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.

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    I think if you get into an engine with DOHC or OHC are potentially rules out. If it had a timing chain, the internal chain tensioner would be on the wrong side and if it had a belt the same would apply. – DucatiKiller Dec 19 '15 at 21:04
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    Because all chains and belts related to engine timing have the slack taken up on the back side of the tension. so, if the crank is pulling the cam along, after the cam you would place the tensioner to take up slack. If it was reversed then your timing changes more as the belt and chain stretch because their is more belt or chain to stretch. Additionally, that pully would now need to be able to take that force AND the tensioner would have a lateral force working against it to push it in and would never automatically take up slack. – DucatiKiller Dec 19 '15 at 21:12
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    Oil pump would probably need modifying. – HandyHowie Dec 19 '15 at 21:53
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    The reluctor ring on the crank would likely need turning over. – HandyHowie Dec 19 '15 at 23:57
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    And I hope when you say "spin the other way" you mean running in the opposite direction than normal. – Pᴀᴜʟsᴛᴇʀ2 Dec 20 '15 at 3:43

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:

enter image description here

  • 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.)
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  • If I've forgotten anything, please call me out on it and I'll add it to my answer. I think I've covered most everything. – Pᴀᴜʟsᴛᴇʀ2 Dec 20 '15 at 13:06
  • Paulster 2 you have forgotten the spring thread throwout clutch on the starter motor .If you have your auto sparkie reverse the motor by say reversing the series field you still have those mechanical issues to deal with. – Autistic Feb 24 '16 at 12:07
  • @Autistic - In a very detailed sense you are right ... I think I was including that under the starting bullet, but in a general sense. Thanks for the addition! – Pᴀᴜʟsᴛᴇʀ2 Feb 24 '16 at 12:10
  • The alternator will still produce power because it has internal rectifier diodes BUT the cooling wont be as good because the airflow is now backwards .Many fans blow better than they suck so you could overheat the alternator if this alternator fan issue is not adressed. – Autistic Mar 28 '16 at 0:33
  • @Pᴀᴜʟsᴛᴇʀ2 I've read somewhere that some reverse rotation engines (GM 5.7) require a different head gasket. Don't know why. – lofihelsinki Jan 17 at 11:57

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.

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  • I did this by mistake with my two-stroke SAAB 96. IIRC it was because I got the timing wrong. Four speeds in reverse was kind of amusing – once I got over the shock of letting the clutch out for the first time and having the car move in the opposite direction from what I expected. – dlu Sep 7 '16 at 4:16

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