The Gen II block.
The Gen II block is different from the Gen I block in that it has aluminum
heads and splayed valves. The stock Gen II camshaft was the same as the "high
output" cam
used in some Gen I blocks. The Gen II block has better flowing manifolds and
the combustion chamber shrank from ~ 48 cc in the iron heads to 28 cc. in the
aluminum heads. The Gen II block made it's debut in the J-body platform in late
1987, just in time for the birth of the "Gen
II" J-body platform. The Gen II block existed in the J-body until 1994.
(the last year for the Gen II J) Only minor changes were made over the course
of those 8 years. In 1990, the 2.8L was stroked to become a 3.1 L. (other platforms
had received this update a year earlier) With the change to 3.1 in 1990, the
location of the vacuum line that goes from PCV to intake moved from the lower
intake manifold to the upper intake manifold. The EGR valve was changed
from a vacuum operated one in 1989 to a digital EGR in 1990. Computer tuning
was changed in 1992 to lower the RPM in which the 3.1 made it's peak torque.
The Gen II block was Multi Port Fuel Injection (MPFI) and was designated with
either 2.8 MPFI or 3.1 MPFI on the plenum.
The Gen III block
The Gen III block never saw action in the J-body platform. GM opted instead
for a 2.3/2.4 L twin cam in their Gen III version of the J-body. What a shame!
In reality, the Gen III block made its debut in 1994 in the L -body (Beretta)
and the N-body (Grand AM) platforms. GM knew that the V6 days were numbered
in the J-body, and did not see it reasonable to make the changes to install
it in their 1994 J-body models. 1994's Gen III engine was an improvement
in that it had remarkably better manifold flow. The oil pan was now made
better with a reinforced cast aluminum. A windage tray had been added to improve
power and oiling. The Gen III engine had an oil galley added below the cam
bore. These engines ran on a new fuel injection system called SFI (Sequential
Port Fuel Injection). This required a cam sensor which is not seen in Gen
II blocks/cams. The new block also got a hydraulic roller cam/lifter system,
reducing friction and allowing for "lumpier" cams,
with higher ramp profiles. The stamped rockers in 1994-95 had a 1.5:1
ratio and oil deflection tabs above the oil feed hole in the rocker where the
pushrod is. Other non-performance related changes include: A different timing
cover, power steering pump, alternator/bracket (except in '94), EGR, fuel
rails, and other numerous less important changes which of course we will add
later
1996 saw the first improvements in the Gen III block. This was the first year
in which the 3400 (3.4 L SFI) was built. the 3400 was a bored 3100 (3100/3400
crank are the same), and had a 1.76" intake valve, 0.004" increase from the 3100.
Both engines were fitted with 1.6:1 roller fulcrum rockers. The 3400's
manifolds were also an improvement over the 3100's. The 3400 was fitted with a
56 MM throttle body a 4 MM increase over the 3100.
In 2000, GM made more changes. The 3100 now shares the same intake
manifolds (with different labeling on the upper of course), and throttle body with the 3400.
The 3100 also got the larger intake valve that the 3400 had already had for 4 years.
Thus making 3100 and 3400 top ends the same from 2000 on. Both the 3100 & 3400
got new exhaust manifolds that have short runners to help flow, and decrease
back pressure over the old "log" style manifolds.