Always Shuts Off At 90°

[bugguy1967]

I've heard before that a healthy engine always lands at 90° when you shut it off. Without looking at the valve positions or thinking about it too much, I'ma guess that 90° ATDC or BTDC is just a point where it's easy for the engine to stop at (friction-wise, as in springs aren't compressed very far, or no pressure is building in a cylinder).

My question purely out of curiosity: is it always 90°? Do higher spring pressures, more compression, wilder cam profiles, etc change everything?
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"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

[vwracerdave]

Maybe one cylinder might be at 90* but there is no way all 4 can be 90* so that is a pure myth.
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2017 Street Comp Champion - Thunder Valley Raceway Park - Noble, OK
2010 Sportsman ET Champion - Mid-America Dragway - Arkansas City, KS
1997 Sportsman ET Champion - Thunder Valley Raceway Park - Noble ,OK

[67 Sunroof]

Maybe he is talking about 90* on the distributor?

[HotStreetVw]

crank is flat plane. All four cylinders would be at 90, either BTDC or ATDC.

I've heard the healthy engine thing before...

Friction from the rings is acting against the crankshaft, so if you assume frictional force is the same anywhere along the bore, when the rod is near perpendicular to the crank (near 90 degrees) it will provide the highest resistance to rotation.

Cam timing and lobe aggressiveness. When your at full lift, the torque required to rotate is low even though the springs at the highest force. You would need spring rate. I would think this is low compared to the tension from the rings (short moment on the cam)
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4inBhore
50 Split - 2724cc NA. Haltech injected
62 Notchback - 2542cc Turbo WIP

[Bugsy61]

[FreeBug]

I think I read someplace that the engine stops on the weakest cylinder. Anyone else ever hear that?

[KTPhil]

Mazda experimented with a stop-start engine based on this concept. It used a computer-controlled valve train and stored compression to kick the engine to restart it, without exercising the starter motor. Clever thinking, but I don't think it ever went into production.

[Steve Arndt]

Doug Berg would always check his pulley after a run. A sign of a valve spring going bad would be if the engine didn't stop in it's usual positions. He posted this on the CLF like 20 years ago.
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Steve
Steve's 87 Syncro project

[Bugsy61]

I'd say when the energy remaining in the rotating mass is less than the energy needed to reach the top of the next compression stroke, regardless of what cylinder that happens to be, the engine will stop.

[Erik G]

I've found it to be true, but I'm not a crusty old man scared to listen to people or learn new things...

I read it on facebook so it must be true... but it was from Clyde and I tend to listen and learn when anyone Berg talks about VW's
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[bugguy1967]

Oh sorry guys. I meant the pulley. It's either 90° ABDC or BTDC. Can't remember which.
_________________
"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

[chrisflstf]

I've noticed that it is easiest to check for end play on a built motor with TDC being at 3 oclock or rotated at 90 degrees CW from TDC. But if end play is normal, you wont feel anything. If its around .010, you will feel it at that position easier. That seems to be where a motor I have now, likes to stop after running

[modok]

[67 Sunroof]

FWIW Briggs and Stratton had a “compression relief valve” on the end of their motors. It would relieve the compression in the cylinder after you stopped the motor.
When you went to re-start the motor it would be easier on the starters
Ask me why I know...I bought a Bobcat zero-turn from a guy for $1200 after he bought it for $6500 and burned up 6 starters and was tired of messing with it. I researched and found that to be the issue and had Briggs replace it for free.
That was 10 years ago-still running like a champ!

[bugguy1967]

[modok]

Ok, but still the same.