean info on this combo, I contacted HPP. I was told the 48 heads have larger cc combustion chambers and a 10.5:1 compression ratio. Mr. Taylor states differently in your Nov. '08 issue. Could you please clarify this for me?Lee CourtneyWhite Salmon, WA
Jim Taylor Responds:
Pontiac didn't offer a R/A-III 350 H.O. engine in a '69 or any other year. The XC-code 350 engine with No. 48 heads was-and is-known as a 350 H.O., having 325 advertised horsepower. This is a very rare engine. The exact same engine with an XJ-code in an A-body was rated at 330 hp. Both had No. 48 heads, and the advertised compression ratio was 10.5:1.
The No. 48 head was also used in the 400 engine. The WS-coded 400 engine was rated at 366 hp, and Pontiac listed the compression ratio at 10.75:1. This is a Ram Air III engine.
You should take the heads off the engine you've had for years and cc the combustion chambers; then you'll know what you have.
I put forth a hypothetical combus-tion chamber size in my response to Mr. Knautz's questions. Sixty-six cc's could be the factory volume of the chamber, or it could have been milled to arrive at that speci-fic number. A blueprint for compression ratio always includes combustion-chamber volume.
There were two versions of the '69 No. 48 head. After 40 years of buying, selling, and building No. 48s, we've seen two different chamber volumes-66 and 70cc versions.
It's the same thing that Pontiac did with the later 6X heads: one chamber size for 400 engines and a smaller size for 350 engines. (You don't even need to read the "4" or "8" stamped on the 6X head, as you can see the intake-side valve-cover rail is much thinner due to the milling of block and intake sides of the head to reduce chamber volume.) The 66 cc's I used was a guideline, not a statement of fact.
Hopefully your engine is untouched, and you can remove and cc the heads. Then you'll know for sure.
Brake Swap On The Cheap
I own a '69 Firebird with drum front brakes, and I have a '74 T/A parts car. Is it possible to interchange these or other brakes? Yes, I would love to have the money to buy new box kits, however I'm disabled, raising four kids alone, and bringing in $21,000 a year. I just can't spend $2,000 on brakes.
I know Hollander used to make a book of interchanges. Can you help a poor guy out? I don't have a computer, but my son does-he says my drum brakes are worse than dial-up Internet.
Rocky Rotella Responds:
I can relate to being on a limited bud-get, but having converted a First-Gen Firebird from front drum to front disc, I believe you'd be better off delaying the swap until you can save enough money to purchase a complete conversion kit from a vendor such as Master Power Brakes (www.mpbrakes.com), Performance Suspension Technology (www.p-s-t.com), or another reputable company.
Our Firebird was converted several years ago. At the time, donor vehicles were scarce but not overly rare, as they seemingly are today. The list of potential donors that allow for a direct front-disc swap includes late-'60s and early-'70s A-, G-, and X-body cars. Some components from your '74 Firebird may be compatible, but I think you'd be much better off starting with a complete donor vehicle instead of mixing and matching components.
We were fortunate enough to locate a donor G-body from which to pirate all the associated power-disc conversion pieces for our swap. However, I was uneasy to trust brake components that were nearing 40 years old and in unknown condition, so we purchased new wheel bearings, rotors, brake pads, rubber brake hoses, and remanufactured calipers. After tallying up the receipts, I found we'd spent almost as much as we would have simply purchasing a complete conversion kit from an aftermarket company such as those mentioned.
While you may get lucky and find a donor that can provide all the necessary components in good reusable condition, a brake system is one area in which I'm uncomfortable suggesting anyone cut cor-ners in an attempt to save a few bucks. You might consider putting up with the front drum brakes until you can afford a quality kit that contains everything you'd possibly need to stop your Firebird in its tracks. And after completing the swap, I can tell you that your son is absolutely correct-you'll find a significant improvement in your Firebird's braking ability.
What's The Best Factory Intake That Will Fit Under The Shaker Scoop?I have a '78 Trans Am. What's the best-flowing factory intake for the 400? I want it to look factory and not alter my Shaker height.
Rocky Rotella Responds:
A detailed comparison of the factory spread-bore intake manifolds used from '67 forward can be found in "Intake Uptake" (HPP, Feb. and Mar. '05). In that two-part story, we recorded airflow measurements in stock form and discussed such physical attributes as accessory bracket mounting and exhaust crossover size at that time.
We learned from the airflow portion of our testing that all factory manifolds contain similar flow capacity, but the cast-iron '73-'74 SD-455, with its enlarged runners, was best in unmodified form. It was followed closely by the cast-aluminum R/A-IV and 455 H.O. units, while the various standard production cast-iron units trailed marginally behind.
You'll be pleased to know that any production four-barrel manifold featured in our test will fit under your Trans Am's Shaker, but it's unlikely that you'll want to spend the hundreds to thousands of dollars required to acquire such a highly desirable piece as a cast-iron SD-455 or aluminum R/A-IV or 455 H.O. unit. If you prefer taking an economic approach, any standard production cast-iron unit from '67-'78 will provide excellent performance at a reasonable cost while working within your stated parameters.
The '67-'72 cast-iron units are generally accepted as the best performers, and with careful hunting one can be purchased for well under $100. EGR was added in 1973, and the same basic casting received additional plumbing for the emissions-control device. The extra material adds weight, but it seems that performance is unaffected. The carburetor flange took on a unique appearance with D-shaped secondary openings in '75, and while it might appear that airflow was significantly affected, airflow testing surprisingly shows otherwise.
After testing various factory and aftermarket intake manifolds, it seems that most moderately built street engines don't recognize the miniscule airflow differences amongst the stock castings. While without modification the SD-455 and cast-aluminum units might possess more potential than a standard-production cast manifold, careful porting such as that outlined in "Intake Uptake, Part 3" (Oct. '05)-which includes enlarging each intake runner to full gasket size to maintain the constant cross-sectional area into the plenum to eliminate restriction-is an easy way to maximize performance from it.
The question then remains, is your engine modified enough that it will recognize the additional airflow at high rpm? If you feel it might but lack the time required to accomplish the task, Dave Bisschop of SD Performance in British Columbia, Canada, has created a CNC-porting program to increase airflow through the factory manifolds. You can learn more about that service at www.sdperformance.com.
If you reach the performance limits of a stock manifold and decide to try an aftermarket unit, consider reading "Shaking Out The Details" (June '07). We gathered several aftermarket intake manifolds and recorded a series of measurements to determine the effects each has on Second-Gen Shaker placement. If you don't have that particu-lar issue, the story is available online at www.highperformancepontiac.com.