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Race Engine Technology Magazine

In some cases of engines with reduction units to drive the propeller, it can be an advantage to have the crankshaft rotate in the opposite direction as the propeller. (For a complete discussion of that subject, see Prop Rotation.)

For example, the GTSIO-520 crankshaft turns in the opposite direction of a standard, direct-drive IO-520, because the gear reduction unit on that engine is a single-mesh type, in which the output shaft rotates in the opposite direction of the input shaft. By reversing the crankshaft rotation, the GTSIO-520 can use the wide range of available propellers designed for clockwise (from the cockpit) rotation.

Reversing the direction in which the crankshaft of a given engine can add considerable extra expense because of the nonstandard pieces needed.

Following is a summary of the changes needed in the standard SBC / BBC / SBF / BBF V8 engines, and others that are similar in architecture.

1. A special crankshaft with the rod-journal oiling passages drilled to the opposite side of the rod throws in order to inject the oil into the bearing cavity on the compression stroke, so that the oil film is fully developed to support the loads during the combustion stroke. For more information on that science, see Hydrodynamic Bearings and the Lubrication section of Crankshaft Design.
   This is a significant issue. As an example, the early versions of the Falconer V-12 that powered the Thunder Mustang used crankshafts from the Falconer parts room, which were drilled for conventional rotation. Those V-12's, in the Thunder Mustang, turning in the "reverse" direction, so that the propeller rotation would be "American Standard", experienced rod bearing problems early on. Falconer fixed the problem by commissioning a run of V-12 cranks that had the conrod-oiling holes drilled properly for reverse rotation.
2. A special camshaft, because:
       (i) reversing the crankshaft reverses the firing order (no, you can’t just use a single-mesh gear-drive to run the cam in its normal direction, and running it backwards opens the intake during the exhaust cycle and the exhaust during the intake cycle);
       (ii) for the engines listed above, which have distributor ignition and the oil pump driven by the helical gear on the cam, those items would turn backwards if the camshaft did not include an opposite-direction helical gear.
   If an external oil pump is used, the distributor problem can be cured by using a distributor engineered for reverse rotation - the centrifugal and vacuum advance mechanisms would need to be modified, and the lubrication scheme either verified for reverse rotation or modified.
3. Special accessory drive provisions:
        (i) the OEM coolant pump will run in the wrong direction, and
        (ii) the alternator(s) will run in the wrong direction.
   The OEM coolant pump will pump a small amount of coolant, but the flow will be significantly restricted by the housing cavity being oriented to capture and direct the flow coming off the vanes when it is rotating in the designed direction.
   There are reverse-rotation coolant pumps available that will bolt into the OEM location and flow correctly. In the reverse-rotation engines I have done, I have used a special, external coolant pump and an external oil pump also.
4. Reverse rotation crankshaft seals.....this is important becahse the "standard" crankshaft seals have tiny helical ridges in the contact surface, with the helix angles oriented so as to draw oil back into the engine. Reverse rotation seals have those helical ridges oriented in the opposite direction, so as to draw il back toward the engine.,
5. specially-engineered oil pan (especially critical in dry sump systems);
6. reverse-rotation starter,
7. and a host of other details depending on the ignition and oiling systems used.

The conversion is a bit more challenging in the newer engines that run the oil pump off the nose of the crankshaft, such as the highly-regarded GM - LS family of engines. The distributor issue is replaced by a need for a reprogrammed ECU that can provide the new firing order.

However, the oil pump issue is more challenging. Here, I am specifically discussing "LS" engines, because those are the only "newer" engines with which I have any hands-on experience.

These engines would need either:
     (i) a specially-engineered gerotor pump to work in the opposite direction, or
    (ii) removing the oil pump but leaving the gerotor pump drive-spline in place {because it provides the axial spacing for the crankshaft vibration absorber (no, it is not a "damper") to be seated against, and, BTW, it also contains the chain sprocket for the camshaft drive}, then blocking off the main oil gallery that opens into the oil pump / cam drive cavity, and installing an external oil pump that turns in the correct direction and provides a pressure regulator valve.

Given those considerations, keeping the standard crankshaft rotation (clockwise viewed facing the waterpump-end of the engine) and using a gearbox with an idler to maintain that direction of prop rotation will appeal to some builders.