 <<Back to pg. 4<< Intake
Manifold:
Z/28 manifold No. 3927610 is an aluminum high-rise tuned-runner type, designed to mount a
single Holley 4-barrel carburetor. Any size Holley from 500-960 CFM airflow can be
mounted. Stock is a 780 CFM Model 4053 or 3943. No manifold porting is necessary, but
opening up of ports to match manifold gasket and head ports is recommended. Do not remove
the center divider from the manifold below the carburetor. Recently released for service
is a 2 x 4 barrel intake manifold with two (2) 600 CFM Holley carburetors under part
number 3940077 (carburetor conversion unit). These carburetors are jetted satisfactorily
for competition as released. A later-design carburetor with accelerator pumps on both
primaries and secondaries has been released for improved performance under part No.
3957859.
Fuel Pump:
If possible, use an electric fuel pump to boost the engine mechanical fuel pump, such as a
Stewart-Warner piston pump. The 1963-65 Corvette high-performance fuel pump No. 6415325 is
the most satisfactory for high-performance usage.
Flywheel and Clutch:
Optionally available from Chevrolet isa 15 lb. nodular-iron flywheel No. 3866735, and a
heavy-duty 10.5” clutch (No. 3886066 cover and No. 3886059 disc). This flywheel and
clutch are presently released for the 427 cu. in. L-88 engine, so are more than adequate
for 302 usage.
Carburetor:
Use the No. 3923289 Z-28 or similar 780-800 CFM Holley carburetor. This carburetor has
1-11/16” throttle bores. Satisfactory jetting for most running conditions is no. 72
jets in both primary and secondaries. If richer or leaner mixture is desired, change all
jets up or down in size. If necessary, the 600 Holley from earlier model mechanical cam
327’s can be used. Jetting should be similar to production for 327 model. Connect
secondaries to operate mechanically by putting a small bolt in the secondary return
quadrant on left side of carburetor if mechanical secondaries are desired. A single 600
Holley No. 3957859 (cross ram carb) with double accelerator pumps and mechanical
secondaries will also work extremely well.
Distributor:
The production Z/28 distributor is a conventional breaker point model. Optionally
available is the transistor system distributor No. 1111267. Also available is distributor
No. 1111263 which is a ball-bearing unit with a tach-drive camshaft. This distributor is
for gear drive camshaft 427’s and requires that you change the distributor drive gear
to a chain-drive model. Do not hook up the vacuum advance. Run 38-40 degrees maximum
advance.
Exhaust System:
A satisfactory tuned open exhaust system is mandatory to extract maximum torque and
horsepower from these engines. Correct dimensions for such a system are 1-3/4” OD by
30” long head pipes collected in a group into 3-1/4” to 3-1/2” collector
tailpipe. 3-1/2” tailpipe is preferred for any installations requiring more than
36” of tailpipe.
Operating Conditions:
The following are several recommended operating specifications and limits that should
ensure long and satisfactory service from an engine built to the foregoing instructions.
Oil - 30 to 50 weight aircraft or other ashless high-performance oil (DA, Valvoline,
etc.). An oil with ashless additives is specified to prevent preignition and burnt
pistons.
Fuel - The best super premium available. Examples are 260 Sunoco 115-130 octane aviation
gas, or Pure Racing gasoline.
Spark Plugs- Champion J6lY, J60R, J86Y, J83Y or UJ60P for track racing; J63Y, J6J, or
UJ64P for drag racing.
Spark Advance - Maximum of 38 to 40 at high RPM.
Valve Lash - .030” intake, .030” exhaust for production cam No. 3849346.
.022” intake, 024” exhaust for service cam No. 3927140 and 3965754.
Maximum Speed for optional cam is 6800-7200 RPM for track racing; 7800 RPM for drag
racing.
Maximum Oil Temp - 300 degrees in oil pan
Maximum Fuel Pressure- 4-5 PSI at high engine speeds. Satisfactory electric fuel pump is a
Stewart-Warner piston-type pump. In operating conditions where fresh air is ducted to the
carburetor (fresh air hoods, plenum air cleaners, etc.), it is necessary that an air
cleaner element or low restriction element be used to diffuse the air entering the
carburetor. If no diffuser
is used, the engine mixture distribution will be upset, causing poor power and misfiring
at high engine speeds.
SPECIFICATIONS FOR 3927140 CAMSHAFT ONLY
Power Team or Option No. RPO Z-28
Model Year 1968
Camshaft Part No. 3927140
Camshaft Casting No 3849347. Lifter Type: Mechanical. Rocker Arm Ratio: 1.5:1
Timing: (See NOTE below) INTAKE EXHAUST
Checking Clearance .020 .020
Opens (BTC) (BBC) 53” BTC 101 BBC
Closes(ABC) (ATC} 100” ABC 65” ATC
Overlap 118”
Duration 333° 346°
Valves:
Head Diameter (Max.) 2.02” 1.60”
Angle of Seat & Face 45° 45°
Lift (Max .) .473” .492”
Springs: “143”
Outer Valve Closed (Max.) 110 @ 17” Same
Outer Valve Open (Max.) 260 @ 1.25” Same
Inner Valve Closed (Max.) N/A N/A
Inner Valve Open (Max.) N/A N/A
NOTE: Timing and lift specifications must be those found at the valve when checking
clearances are set as specified above.
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