Why is it bad to turn an engine backward?

Shel_B

Shel_B

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What damage may result from turning an engine backward, opposite in the direction it is designed to run?

Do all engines, when running, rotate in the same direction? What is the reason engines are designed to rotate in the direction they do?
 
4 cycle engines run like this: 4 modes. Intake, compression, combustion and exhaust..

An engine can be designed to run either direction. However, you can not have the exhaust valve doing it's thing before the intake and combustion takes process. I'm not good on explaining this...sorry!
 
Shel_B said:
Do all engines, when running, rotate in the same direction? What is the reason engines are designed to rotate in the direction they do?
Click to expand...

How do you define the direction an engine rotates in? A Honda F23 engine, as found in 90s 4-cylinder Accords, rotates counterclockwise when viewed from the cam pulley end of the crankshaft. Most engines are clockwise. For these Hondas, the engine is mounted on the (US) driver's side of a front-drive car, with the transmission on the passenger side.

I have read that some Mistubishis had CW engine rotation, but the engine was also mounted on the driver's side. So from another perspective that was a reverse-rotating engine. They had a transmission with an extra reversal in it to get the axles going the right way. But that is a less efficient transmission.

Some old two-strokes were made to run either direction...
 
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My Cobalt has occasionally made one chug back due to a rather large number of parts needing replacement to fix the parking brake, I generally predict the direction of roll but have occasionally misstepped

Oops still runs
 
Rmay635703 said:
My Cobalt has occasionally made one chug back due to a rather large number of parts needing replacement to fix the parking brake, I generally predict the direction of roll but have occasionally misstepped

Oops still runs
Click to expand...
It would generally take more than that to jump time
 
brages said:
How do you define the direction an engine rotates in? A Honda F23 engine, as found in 90s 4-cylinder Accords, rotates counter-clockwise when viewed from the cam pulley end of the crankshaft.

Some old two-strokes were made to run either direction...
Click to expand...
That would translate to CW rotation from the flywheel end. I have read some (all?) Honda with engine on the DS are this way.

Looking from the "flywheel" end, a longitudinal mounted engine should turn A-CW. From the front "balancer" end" it will of course spin CW.

Modern I.C. automotive engine direction of rotation was standardised by SAE along with engine cyl numbering that has been different between manufacturers. But it looks like Ford and GM just kept doing what they were doing all along :)

Aircraft and marine using twin screw engines may have reverse rotation on one engine.

I will say this CW D.I.R. is not failsafe; when I broke the motor mount on my'65 Buick the engine torqued over clock-wise and pulled the throttle wide open ... Hang ON !
 
To answer the OP question. The Oil Pump will spin backwards, for one.

On a V8 pushrod engine, I don't see a problem if you are doing it to adjust valve lash or move the timing mark into position; just finish turning in the correct direction of rotation when sneaking up to your hash mark.

I would not roll down a hill backwards for a distance with the car in top gear. That could mess up a modern engine I suppose,

- Ken
 
I suspect the original direction of engine rotation was chosen back in the early 1900's before electric starters were born and they picked CW rotation viewed from the front to make hand cranking the engine easiest for right handed people. From there it got passed along each year.

I have had one car that the engine rotated CCW when viewed from the front of the engine. It was a 1990 Accord and the drive train was installed reversed from every other FWD car I've owned. The transmission was on the passenger side and the engine was on the driver side. I suppose that is why the engine rotation was reversed too.
 
I'm not sure it's really that bad for it, to turn it backwards with a wrench on the crank (plugs out) at least for older engines. Certainly I did it many times with pre-2005 autos to bring it back to TDC and set ignition timing (mostly Euro & Japanese). I actually used to push start my Datsun B210 in reverse (bad ring gear, I would park going uphill and let it roll back on it's own and pop the clutch) but of course the engine was still spinning forward lol.
 
ARCOgraphite said:
To answer the OP question. The Oil Pump will spin backwards, for one.

On a V8 pushrod engine, I don't see a problem if you are doing it to adjust valve lash or move the timing mark into position; just finish turning in the correct direction of rotation when sneaking up to your hash mark.

I would not roll down a hill backwards for a distance with the car in top gear. That could mess up a modern engine I suppose,

- Ken
Click to expand...
Not only that but with the old school piston stop tools you'd rotate the engine backward A LOT until it hit the stop in both directions. Seems fine on old pushrod motors but I cringe thinking about someone trying this on most modern OHC/DOHC motors
 
The great Smokey Yunick apparently designed an engine to run in the opposite direction to take advantage of the torque reaction on the chassis. Legend has it when the tech inspector showed up, Smokey slammed the camshaft on the concrete floor and let them have two lobes to measure, but they didn’t notice the ramps in the lobes were a mirror image.
 
Snagglefoot said:
The great Smokey Yunick apparently designed an engine to run in the opposite direction to take advantage of the torque reaction on the chassis. Legend has it when the tech inspector showed up, Smokey slammed the camshaft on the concrete floor and let them have two lobes to measure, but they didn’t notice the ramps in the lobes were a mirror image.
Click to expand...

Smokey Yunick a true legend. Way too smart for his time!

Just my $0.02
 
There are both clockwise and counter clockwise turning engines. Even unmodified some will run in the wrong direction.
It is not good practice to turn any over head cam belt drive or chain drive engine in reverse direction, the tensioner could compress and would allow some slack and possible slippage.
 
Interesting topic. GG's explanation about hand-cranking is correct. Many Honda engines ran "backward", which required not only orienting the engine opposite usual to avoid an extra gear stage, but also required counterclockwise alternators and water pumps .
I have trouble believing that rotating an engine backward a little by hand will screw up a well made cam drive. I've done it many times on my previous cars. Obviously you shouldn't do it many revolutions at higher speed because the oil pump wouldn't function.

Large ship engines that drive the propeller directly have to be designed to run both ways.
 
I miss my old VWs with the timing belt tensioner a simple eccentric pully with a jam nut. Bullet proof, and reverse spin proof. No hydraulics!
 
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