|High compression is generally associated with better gas mileage, because it is a well-known fact that it improves thermal efficiency. This naturally gives rise to the question, what does the 11.25-to-1 compression ratio do for the "409's" economy of operation? Obviously it doesn't do what would be popularly expected of it. in order to understand why this is so, we must again look at valve timing specifications.
Although the compression stroke is normally thought of as starting the instant the crankshaft passes bottom dead center and starts to push the piston upward, the 106-degree late closing of this engine's intake valves prevents actual compression from taking place at low rpm until the piston has moved a considerable distance upward. At high rpm the situation changes, because inertia ramming causes fuel mixture to continue entering the cylinder even though the piston is apparently trying to pump it back into the intake manifold. The resulting improved breathing at high rpm is partially responsible for this engine's outstanding performance in the 5,000-to-6,000-rpm range.
Optional sintered iron brake linings are a part of the 409's running gear kit. Note the fine honed finish on the inside of the brake drum, which prevents squeal.
The flywheel housing, transmission housing and the transmission tail-shaft housing are all made of aluminum. Pencil points out area of removed paint.
|Another factor, or maybe we should say lack of a feature, contributes to fuel consumption when the car is driven conservatively - the lack of a distributor diaphragm. This useful little device is left
off the distributors of many high-performance machines produced in the U.S. today because of the high cost of building a satisfactory mechanism for high-rpm operation. A diaphragm is absolutely unnecessary when an engine is operated at full throttle, whether at 1,000 or 6,000 rpm, but whenever it is running at partial throttle, the extra advance provided by this vacuum-controlled device improves efficiency.
In addition to reducing gas mileage, the absence of a diaphragm revealed another effect when the quarter-mile tests were made. Each time we took the car to the Lions Associated Drag Strip, this phase of the 'testing was preceded by 25 miles of conservative freeway driving. These partial-throttle trips would cause enough carbon, in the form of soft soot, to become deposited on the spark plug insulators so that the first couple of runs would be inferior to later efforts. This light fouling of the plugs, though it caused a pronounced high-rpm miss for one or two runs, would clear up and allow consistently good runs thereafter.
Pro drag racers avoid this difficulty with high-performance machines by either towing the car to the strip or changing plugs on arrival if the car must be driven.
Our first quarter-mile test was made with the 3.36-to-1 rear end ratio and quickly proved that it wasn't the gear for drag racing. Many bystanders thought we were mentally retarded for even making an attempt. Of course, the engine bogged down in first gear, as it did in the low-speed testing. At the quarter-mile marker, the transmission was still in third gear, with the engine taching only 5,300 rpm. The speed at the end of the quarter was electronically timed at 94.24 mph with an, elapsed time of 15.31 seconds, despite the gearing handicap. Mickey Thompson, who manages the Lion's Drag Strip, took it through the lights a couple of times. Mickey bettered our time by one-tenth of a mile per hour and dropped the ET by .06 of a second.
|After switching to the 4.56-to-1 gears, leaving the starting line required judicious use of the throttle to avoid excessive wheel spin. After travelling about 75 feet, traction was no longer a problem and those 409 cubic inches really made themselves felt. This time the tach read the same at the finish line as it did with the other gears, but the transmission was in fourth. The clocks recorded a speed of 98.14 mph and an elapsed time of 14.02 seconds. Since we were running into a head wind the potential of the car was reduced between 11/2 and 2 mph. Better tires would have been a help too, but to become a
record-breaker more horsepower would be necessary.
The simplest way to boost the power of our test car would have been to switch carburetors.
Thumb and forefinger span the width of the pinion yoke on the 409's heavy-duty universal joint.
|Though the AFB letters that appeared on the float bowl usually create the impression that this is Carter's biggest carburetor, it was not true in the case of the test car. We were probably provided with the smaller of the carburetion options, so that inexperienced drivers wouldn't complain about poor throttle response when flooring the pedal at low rpm.
Profile views of both carburetors look the same, except that fuel enters the float bowl at two points on the big one. Looking down the throats, the two are easy to identify; the smaller one reveals a much bigger difference between venturi sizes of the primaries and secondaries - 1.25 inches and 1.56 inches, respectively. The bigger carburetor has primaries that are 1.62 inches in diameter and secondaries that measure 1.68 inches-only one sixteenth of an inch difference between these two makes them look nearly the same.
Regardless of which carburetion option may appear on a "409" at delivery, the large aluminum intake manifold is the same. The fact that they are aluminum may not be readily apparent, because they are painted red for some elusive reason. Since there is no base coat on the aluminum, it will eventually peel off and later prevent anyone from mistaking the large-passage lightweight unit for the internally smaller cast-iron part used on the various 348-cubic-inch Chevrolet engines.
