Calling Mr. Cosworth Vega

[Chuck R.]

Hi Duke,

I have a four cylinder power plant related question.

If a cylinder was not firing at highway speeds, could that cylinder as it's not under what I'll call a combustion load, possibly produce a bearing knock?

I hope it's one of those crazy Chuckster yes or no posers that won't require alot of key board work.

Thanks,

Chuck

[Mike Cobine]

Re: Calling Mr. Cosworth Vega

Since you are not in combustion, the downward stroke would pull a vacuum, so instead of the top of the journal seated against the crank as on the upward stroke (compression), the bottom of the journal would seat against the crank.

If you have slop, this would maximize the movement and get a knock.

Kind of like bearing rattle on an old worn engine when you pull off the gas runnign down a hill.

Of course, let's hope that the oil pressure is still floating the journal enough you are not really seating against the crank but just close.

[Mike Cobine]

Re: Calling Mr. Cosworth Vega

Since you are not in combustion, the downward stroke would pull a vacuum, so instead of the top of the journal seated against the crank as on the upward stroke (compression), the bottom of the journal would seat against the crank.

If you have slop, this would maximize the movement and get a knock.

Kind of like bearing rattle on an old worn engine when you pull off the gas runnign down a hill.

Of course, let's hope that the oil pressure is still floating the journal enough you are not really seating against the crank but just close.

[Duke W.]

Re: Calling Mr. Cosworth Vega

No. Regardless of what's happening in the combustion chamber you still have full hydrodynamic lubrication at the bearing, so a misfiring cylinder should not cause bearing knock regardless of the number of cylinders.

Duke

[Duke W.]

Re: Calling Mr. Cosworth Vega

No. Regardless of what's happening in the combustion chamber you still have full hydrodynamic lubrication at the bearing, so a misfiring cylinder should not cause bearing knock regardless of the number of cylinders.

Duke

[Mike Cobine]

Re: Calling Mr. Cosworth Vega

It could be he has it already, but under normal load conditions, he doesn't hear it knock. Under these loads (compression, then vacuum), the slop becomes readily noticable.

It won't cause knock, but it will let you hear it if it already exists.

[Mike Cobine]

Re: Calling Mr. Cosworth Vega

It could be he has it already, but under normal load conditions, he doesn't hear it knock. Under these loads (compression, then vacuum), the slop becomes readily noticable.

It won't cause knock, but it will let you hear it if it already exists.

[Chuck R.]

Re: Well!

Sounds like the ole beast (1980 Honda Goldwing) will have to go under the knife this winter

Apprieciate the help,

Chuck

[Chuck R.]

Re: Well!

Sounds like the ole beast (1980 Honda Goldwing) will have to go under the knife this winter

Apprieciate the help,

Chuck

[Duke W.]

Re: Calling Mr. Cosworth Vega

As long a bearing has sufficient oil supply, it can develop several thousand psi hydrodynamic pressure at typical operating speeds. If the bearings are off sufficient size that the total bearing load does not exceed hydrodynamic pressure that the bearing can generate, the bearing will never contact the journal. If there is no load on the bearing the journal will center in the bearing bore. As load increases the journal offsets slightly, but the more it offsets the higher the hydrodynamic pressure it will generate, which limits the offset to a few ten-thousands of an inch for typical engine bearings.

Typical engine bearings are sized to limit unit loading on the bearing to achieve high fatigue life. Hydrodynamic considerations are rarely the limiting issue.

If the bearings ever did touch the journal at normal road speed revs, the bearings/journal would quickly score and possible seize. The only time you might have any jounal/bearing contact is during cranking, but the boundary lubrication of the oil film in the bearing is usually enough to prevent significant contact if the surfaces are smooth and at cranking speed the load is very low.

At high revs the greatest bearing load is at the top of the exhaust stroke since there is no compression or combustion pressure to offset the inertial tension load on the rod as the piston goes over the top.

Duke

[Duke W.]

Re: Calling Mr. Cosworth Vega

As long a bearing has sufficient oil supply, it can develop several thousand psi hydrodynamic pressure at typical operating speeds. If the bearings are off sufficient size that the total bearing load does not exceed hydrodynamic pressure that the bearing can generate, the bearing will never contact the journal. If there is no load on the bearing the journal will center in the bearing bore. As load increases the journal offsets slightly, but the more it offsets the higher the hydrodynamic pressure it will generate, which limits the offset to a few ten-thousands of an inch for typical engine bearings.

Typical engine bearings are sized to limit unit loading on the bearing to achieve high fatigue life. Hydrodynamic considerations are rarely the limiting issue.

If the bearings ever did touch the journal at normal road speed revs, the bearings/journal would quickly score and possible seize. The only time you might have any jounal/bearing contact is during cranking, but the boundary lubrication of the oil film in the bearing is usually enough to prevent significant contact if the surfaces are smooth and at cranking speed the load is very low.

At high revs the greatest bearing load is at the top of the exhaust stroke since there is no compression or combustion pressure to offset the inertial tension load on the rod as the piston goes over the top.

Duke

[Michael W.]

Now that you've mentioned that it's a Gold Wing

it's probably the primary chain that's hitting the inside of the engine casing, due to stretch. My '79 has been doing the same thing since I passed the 150,000 km mark sometime in the early 1980s. I'll get around to fixing it when the rest of the engine needs work (ie likely never)

I guess pulling a heavy camping trailer all over North America and crossing over top of the Alps several times while riding two up was just a tad more than Honda expected. And they call it cheap junk.

If you do have a cylinder misfiring at idle but clears at moderate engine speeds, you may find the intake valves are slightly crudded up with deposits. A good midnight run at redline usually shakes things loose.

Amazing what info you can get on a Corvette website, huh?

[Michael W.]

Now that you've mentioned that it's a Gold Wing

it's probably the primary chain that's hitting the inside of the engine casing, due to stretch. My '79 has been doing the same thing since I passed the 150,000 km mark sometime in the early 1980s. I'll get around to fixing it when the rest of the engine needs work (ie likely never)

I guess pulling a heavy camping trailer all over North America and crossing over top of the Alps several times while riding two up was just a tad more than Honda expected. And they call it cheap junk.

If you do have a cylinder misfiring at idle but clears at moderate engine speeds, you may find the intake valves are slightly crudded up with deposits. A good midnight run at redline usually shakes things loose.

Amazing what info you can get on a Corvette website, huh?

[Michael H.]

Re: Calling Mr. Cosworth Vega

Gotta agree with the Duke on this one. A cylinder missfire cycle is no different than the very next revolution or cycle where there's pressure on the upstroke followed by by vacuum on the downstroke and that causes no abnormal knocking. The compression and power stroke would normally both be under high pressure but the next cycle, the exhaust and intake cycles, would first act as compression on the connecting rod/bearing, then tension, the same positive/negative forces that would apply during a misfire.

[Michael H.]

Re: Calling Mr. Cosworth Vega

Gotta agree with the Duke on this one. A cylinder missfire cycle is no different than the very next revolution or cycle where there's pressure on the upstroke followed by by vacuum on the downstroke and that causes no abnormal knocking. The compression and power stroke would normally both be under high pressure but the next cycle, the exhaust and intake cycles, would first act as compression on the connecting rod/bearing, then tension, the same positive/negative forces that would apply during a misfire.