# Can you reverse the turn of a Car Engine **URL:** \ **Category:** Factual Questions\ **Created:** [July 28, 2009, 12:36pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537 "2009-07-28T12:36:19Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![The\_Flying\_Dutchman](https://avatars.discourse-cdn.com/v4/letter/t/9fc348/32.png) [@The\_Flying\_Dutchman](https://boards.straightdope.com/u/The_Flying_Dutchman)\ **Post date:** [July 28, 2009, 12:36pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/1 "2009-07-28T12:36:19Z")
by changing the order of the firing of the spark plugs ? ---
**Author:** ![J-P\_L](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/j-p_l/32/3156_2.png) [@J-P\_L](https://boards.straightdope.com/u/J-P_L)\ **Post date:** [July 28, 2009, 1:42pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/2 "2009-07-28T13:42:33Z")
I know it is sometime done, for example, for tractor pulling, where they have multiple side-by-side engines, it is done to counteract the torque. I have no idea how it is done, it is most likely a very specialized application. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [July 28, 2009, 1:53pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/3 "2009-07-28T13:53:20Z")
On a two- stroke engine (boats, mowers, etc), you can get it to reverse cycle by cranking it the other way when starting it, but on a four-stroke (“car”) engine, the valve train will only work in one order. Switching plugs’ firing order doesn’t matter until you can set up the fuel delivery and exhaust expulsion and crank the engine to start in reverse rotation. You need the fuel delivered when the piston is coming up, so that it can be compressed with the valves closed. That is not gonna happen by switching firing orders. ---
**Author:** ![Rocketeer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocketeer/32/4437_2.png) [@Rocketeer](https://boards.straightdope.com/u/Rocketeer)\ **Post date:** [July 28, 2009, 1:56pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/4 "2009-07-28T13:56:11Z")
On a normal auto engine (four-stroke) you need a new camshaft, ground opposite to the normal one. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [July 28, 2009, 3:16pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/5 "2009-07-28T15:16:45Z")
> [@Rocketeer](#): > > On a normal auto engine (four-stroke) you need a new camshaft, ground opposite to the normal one. This is correct. If the original camshaft is used, the engine will pump air from the exhaust system to the intake system, and so fuel (either from a carburetor or fuel injection system in the intake manifold) will never reach the cylinders. Moreover, if there is a tensioner on the timing chain/belt, it will now be on the wrong side (the “tension” side instead of the “slack” side), and the chain/belt may experience some weird running dynamics/vibrations. A diesel engine, which injects fuel directly into the combustion chamber near TDC, could conceivably run backwards. Fuel is typically delivered shortly before top-dead-center, so when the engine is spinning backwards the fuel will be delivered shortly _after_ TDC, which will result in a loss of efficiency and possibly a hot-running engine. Having said all that, if we’re talking about a modern engine with electronic control, it’s likely the computer will not run anything if it sees the crank spinning backwards. Any engine blowing hot exhaust backwards through its intake manifold (which nowadays is typically plastic or aluminum, as opposed to the cast-iron exhaust manifold) will likely melt the manifold and also destroy sensitive instruments (pressure sensor, air flow sensor, intake temp sensor designed for low temp range, etc.). As was noted earlier, two-stroke engines do not rely on valves for breathing; instead, the piston covers/uncovers intake and exhaust ports in the cylinder wall, and it does this at the appropriate time whether the crankshaft is spinning clockwise or counterclockwise, so it will breathe in the correct way no matter which way the crank spins. It’ll have spark timing issues similar to the aforementioned diesel. Short version: changing the firing order of the spark plugs on a multi-cylinder engine won’t do anything to facilitate reversal of crankshaft rotation. ---
**Author:** ![Nunzio\_Tavulari](https://avatars.discourse-cdn.com/v4/letter/n/b9bd4f/32.png) [@Nunzio\_Tavulari](https://boards.straightdope.com/u/Nunzio_Tavulari)\ **Post date:** [July 28, 2009, 3:45pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/6 "2009-07-28T15:45:29Z")
I believe that even if you were able to get the engine going backward, the gears in the transmission and differential would be cut to turn in a specific direction. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [July 28, 2009, 5:16pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/7 "2009-07-28T17:16:57Z")
