CC Tech: Pontiac Royal Bobcat Kit – “The Screwdriver Tune-Up” For Hot 389s And 421s

 

Most fans of the Pontiac GTO have heard of the legendary Royal Bobcat tuning kit for Pontiac 389 and 421 engines. In the June 1963 Car Life, automotive writer Roger Huntington explained what the Bobcat treatment involved — and what it could do in a hot Pontiac Catalina hardtop with the triple-carburetor 421-HO engine.

Bobcats by Royal Pontiac

The Pontiac GTO made Ace Wilson’s Royal Pontiac dealership a legend, and Royal Pontiac played no small part in the legend of the GTO. Many early GTO press cars had the benefit of a little extra under-hood attention from Royal Pontiac mechanics, giving their performance more sparkle than the Pontiac assembly lines normally supplied. The infamous 1964 Car and Driver GTO allegedly had a Tri-Power 389 with a Royal super-tune, although in fact the red car used for their acceleration runs (which still exists today) actually had the bigger 421-HO engine, and was in what Motor Trend (called upon a few years later to explain a stunt pulled by a rival magazine) politely called “an advanced state of preparation.”

Such was the notoriety of the C/D test that Royal Pontiac even referred to it in print ads for their Pontiac tuning kit:

However, the Royal Bobcat kit, like Asa Wilson Jr.’s performance Pontiac dealership, was born before the GTO was ever conceived. As I’ve previously explained, in 1961, Wilson and Pontiac ad exec Jim Wangers had come up with a performance-oriented dealer special called the Royal Bobcat, which was essentially a Pontiac Catalina with a load of factory performance options dressed up with a few cosmetic addenda and new “BOB CAT” badges:

 

The original Royal Bobcats didn’t necessarily have specially tuned engines — you could buy the cosmetic pieces with a stock powertrain — but for about $55 to $60 extra, Royal Pontiac would also give the engine a set of performance modifications developed by chief service technician Frank Rediker, who also prepared the dealership’s drag racing cars. This engine package was originally known as the “Superchief” engine, but this was a little corny even for 1962, and it soon became more commonly known as the Bobcat kit, even at Royal Pontiac.

For the real lowdown on this kit, we turn to the June 1963 issue of Car Life and a three-page item by Roger Huntington entitled “The Screwdriver Tune-Up.” The title was chosen advisedly: Rediker and his colleague Jim Stewart were emphatic that neither the kit nor its installation required any arcane secrets or special equipment. In the road test of a Royal Bobcat in the July 1962 Car Life, Stewart had emphasized, “we don’t use any tools that the average small garage wouldn’t have. Nothing more exotic than a dwell meter.”

 

The Bobcat Car Life tested in 1962 had the 389 cu. in. (6,372 cc) engine, but the following description involves the bigger 421 cu. in. (6,902 cc) engine, which was externally almost indistinguishable. A 421-HO engine had certain internal changes, such as lighter forged connecting rods and bigger main bearings, but essentially anything that could be done with a 389 could be done with a 421 and vice versa.

Huntington begins with a rundown of the cosmetic aspects of the Bobcat trim package and the factory heavy-duty equipment that was ordered with it, including heavy-duty suspension, radiator, battery, and alternator, plus a limited-slip differential. Since that’s covered in the earlier article linked above (which also has some color pictures), I’ll skip over that to get into the nitty-gritty.

The first part of the “Bobcatting” process was a modified distributor. According to the 1963 Pontiac AMA specs, a high-compression Pontiac 389 or 421 had an initial spark advance of 6 degrees at hot idle, with a dwell angle of 30 degrees plus or minus 2 degrees. Centrifugal advance was a maximum of 16 to 20 degrees at 2,800 rpm, with intermediate advance of 14 to 18 degrees at 2,000 rpm, plus vacuum advance of up to 20 degrees at 15 to 17 inches of mercury. Royal mechanics increased the dwell angle to about 36 degrees (decreasing the breaker gap), advanced the initial spark setting to around 16 to 18 degrees, and replaced the centrifugal advance springs for a much faster spark advance curve:

Pontiac 421-HO Centrifugal Spark Advance

RPM	Stock Pontiac Distributor	Modified Royal Bobcat
500 rpm	0 degrees	0 degrees
1,000 rpm	4 degrees	7.5 degrees
1,500 rpm	5.5 degrees	9 degrees
2,000 rpm	6 degrees	11 degrees
2,500 rpm	6.5 degrees	12.5 degrees

 

With this much initial and centrifugal advance, Royal disconnected the vacuum advance completely. As Huntington explains on the following page:

Frank Rediker says there’s enough lag in these [vacuum] mechanisms to sometimes give a second of severe detonation when you punch it hard when cruising along on high vacuum advance. Royal feels this can be unnecessarily rough on the engine.

