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!

Speed Warning

I have a Borg Warner Super T-10 four-speed out of a 1979 Firebird and need to change the speedometer drive gear to accommodate a larger driven gear. I have a 4.10 gear in the rear. I am using the gray 22-tooth driven gear. The internal gear is green and I believe it to be the 8-tooth large. Is there a 8-tooth small to fit the 32-spline output shaft? I have the 24-tooth yellow that is called for in the speedometer calculators, but cannot use it with the large-diameter drive gear.

Wayne Scott
Port Colborne, Ontario, Canada

Rocky Rotella responds: For street use, I consider a speedometer as important as any other instrument within a vehicle's dash panel. What good is an inaccurate fuel gauge when you venture out on a leisurely trip away from home or an inaccurate coolant-temperature gauge should your engine boil over? An inaccurate speedometer falls into this same category. It not only incorrectly displays the speed at which you travel, making you susceptible to speeding tickets or becoming a possible traffic hazard, but it also alters the rate in which your odometer racks up miles.

It sounds as if you're experiencing the frustration most hobbyists face when swapping transmissions, rear-axle gears, and even rear tires. Pontiac achieved a specific speedometer drive ratio using specific drive- and driven-gear combinations in the transmission's tailshaft. Changing a single driveline component can alter that ratio and leave you wondering just how accurate your speedometer really is!

As you noted, simply changing the driven gear—which resides in a housing that's inserted into the side of the transmission—to one with greater or lesser teeth can improve accuracy in many instances. However, in certain cases, it also requires replacing the drive gear with a unit containing a different tooth count. Since the drive gear is pressed or clipped onto the transmission's output shaft, replacing it can require some transmission disassembly, and that's not always convenient.

Transmission speedometer gear calculators readily available on the Internet can tell you exactly how many teeth are needed on the drive and driven gears to attain proper speedometer output, but actually finding the gears today can be challenging. What complicates this seemingly simple swap is that while most General Motors speedometer gears appear identical, the actual gear diameters can vary with transmission application. In other words, automatic and manual transmission gears can be different, and not all Muncie speedometer gears will fit Borg-Warner transmissions. So while they look alike, sometimes they do not fit exactly as intended, and I believe that's exactly what you're seeing with your Super T-10.

I spoke with Brian Higgins in the Manual Transmission Shop at S-K Speed in Lindenhurst, New York (, and he confirmed that the large and small drive gears you referenced in your question are common in Muncie applications, but that 32-spline Borg-Warner transmissions used a single-sized drive gear. Brian has an assortment of Super T-10 drive and driven gears available and can help you acquire a correct set to get your speedometer performing accurately at a reasonable cost. He's available at (631) 957-9427, and when you call, be sure to have the correct diameter of your rear tires handy for easy calculation.

It's also worth noting that Pontiac sometimes used a mechanical transducer positioned between the transmission and speedometer. It contained an additional set of internal gears which further altered the ratio to attain an accurate speedometer reading when using common drive- and driven-gear combinations in the transmission. While I've had good results simply finding the correct drive and driven gear combos for my applications, there are sources that sell new speedometer ratio adapters for about $70 that can adjust the output by a fixed percentage based on how far your speedometer is off. I have no experience with them, but a simple search on the web for a speedometer ratio adapter should provide a number of potential sellers.

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.

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