> [@Nunzio\_Tavulari](#): > > I believe that even if you were able to get the engine going backward, the gears in the transmission and differential would be cut to turn in a specific direction. The gear teeth in a drivetrain are cut to carry a load in either direction; this is why you can safely leave your car/motorcycle in gear when going down a hill. In fact, [engine braking](http://en.wikipedia.org/wiki/Engine_braking) is absolutely necessary for OTR trucks to keep their speed in check when descending long/steep grades; without it, they’d soon overcook their brakes, lose control, crash, burn, and die horribly. It’s also why you can have a reverse gear that will propel the car backwards. Sure, it’s a different gear from all the other normal “forward” gears inside the gearbox, but the gearbox output shaft (and differential) are spinning (and carrying load) in the reverse of their normal “forward” direction without complaint. One other issue that just occurred to me, with regard to the engine: If a four-stroke engine is spinning backwards, then so is the oil pump. The pressure-fed oil-film bearings throughout the engine (and also piston underside squirters, if this is a diesel) will not be pressure-fed, they will be sucked dry. If you’re able to get the engine to run backwards at all, it won’t run backwards for long unless you’ve devised a way to spin the pump in the correct direction. For lubrication, small two-stroke engines generally rely on oil mixed with the fuel; there is no oil pump to be spun backwards, so this is not an issue. Note that this is probably not true of the collossal two-stroke marine diesels, such as [this one.](http://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96-C) OTOH, a diesel likely has a high-pressure common-rail pump that is belt/chain/gear-driven, and so when spun backwards it wouldn’t develop pressure for the injectors anyway… ---
**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\ **Post date:** [July 28, 2009, 5:20pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/8 "2009-07-28T17:20:46Z")
> [@Philster](#): > > On a two- stroke engine (boats, mowers, etc), you can get it to reverse cycle by cranking it the other way when starting it, but on a four-stroke (“car”) engine, the valve train will only work in one order. This was actually the answer to a _Car Talk_ puzzler a while back. [Puzzler](http://www.cartalk.com/content/puzzler/transcripts/200850/index.html) [Answer](http://www.cartalk.com/content/puzzler/transcripts/200850/answer.html) ---
**Author:** ![Rocketeer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocketeer/32/4437_2.png) [@Rocketeer](https://boards.straightdope.com/u/Rocketeer)\ **Post date:** [July 28, 2009, 5:33pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/9 "2009-07-28T17:33:56Z")
> [@Joe Frickin Friday](#): > > The gear teeth in a drivetrain are cut to carry a load in either direction; this is why you can safely leave your car/motorcycle in gear when going down a hill. In fact, [engine braking](http://en.wikipedia.org/wiki/Engine_braking) is absolutely necessary for OTR trucks to keep their speed in check when descending long/steep grades; without it, they’d soon overcook their brakes, lose control, crash, burn, and die horribly. > > It’s also why you can have a reverse gear that will propel the car backwards. Sure, it’s a different gear from all the other normal “forward” gears inside the gearbox, but the gearbox output shaft (and differential) are spinning (and carrying load) in the reverse of their normal “forward” direction without complaint. I had heard that gear teeth in a gearbox are heat-treated on one side only, so that spinning them in reverse would result in accelerated wear, since now the load was on the un-heat-treated side. :shrug: ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [July 28, 2009, 5:45pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/10 "2009-07-28T17:45:13Z")
You could keep the same firing order and mirror-image the crankshaft and camshaft instead. You’d have to redesign all the accessories (starter and alternator too) to run in reverse as well. For competition tractors, aren’t those engines usually Allison V1710’s? Those were built in both clockwise and counterclockwise versions for use on opposite sides of the P-38 fighter, so maybe they don’t need rebuilding at all. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [July 28, 2009, 6:15pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/11 "2009-07-28T18:15:45Z")
