The slant six Hyper-Pak
In 1960, in building #135 of the Chrysler complex in Highland Park, Michigan, the very first high-performance slant six was born. The 170 cubic inch slant six was originally designed in 1958 to replace the flat-head six, which was big and bulky, and underpowered for the new Valiant compact car line for model-year 1960.
With the release of the compact 1960 Plymouth Valiant, NASCAR sanctioned a special race for the new six-cylinder compact car class, which would involve all three major automakers.
Located in building 135 was a special group of Chrysler racing engineers that worked in Dyno Cell 13. With NASCAR putting out word of the special race, the engineers went to work on the 170 cid slant six. This special group would be responsible for taking an ordinary 170 cubic inch six and making it put out extraordinary horsepower, that would earn these Chrysler racing engineers the title of most powerful and feared in-line six cylinder ever built in America.
With these special group of engineers, the 170 slant six would be transformed from economy to high performance and the question of the slant six durability and performance would be put to the test in front of millions of TV viewers.
The 1960 Plymouth Valiant was only available in a 4 door model; this was given a standard block with a set of special domed pistons for a higher compression ratio. The dome height was .250” above the block and the dome volume was 13.7 cc. The camshaft was a racer brown ST-21 which has a lift of .520, duration of 286°, and a centerline of .108; that was the race version that was used in the Daytona race. The public version of the camshaft was a Direct Connection camshaft with specs of 276° duration with a .430 lift (part 2205620). The next step was valve springs (1944554) and a special set of pushrods (2129619). [Originally we had listed STX-21 which has a lift of .560 and duration of 306°. Thanks, Jim at RB Cams.]
With the intake system, Chrysler racing engineers specifically designed the long ram intake manifold to give more power around 4-4,500 rpm to help with coming out of the turns. The carb was hung out 21 inches from the cylinder head to force the fuel to give a ram effect for even more power; the carb was a special four-barrel Carter AFB model 3083, which carried part number 2129881.
A set of special cast iron manifolds called the Hyper-Pak header split the manifolds; the front three standard manifold carried P/N 2129899 and the rear manifold carried P/N 2129900. These dumped into a single 2-1/2” o.d. outlet pipe followed by a bigger muffler from an Imperial. The exhaust and muffler carried P/N 2298350. The special air cleaner assembly carried P/N 2129992 with a special air cleaner gasket (P/N 1821170).
A high performance manual 3-speed was used behind the race version Valiant with a special clutch driving disc that carried P/N 2201219, with a clutch cover and pressure plate with P/N 2201223.
The name Hyper-Pak was given while the engineers were doing the dyno runs with their new high performance 170 cu. inch six cylinder; once it fired up the motor would shake and lope badly and some of the engineers commented, “that thing is so hyper!” The rest is history.
With the slant six introduced in 1960 in two versions, a 101 hp, 170 cu.in and a 145hp, 225 cu. in. version, the slant six built its reputation for performance and reliability. The 148hp, 170 cu. in Hyper-Pak introduced in the 1960 Valiant was the next step and the one that was to prove the new engine worthy of any performance challenge.
Seven slant six valiants entered the new NASCAR race; when the race ended, not only had all seven Hyper-Paks finished, proving their reliability and durability, but they won the first seven places!
This type of performance speaks for itself.
In a first, CBS television had aired the compact sedan races as well as the qualifying for the Daytona 500 race in a live TV special for their “sports spectacular;” it was estimated that 17 million people were watching. The compact sedan race quickly turned into a dull affair as lap after lap Lee Petty managed to turn away all challenges. It was made even duller by the fact that besides his Valiant, the next 7 positions were also hyper-pak valiants.
The finishing order was Valiant 1,2,3,4,5,6,7 ! Did Chrysler use that for advertising fodder? Nope. Go figure! NASCAR agreed with CBS on the dull part of the race, and quickly cancelled any other plans for a compact sedan race.
Not the first time a sanctioning body has changed the rules, because Chrysler showed up ready to race!
Creating a new Hyper-Pak (by Jerry Engle)
First, hot tank the block and head as well as the connecting rods. Once everything is cleaned, mage the the block and check for core shift as well as cylinder wall thickness. The best way to check for core shift between the combustion chamber and cylinder bore (as much as 0.020-0.030 inch can be caused by manufacturing) is to mount the head to an empty block and outline the cylinder bores with a scratch awl - another thing to use is machinist dye for obvious core shifts. If possible you can use an abrasive disc to get the protrusions out. The slant six head is extra thick, therefore the heads can be safely milled up to 0.100 inches, which will give your compression a dramatic boost from 8.4 to 9.2:1 ratio.
The crankshafts from 1961-1968 used a small pilot 1.960 od/ 1.530 id. They were a forged casting as well. Crankshafts after 68-76 used the large pilot 2.160 od/ 1.805 id and are all forged castings. Crankshafts after 1976 were all cast crankshafts and are not recomended for high RPM engine builds.
