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 10 November 2015

- "LS"-based Crate Engines -

LS-2, LS-7

LS-2 Crate Engine

The GM LS-2 crate engine makes an interesting candidate for a lightweight liquid-cooled aircraft powerplant, which might provide reasonable reliability and power in the factory configuration. The LS-2 weighs less than 390 pounds complete (less flywheel, alternator, and accessory drives).

The curves below show the power and torque for the stock (green) LS-2 (364-CI) engine compared to a (black) 445 HP EPI developmental (402-CI) LS-based engine and the (red) 500 HP EPI Gen-1 (427-CI) engines.

LS-2 Performance

Note that the power peak for the LS-2 is 400 HP at 6000 RPM, but keep in mind that the engine is tailored for high-performance street vehicles (and for the ability to advertise 400 HP). At 6000 RPM, the mean piston speed is 3620 fpm. Sustained operation at that high a crankshaft speed puts far too much loading on valvetrain and reciprocating components for acceptable aircraft reliability, without even discussing the use of a cast crankshaft and powdered-metal conrods.

(It is interesting to note that every aircraft piston engine designed to operate at sustained mean piston speeds over about 3100 fpm have been short-lived.)

Because of the rather flat torque curve, a stock LS-2 might be a reasonable aircraft powerplant while operating substantially below peak power. If the takeoff and cruise RPM are set for 4750 and 4000 respectively, the engine provides a respectable sea-level power of 360 HP for takeoff (4750 RPM, mps = 2865 fpm) and 300 HP for max cruise (4000 RPM, mps = 2413 fpm). We have demonstrated with previous engines, that mean piston speeds (mps) of 2600 and 2900 fpm for cruise and takeoff respectively are very reasonable for extended life (all other things being right).

For the 360 HP range, a propeller which can turn in the vicinity of 2600-2700 RPM might be suitable (see Selecting the Right Propeller). For that speed range, the Mark-15 gearbox with a reduction ratio of 1.724 or 1.821 would make a reasonable powerplant. For greater power, the prop would probably like to turn slower (and higher aircraft speed would demand it), so the large selection of ratios in the Mark-15 give great flexibility of design and implementation.

HOWEVER, before running off to buy an LS-2, take a close look at the Engine Conversions section of this site, espacially the page titled Evaluating a Conversion. Bottom line: does 360 HP justify an engine package in excess of 750 pounds?

And, keep in mind, the engine uses a cast iron crankshaft (instead of the high-grade Nickel-Chrome-Moly steel used in certified and high-performance engines), powdered-metal connecting rods (no comment necessary), cast aluminum pistons, and a plethora of other components designed for the automotive engine load scenario.

LS-7 Crate Engine

During late 2006, we had a new LS-7 engine in our shop for evaluation as a potential aircraft powerplant. This engine represents a new era in factory-crate engines. It is based on the GM Gen-4 LS series, with 427 cubic inches of displacement (4.125 x 4.000) and its own special block, crankshaft, rods, cylinder heads and valves.

The block and heads are all-aluminum. The ports and combustion chambers are CNC-machined and provide very high flow numbers. The crankshaft is forged-steel instead of the currently-popular castings. The conrods are titanium (in an unspecified alloy).; The pistons are castings; The 1.8 ratio rocker arms have roller trunnions. The engine has 2.20 inch titanium intake valves, 1.61-inch sodium-filled exhaust valves, a dry-sump oil system, and much more.

Techniques which were previously limited to custom engine shops are used in the production of this engine. For example, the final honing on the (special) block is done with deck plates torqued in place and with the main caps installed and torqued. Each engine is assembled by one technician, who signs the worksheets. Each engine is test-run at the GM engine factory before shipping.

This engine produces 505 HP at 6300 RPM and 470 lb-ft of torque at 4800 RPM. As pictured (complete) below, it weighs less than 380 pounds! It is probably as close to a race-engine as one will ever see in production. (NOTE the standard, hydroformed, double-wall stainless steel headers).

LS7 Right Side

LS7 Left Side

The blue lines in the graph below shows the stock LS-7 torque and power curves. Included in this graph are the torque and power curves provided by the 427-CI EPI Gen-1 aircraft engine (red), and initial projections of the EPI-modified aircraft LS-7 (black).

These EPI engines are tailored to aircraft operation (sustained high output at max mean piston speed of 3000 fpm (15.2 m/s).  As shown in the graph, the stock LS-7 torque curve peak is 475 lb-ft. at 4800 RPM and the power curve peak is 505 HP at 6,300 RPM, at a mean piston speed of 4200 fpm (21.3 m/s). That, like most high-performance automotive engines, is too high for sustained aircraft operation.

LS7 Comparative Performance

The pictures below show some of the high-tech (for production engines) componentry used in the LS-7 engine.

