Top-End Time

I am working on a project (and a budget). I have a 1978 W72 short block with stock pistons and a nice set of 1969 No. 46 heads that I would like to use. I know that 6X heads are the factory match and that should be about an 8.1:1 compression ratio. I think the No. 46 heads will result in closer to 10:1, but I have not cc'd them.

The vehicle will not see more than a couple thousand miles a year at most, and I don't mind spending a little more on fuel. My question is what should I do to keep it streetable? I was planning to use a cam similar to the factory 549431 that came with the W72.

Via Internet

Rocky Rotella responds: When compared to the 6X4 heads that were originally installed on your T/A 6.6, the 1969 No. 46 castings (not to be confused with 1973-74 No. 46 castings) have smaller intake valves that measure 1.96 inches instead of 2.11 inches. I've had both castings on my flow bench and accurately recorded peak airflow at 28-inches of pressure at about 190 cfm for the No. 46 and 210 cfm for the 6X. The resulting 15-cfm difference really only shows up at high rpm, and even then it's usually only 12-15 hp on a relatively stock engine. Installing the No. 46s on your 400 to boost compression should more than offset any performance loss associated with the smaller valves.

The No. 46 casting has a combustion-chamber volume that measures around 75cc in stock form. Any surfacing or valve jobs can alter that. If yours are stock, your guess that it would equate to a compression ratio around 10:1 on a typical 400 is fairly accurate. However, one unique characteristic of Pontiac's 350s and 400s produced during the mid-and-late 1970s was that the otherwise normal piston design contained a chamfered edge. It served to increase cylinder volume and decrease compression slightly. Based on the fact that your short block is original, this should work in your favor.

Assuming that your 400 remains at its original dimension, the pistons are approximately 0.010 inch down from the deck surface, and that you'll use a stock replacement head gasket such as the Fel-Pro 8518 that compresses to 0.041-inch thick, your compression ratio calculates to approximately 9.6:1. The actual compression number could be more or less based on other variables, but you're likely in a range that's livable on premium-grade pump gas (91-plus octane). I highly suggest having a local machine shop pour a chamber to verify its actual volume. You can then find a compression calculator on the web to determine what your compression ratio calculates to.

Personally, I wouldn't reuse the stock T/A 6.6 camshaft. It served its purpose back when it was developed, and it was specifically designed for a low-compression application. In my opinion, a modern hydraulic roller would be a better choice. With the compression ratio the No. 46 heads provide on your 400, you should have no problem tolerating a grind with 220-225 degrees of 0.050-inch intake duration and 10 additional degrees of exhaust duration.

You can find some catalog cams with specs in this range, but I believe most, if not all, are ground with a Lobe Separation Angle (LSA) of 110. Idle quality and the engines tolerance to octane-induced detonation may be a bit better with an LSA of 112 to 114 degrees and advancing the Intake Centerline (ICL) by four degrees. This will likely require a custom-ground camshaft, but most any cam company can make one for you quickly and easily.

I recommend selecting lobes that limit gross valve lift to 0.500-inch or less. Your cam company can provide proper valvesprings to match the pressure requirement and install height to match the cam. You will, however, need to convert the No. 46 castings from pressed rocker studs to threaded. That's a fairly easy process, which we outlined in our June 2011 issue (http://www.highperformance

I think you'll find the engine will perform quite well and be manageable on premium-grade pump fuel if the carburetor and distributor are properly calibrated, but a splash of racing fuel can't hurt if it's in your budget!