In 1967, Chevrolet was
busy promoting their new pony car,  the Camaro, and part of the promotional efforts included
racing the Camaro in the SCCA Trans-Am series. In order to make the
Camaro competitive, Chevrolet introduced the Z/28 option package which
included among other things, a special 302 cubic inch small block. The engine size was a result of the
SCCA's 305 cubic inch displacement limit in the Trans-Am series at the time. The 302 turned out to be
one of Chevrolet's finest small block offerings, and the engine stood in stark contrast to the ever
increasing size of the big blocks used in the muscle cars of the day.
To
arrive at the SCCA legal 302 cubic inches, Chevrolet used a 4.00" bore and a 3.00" stroke
resulting in a very oversquare (the bore is larger than the stroke) combination. In overly simple terms and
ignoring such important design factors such as bore / stroke ratio, an engine with a short
stroke has the ability to rev higher due to slower piston speeds. As an example, think of
two engines, A and R. Engine A has a stroke of 3.00", while engine B has a
stroke of 4.00". If both engines are turning at a speed of 4000 rpm, the pistons of engine A
have less distance to travel than those of engine B. Since both engines are
turning at the same rpm, the pistons of engine R have to cover more distance in the same amount of time as
the pistons of engine A, resulting in higher piston speeds. Short stroke engines,
therefore, can run higher rpm with greater reliability and less stress on the reciprocating assembly.
This is the approach Chevrolet took when designing the 302 for SCCA competition.
The
blocks used in 1967 were casting number 3892657. These small journal blocks
were also used for 327 and 350 cubic inch engines as well (all three engines used a
4.00" bore). 1968 models used block casting number 3914678 and featured the new style large
journals. The 1968 block was also used for the 327/210 hp and 350 295 hp SS engines. The blocks used in 1969
featured thicker webbing around the mains and used nodular iron 4 bolt caps. A common misconception is
that 1967 and 1968 302's were 4 bolt blocks, while actually the only engine to use 4
bolt main caps was the 1969 version.
The
crankshafts used were forged steel, tufftrided pieces in all three years. 1967 models used a small
journal crank with 2.000" rod journals and 2.299" main journals. 1968 and 1969 models used
a large journal crank featuring 2.100" rod journals and 2.449" main journals. The forged cranks were
deemed necessary due to the high rpm the 302s were expected to see.  The connecting rods varied considerably from year to year. 1967 models used what was
the standard small journal rod of the time with a pressed in wrist pin and 5/16" rod bolts. Two styles of rods were
used in 1968, the first being a strong large journal rod using a pressed pin and larger
3/8" rod bolts. Middle production 1968 rods were changed to a floating wrist pin design.
Both early and late style 1968 rods were shot peened (a stress relieving process) at the factory. 1969
engines continued to use the late 1968 style floating wrist pin rod.
All
302s used a special baffled oil pan (the baffles prevented the uncovering of the oil pump
pickup as a result of the g-forces generated during acceleration, braking and cornering) and a high
pressure oil pump.
All
1967 and early production 1968 302s used a pressed pin design forged
aluminum piston with an 11.0: 1 compression ratio. Late 1968 production pistons switched to a
floating pin design. The 1967 and early 1968 pistons used a dome design taken from the
327/350 hp engine, and featured two individual valve reliefs in the dome. Late 1968 piston design
has a long notch-type valve relief cut across the entire dome. 1969 models featured a new 11.0:1
compression impact extruded piston with slipper skirts and a floating wrist pin. The dome design
was the same as the late 1968 pistons.
Cylinder
heads used on the 302s can be a bit confusing. In 1967, two different
cylinder head castings were used on the 302, 3917291 and 3890462. Both heads used 2.02" intake and
1.60" exhaust valves. The confusion arises from the fact that the 3890462 casting was also produced using
smaller 1.94" intake and 1.50" exhaust valves, although this head was never used on the 302. 1968
heads were also produced using casting number 3917291, but 1968 heads included a provision for a water
temperature sensor not included on 1967 heads. Valve sizes were again 2.02" and
1.60" intake and exhaust respectively. To further complicate matters a small valve
version of the 3917291 head using 1.94" and 1.50" intakes and exhausts was used on the SS 350
engines in 1968. So far that's two different versions of the 3890462 cylinder head and
three versions of the 3917291 cylinder head! Both castings had the
familiar "double hump" machined pad located on each end of the head. 1969 engines used head
casting number 3927186 and featured the large 2.02" intake and
1.60" exhaust valves of the earlier heads. 1969 heads also have holes drilled and tapped in the ends due to
a change in the alternator mounting method. All of the 302 cylinder
heads, regardless of casting number, had the same characteristics. Large port volumes and
large valves were used to facilitate breathing at high rpm, at the expense of low to
midrange efficiency.