Connecting rods from 1961 to pre 1976 are all forged and are excellent for a hp build up. A lot of horsepower can be obtained by installing oversize valves and blending the bowl areas. I used the bigger valves 1.70 intake and 1.44 exhaust which will give you roughly 5% over the stock valves. 1.62 intake and 1.36 exhaust. on the camshaft you can use the 71-76 camshafts which have a 244 duration and a lift of .406 intake and .414 exhaust. The settings for this camshaft are .010 intake and .020 exhaust.
I used a Clifford .464/276 duration camshaft. Remember that when choosing the more radical cams anything over the 280 duration will require the pistons to be notched.
The valve springs are Mopar Performance part # p4120249; these are single springs with a damper installed ht.1.67/1.70 o.D 1.50' wire diameter is .206 lift range for these springs are .430/.500" and are chrome-van. The intake is a Clifford water cooled 4-barrel; the original Hyper-Pak intake will not clear the power booster and master cylinder in the Dodge pick ups that have disc brakes. The carb is a Holley 4 barrel 4160 series which is recommended for engines in the 150-300 displacment; it has a 390 cfm rating which will work well with street and strip usage.
I used a Chrysler big block lower gear and chain and ordered a sealed power top gear pn-223-338. A windage tray from J&M Engines will free up close to 15 extra horsepower. A Clifford high-volume oil pump will give you 50psi at all times which is well worth the money.
For the exhaust I used Clifford split headers with dual 2/1/4 inch pipes that feed into Dynomax glasspacks. The A-833 4-speed backs the Hyper-Pak along with an 8/1/4 3:91 sure-grip.
The best time so far with the addition of a nitrous set-up is 13:90 in the quarter. The best thing after all this work is pulling up to a 5.0 or a Camaro and having them have no choice but to see what the rear tailights of a Dodge Ram look like.
Other Hyper-Pak notes
Mopar's own Web site refers to the Hyper-Pak:
The Hyper Pak used a long runner, 4-barrel intake package, which is almost impossible to find today.
The aftermarket version of this intake manifold is a short runner 4-barrel. It should only be used in racing applications ... the large runner, aluminum 2-barrel intake (P4529115)... is no longer sold by Mopar Performance. However, it was sold for 10 to 15 years.
The small 4-barrel carbs available today are much too large for 225 cubic inches, so you want to stay with a 2-barrel carb with 350 cfm or less. ... The production cam was very small, so upgrade to a mechanical cam (P4120243) which has 244 degrees duration and .436" lift.
All of the production heads used 1.50" intake valves and 1.36” exhaust valves. The hot tip in the old days was to cut down 392 Hemi valves for high-performance applications, but that is no longer required. Use 1.70" intake valves (P4286785) and 1.44" exhaust valves (P4286786) — both are stainless.
From mid-1961 to 1963, 50,000 225-cid Slant Six engines were produced with an aluminum engine block. A competition engine, the Hyper-Pak featured a much more radical cam, intake, and carb; steel tubing exhaust headers; higher compression pistons; and a special tuned exhaust system. Advertised at 195 hp, Hyper-Pak engines are often said to put out even more power. They were the rulers of the lower stock classes at the drag races. The Hyper-Pak was available as a dealer-installed or user-installed option.
- Overhead valve. Cast iron block.
- Compression ratio: 11.5:1 (225), 10.5:1 (170)
- Brake hp: 195 at 5200 rpm (225), 148 (170)
- Four main bearings.
- Solid valve lifters.
- Carburetor: Carter AFB 3083S four-barrel.
- Larger exhaust system using cast iron hearders.
Bob Scott wrote:
The word got out among Chrysler bigwigs that the slant six was going to be very effective. When it was announced that Daytona was going to have a Compact Car class for their races, all three major auto companies started designing variations of their eligible compacts for the race. Ed Cole was head of engineering and research and a bit overconfident about their Corvair. He offered to bet one of the higher ups at Chrysler that the Chevys would dominate the Daytona Compact race. George Huebner, Director of Research at Chrysler [most commonly associated with the turbine cars], said "I'll take that bet."
All of a sudden, the research department had an infusion of a couple of million dollars to find ways to make the Valiant faster. They developed a legal Daytona car with the hyper pak engine and various other modifications, such as thinner, lighter glass and sent them down. They dominated the race, taking first through third place, and Ed Cole lost his bet.
George also was working at Detroit Dragway as the head guy who inspected cars and determined what class they had to race in. Right around the same time the Nationals were held at Detroit Dragway. About twenty guys showed up with hyper pak Valiants and blew away everyone in their class there too. Chevrolet was humiliated when during flying mile tests their 283 fuel injected car was surpassed in top speed by a Valiant with the hyper kit.
Later note: Clifford Performance sells an original specification long runner Hyper-Pak intake manifold for the 170 Slant Six and mentions a certain conventional bodied Six Pack car that turned in 12.34 seconds with their equipment.
- Interview with engineer Pete Hagenbuch
- The Valiant (and Dart/Duster/etc) Page
- Other stories of the slant six's development (courtesy Valiant Varieties)
- Slant Six Performance Upgrades (courtesy Valiant Varieties)
- Modifications - one person's experience
- Inliners International
- Slant six.org
- Slant Six Club of New York and New Jersey