Oil Pressue Gauge Damping

[posplayr]

Don't know if anybody else has the same problem but, with my GS1100ED I have GS750 gears and a new pump and there seems to be a lot of variation in the oil pressure as the engine pulses. Part of it could also be due to engine crankcase pressure but regardless there is a lot of needle bounce. I think it may be contributing to movement of the zero set point of one of the guages I got from RenoBruce.

I was looking for a simple way to reduce the bounce and found something that work pretty well. Running down the road it was not unusual for the oil pressure needle to bounce +/- 2-4 PSI. This makes it hard to see any changes.

Bruce provides a right angle fitting that converts from the metric thread in the block to thread on the back of the guage. I used this little elbow to create an accumulator.

On the guage side I stuck in some paper, so that when I shoved some JB weld into the other side (actually another automotive two part epoxy) I contained a plug into the part that went into the engine. This creates a small chamber on the guage side. I then drilled out the plug using a small drill. (about 0.070") . This is still large enough to get plenty of dynamic response but now the bounce is less than about +/- 0.5 PSI. In hydraulic terms this is a small accumulator which low pass filters (averages)the hydraulic pressure reducing needle flutter.

Pos

Edit: 4/16/09 After the bike got warm, the oil thinned and the bounce increased ( 3 PSI total spread e.g. 6-9 psi). I went to a Size 60 drill which is about 0.039" which should drop the bounce down to less than 1/3. Have not had a chance to run it to hot to see that improvement though. It moves pretty slow when the oil is cold. Takes about 3 seconds to go from 6 psi to 0 after shutting the motor off (when cold).
FWIW, theoretically this should be an decaying exponential response so it is below 3 psi within about 1 sec.

[Dogma]

Sounds cheaper than a snubber.

[posplayr]

Update

I needed to go to a smaller drill; see the first post. I'm hoping the new size will be acceptable. My guage has always bounced wildly when warm. It is improving. Worst come to worst I can increase the size of the chamber with a brass extension.

I don't have the equation in front of me but the damping ratio (1st order exponential decay time) should increase inversely to the area of the pin hole or proportionately to the volume in the little chamber. That is ignoring any viscosity effects. There will likely be too much change due to viscosity (cold to hot) to keep dropping the pin hole down further in size (0.039" size 60).

I was thinking about mounting the guage on the handle bars and I could integrate the larger chamber into the mount.

more later

Pos

[reddirtrider]

You're correct that the oil viscosity and the port diameter have a relationship to the needle bounce.

Just a suggestion, but if you have either a gas charged or spring loaded accumulator you might be able to dampen the needle bounce without changing orifice size. The gas charged one would be the simplest as it would be nothing more than a chamber that holds some air so that the oil compresses it during fluctuations.

Anyway, you're definetly on the right track.

[posplayr]

Red

Just a suggestion, but if you have either a gas charged or spring loaded accumulator you might be able to dampen the needle bounce without changing orifice size.

I was trying to create an accumulator with near zero space and cost. With the wide open fitting the needle would just flail . Like 1/2 the guage range for 0-15 psi.

With the 0.070 drill hole it is +/-1.5 psi hot

Figuring 0.039 drill hole will be 0.5 psi hot (0.039/0.70)^2=0.31 => less than 1/3 the bounce.

As mentioned before, the next step is to increase the chamber volume behind the orifice.

If I extend a line from the rear adapter plate, to the handlebar and use an braided SS brake line I should have plenty of volume to improve damping. The orifice will go at the engine port rather than as close to the guage as it is now. I really want to get away from looking between my legs to see oil pressure.

Pos

[reddirtrider]

I was trying to create an accumulator with near zero space and cost. With the wide open fitting the needle would just flail . Like 1/2 the guage range for 0-15 psi.

With the 0.070 drill hole it is +/-1.5 psi hot

Figuring 0.039 drill hole will be 0.5 psi hot (0.039/0.70)^2=0.31 => less than 1/3 the bounce.