Fortunately,
identifying the camshafts used in the 302s is much more straightforward than the identifying the
cylinder heads. All three year model 302s used the famed solid lifter "30/30"
camshaft, so named because of the .030" intake and .030" exhaust valve lash
adjustments. This camshaft was also used in the 1964 and 1965 special high performance and
fuel injected 327's installed in the Corvette. Again, due to the high rpm nature of the 302,
a solid lifter camshaft was chosen. Solid (or mechanical) lifters require maintenance more
often than hydraulic lifters, but are more reliable at high engine speeds
than the hydraulic lifters. Hydraulic lifters tend to "pump-up" at higher rpm, which leads to valve
float. This is not as much of a problem today due to the advances in valvetrain design, but
was quite a concern in the mid-'60s. Specifications for the cam are .452" intake and
.455" exhaust lift, 229 degrees intake duration and 237 degrees exhaust
duration (both measured at .050" tappet lift) and 78 degrees of overlap (at 0 lift).
Chevrolet
needed an appropriate intake manifold to take advantage of the other
high performance engine pieces, and they settled on a single four barrel aluminum high rise design.
The basic design of the manifold remained unchanged from 1967 to 1969 although two different
casting numbers were used. 1967 and 1968 engines used casting number 3917610, and the intake featured
the thermostat hole located off-center toward the drivers side of the car. The engine temperature
sensor on 1967 models was located in a drilled and tapped hole next to the thermostat opening. On
1968 intakes, this hole is plugged due to the relocation of the sensor to the head. 1969 intakes, casting
number 3932472, centered the thermostat hole and are otherwise unchanged from the earlier
intake. As
an over the counter option in 1969 (available through the parts department, never
installed by the factory), the Z/28 buyer could order a dual four barrel aluminum cross ram
intake manifold (casting number 3940077). This intake was designed so that longer intake runners and two
carburetors could be used while fitting under the stock hood. In 1969, the intake came with a
special ZL2 cowl induction hood and air cleaner. This intake performed
poorly on the street, but when used on high rpm competition engines (the engine was designed for SCCA racing,
after all) really came into it's element.
Holley 800 cfm dual-feed carburetors were used on all 302s, although list numbers
differed somewhat on 1967 models. Cars built at the Norwood, Ohio assembly plant used list number 3910
carburetors only, while cars built at the Los Angeles plant used list
number 3910 and list number 3911, the latter being used on cars equipped with an
AIR (Air Injection Reactor) emissions system. California laws mandated
that any new car sold within the state must be equipped with an emissions
control system, thus the difference in carburetors.
 All 1968 and 1969 302's used an AIR system thereby allowing the use of the same
carburetor (list number 4053) regardless of assembly plant or car destination. The use of Holley
carburetors on the 302's followed a Chevrolet tradition of using Holleys on their high performance engines. In
fact, the 4053 carburetor also saw duty on the 396/375 hp big blocks of 1968. Holleys are still
used on the vast majority of carbureted competition engines and offer a high degree of adjustability,
performance and reliability.
The
exhaust manifolds used on 302s in 1967 and 1968 were little more than the standard "log"
type found on most small block equipped passenger cars. The only major difference between years is non-AIR equipped
1967 versions have no provisions for smog tubes. Chevy must have felt that the majority of Z/28
owners would bolt on their own headers if so desired. In 1969, Chevrolet made available a tube header
option for the 302 engine (when this option was ordered, the engine in the car came equipped with exhaust
manifolds while the headers were shipped in the trunk of the car) along with a low restriction chambered
exhaust system. The chambered exhaust was discontinued in May of 1969 due to problems with
passing noise laws.
All
302s used a standard single point type distributor, although the advance
curves were
optimized for the needs of the 302.
What
all of these various pieces added up to was a healthy small block rated very conservatively at 290
horsepower at 5800 rpm and 290 ft. lbs. of torque at 4200 rpm. Rumor has it, however, that the same
engine produced 350 horsepower at 7000 rpm on the dynamometer! Why would Chevy underrate
the engine? Certainly insurance reasons come to mind, along with the desire to understate what
the engine was capable of lest the various racing sanctioning bodies penalize the teams that chose to
run the Z/28 in competition.
Road
tests of the day praised the engine that Chevy put together. While most testers found the lack
of "bottom end" power a nuisance, the 302 more than made up
for that shortcoming with a very strong top end rush. One tester likened the pull of the Z/28 in
the upper rpm range as being similar to a 426 Hemi! That's high praise indeed for an engine with
two-thirds the displacement. Quarter mile times were in the low to mid 14-second bracket, depending upon the
conditions the test was performed under. Modified 302s used in Trans-Am racing generated in the
neighborhood of 450 horsepower, which is an amazing amount of power from 302 cubic inches with
enough
reliability to win SCCA championships in 1968 and 1969.
The 302
powered Z/28 was very successful in both NHRA drag racing and SCCA Trans-Am racing from 1967 to 1969. The
Penske team with Mark Donahue dominated the Trans-Am series in 1968 and 1969 (winning
manufacturer's championships for Chevrolet), while in 1968 Dave Strickler won the IHRA Super Stock world
title in a 302 powered Z/28. That the engine and thus the car was so successful so soon after
its release is a testament to the effort that went into the project by Chevy's engineers. It is,
without a doubt, one of the all-time great small blocks produced by Chevrolet. |
