The differences in air and fuel management between the LT1 and LT4 engines are relatively minor. The major components of this aspect of the engine are the intake casting and the cylinder head casting. This chapter covers the different sensors required to keep the engine alive and some minor changes in the development of the LT series.
Intakes: LT1 vs. LT4
The main engineering difference between the LT1 and LT4 intake manifolds are that metal was added to the top of the gasket surface of the LT4 intake because the LT4 cylinder heads have a raised intake port location. But, in raising the port, the factory did not enlarge the LT4 port; the location was simply changed its location. Because of this, the ports do not ﬂow as well as the original LT1 version.
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As mentioned, metal was added to the LT4 intake manifold to ensure proper gasket sealing. General Motors did not change the shape of the port interior; it changed the shape of the mounting ﬂange where it mounts to the head. After many airﬂow tests, we have noticed no discernable difference between the LT1 and LT4 intake manifolds. The performance of the two are exactly identical, given they are mounted to the exact same cylinder head conﬁguration.
The bolt patterns on the LT1 and LT4 intakes are vastly changed from the previous Gen I (1955-up 265- to 400-ci) small-block. It goes without saying that early-style intake manifolds simply do not ﬁ t on Gen II LT-series heads. Both LT1 and LT4 intakes feature a one-piece cast- aluminum design with no coolant crossover passages.
Several intakes are available to help you swap out the fuel injection for a 4-barrel conversion. again, make sure that you obtain the correct intake, as the Gen I, early- style intakes do not bolt on.
It is very important to note that the LT4 intake does, in fact, bolt onto a set of LT1 heads However, the ports do not match, and there is interference with the valve cover gasket rail on the LT1 heads.
The 1992–1993 LT1 uses a 22-pound/hour injector, the 1994– 1997 LT1 uses a 24-lbs/hr injector, and all LT4s use a 26-lbs/hr injector.
How to Size a Fuel Injector
When you increase your LT engine’s power, it may require fuel injectors capable of ﬂ owing more fuel to adequately support the engine’s new-found capability. Fuel injectors are offered in a wide range of sizes, and they are rated in pounds of fuel per hour that they are capable of ﬂ owing. This is represented as lbs/hr.
If you have a target horsepower number in mind, or if you are following the blueprint of another engine that has been built before (that you have horsepower ﬁgures for), you can use the following mathematical formula to determine the proper injector for your engine:
Injector Size = (horsepower X BSFC) ÷ (number of injectors x .8)
Where: BSFC = brake speciﬁc fuel consumption
.8 represents the duty cycle of the injectors, since injectors can comfort- ably function at 80-percent duty for an extended period of time
As an example, let’s use a 500-hp engine, running at .40 BSFC (which is within a normal range for an efﬁcient engine), and using 8 injectors:
(500 x .4) ÷ (8 x .8) 200 ÷ 6.4 31.25 lbs/hr (per injector)
When purchasing injectors, you want to round up to the next higher number to ensure the injector you end up with has sufﬁcient capabilities. Typical injector sizes jump are 24 lbs/hr, 36 lbs/hr, 42 lbs/hr, and 55 lbs/hr, so we’d choose 36 lbs/hr injectors for this particular engine.
If you are serious about performance, exhaust headers are mandatory equipment. Factory exhaust manifolds are not very efﬁcient, and upgrading to headers is a sureﬁre way to gain power and performance throughout the RPM range. Freeing up the exhaust also contributes to improved fuel economy ﬁgures, due to improved effciency.
It’s generally understood that a long-tube header aids in lower-RPM power more than a comparable short-tube header. It’s also believed that a short primary tube length has a power advantage over the longer tubes at higher RPM. A properly-sized header helps throughout the RPM range, and determining the right length of header your engine wants before purchasing a set is a wise move.
So, how do you determine the best possible header diameter/length for your engine/application?
Input the cubic-inch displacement of the engine for “CID.” Input the maximum RPM of the engine into the equation. Input the Out- side Diameter of the header tube for “OD” in the equation. Multiply the CID by 1,900 and note the result.
Multiply the maximum engine RPM by the OD squared, and note the result. Divide the ﬁrst result by the second result to obtain the size of the pipe header.