> [@Rocketeer](#): > > I had heard that gear teeth in a gearbox are heat-treated on one side only, so that spinning them in reverse would result in accelerated wear, since now the load was on the un-heat-treated side. :shrug: This can’t possibly be true for OTR trucks. Engine braking is a physical requirement in hilly terrain; a big-rig headed west out of Denver on I-70 will not survive the descent from the first pass unless he’s using engine braking. Moreover, they employ various devices ([Jake Brakes,](http://en.wikipedia.org/wiki/Jake_brake) or [exhaust backpressure valves](http://en.wikipedia.org/wiki/Exhaust_brake)) to get the engine to absorb a LOT of torque in this manner, much more than you would get with simply taking your foot off the gas. While this does not spin the gearbox backwards, it does load the backside of the gear teeth; if these surfaces were not heat treated, they wouldn’t last long. As for cars, the same advice is given (use engine braking) when towing a trailer, and for the same reason, i.e. so you don’t fry your brakes when trying to decelerate all that extra mass. Presumably this advice would not be given if the backsides of the gear teeth were not up to the task. Last month I sold my motorcycle, with 135,000 miles on it. Bought it new ten years ago, and in all that time I was not shy about using the engine for decel. Last summer (at 130K miles) there was a problem with the gearbox’s input shaft bearings (long story, but it was unrelated to engine braking). I had the gearbox apart, and in the course of casually inspecting all the innards, I never saw any unusual wear on any faces of any of the gear teeth. Note that in any vehicle - whether it’s got a manual or an automatic gearbox - if you’re not putting it in neutral or putting the clutch in, you are loading the backsides of the gearteeth when you decelerate. This is true even if you don’t downshift (you just won’t load them quite as much as if you did). Yet another issue with running an engine backward: modern engines typically offset the wrist pin bore in the piston. The piston loads one cylinder wall during compression (because the con-rod is pushing at an angle), and loads the opposite wall during expansion (because the con-rod is now pushing the piston from the opposite angle). With no wrist pin offset, this loading transition happens pretty much at TDC, when combustion has already signficantly increased the pressure, resulting in a more violent transition (google “piston slap”). With the offset, the loading transition happens slightly before TDC, before combustion has built up a whole lot of pressure. The result is quieter running and better durability. If you’re running the engine backwards, you are effectively using _reverse_ wrist pin offset, which will result in noiser operation and reduced durability. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [July 28, 2009, 6:36pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/12 "2009-07-28T18:36:04Z")
> [@Joe Frickin Friday](#): > > Having said all that, if we’re talking about a modern engine with electronic control, it’s likely the computer will not run anything if it sees the crank spinning backwards. Does a modern engine control even have the ability to check for the direction of roatation of the crankshaft? I know it has a crankshaft position sensor, but why would someone program in a test for condition that will never occur during operation of the engine? > [@Rocketeer](#): > > I had heard that gear teeth in a gearbox are heat-treated on one side only, so that spinning them in reverse would result in accelerated wear, since now the load was on the un-heat-treated side. :shrug: Given the small size of the metal being heat treated, is it even possible to heat treat one side of the gear? Even if it is, it seems it would be easier to heat treat the entire gear. ---
**Author:** ![Deereman](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@Deereman](https://boards.straightdope.com/u/Deereman)\ **Post date:** [July 28, 2009, 7:12pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/13 "2009-07-28T19:12:26Z")
Certain two cylinder diesel engines in old JD tractors can run backwards, they are four strokes too. Was common when the tractor was lugged down to NEARLY stalling…they would cough and run backwards until it was shut down…blowing oil out the aircleaner, sucking in the exhaust. ---
