- 620 to 725 HP Big-Block Based Engine -
Ideal Cropduster Engine
EPI has developed a high-power, relatively low-cost GM big-block-based engine for aircraft applications in which the existing engine is very heavy (such as the Pratt & Whitney 1340 radial used in several cropduster applications). The components used by the factory in this engine are the same or similar to the components which a competent engine builder would use, but they come already assembled in a complete engine. The list of components is amazing: AQ-4340-non-twist-forged crankshaft with full counterweights, AQ-4340 H-beam connecting rods, forged aluminum (4032) pistons, excellent high-flow aluminum cylinder heads, etc.
In order to determine the suitability of a virtually unmodified engine for demanding aircraft usage, the first version of this engine was relatively unmodified and is being readied for testing in a cropduster application.
EPI developed an aircraft-suitable accessory drive system and an aircraft-suitable 18-quart wet sump oil system. The oil system can maintain oil pressure in any pitch / roll attitude up to 50 degrees away from straight and level. This configuration is shown below, with the EPI Mark-15 gearbox.
The EPI accessory drive for the Stage-1 engine is shown in the next picture. It features self-adjusting dual-serpentine drives, with both belts driving the coolant pump at the optimal speed for the required heat rejection. The coolant pump pulley features a fence between the two belts, so that if either belt fails, it will not derail the other belt, and the coolant pump will continue to be driven by the remaining belt. The alternator and air-conditioning compressor (it is usually HOT in cropdusting country) are driven at speeds suitable for long life.
In the Stage 2 version, EPI replaced the carburetor-based fuel system with a mechanical fuel injection system, a specially-developed intake manifold, and specially developed valvetrain components to make the engine more suitable for aircraft applications. The following curve shows the power and torque for the engine in factory-delivered (blue) configuration, and the (green) Stage-1 and (red) Stage-2 EPI configurations.
Note that the Stage-1 EPI version found a significant power increase over the factory configuration in the upper RPM range. This was mainly due to (a) the EPI oil system substantially reducing windage losses, (b) the tailored header design, and (c) some small but significant changes in the valvetrain.
In contrast, note the shape of the (red) power and torque curves for the Stage-2 EPI engine. This version was specially developed to produce the torque curve shown, with a high, narrow peak in the cruise-to-takeoff range, while maintaining an acceptable mean piston speed in cruise (2900 fpm). The other two versions (blue & green) are based on the factory cam profile and manifolding, which GM developed with street-vehicle usage in mind, with the preponderance of it's life spent around 2500 RPM at less than 100 HP, and occasional full-throttle bursts to the 6000 RPM (4375 fpm mean piston speed) redline.
The takeoff RPM for both the Stage 1 and 2 engines is 4500 RPM, for a mean piston speed of 3280 fpm. That is higher than we like to use, but in the targeted application, takeoff power is a very low percentage of the operating model, and the Stage 1 engine is a test anyhow.
The installation weight of this powerplant is such that it is only suitable for an aircraft which is equipped with a heavy powerplant and propeller to begin with. The complete powerplant as pictured above (with accessories and gearbox) weighs 763 pounds. Add a 160 pound propeller & spinner, a 56-pound engine mount, heat exchangers, other cooling system components, air intake system, ductwork, exhaust system, oil filter and plumbing, cowling, etc. and the entire installation weight is approaching 1250 pounds.
This site will be updated with current information on the project as it develops.
