Learned Something

Ron Bartell

Ron Bartell

Well-known member
For anyone who might be aware of my oil pressure issues over the last year or so, including the Runoffs, I found something that is worth sharing with those that may not know this. I use an external oil pump and a specially designed pan on my Spridget because with the stock pump and even a special "racing" wet sump pan they caused me to see the oil pressure light six or seven times a lap when we were at Kansas. The stock pump is also unreliable but I am not allowed a dry sump in my configuration. The system I had was in place for three years but suddenly started giving me problems - big problems.

I had been told that my oil cooler set-up was wrong, that I should have opened up this or that oil galley in the block (while others told me that what I was doing was correct). Over the winter I replaced and replumbed the cooler, opened up oil galleys, replaced the pressure relief valve, and a number of other things that could have led to the problem. I really thought I had found it when I learned that some people had collapsed the suction line to an external pump, so I put in some flat-wound spring to prevent that. Still had the problem.

Just for the heck of it I sent the single stage TDC pump back to be checked. I didn't think it was a problem because it could easily put out 80 lbs of pressure or more if I adjusted the pressure relief valve on the pump. The results were that it was down from 8.6 to 7.5 GPM, way more than I needed for a little 1275 motor, so it seemed ok, but I installed my new spare anyway and had that one rebuilt.

Well, it turns out that what must have been biting me is that I pulled air into the oil pick-up (which believe me was well-designed) under cornering, and with the large dash 12 line and a marginal pump, it couldn't reprime itself, the Accusump would kick in for as long as it took to empty 3 quarts, and then I had no oil pressure and if I continued a broken motor.

Two things to remember: 1) Suction lines should not be any bigger than they need to be - bigger is definitely not better on the suction side; and 2) Oil pumps can still be marginal even if they are capable of putting out good pressure - they need to be able to create good suction if you pull enough G's to expose the pick-up.

BTW if anyone needs flat-wound spring for dash 12 lines I have plenty and it is normally very expensive.
 
Ron:

Glad to hear you found the problem, and thank you for sharing the info. Hopefully, this will help a few more drivers chasing similar problems.

I suspect your competitors in HP are less enthusiastic about this though... :wink:

Rick
 
Ron,

I ought know better than to disagree with a past National Champion, but pump inlet side tubing size is very critical.

Inlet (suction) side pump operation is contrary to conventional thinking. The most vacuum (negative pressure) any pump can create on the inlet side is about 30" Hg (atmospheric pressure - about -15 psi), and wait for atmospheric pressure to "push" the oil to the pump. Once the pump has the oil, it can create great positive outlet pressure, but inlet suction (negative pressure) will never exceed -atmospheric pressure. The definition of "vacuum".

Most pump inlet side tubing is much larger (cross sectional area) to permit displacing large volumes at low pressures. Basic P1V1 formula. Pumps without adequate inlet size tubing will cavitate.

Most of my 948 oil pressure issues ceased after I fit 1275 oil pick-up tubes to the blocks, more than doubling the cross sectional area.

Regardless of inlet tubing size, pumps can "lose their prime" for various reasons, and fail to create any vacuum (negative pressure) on the inlet side.

Flexible vacuum tubing is a special construction as high pressure tubing capable of withstanding 3-5000 psi will collapse under minimal vacuum. Defies logic.

Industrial vacuum systems are very special. I've worked on a few. Defy logic.

Player pianos are a great study in miniaturized vacuum systems.

Care to comment Greg G. ? (a real hydraulic system engineer)

RJS
 
Ron, you didn't indicate how you vented the engine. Venting properly allows atmospheric pressure to assist the pump. If the vent filter is plugged, obstructed or constricted, you loose air flow to the pan. Did you check for vacum leaks in the suction lines? Have also seen "flapper valves" created inside an oil line, a result of assembly errors when constructing the lines. A small rubber flap can be a tear in the interior of the oil line, with suction conditions causing it to rise up and impede flow. I have even used the shop vac to draw a vacum, trying to simulate the conditions.

