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 Why oil and what does it do?

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Pete Roper
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PostSubject: Why oil and what does it do?   Mon Jun 18, 2018 7:53 pm

Since there seems to be a bit of confusion about what the oil warnings are and how they should be interpreted perhaps it would be a good idea to actually have a squizz at what the oil actually does and how.

First up lets look at the three functions oil has in a motor. There are

1.) To LUBRICATE. This means preventing bits of the engine rubbing against each other, be they rolling element bearings, plain bearings or simply sliding surfaces. At all times any two parts should have a film of oil between them.

2.) To CLEAN. Very simple, a flow of oil will wash crap away to somewhere where it can be either filtered or drained out of the motor.

and 3.) To COOL. Probably the most important aspect of the oil's function is to remove heat because if heat isn't removed, parts will fail.

Now in your engine there is a fair bit of oil. 3.5 litres in a big block guzzi. This sits in essentially a big bucket at the bottom of the engine and its from this bucket that oil is drawn by means of a pump, (Or on the 8V two pumps.) and pushed through the engine. But why does it need to be pushed? Surely when the crank is spinning around ten to the dozen and the pistons are flashing up and down and all the other bits are thrashing about everything is going to be whipped up into a frenzy and oil is going to be getting thrown around everywhere. That will make everything slippery enough won't it?

Well. No, it won't. To understand why you need to understand how different sorts of bearings work.

If we start off with what are known as 'Rolling Element' bearings. These are versions of what most people think of when you mention a 'Bearing'. Usually spherical balls or tubular rollers running around between an inner and outer track known as races. Now these have a lot of advantages. They are very strong and because the balls and rollers *Roll* there is little need for heavy duty lubrication BUT because the inner race is smaller than the outer race the outer parts of the rolling element have to travel further than the inner parts so there is inevitably a degree of sliding and scuffing and for this reason there has to be a large throughput of oil to COOL.

The other type of bearing you will find in most modern motors are known as 'Plain' or 'Slipper' bearings and these actually work in a slightly more complex fashion.

With these you have a shaft known as the 'Journal' that rotates within a circular bearing with a small clearance between the two. To keep the two surfaces separate there has to be a film of oil between the two and the way that this is achieved is by feeding oil under pressure through galleries, (Usually in the journal.) to the gap between journal and bearing.

Now hold that thought because we'll be coming back to it but first we have to look at how a plain bearing actually WORKS.

Having a spinning bearing on a shaft is all fine and dandy but the oil film within the bearing has to be strong enough to prevent the journal and bearing touching. Now in a motor there are some fairly serious forces being exerted on the journal and bearing from a number of directions. Just take for example the big end of a connecting rod.

When the piston is rising against compression it will take a theoretical 14PSI in the cylinder and by the time its at TDC compression, if the engine has a 10:1 CR there will be a pressure above it of 140PSI. Now burn the charge and the heat tries to make the gas expand, but it can't because its in a confined space so what? The pressure goes up say by a factor of ten, so now you have 1400PSI trying to force the piston down and likewise force the bearing of the connecting rod through the oil film so it will rub on the big end journal.

Now oil is only being delivered to that bearing at say 50PSI but there is this huge force pushing on a very small area in the bearing so why doesn't it just force the bearing through the oil film and into the journal creating what is known as 'Boundary Lubrication'? Well this is where it gets very clever.

By having only a small clearance between the bearing and the journal as they spin, at the point where the greatest force is being exerted a rolling pressure front forms in the oil film known as the Hydro-dynamic wedge. Depending on the clearance this multiples the pressure of the oil manyfold thus protecting bearing and journal by keeping them apart.

As a 'Rule of thumb' in automotive there will be one thousandth of an inch of clearance for every inch of journal diameter but the smaller the clearance the greater the strength of the wedge. The problem is that you can't make the clearance too small otherwise there isn't enough throughput of oil too take away the heat generated by the forces acting within the oil itself. As the bearing tries to squeeze the oil out of the clearance it imposes massive loads on the oil itself. So much so that the molecules get forced closer together and heat up! If there isn't enough throughput the oil and bearing will overheat. The oil will break down or burn and the bearing will melt! Like all things, clearance is a compromise and a choice has to be made carefully about how much to give. Also vital for the formation of a safe wedge is the shape of the bearing and journal. They both have to be as close to perfectly round as possible to prevent variations in flow and pressure.

OK, so lets return to the oil in *our* motors. The actual quantity required in the sump is pretty much immaterial as long as there is *Enough* for it to be continuously picked up, do its job and be returned to the sump without exposing the pick up. A fine case of evidence for this is Mark's 1400 motor. It drank the vast majority of its oil and in fact when we drained out the remnants there was only about 600ml left in the sump! But, despite this, it had still been able to keep it circulating and prevent boundary lubrication. Yes there was a small amount of scuffing on the big end shells and interestingly clear evidence of damage to the shells due to vaporisation but luckily it hadn't had the chance to run itself completely dry so the pick-up was exposed otherwise we'd be looking at a new motor!