As might be suspected, the "409" is a bored-and-stroked version of the 348-cubic-inch engine. It is not made from the same block casting but is cast with a different water jacket core, so that the cylinders can be bored V8-inch larger without making the walls thinner. A stroke 1/4-inch longer than that of the "348" engine accounts for about half of the 61-cubic-inch difference in displacement.
Cockpit looks that of a conventional Impala, except for the four-speed gear-shift lever on the floor and the Sun tachometer mounted on the steering column.
|Instead of moving the wrist pin 1/8-inch closer to the crown of the piston to compensate for the longer stroke, as is the custom in hot rodding circles, Chevrolet chose to use 1/8-inch-shorter connecting rods. This has definite advantages over the former system. The pistons are not of the autothermic design used in conventional Chevrolet engines, but are solid skirt racing types. They are not die-cast as are conventional parts but are made by an impact extrusion process often referred to as forging. The aluminum alloy used in this process and the densification that results from the high|
|impact pressure that forms these pistons, makes them much stronger than die-cast types.
Solid skirt pistons, unlike the more common autothermic type, expand and contract considerably as they heat and cool. Because of this size change with variations in temperature, piston clatter is heard in the engine, in addition to the noise made by valve lash.
In addition to the powerplant applications of aluminum, this light metal is also found in the power train. The flywheel housing and the complete case of the four-speed transmission both employ this material.
Although the four-speed gearbox is patterned after the unit used successfully for some time in Corvettes, it wasn't quite as easy to shift. It, too, mounts the shift lever on the tail shaft housing of the transmission and employs rod linkage to the levers on the side of the transmission case proper. Not having a reverse lockout like the Corvette, there is occasionally some doubt in the selection of first gear, since reverse is adjacent to it.
Aside from minor difficulties with the shift lever, no other problems were experienced in the manipulation of controls. The power steering and power brakes both performed very well and made the car easy to control. The brakes were especially good and behaved very well after numerous stops from high speed. Later we found out why, instead of using conventional asbestos brake lining, the shoes were fitted with segments of sintered iron similar to the type used in racing Corvettes. Squeal, formerly associated with sintered iron linings, never became apparent. Possibly this is because the drums used with these shoes are honed to a mirror finish instead being merely ground.
Cornering ability and high-speed stability were both better than are experienced in a standard Chevrolet, thanks to the inclusion of a complete Police-option under-carriage. The stiffer springs and heavy-duty shock absorbers that are a part of this RPO Number 1108 make the ride a little rougher, but the added safety is worth the sacrifice. A heavier torsion-type stabilizer bar is also a part of the kit. This bar could have been even heavier and will doubtless be replaced with one that is when these cars are raced on tracks.
A limited-slip differential adds the final touch to the "409's" running gear package. Not only does this device provide better traction when accelerating from the drag strip starting line, but it helps cornering ability considerably. On an oval race track the limited-slip unit will produce rear end behavior that might be considered half-way between that of an open differential and a totally locked assembly.
Though our test of the "409" Super Sport presents a fairly accurate picture of what one of these machines will do in showroom condition, it does not indicate what they will later do in competition. Certain further modifications are allowed both under NASCAR rules and NHRA regulations that result in considerable improvement. The stock exhaust system would be replaced with paired headers that follow the firing order, and the large carburetor would replace the smaller unit before any pro would enter it in competition at a drag strip.
|MOTOR TREND TEST DATA|
|TEST CAR:||1961 Chevrolet Impala Super Sport|
|BODY TYPE:||Two-door hardtop|
|BASE PRICE:||$ 3273|
|ENGINE TYPE:||Ohv V-8|
|DISPLACEMENT:||409 cubic inches|
|HORSEPOWER:||360 @ 5800 rpm|
|REAR AXLE RATIOS|
|(two tested):||3.36 and 4.56|
|GAS MILEAGE:||10 to 14 miles per gallon (3.36 axle ratio)|
|9 to 13 miles per gallon (4.56 axle ratio)|
|WEIGHT-POWER RATIO:||10.38 lbs. per horsepower|
|HORSEPOWER PER CUBIC INCH:||.88|
|3.36 AXLE RATIO|
|0-30 mph in 4 seconds, 0-45 mph in 5.8 seconds and 0-60 mph in 7.8 seconds.|
|Quarter-mile: elapsed time -- 15.31 seconds; 94.24 mph|
|4.56 AXLE RATIO|
|0-30 mph in 3.2 seconds, 0-45 mph in 4.8 seconds and 0-60 mph in 7 seconds.|
|Quarter-mile: elapsed time -- 14.02 seconds; 98.14 mph|