Have also used the shop vac to draw silicone into an oil pan gasket leak. Hooked the shop vacum up to one of the valve cover vents and closed off the other vents. It was too much trouble, and not enough time, to pull the pan and replace the gasket. Some times you do strange things when there is not enough time at the track to fix it properly and get back on the track.

This is one of those conversations, every one has an opinion. Only your finding will confirm someone was right.

Good luck, sometimes it can be tough to resolve a problem. That's the part that I enjoy. It makes you think, or get very frustrated!
 
R. J. Sorensen":14b6z9ui said:
I ought know better than to disagree with a past National Champion, but pump inlet side tubing size is very critical.
Click to expand...
Didn't say it wasn't critical. Probably should have said that bigger isn't necessarily better. The pump expert says to keep it under dash 10. I also understand that you can't do better than to create a vacuum which means atmospheric pressure pushes it up the pipe. But if the pump is marginal, and the line is big enough, and the distance it has to be pushed up the pipe is high enough, it doesn't work. That much I know.

Anyway I have never had this problem before. The suction line inside the pan is under the oil the whole time, so if there was a leak it would only draw oil not air. The suction line outside the pan was replaced along with the fittings. The motor is properly vented with big lines coming from the timing cover, valve cover, and the distributer hole.

I suspect that a two-stage pump that was run under the same conditions would yield two stages that had lost prime instead of one, unless you didn't allow the pump to draw from the side with no oil. We actually thought of that one but dismissed it as unworkable without special valving.
 
Ron, All,

One other issue is the braided stainless lines themselves (nitrile). On more than one occasion I have seen the inlet side lines fail and pass air. This has been on oil and fuel lines. The worst thing is that there is no leak or weeping of the line, nothing.
I have stopped using braided lines on inlet/pickups and gone to solid aluminum or teflon lined style with internal spring.

J
 
Unfortunately I don't have a lot of experience with gerotor style pumps as we don't use them in our industrial hydraulic applications.

That said I did pick the brain of one of our hydraulic application engineers and he thought of two possible problems in addition to the hose material issues described by others.

1 - Oil line routing. The oil line for pump inlet should be routed so that the pump is the highest point. If the hose has a 'high point' somewhere between the hose inlet and the pump inlet, this could trap air and prevent the pump from priming. Think about where the stock oil pump is located in relation to the stock oil pick-up...the pump is the highest point in the system with a relatively short, straight run from the pick-up to the pump inlet.

2 - Pump Wear. You indicated the pump was tested at 7.5gpm vs its rated flow of 8.6gpm. This is quite a bit of leakage, approx 13%. It's possible that the pump is so worn out and has so much internal leakage that when it looses its prime, it's actually sucking air from the pressure side thru the internal leakage path and just circulating air. I would suspect this problem would be worse with hot oil as the internal pump clearances open up as the pump heats up, plus the oil is thinner.

The only other suggestion is to pay attention to your the size of your strainer on your oil pickup. If it's too small or has too fine a mesh, this could contribute to problems with cavitation, but this would be more of an issue on start-up with cold thick oil.

You could have issues due to leakage at your various hose fittings. I would think you could come up with a method of simulating this problem by dropping the oil pan and spinning the oil pump with a drill, and experiment with uncovering the pick-up to see what issues prevents the pump from repriming. If you think it might be due to a leak at a hose fitting, try smearing the fittings with heavy, thick grease one at a time as a test. The grease will help provide a temporary seal against air leaks.
 
In response to what John said about braided lines leaking air into the system: In 88 at Road Atlanta during qualifying I had an off at turn 1 and saw no apparent damage. During the week I added two quarts of oil as I seemed to be using oil with no apparent leaks. It turned out that I had an air leak in my suction line from the pan and wasn't pumping all the oil out the the motor, eventually hydrolocking it as the block filled with oil. All the time my oil pressure was fine.
 