Header Tube Length = (CID x 1,900) ÷ (peak RPM x tube OD2)
Average Header Pipe Diameter(s) Pipe diameter is determined by cubic-inch displacement and peak RPM. There is a complex formula that can determine the optimal pipe diameter, but for LT1/LT4 engines, we only need to determine the best- possible choices for engines from 350 to 406 ci, spinning from 6,000 to 7,000 peak rpm.
Based on the headers offered for LT1-equipped cars, we can determine that 350-ci engines beneﬁt from
1.5-inch head pipes, 383-ci engines beneﬁt most from 1¾-inch head pipes, and 396-plus-ci engines need 2-inch-diameter head pipes. These are based on 6,000-rpm redlines. Raising the RPM limit is justiﬁcation for bumping up to the next larger pipe diameter offered.
Electronic Control Module
This is, of course, the dreaded “Black Box” of doom that strikes fear into the hearts of the bravest me chanics. Housed in an aluminum case, the “brains” of the electronic control module (ECM) house a variety of electric parts. The most well-known component of the ECM is the PROM (also called a chip). The ECM also contains a Cal Pak, or calibration package.
Many people are surprised to learn that the ECM has a “self-learn” capability. This allows the computer to make slight calibration tweaks to accommodate variations in operating conditions such as gasoline octane level, air temperature, and driving mode. One of the problems when testing new items for GM’s TPI system is that all ECMs do not seem to learn at the same rate.
When making changes, you will be instructed to disconnect the battery. This is for many reasons, chieﬂy safety. This also clears (or “dumps”) the ECM’s memory of learned data. When power is reconnected, the ECM has to learn your driving habits all over again. When driving for the ﬁrst time after power has been re-established, the vehicle may seem a little dead during part-throttle operation. It learns very quickly, so it doesn’t take very long for a good portion of its ﬁnal performance potential to return. We have found that this can occur as quickly as within 1 to 2 miles of driving. However, we have tested some vehicles that have not reached their full potential for 50 to 75 miles! It all depends on which ECM you have.
It deserves to be mentioned that newer ECMs (both factory and after- market) have much faster learning functions. As computing power has increased in these engine control units, the time they take to “learn” has decreased in kind.
Let us reiterate a major point: When you have a road test after a modiﬁcation such as an airfoil, adjustable fuel pressure regulator, air ﬁlter, spark plug wires, PROMs, or others, you deﬁnitely see gains immediately, but it may take time to realize their full potential. There have been a few reports from drivers who can feel gains of .10 second in the 1/4-mile. We all become accustomed to the performance of our cars, so minor differences may not be noticed immediately. Additionally, it always helps to have the proper testing equipment to keep an objective eye.
There are ﬁ ve basic ECU part numbers for the LT1/LT4 engine family. We typically use the last three or four digits of the full part number to identify a given ECU. As an example, a 1992–1993 Corvette ECU part number is 16163993, but we would call it a 3993. A 1994–1995 F-Body uses an 8051 ECU; a 1994–1995 Corvette uses a 133. For 1996–1997 a B-Body
uses a 4399 ECU; an F-Body uses a 921 ECU; and the Y-Body is 2148. You get the idea. Each of these ECUs has its own characteristics.
Modes of ECU Operation
There are three basic modes of operation: limp home, open loop, and closed loop. They all apply to both Mass Air Sensed and Speed/ Density systems.
This mode is designed to get you home in the event of a major system or sensor malfunction. It is also the mode that street rodders tend to use because of improper wiring. In limp- home mode, the TPS controls the fuel curve much like a Hilborn injector, and the timing is ﬁxed at about 22 degrees total. This is not very efﬁ – cient but it does maintain drivability and allows you to get home or to a repair shop if you have problems. The TPIS Adjustable Fuel Pressure Regulator is most helpful in improving the performance of engines running in limp-home mode.
This means that the system is not under the control of the oxy- gen sensor. This occurs during warm up and during wide-open-throttle (WOT) operations. The system uses spark and fuel tables that have been programmed into the PROM or the memory of the ECU.
This occurs when the oxygen has come up to temperature (about 600 degrees F) and is providing a signal back to the ECM. This is the mode that makes the TPI so nice to drive. Good throttle response, great idle, and good gas mileage all happen once the engine enters closed-loop operation and can rely on its programming.
Closed-loop operations are effective only during idle and part- throttle operation. At wide-open throttle, the ECM automatically switches to open-loop mode.
Written by Eric McClellan and Posted with Permission of CarTechBooks