**Author:** ![SanDiegoTim](https://avatars.discourse-cdn.com/v4/letter/s/85e7bf/32.png) [@SanDiegoTim](https://boards.straightdope.com/u/SanDiegoTim)\ **Post date:** [July 28, 2009, 8:03pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/14 "2009-07-28T20:03:12Z")
Anyone know why marine versions of the same engine run “backwards?” ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [July 28, 2009, 8:09pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/15 "2009-07-28T20:09:01Z")
> [@Philster](#): > > On a two- stroke engine (boats, mowers, etc), you can get it to reverse cycle by cranking it the other way when starting it, but on a four-stroke (“car”) engine, the valve train will only work in one order. This is true of two-strokes that use piston or reed induction valving, which is most of them. Rotary valves are common on model airplane engines, and show up on some motorcycles. Most of those won’t run backward, but there are exceptions. Two actual examples that speak to the OP: Because reversing gears would be gigantic, and a possible point of failure, large ship engines frequently have two sets of cam lobes, and to reverse, the engine is stopped, then started (using compressed air) in the opposite direction…a rather involved process carried out by the crew in the engine room…which is why the bridge uses a semaphore device instead of a throttle linkage connected to the engine itself. One of the Allison V-12 engines used on the P-38 lightning was reversed by end-for-ending the camshaft. This would not have worked on the alternative Rolls-Royce Merlin engines, and was the reason the inferior Allisons were used (well, that and to leave more Merlins available for the P-51s) ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [July 28, 2009, 8:13pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/16 "2009-07-28T20:13:04Z")
> [@SanDiegoTim](#): > > Anyone know why marine versions of the same engine run “backwards?” \ Propellers are directional. Maybe only one direction is available, so you build the engine to match the desired rotation of the prop? This wouldn’t make much sense, as it’d be cheaper to make a prop that spins the other way instead of an an engine. Alternative: two engines side by side, you want counter-rotating props so the torque from the two cancel instead of summing. Maybe not a major deal on a small pleasure craft (just trim the steering), but on a big vessel where efficiency makes a difference, you don’t want to waste energy fighting prop torque with rudder input. \ ---
**Author:** ![FE3O4ENAIL](https://avatars.discourse-cdn.com/v4/letter/f/ba9def/32.png) [@FE3O4ENAIL](https://boards.straightdope.com/u/FE3O4ENAIL)\ **Post date:** [July 28, 2009, 9:51pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/17 "2009-07-28T21:51:41Z")
The teeth of car tranny gears are induction hardened. The hardness at the pitch circle will be the same on driving/driven faces. The root of the tooth… now that is a differant story. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [July 28, 2009, 10:06pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/18 "2009-07-28T22:06:33Z")
> [@FE3O4ENAIL](#): > > The teeth of car tranny gears are induction hardened. Ah, that makes sense. Thanks. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [July 28, 2009, 10:07pm UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/19 "2009-07-28T22:07:48Z")
> [@FE3O4ENAIL](#): > > The teeth of car tranny gears are induction hardened. The hardness at the pitch circle will be the same on driving/driven faces. The root of the tooth… now that is a differant story. /aside I just figured out your username. 😃 ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [July 29, 2009, 12:45am UTC](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537/20 "2009-07-29T00:45:31Z")
Smokey Yunick, a legend in race engine and stock car building, built several cars that had the engine run backwards. This helped plant the right rear tire as the car exited left hand turns. As was mentioned upthread, this required custom camshafts, oil pumps and/or distributors (It seems to me that the oil pump and distributor could be stock so long as the distributor gear was cut for reverse rotation.) It is my understanding that stock car racing rules at the time required that the engines be built using factory parts, so he had the cams custom ground by the factory, which then stamped them with the production part numbers. [Next page](https://boards.straightdope.com/t/can-you-reverse-the-turn-of-a-car-engine/504537.md?page=2)