This picture shows the roller rockers and the special intake ports used in the LS-7 head. The new LS-7 head design has been rolled over to 12 degrees (LS-2 and 6 are 15°), the intake ports have been raised and reshaped from the familiar "cathedral" shape of the other LS family engines. The LS-7 uses special, 1.8 ratio roller-trunnion rocker arms. The exhaust rocker is in-line, but the center of the pushrod socket in the intake rockers is offset about 0.300" to the right, which tilts the intake pushrod enough to clear the wider intake port. The new intake ports flow in excess of 340 CFM. (For reference, the best of the Gen-1 smallblock heads we developed for our 500 HP engine flowed about 320 CFM while maintaining an excellent velocity profile).

LS7 Head Intake Port

The heads have 2.20-inch titanium intake valves and 1.61-inch, hollow-stem, sodium-filled exhaust valves (for substantially-reduced valvetrain effective mass), beehive valve springs which avoid the problems associated with focused spring resonant frequencies.

LS7 Valves & Springs

The LS-7 has a forged steel crankshaft (opinion from GM seems to vary as to the material: we have been told both 4140 and 4340 by different sources.)

LS7 Crankshaft

This picture shows the LS-7 piston and connecting rod. The pistons are high-grade castings (by Mahle) in an allegedly hypereutectic alloy, and provide the engine with a static compression ratio of 11:1. The connecting rods are made from a titanium forging. We have been unable to determine the titanium alloy used. We asked several highly-placed GM sources, but none would reveal the alloy. However, when we mentioned the reason for the deep interest in the LS-7 was the potential use in an aircraft, one very knowledgeable GM tech source said he would DEFINITELY recommend replacing the OEM rods with a stout set of aftermarket H-beam rods.LS7 Rod and Piston-2 LS7 Rod and Piston-1

The LS-7 has a dry-sump oiling system with an internal two-stage oil pump (pressure and scavenge) driven off the nose of the crankshaft (same location as the single-stage LS-1&2 pump). The scavenge pump has approximately 1.5 times the capacity of the pressure pump.

LS7 Dry Sump Pump

LS7 Dry Sump Pump Pieces

For all the exotic equipment and specifications this engine has, we are somewhat disappointed by the relatively low 11.6 bar BMEP (168 psi) which the stock engine produces. That BMEP is undoubtedly the by-product of the optimizations required for modern automotive applications, including (a) emissions requirements, (b) fuel economy standards, and (c) the astounding wide-range operating response and flexibility which modern engines provide.

It is our current opinion that a stock LS-7, in normally-aspirated form, limited to 4500 RPM for takeoff (400 HP at sea level) and 3900 RPM for cruise (330 HP at sea-level), and operated exclusively on a diet of 100-LL avgas, might be used as a reliable aircraft powerplant. That is only an opinion, and there are no test data to back it up.

We have been unable to determine the alloy of steel used in the crankshaft. Of greater concern, we have been unable to determine the titanium alloy used in the conrods. BUT, in view of the fact that a set of high-grade, performance titanium rods cost roughly half as much as the whole LS-7 engine, we cannot help but doubt that this engine contains that quality of conrod, and therefore, we seriously question the use of these rods in an aircraft application, just as we question the use of cast pistons and other production automotive pieces.

Note that we could easily carve a sample piece of metal off an LS-7 crank and rod and have the material analyzed in a mass-spectrograph. However, we aren't several-thousand dollars curious, and neither are any of our clients.

The reason we think the stock engine should be limited to avgas is that the high 11:1 static compression ratio and sustained full-throttle operation near peak torque will produce very high cylinder pressures which will invite detonation on mogas. The ECU strategy for dealing with detonation is based on the occurrence of transient events, and is handled by removing spark advance until the incipient detonation subsides. That is fine for an automotive transient, but causes EGT to rise dramatically, which is not terribly good for exhaust valve life.

In order to provide both normally-aspirated and supercharged versions of an LS-based (see EPI Gen-2 engines) aircraft engine producing in the 500-hp range at a mean piston speed less than 3000 fpm, we determined that several of the internal parts of the LS-7 engine would need to be replaced in order to achieve both the required performance and adequate reliability.

Although the LS-7 is a remarkable product, the crate engine is quite expensive (GM factory list price in 2006 was $17,495) and the availability has been somewhat limited. The base cost of the engine, coupled with the limited availability and the extensive modifications needed to transform it into an aircraft powerplant with the performance and reliability we require, make it less interesting as the basis for a 427 CI engine. The same effect can be achieved at a lower cost by purchasing the required components at the beginning, rather than spending a huge sum for a crate engine and then subsequently replacing many of the expensive parts.

The Bigger Issue

A major issue with these engines is the fact that all the LS engines are electronically controlled by a dedicated computer system (aka Engine Control Unit, or ECU). That system (computer, sensors, injector solenoids, spark coils, and wiring) completely controls the engine's fuel delivery and the spark timing, and a host of other variables as well. Many of the newer engines have "fly-by-wire" throttles (no direct connection between the gas pedal and the butterfly). Although the level of control provided by these systems is remarkable, there remains the problem of reliability, fail-safe operation, and the question of "What do I do when the engine fails (or even hiccups) at when I am 300 feet off the ground on takeoff at Vx ?" CLICK HERE for an expanded discussion of electronic engine controls in an aircraft application.

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