(If you feel like you’ve missed a beat here, distributor vacuum advance was designed to improve mid-range performance and fuel economy by increasing the spark advance at part throttle. Vacuum advance was ineffective at idle and diminished as intake manifold pressure increased at higher rpm. The problem, as Rediker saw it, was that it could take a beat or two for the vacuum advance mechanism to react to a sudden change in engine vacuum, with potentially expensive results in a highly tuned engine.)

 

Increasing spark advance like this provided more power and sharper throttle response, but it also increased the engine’s octane requirements, making super premium fuel a must. Royal mechanics actually wanted a little bit of spark knock at 2,000 with the throttle wide open, even on Sunoco 270, which I think had a RON of about 110 (Sunoco 260 was in the realm of 102 to 104 RON) and was not available in all areas. The reason Pontiac didn’t set the advance curve like this in the first place was that they had to make allowances for the varying quality of pump gasolines and the fact that super premium wasn’t always widely available, lest owners blow up their engines on a tank of lower-octane fuel. Royal could presume that a Bobcat owner understood the difference between premium and super premium gasoline and was prepared to assume the risks.

 

Royal also installed different spark plugs with a smaller gap and no gaskets, and then got to work enriching the carburetor mixture settings. With a stock Tri-Power setup, Huntington explains:

The standard Tri-Power jets give more richness on the end carburetors than on the center (which feeds all the gas about 90% of the time). This gives decent gas mileage and adequate mixture richness at full throttle. But the Royal doesn’t feel the customer gets really hot throttle response when he whips open the end carburetors—if the center one is cruising lean just the instant before. The jet change actually leans down the end carburetors but considerably richens up the center unit. The overall mixture richness at full throttle is not a lot different but the response is 100% better.

As explained more fully on the following page, a further improvement in response was available by replacing the Tri-Power carburetors’ vacuum linkage, with a progressive mechanical linkage. The latter was actually a factory item, but the added labor of installing and setting it up discouraged Pontiac from offering it in regular production, so it was a dealer-installed replacement option (DIRO), listing for about $40. This was not part of the Bobcat tuning package, but it was a worthwhile addition, since with the vacuum linkage, the front and rear carburetors were an all-or-nothing affair: impressive for a sudden burst of speed, but not very convenient in really hard driving.

 

Two further DIROs included with the Bobcat kit were new intake manifold gaskets and a low-pitch engine cooling fan. The gaskets blocked off the heat riser passages, which used exhaust heat to improve fuel vaporization, especially on a cold start, and warm the control coil of the automatic choke. Eliminating manifold heat increased power by about 4 percent. (Incidentally, this is part of why port fuel injection generally produces more power than a comparable carbureted or throttle-body set-up — since the manifold is “dry” until the intake air reaches the ports, there’s no need for manifold heat.) However, without the heat risers, Royal had to disconnect the choke completely. Huntington felt it was unneeded for cold starting, thanks to the richer carburetor settings, but I wonder how well that would go after an overnight soak in below-zero temperatures, making this a change best limited to knowledgeable owners who were very clear on what sacrifices they were prepared to accept for performance.

The same went for the low-pitch fan. As the text above notes, this saved about 7 hp at 5,000 rpm compared to the standard fan, but it made the fan less effective at its job at lower speeds. In theory, the heavy-duty radiator compensated, but when crawling in slow traffic on a 100-degree day, I have a feeling a close eye on the temperature gauge was in order.

An addition that I think was original to Royal was to use fiber-sleeve lock nuts on the rocker studs to lock the hydraulic lifters at their limits of travel, to prevent lifter “pump-up” at high rpm.

 

A last touch was individual low-restriction air cleaners with paper filter elements. The Car Life article doesn’t picture them in place, but the photo above shows them in a 1964 GTO (albeit with a 389 rather than a 421).