So as long as there is SOME oil in the sump you're pretty safe on that front. What the warning lights and error codes in the dash will tell you is if you have a PRESSURE problem. Nine times out of ten if you get a warning light or error code it will be a false flag due to the unbelievably crappy oil pressure sender switches but if you get the 'Oil Can' icon showing up its time to start getting serious. There can be several reasons why oil pressure can be lost and on the 8V's, early ones anyway, it tends to be the gasket between the sump spacer and the block blowing out. This gives no outward signs as the *Leak* is internal. Rather than the oil being delivered to the mains and big ends some of it is simply returning to the sump. Other causes can be blocked pick-ups, the oil pressure relief valve being held off its seat or problems with the oil pumps but these are incredibly rare.

So the long and the short of it is. If you get an oil warning via an OBD code don't ignore it, but don't panic either. Sure check your oil level but as long as its visible on the stick you've got lots of oil in there. Just clear the codes and see if they come back, they probably won't. Otherwise? Chances are it will be the shitty switch, (Throw a new one at it.) or, if you've recently changed your filter, even if you pre-filled it, it may of held an air bubble and the dash has detected a low pressure event on start up.

Anyway I hope that has explained a bit and hopefully put peoples minds at ease. Our bikes are pretty bloody tough. You don't need to treat them like a Faberge Egg, just use common sense.
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PostSubject: Re: Why oil and what does it do?   Mon Jun 18, 2018 8:13 pm

Thanks for the detailed explanation Pete! I checked my oil on the weekend and was a bit horrified to find it at the bottom mark, but it’s good to know this in itself should not cause any problems. I added enough to get half way between the marks. The bike is running so great right now, I just want to “do no harm” and let it keep doing what it’s doing Smile
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PostSubject: Re: Why oil and what does it do?   Mon Jun 18, 2018 8:20 pm

Thanks for sharing Pete, very well written especially the hydrodynamic lubrication part
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PostSubject: Re: Why oil and what does it do?   Mon Jun 18, 2018 8:21 pm

Thanks Pete - I love being educated!
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LBC Tenni
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PostSubject: Re: Why oil and what does it do?   Mon Jun 18, 2018 11:04 pm

Now this is an oil thread. Good stuff Pete. Thanks for sharing.
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anguscameron1966
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PostSubject: Re: Why oil and what does it do?   Mon Jun 18, 2018 11:39 pm

Pete - Interesting and informative read, appreciate you sharing your know how Smile
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McCarthy1983
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 2:38 am

Great write up Pete, very much appreciated and my mind is now at ease.
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Pete Roper
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 2:43 am

Do note that the figures quoted for pressures are very simplistic. Because of the speed that the engine is operating, even at idle, cylinder fill and therefore pressures are lower but using those figures makes it easier to understand. The principle is accurate though.

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DeepSpace
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 9:49 am

Interesting read! I do have a question. I know it's pretty clear that your oil pump needs to produce a certain amount of pressure to be able to squeeze it where it needs to go, but what i always wondered is, what about the flow. I know on some older guzzi's, the big blocks, some liked to increase the size of the pump to increase the oil flow. I know the oil pumps are directly driven by the crank, so the higher the revs the higher the flow.

So, what about the flow? Do you need a higher flow at higher revs. Would a constant flow also work? I understand it also cools, and the faster the flow, the more heat can be "moved" into the oil, but what about lubrication?

There is one thing i do like to add. Even though you still have some lubrication at say 400ml of oil left, you may not have enough oil to cool the oil down. (and maybe even to "unbubble") Hot oil will loose its lubrication effect at one point.
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motor-timothy
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 10:11 am

Thanks, very informative topic! I’ve read on another forum (cannot find the topic anymore) that sometimes the red alladin lamp shows in combination with the SERVICE, after short stops due to the oil still being under pressure upon startup of the bike, which results in the oil sensor giving a faulty value. Is there any truth to this?
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Pete Roper
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 11:47 am

DeepSpace wrote:
Interesting read! I do have a question. I know it's pretty clear that your oil pump needs to produce a certain amount of pressure to be able to squeeze it where it needs to go, but what i always wondered is, what about the flow. I know on some older guzzi's, the big blocks, some liked to increase the size of the pump to increase the oil flow. I know the oil pumps are directly driven by the crank, so the higher the revs the higher the flow.

So, what about the flow? Do you need a higher flow at higher revs. Would a constant flow also work? I understand it also cools, and the faster the flow, the more heat can be "moved" into the oil, but what about lubrication?

There is one thing i do like to add. Even though you still have some lubrication at say 400ml of oil left, you may not have enough oil to cool the oil down. (and maybe even to "unbubble") Hot oil will loose its lubrication effect at one point.

Once the engine is spinning above a fairly speed the oil pressure will be being controlled by the OPRV. You're of course right in that the less oil there is the harder it gets worked and there does come a point where it can't protect any more due to overheating and vaporisation within the bearing itself but modern oils are remarkably tough!

Your observation about the increase in size/volume of the gear type pumps in older Guzzi's is interesting. It actually occurred not long after the adoption of the 8/33 final drive and my guess is it was done partly to ensure sufficient volume and pressure at lower crank speeds as the tall geared Cali 11's chunter along at or close to the legal limit in top at the sort of engine speed where the OPRV may not in fact be the governing factor in pressure.

