I’ve read your magazine for years, and please keep up the good work. I’ve been restoring an ’81 Turbo Trans Am and I have everything done but the motor. The original drivetrain was long gone, so I sourced a ’70 400 and Turbo 400.
I’m having difficulty finding pistons that don’t have those “8” valve reliefs. I’m aiming for a compression ratio of 10:1 to 10.5:1, and I plan on using No. 13 cylinder heads. This is just going to be a solid street car. Do you have any suggestions?
Rocky Rotella responds: Thank you for being a loyal HPP reader. It sounds as if you have the Pontiac pistons included in typical “rebuilder” sets.
Replacement pistons with multiple valve pockets are designed to accommodate essentially all regular production Pontiac cylinder heads. Through ’67, the valves were rolled 20 degrees from the centerline of the bore. Pontiac decreased the valve-inclination angle to 14 degrees in ’67 performance applications, and carried that into all applications from ’68 forward. The valve pocket (or relief) was revised accordingly on production pistons at that point.
Some entry-level replacement pistons for the 400ci Pontiac feature valve-pocket types for both valve-inclination angles to increase application coverage. While that may not be a concern for hobbyists looking to simply rebuild their engine, the pocket design increases volume, which reduces compression slightly, and that can be a concern for performance use. That’s why I prefer using pistons more specialized for the application I am working on, and fortunately many choices are available.
Replacement Pontiac pistons are commonly available in aluminum and hypereutectic castings, and aluminum forgings. While a cast piston can certainly provide a long service life, I prefer a forged piston for performance use because of its improved durability. JE, Icon (Keith Black), and Speed Pro each produce high-quality, off-the-shelf forgings that should fit your 400. If none of these meet the exact requirements for your particular application, you can always source a custom-spec piston from companies such as Diamond or Ross.
In applications such as yours, I have used the high-quality Speed Pro forging (PN L2262NF) with great success. It contains the appropriate valve pocket for your particular cylinder heads and comes in a number of popular overbore sizes. You can find them readily available from sources, such as Summit Racing. These or similar-quality units can also be sourced from practically any Pontiac builder with advertisements found throughout this issue. I’m confident such a piston will perform suitably for your intended performance level.
I am a bit confused when it comes to factory D-port cylinder heads. I have a mild 455 and want to know what castings are best for me.
I plan to run on pump gas and would like to use an “068” cam for good street manners. I have an automatic transmission and 3.08 gears. What casting might give me the best bang for my buck?
Rocky Rotella responds: Pontiac’s basic D-port cylinder head was introduced for ’55. Those castings from ’67 and later generally contain the most desirable characteristics that work best with modern builds. That includes large-diameter intake valves and threaded rocker-arm studs for certain applications. Knowing the right set to look for can be confusing and overwhelming, but if numbers aren’t important to you, then you can narrow your search down to a specific era.
Generally speaking, castings produced through the ’70 model year contain a combustion chamber volume intended to produce a compression ratio near 10:1 or more on the original application. That limits the effectiveness of such castings for pump-gas applications today, since 10:1 would be the absolute maximum I suggest for 91-octane fuel. In addition, the vehicles they were typically installed on are generally some of the more desirable models of the era, so they tend to be quite costly. In my opinion, leave them for the restorers.
There is an affordable solution that will give you the performance you desire without breaking the bank. During the mid-to-late ’70s, Pontiac introduced the 4X, 5C, and 6X castings. Again generally speaking, they produce a compression ratio around 8:1 on their original applications. When combining a casting originally intended for a ’73-and-later 400 (such as the 4X-7H, 5C-8, or 6X-8) on a 455, it usually produces a compression ratio of slightly more than 9:1 on a typical 455 rebuild.
These particular castings are often quite affordable. By that I mean less than $200 for a virgin pair. They also contain the most desirable qualities and provide sufficient airflow (around 210 cfm at 28 inches) to achieve at least 350 hp and maybe as much as 400 with the correct combination of components.
If you desire greater horsepower, they can easily be ported to 250 cfm or slightly more, raising power potential to nearly 500.
I’m having difficulty finding pistons that don’t have those ‘8’ valve reliefs
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