Most U.S. automakers of this time didn’t publish net power and torque ratings (the principle exception being Chevrolet, and only for its bread-and-butter engines), but Huntington developed some formulas for estimating net horsepower at the flywheel based on accelerometer readings. The factory gross ratings for the 421-HO engine were 370 hp @ 5,200 rpm and 460 lb-ft of torque @ 3,800 rpm. Huntington estimated that its net output in stock condition (pre-Bobcatting) was 310 hp @ 4,700 rpm and 415 lb-ft @ 3,400 rpm.

 

With the Bobcat, Huntington estimated that net output increased to 335 hp @ 5,000 rpm and 445 lb-ft @ 3,600 rpm — in other words, a gain of 30 real horsepower and 30 lb-ft of torque. More significantly:

It’s easy to feel the difference in the car. Throttle response is entirely different with the Bobcat. It produces instant acceleration —anywhere in the rpm range. The wild spark advance does a lot for low-end torque, too, even as low as 1200 rpm. With the stock setup, we had to burn off the starting line to keep the engine from bogging slightly. With the Bobcat treatment we could lug off, with hardly a whisper of wheelspin, and just let the brute torque take over. … The engine was extremely quiet under all conditions; it idled smoothly at 800 rpm, started quickly and would pull smoothly from 1500 rpm.

Huntington makes little mention of the tradeoffs, but they’re not hard to guess. Fuel consumption was undoubtedly higher, and super premium fuel was presumably mandatory, as were regular tune-ups, since the spark advance was set right to the edge of audible engine knock even on the highest-octane fuels available. The rocker lock-nut trick would also effectively prevent the lifters from self-adjusting, so you’d have to periodically adjust the valve clearances by hand. Winter starting might have been more troublesome than a brief acquaintance suggested; likewise engine cooling in sustained low-speed driving.

For brief bursts of speed, either on an organized drag strip or in risking tickets for “display of speed” on public roads, this package was probably just the ticket, but I can see why Pontiac would have been reticent to sell cars this way from the factory.

 

How well did the Bobcat-tuned Catalina go? Sorry you asked:

Catalina 421-HO Acceleration, Stock vs. Modified

Time in seconds	Stock 421-HO	Royal Bobcat Modified
0 to 30 mph	3.0	2.8
0 to 60 mph	6.9	6.3
Standing 1/4 Mile	15.5 @ 91 mph	15.0 @ 95 mph

 

Now, in 1963, no one was going to scoff at 0 to 60 mph times under 7 seconds, and there’s no question that the Bobcat was quicker, but those are not spectacular figures for 300+ net horsepower, especially with a close-ratio four-speed and 3.90 axle. The problem was that the Catalina was just too heavy: The ’62 Bobcat Car Life tested before had a curb weight of 3,960 lb; the ’63 421 car was 4,040 lb with a half-tank (and thus around 4,120 lb with a full tank), making it about 160 lb heavier than a comparable ’62. Test weight of the 421 Catalina, with Huntington, his driver (Huntington couldn’t drive), and their gear aboard, was a hefty 4,380 lb, which cost too much performance even with the most potent stock or mostly powertrain you could get on a new Pontiac in 1963. (The rare 421 Super Duty was gone by the time the CL article appeared.)

 

You can see why full-size performance cars, even massaged ones like this, fell out of favor by the mid-’60s. The hot intermediates weren’t really light, but they were definitely lighter: A 1964 GTO pillared Sports Coupe was under 3,500 lb at the curb, and was easily a match for the hotted-up 421 Catalina.

 

Of course, Pontiac couldn’t sell the GTO with the 421-HO engine — corporate policy and all that — but buyers could apply the Royal Bobcat kit (or essentially similar modifications) for the same price, with better results. That’s why the original Catalina-based Royal Bobcat cars generally draw blank looks even from fairly hardcore Pontiac fans, but the word “Royal Bobcat GTO” are still a siren song.

Related Reading

Vintage Car Life Road Test: 1962 Pontiac Royal Bobcat – Dry Run for the GTO (by me)
Vintage Car & Driver Review: 1964 Pontiac GTO – The Ultimate Ringer (by Paul N)
Vintage Magazine Ads: April 1964 Car and Driver – With Full Color 1964 GTO Insert (by Paul N)
Curbside Classic: 1965 Pontiac GTO – How To Create a Legend and Build a Brand (by Paul N)