Also known as the Vortec 7400, the L29 was the final and most modern iteration of the venerable big block Chevy. It was available from 1996 to 2000 in 2500/3500/3500HD trucks and 2500 Suburbans. Factory power rating was 290 HP at 4000rpm and torque was 410 ft-lbs at 3200rpm.
The L29 is considered the Gen 6 big block, with 4 bolt mains and a 1 piece rear-main seal. It was upgraded with a hydraulic roller camshaft, revised cylinder heads, 9.4:1 compression ratio, and a long runner multi-port fuel injection system.
The cylinder heads are unique to the L29/L21, featuring small oval ports with a swirl inducing “ski ramp”. The small 100cc combustion chambers allow a decent 9.4:1 compression ratio with flat top pistons, quite an improvement over the 7.9:1 compression of the previous generation L19. The valve train consists of non-adjustable, bolt down 1.70:1 rocker arms and weak valve-springs the are barely adequate for the stock camshaft. While not as high flow as the large ovals and rectangle port heads of older big blocks, they are a marked improvement over the previous peanut port heads.
Modifications
The factory calibration on these engines is considered to be very conservative, with a good bit of power to be found in aftermarket tuning.
The Gen 6 block is considered by many to be a good starting point for a nice street 454, since it comes already set up to support a hydraulic roller camshaft, and will accept most aftermarket heads without much difficulty.
The L29 responds well to a small cam, matching valve springs, headers, and a tune.
Common issues are the injectors and or regulator leaking. 19lb Bosch “Yellow Top” injectors are a drop in upgrade. They are commonly available as they were installed in the 5.0HO Fox Mustangs and many other Fords. Be careful as the plastic injector clips are easy to break. The fuel injectors are located under the upper manifold, but not inside the manifold as with the 5.7L vortec “spider” CFI injection.
This makes the L29 much more flexible for modifications as larger injectors are readily available.
Another common upgrade is swapping the factory ECU for the more tunable 411 ECU found in later model GM’s. It has much wider aftermarket support and can be tuned with common packages such as HP tuners and EFI live. Installing one involves carefully re-pinning the ECU connectors to the new ECU, and having the 411 tuned to support your engine and transmission combination (as there are no completely factory tunes for the L29).
The L21
The L21 is a different version of the Gen 6 that can be found in medium duty trucks like the Kodiak, Topkick, and Gm motorhomes. It is basically the same as the L29, and parts can physically be swapped between the two. The L21 has coil near plug ignition like the later 8100 vortec and LS based engines. This gives it a unique set of valve covers with bosses to bolt the individual coils to. It also has a special distributor stub that is used as the cam sensor, but has a smaller cap since it is not needed distribute the spark to the spark plug wires. The L21 has a different ECU to support the individual coil ignition system and different cam/crank signals. Finally, it is reported to have a forged crankshaft and rods.
Detailed Specifications
| Basic Description | |
| Displacement | 7.4L (454ci) |
| Bore | 4.25in |
| Stroke | 4.00in |
| Compression Ratio | 9.4 : 1 |
| Firing Order | 1-8-4-3-6-5-7-2 |
| SAE Net Horsepower @ RPM | 290 @ 4000 |
| SAE Net Torque @ RPM | 410 @ 3200 |
| Intake System | |
| Induction | Long Runner MPFI |
| Fuel Injectors | 19lb/hr(21lb/hr at GM installed pressure) |
| Camshaft | |
| Intake Duration | 204 @ .050 |
| Intake Lift | 0.480″ |
| Exhaust Duration | 209 @ .050 |
| Exhaust List | 0.483″ |
| Lobe Seperation | 120 degrees |
| Valvetrain | |
| Туре | Nonadjustable, push rod actuated |
| Lifters | Hydraulic Roller |
| Rocker Arm Ratio | 1.70:1 |
| Cylinder Heads | |
| Туре | Cast Iron Small Oval |
| Chamber Volume | 100cc |
| Intake Volume | 235cc |
| Intake Valve Diameter | 2.06in |
| Exhaust Volume | 82cc |
| Exhaust Valve Diameter | 1.72in |
| Head Flow Chart | |
| Lift | CFM (I/E) |
| .100 | 63/49 |
| .200 | 121/91 |
| .300 | 165/120 |
| .400 | 198/133 |
| .500 | 226/140 |
| .600 | 242/142 |