As mentioned before, the next step is to increase the chamber volume behind the orifice.

If I extend a line from the rear adapter plate, to the handlebar and use an braided SS brake line I should have plenty of volume to improve damping. The orifice will go at the engine port rather than as close to the guage as it is now. I really want to get away from looking between my legs to see oil pressure.

Pos

I don't have a lot of experience with hydraulics, but does an accumulator have any effect with non-compressable liquids? This is why I suggested air.

[posplayr]

Red

I don't have a lot of experience with hydraulics, but does an accumulator have any effect with non-compressible liquids? This is why I suggested air.

Good question, but not sure if I have a complete answer.

We know that a compressible fluid (air) will have a damping effect if there is a volume space which is feed and supplies a pair of constrictions (smaller lines).

The question is whether an incompressible liquid would have the same behavior?

I would have to pull out my undergraduate fluid mechanics book (circa 1978) to be for sure, but I think the low pass filter mechanism relates to flow rates through the constriction in ratio to the accumulated flow in the volume. At a given pressure in the line, the volume will fill as a constant rate and the time required Volume area divided by the flow rate ,Q.
The smaller the constriction the lower the flow and the longer the time. This is like changing the gain on an integrator which is what a Llow pass filter is in the limit where teh pole goes to zero (infinite time constant).

If there is a pulsing of the pressure, then the instantaneous pressure at the input orifice will move above and below the nominal at the guage and back and forth flow will occur in the line (between guage and engine).

This is where it gets a little sketchy. I'm not sure that the issue is really compressible v.s. incompressible flow but rather if there is any compliance in the system. If it were a perfect hydraulic system there may not be much compliance. But alas what potential imperfections provide for the damping I'm observing?

I have a suspicion that part of the pulsation is actually air pressure, blow by or engine vacume. Any air in the lines would provide compliance. I did not bleed my guage line. I have seen indicated oil pressure drop off as I opened up the throttle.

With the guage mounted between my legs, I have not studied as much as if it was on teh handlebars.

The guage itself has a geared mechanical spring inside of it that has compliance.

If I ran a long plastic line, the line would exhibit compliance as the pressure raised and lowered.

An compressible fluid accumulator probably has better damping properties over temperature because the spring allows larger orifices to be used and is not as sensitive to viscosity change.

I'll think about it some more, especially if I don't get an improvement as predicted by 1/3.

Pos

[salty_monk]

Take the gauge off & put the original bolt back, that's the cheapest & easiest way to cure it. The needle won't bounce sitting on your workbench & you'll no longer have to look between your legs to see it

Didn't Bruce already have some kind of bar mount for these gauges? I remember seeing a pic of something....

[posplayr]

Take the gauge off & put the original bolt back, that's the cheapest & easiest way to cure it. The needle won't bounce sitting on your workbench & you'll no longer have to look between your legs to see it

Gee Dan, why didn't I think of that.

Didn't Bruce already have some kind of bar mount for these gauges? I remember seeing a pic of something....

Yes but he is using 2" guage i think and more than likely with a electric sender which is mounted under his handle bars or the fork.

I'm looking to use a small 1.5" guage that mounts directly to the bar (lited if I could find one), and I'm trying to decide on a configuration for the lines and any fittings.

But first I'm trying to see if the guage holds up better when the needle is not being beat to death.

[Nessism]

I’m with Salty Dan. I removed the gauge on my 850 because, a) there is already an idiot light to tell me if oil is flowing and, b) those undamped VDO gauges are not suited for the application (mine doesn’t go back to zero anymore, and I’ve seen others worse than mine). I don't mean for this to be a knock against Bruce who sells these kits (I didn't buy mine from him, made it myself).

[posplayr]

Ed

b) those undamped VDO gauges are not suited for the application (mine doesn’t go back to zero anymore, and I’ve seen others worse than mine).

I opened mine up, pulled the needle off and stuck it back on with Red loctite. I has a 6 PSI offset.

I'm not convinced that the damage is coming from engine vibration as much as oil crankcase pressure pulsations that whip the needle frantically.

