What's involved in getting a piston engine to spin the other way?

Question

I've been thinking about this problem ever since I had to slightly crank my BMW's engine the opposite way to insert a crank-locking pin.

What are the fundamental changes required to make an engine spin the other way?

Here are my thoughts:

• the short-block would be unchanged since there is nothing on the crank, connecting rods or pistons that are direction-sensitive.

• the cylinder head(s) may improve efficiency if reworked but the engine should still run without having to change or flip the positions of the intake and exhaust valves.

• I believe new camshafts would be needed, but am unsure since it could be that the intake and exhaust camshafts just need to be swapped over (it's a bit difficult to simulate that in my head).

• Firing order may have to change in order to maintain identical harmonics. If so, would it be sufficient to just flip the crankshaft around?

To clarify, I'm not interested in other auxiliaries like the alternator, water pump or power steering pump - just what needs to be engineered on the engine in order to get it to run the other way.

Answer 1

There's actually quite a bit involved in making an engine run correctly in reverse. Here's the run down as far as I can tell (hopefully I won't miss something obvious):

• Block/Crank/Connecting Rods: These should be standard between either Counter Clockwise Rotation (CCWR) or Clockwise Rotation (CWR) engines.
• Pistons: Looking at a V-style engine, when you look at a piston and how it's made, you'll understand it's made shifted a little towards the thrust side. This means that the piston pin boss is offset to one side slightly to make up for the V and rotation of the engine. This offset helps keep the piston square in the bore and reduces piston/cylinder wear when running under load. With this in mind, you'd need to run your pistons offset to the opposite side. I'm sure this would take specially built pistons, because on a lot of V-type engines, valve reliefs in the piston face are for one direction. I believe even inline engines have pistons offset to compensate for thrust load because the angle of the connecting rod will always be anything but inline with the cylinder bore (on any production engine, anyway).
• Camshaft: Since the camshaft is the "brain" of the engine, it makes sense that you need a cam shaft which is built to run the engine in the opposite direction. Since custom cams are common place these days, getting one built should not be an issue. On most engines which are built to run CCWR, the only thing which runs in reverse is the crankshaft. While the lobes are cut differently with timing, the only thing which needs to change is how it's timed. With Chevy V8's, they usually run a direct drive (helical gear cut with direct drive instead of a timing chain). Here's a picture of one:

• Oil Pump: You'd need to have the oil pump do its thing in the opposite direction. If the pump is driven off of a camshaft like the old school Chevy V8, since these are usually run in the correct rotation, it will run as normal. Others would require a whole different pump setup where the oil passages could be reversed to allow the gears to work correctly. A dry sump may be an easy fix for this, or a bunch of machining/rewelding of an existing pump to make the inlet/outlet holes go in opposite directions.
• Distributor/Ignition system: Modern engines with electronic control units (ECU) ignition systems require new programming. You just have to tell the computer what's going on, when to expect an even to occur, and when to deliver the spark. The reluctor ring just tells the computer where the crank shaft is at. As long as it knows where it's at, it will know what event should take place when as long as it's programmed correctly.
• Starting: This is an obvious one ... you need to either get the starter to work in reverse (rewired to work in the opposite direction) or turn it around somehow so it will work from the opposite side (usually a nose cone on the starter which allows you to mount the starter on the opposite side). There are options for many standard engines which are worked to run CCWR which allow you to do either.
• Timing Belt: If the engine were designed originally with a timing belt, as @DucatiKiller stated, you'd have to run the tensioner on the opposite side, or somehow compensate for it, otherwise the belt would be prone to slippage due to it not providing the amount of tension in the right places. Slack would appear where it wasn't intended to.
• Windage Tray: If the engine is equipped with a windage tray, you'll have to figure out some way to collect the air the other way. This might not be too hard if the tray is louvered. Pound down the louvers so they are flat, then punch out the opposite side so it will then collect the wind/oil as it flings towards it. (NOTE: this was an addition in edit.)

Answer 2

If it's a two stroke gasoline engine, you can just start it backwards. Occasionally, two-strokes will start incorectly and run backwards on their own. See this video.

Two-stroke boats will often reverse polarity on the starter to run the engine backwards to reverse the boat then kill the engine and restart to run normally.

From this Wikipedia article.

Regular gasoline two-stroke engines will run backwards for short periods and under light load with little problem, and this has been used to provide a reversing facility in microcars, such as the Messerschmitt KR200, that lacked reverse gearing. Where the vehicle has electric starting, the motor will be turned off and restarted backwards by turning the key in the opposite direction. Two-stroke golf carts have used a similar kind of system. Traditional flywheel magnetos (using contact-breaker points, but no external coil) worked equally well in reverse because the cam controlling the points is symmetrical, breaking contact before top dead center (TDC) equally well whether running forwards or backwards. Reed-valve engines will run backwards just as well as piston-controlled porting, though rotary valve engines have asymmetrical inlet timing and will not run very well.