Ron Bartell":3qotq4rr said:
The pump expert says to keep it under dash 10.
Click to expand...
I just realized that I inadvertently misspoke here and need to correct any misconception. The pump expert actually said to keep it "no bigger than dash ten".

Sorry about that.
 
Ron,

My dad has been using an external wet sump pump on his MG engines for years. Might be worth a call to him to just discuss your system. I know he uses -10 on his engines. Can't remember what pump he's using but it's not a TDC.
 
Jesse Prather":twg8lnkg said:
My dad has been using an external wet sump pump on his MG engines for years. Might be worth a call to him to just discuss your system.
Click to expand...
Thanks Jesse. I have spoken to him recently on this. Since he was trying to help me starting with the Runoffs last year, I called him to bring him up to speed with what I had found when I was sure that the suction line had collapsed and he told me that he did use the flat spring to keep it from collapsing. I didn't tell him that that wasn't it yet, but I will.
 
I know lots and lots of racers that make G rotor style pumps work well, and that I could have eventually figured it out. But I lost a whole season changing lines and plumbing and pumps and almost just quit racing in frustration over trying to make a pump work. I know they work, and that I am just a sample of one, but a Rootes style pump solved whatever my problem was.

When I bolted on a Dailey Rootes style pump, it just worked. Since that time I have learned that I don't have to run it at 1/2 engine speed like a G rotor so a smaller pulley has made packaging easier and it seems to lose less pressure/capacity when the oil gets really hot than any of the several G rotor style pumps I tried. Mine is a dry sump application however I know Dailey will also makes external wet sump pumps because I explored that possibility for a vintage race engine.

Good luck,
 
Some basic pump info

Running a pump dry, really isn't good for the pump, but limited prep wet sump, it will sometimes run dry.

The reason the inlet hoses or lines tend to be larger, pressure drops as velocity increases, and there tends to be very little pressure to work with on the inlet side. Typical hydraulic pump installs have larger lines on the inlet side.

If the inlet side hoses collapse, first thing that happens, basically the hose is smaller, so the velocity goes up and the pressure drops, even is it just collapses a little. Of course if it completly collapses, then there is no oil flow, and that just won't work....

If the pump is worn just a little, it might work fine while wet, but it will have a hard time working when run dry. The wear grooves will be filled with oil, but they will create leak pathes when dry, making it harder to draw air and re-prime. Any air leak anywhere on the inlet side will make it harder to draw air and reprime.
When run dry, it has to work as a air pump, until it gets oil again. Some minor wear grooves and it won't easly draw air.

Any pump should be mounted as low as possible, so it has more inlet pressure.

If the pump isn't pumping as much pressure, that indicates excess wear. Loss of pressure is leakage. Pumps don't really pump pressure, they move volume, and pressure is created by a restriction in the flow. For the most part in a engine the pressure is from oil flowing thru the bearing clearances, and controled by a relief valve. If it takes more RPM to come up the the relif valve pressure, that indicates wear somewhere, either the pump isn't moving as much volume or more of the pumped volume is leaking thru the bearings or somewhere else.

I set up the accu-sump with just a T fitting and no flapper valve, so oil goes to the bearings and to the pump. With the flapper valve it would hold pressure longer, but without the flapper valve oil goes to the pump. Seems like keeping the pump wet is a good idea while it is drawing air on the inlet side??
 
Thanks Ted - I removed the check valve for the NHMS race and still had the problem. A worn pump apparently won't be able to reprime a large line when it goes dry. Since it is a big job for me to change the size of the line I willl go with it for now and make sure that the pump is up to snuff all the time.

Curtis - Believe me, I know how frustrating it can be. I was on my last try when I finally solved it as well. If I hadn't, I would have been done myself. There is nothing worse than having the car let you down time and time again, especially after you have been through vitually everything that can go wrong over as many years as I have been at it. I thought that I had seen it all.

I used the G rotor style pump because it was just a smaller version of my dry sump pump that I used on the full-prep 948. It allowed me to use the same brackets, belts,etc. as the dry sump setup.
 
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