Before I buy another, I'm going to figure that out as well as an effective method to dampen without too much viscosity sensitivity or cost.

Jim

BTW, even my OEM (supposed) electronic temperature gauge will occasionally exhibit oscillation that looks like the same thing from the oil pressure guage.

[renobruce]

those undamped VDO gauges are not suited for the application (mine doesn’t go back to zero anymore, and I’ve seen others worse than mine).

I'm not sure where you are getting your information, but this is from VDO's website:

A space-saving way to monitor oil and fuel pressure.
These VDO mechanical pressure gauges are a great way to monitor your fuel and oil pressure systems and save space, too. They feature a 1 1/2 in. diameter steel case, precision-machined brass movements, and built-in restrictors to give you the accuracy and durability you want.

So yes, they are for oil pressure, and yes, they are "dampened".

I'm not convinced that the damage is coming from engine vibration as much as oil crankcase pressure pulsations that whip the needle frantically.

I've had the same gauge on my 1000 for two years and it still works perfectly. The original black gauge I had on my 1150 got stuck on 3 psi after I blew a shift on the Dynodrags and took it to 11500 rpm. So I learned that running the gauge past it's capacity will ruin it. I now include an instruction sheet with the kits to warn people not to over-rev their motors when the oil is cold. It may also be that bikes with the 750 gears might need the 0-30psi gauge? I don't know how much higher the pressure is.

I've had some of the 0-15 psi gauges registering 3-6 psi right out of the box, but not many. I've also sold probably 400-500 of these kits on ebay, and I've only had maybe 6 or so returned to me because of a stuck needle.

I like your idea of resticting it further. I wonder if I could come up with some kind of insert that I can include in the kit?

[salty_monk]

Couldn't you put a brass washer into the elbow to restrict the flow?

Dan

[renobruce]

Couldn't you put a brass washer into the elbow to restrict the flow?

Dan

I'll bet I could come up with something, and you could get it made for me.

[posplayr]

Bruce

I now include an instruction sheet with the kits to warn people not to over-rev their motors when the oil is cold. It may also be that bikes with the 750 gears might need the 0-30psi gauge? I don't know how much higher the pressure is.

Yea, it is pretty obvious that if you peg the needle hard it could twist shaft. I generally don't rev excessively as I'm looking right at the oil pressure guage on startup.

It may also be that bikes with the 750 gears might need the 0-30psi gauge? I don't know how much higher the pressure is.

I have only had my guage after having put in a new oil pump and new 750 gears, so don't know where stock is. When cold I can easily get to the top of the pressure range on some moderate reving (2500+ RPM)

When warm I can still see only 3 psi idling

Running down the road at 65 mph in 5th the needle averages about 7 psi but will bounce wildly depending (+/-3 psi) upon what I use to dampen it (I'm trying to get to +/- 0.5 psi.

I think the guage is simply reading the pulsing pressure which is surprising as I used to run for years with a Steward Warner 80 psi mechanical guage on my 64 Chevelle (with hypo oil pump) and it was always a comforting site to check it. It never bounced like my GS. Pressure would vary 2:1 with temperature.

A 30 psi guage could handle the variation better, But generally pressure is under 15 psi 95% of the only exception being reving at a cold startup.

The mods I'm doing so far are right in the elbow adapter you provided.

The main question in my mind is, once the pressure pulses are damped, how much is engine vibration effecting the guage. I guess I would also have a concern if most GS's don't have pulsating oil pressure?

I have the top end oiler as well as oil cooler so the 750 gears should simply be compensating for that.

I have another oil cooler coming with larger 3/8" lines rather than 5/16". I wonder if small lines on the cooler could be causing turbulent disturbed flow that causes the pulsating? I might try and run without the cooler connected to see if that changes the guage behavior.


Another wild a$$ theory is that the two top end oiler lines exaggerate pulsing due to having slightly different lengths. I would not be surprised if one of the other of these devices is causing pulsating.