OIl pressure insights

[pamcopete]

Some time ago I removed the spring loaded pressure bypass from my oil filter because I didn't want un filtered oil to by pass the filter during start up when the pressure is high.

I installed a oil pressure gauge because I wanted to be able to monitor the pressure at start up and keep it below the nominal 15 psi that the bypass valve was supposed to maintain.

I changed the oil this weekend and refilled with Valevolene 20W-50, as recommended by Yamaha. Upon initial start up, the pressure was well in excess of 15 psi, much more than after previous oil changes, but after a lengthy warm up, the pressure came down as expected, but what this indicated to me was the fact that the viscosity on the bottle of oil can vary. The pressure was so high and lasted so long that I decided to drain the oil and go with Valveoline 10W-40. The results were much better.

So, what can we conclude from this experience? That the bypass valve will allow unfiltered oil to enter the engine upon initial startup and for some time after until the engine warms up sufficiently to reduce the viscosity of the oil. This is most prevalent in cold conditions, so I will probably change back to 20W-50 in March or April.

But for most XS650's with the bypass valve installed, it is very important to warm up the engine before riding because the by pass valve will open and allow unfiltered oil into the engine. Of course, we do have the sump filter, but that is notorious for developing a hole and allowing unfiltered oil into the oil pump and subsequently passed the by pass valve and into the engine. I personally believe that the bypass valve when open increases the flow of oil from the sump, and that is what causes the hole in the sump filter. I removed the by pass valve in my trusty '78/E as well and the sump filter in that engine lasted for 75,000 miles and was intact when I sold the bike.

 

[CDNTX650]

Does the bypass open for sure? Isn't that 15 psi differential....so if the pressure is fairly even through the filter itself it shouldn't bypass should it.?

 

[TwoManyXS1Bs]

Hey, Pete. Been collecting info to put together an experiment for a similar thread, but for temp/viscosity issues. So, just a few random points to ponder:

Your oil pressure gauge is displaying differential pressure between 'wherever' it's plumbed-in and the atmosphere.

The side-filter bypass valve opens based on differential pressure before and after the valve. It just protects the filter and the pump.

The oil pump (if in good condition with little internal leakage) is a positive displacement pump, and could theoretically produce thousands of PSI if the system is blocked (or until something breaks). The hyd pump in my tractor, and many other hyd cylinder machines, also use positive-displacement pumps, and use a lower viscosity hyd oil, and typically have pressures around 2000 psi, the relief valve setting.

The oil pump pressure relief valve (that dumps to the sump) on our XSs was dropped after 1971, so we're getting everything the pump can possibly put out, full flow at all times, about 1 gpm @ 3000 rpm, for the 8mm pump.

Being a 'non-relieved' positive displacement system means that the flow rate thru the sump screen = flow rate thru pump = flow rate thru side screen = flow rate thru front main oil gallery entryway.

You can have different pressures at each of those zones, but the flow rates will be the same.

Kinda like having a 'constant current' power supply instead of a 'fixed voltage' power supply. So, the logic here is different.

I feel that your oil pressure gauge variations may be more related to the oil kinematic viscosities (CentiStokes). Here's the chart I'm referencing for that 'other' experiment.

 

[cruzin]

The oil pump pressure relief valve (that dumps to the sump) on our XSs was dropped after 1971, so we're getting everything the pump can possibly put out, full flow at all times, about 1 gph @ 3000 rpm, for the 8mm pump.

only 1 gallon per hour? that's not a lot of oil is it?

 

[TwoManyXS1Bs]

Here's an extract from the manual that states the oil pump flowrate.


The cumulative orifice openings in the oil galleries and sprayers, along with oil viscosity, will determine what pressures will be realized at various points in the system. This diagram of the XS650 lubrication system shows those points. My concerns (very high oil temps) involve very low oil pressures. Note that it takes at least 1/2 psi just to get the oil up the front tube to the head, due to gravity.

 

[TwoManyXS1Bs]

...only 1 gallon per hour? that's not a lot of oil is it?

Whoops! Thanx, eagle-eye cruzin. Edited above post to show 'gpm'.

 

[cruzin]

no problem, see my latest post question about rectifier question and let me know if you have a answer. thanks

 

[pamcopete]

Does the bypass open for sure? Isn't that 15 psi differential....so if the pressure is fairly even through the filter itself it shouldn't bypass should it.?

Yes, I know that I am measuring gauge pressure whereas the bypass opens on differential pressure but I think the filter element does create a significant obstruction with cold oil and could create enough differential to open the bypass valve. If and when the bypass valve opens, the flow of cold thick oil would increase and that is where I think the source of the sump filter holes comes from. In other words, the sump filter does not have a bypass valve so when the oil filter bypass opens there would be in increase in differential pressure affecting the sump filter.

Added to this theory is the fact that as far as I know there is no accepted way to adjust the bypass valve for the nominal 14 psi differential so it could be greater or less than 14 psi.

I removed and blocked the bypass valve on my '78/E as well and it went 75,000 miles with the same sump filter and that was before the introduction of the improved sump filter. I also noticed that there are no elephants in the trees since I removed and blocked the by pass, so it must be working!