With the Hydros there was also the need for higher flow to ensure the lifters stayed pumped up!

Pete
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Pete Roper
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 11:48 am

motor-timothy wrote:
Thanks, very informative topic! I’ve read on another forum (cannot find the topic anymore) that sometimes the red alladin lamp shows in combination with the SERVICE, after short stops due to the oil still being under pressure upon startup of the bike, which results in the oil sensor giving a faulty value. Is there any truth to this?

Highly doubtful. I'd be looking at the spacer to block gasket.
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 12:06 pm

Pete Roper wrote:
motor-timothy wrote:
Thanks, very informative topic! I’ve read on another forum (cannot find the topic anymore) that sometimes the red alladin lamp shows in combination with the SERVICE, after short stops due to the oil still being under pressure upon startup of the bike, which results in the oil sensor giving a faulty value. Is there any truth to this?

Highly doubtful. I'd be looking at the spacer to block gasket.

Thanks, will look at that when I get back home in 6 weeks. Its just odd that when I leave the bike parked for a longer time it won’t show any warnings/errors when I turn it on. Only after riding it for hours and turning the key on after a quick stop like refueling does it happen. And even then, after I start it, turn it off, then turn it on again the warning is gone.
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DeepSpace
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 12:30 pm

Pete Roper wrote:
DeepSpace wrote:
Interesting read! I do have a question. I know it's pretty clear that your oil pump needs to produce a certain amount of pressure to be able to squeeze it where it needs to go, but what i always wondered is, what about the flow. I know on some older guzzi's, the big blocks, some liked to increase the size of the pump to increase the oil flow. I know the oil pumps are directly driven by the crank, so the higher the revs the higher the flow.

So, what about the flow? Do you need a higher flow at higher revs. Would a constant flow also work? I understand it also cools, and the faster the flow, the more heat can be "moved" into the oil, but what about lubrication?

There is one thing i do like to add. Even though you still have some lubrication at say 400ml of oil left, you may not have enough oil to cool the oil down. (and maybe even to "unbubble") Hot oil will loose its lubrication effect at one point.

Once the engine is spinning above a fairly speed the oil pressure will be being controlled by the OPRV. You're of course right in that the less oil there is the harder it gets worked and there does come a point where it can't protect any more due to overheating and vaporisation within the bearing itself but modern oils are remarkably tough!

Your observation about the increase in size/volume of the gear type pumps in older Guzzi's is interesting. It actually occurred not long after the adoption of the 8/33 final drive and my guess is it was done partly to ensure sufficient volume and pressure at lower crank speeds as the tall geared Cali 11's chunter along at or close to the legal limit in top at the sort of engine speed where the OPRV may not in fact be the governing factor in pressure.

With the Hydros there was also the need for higher flow to ensure the lifters stayed pumped up!

Pete

yeah, that makes sense, i know my 1100 cali runs at low(er) rpms

I wonder what will happen if the OPRV stops relieving. May that have any negative effects? I should ask my hydraulic buddy from work, pretty sure he might know Razz
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Pete Roper
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 12:45 pm

The internal friction of the oil can cause enough drag to spin the bearing shells in the rods. Over pressure is an ugly thing.
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 3:08 pm

Great write up Pete, as always.
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 3:41 pm

Thanks all, interesting and educating thread .......
Have a safe day
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Street
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 6:09 pm

Great article, Pete, thanks for such a thorough explanation.
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Pete Roper
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 6:17 pm

Interesting pic of a couple of the shells out of the 1400.

If you look at the surface of the one uppermost in the picture you can see where the babbit coating has been 'Picked off' by the over-taxed oil vaporising under pressure.

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The striations on the lower shell are also typical of oil that is working too hard.

It came a bit too close for comfort to disaster.....

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Very Happy
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 6:22 pm

Pete Roper wrote:
The internal friction of the oil can cause enough drag to spin the bearing shells in the rods. Over pressure is an ugly thing.
We call It, washing out the bearings, and yes not at all pretty.
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PostSubject: Re: Why oil and what does it do?   Tue Jun 19, 2018 7:40 pm

Thanks for the write up Pete. This inspires me to look at that Guzzi 8v motor course presentation again and try to follow the oil system a bit more closely.
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PostSubject: Re: Why oil and what does it do?   Wed Jun 20, 2018 12:40 am

An oil thread worth reading. Thanks!
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PostSubject: Re: Why oil and what does it do?   Wed Jun 20, 2018 3:35 am

Thumbs Up Thumbs Up Thumbs Up
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PostSubject: Re: Why oil and what does it do?   Wed Jun 20, 2018 4:00 am

As a comparison, for those like myself looking at this for the first time, what does a bearing that is functioning correctly look like ?
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Pete Roper
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PostSubject: Re: Why oil and what does it do?   Wed Jun 20, 2018 4:36 am

Very Happy very good question and one that might be hard to answer because you don't replace one's that aren't fucked!

I do think I may have a couple of rods with nice bearings in from one of the wrecks lying around. If I can find them easily I'll get some pics.

Pete
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