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Drysump pressure drop

Drysump pressure drop

Drysump pressure drop

Morning folks, first post. I've spent a couple hours digging around and didn't see my question answered before so I thought I'd post a new thread.

I am racing an EProduction mazda miata in SCCA club racing. As the stock miata oil pump regulator is proned to problems, I've "made the plunge" and switched over to drysump.

On startup, I have adequate pressure - about 40psi at 2K rpm idle. As RPM increases, so does oil pressure- up to 75 or so when the regulator kicks in.

Here's my issue: After startup, letting the car idle for say, 60-120 seconds, the pressure starts dropping... down to say 15psi where I get chicken and shut it off. The oil temp is maybe 100-110 at that point - way before anything is warm.

If I let the car sit for 30 minutes, and try again- voila my oil pressure is back to 40 ... but then the dreaded pressure drop starts all over again.

My system is the following: four-stage TDC pump, two -10 scavenge lines from the pan, one -10 scavenge line from the back of the head (in the valley with the valve train). The pump section for the head scavenging is much smaller than the two pan scavenge sections. Each of those lines is about 18" in length.

There's a -12 line heading back from the engine compartment to the trunk area; that line enters the top of the 24x6 tank. Another -12 line returns along side that line back to the pump.

The pump outputs into a -10 line, into a filter, into a cooler, and into the block at the stock oil filter input location. Just above that input is the stock pressure sender location, where I have my current gauge attached.

After speaking with several people, I believe one of several problems are possible - but I am looking for input / thoughts / whatever learning I can get.

1) The tank lines are too long, too far away from the pump, or simply the tank is too low relative to the pump for it to suck oil from the tank.

2) The head scavenge line is never below the oil level in the head. Specifically, it's basically in free air as far as I can tell. This seems like it's just sucking air into the system (minimal oil) and is asking more as a vacuum pump. Unfortunately for me another several cars have this setup but do not have my problem.

3) Some other problem I haven't thought of.

Diagnostics I've done so far:

1) Removed the line going to the tank, and verified that the pump is sending oil down the correct line. Simply turning the pump at starter speed shows the lines are hooked up correctly...

2)... however when the car is running, there really isn't that much fluid coming out. Yes with 20 seconds of cranking I can fill a 1-qt bottle, but the stream doesn't seem that consistent to me anyway.

3) When the pressure has dropped, I removed the valve cover and ran the starter again. You can clearly see a few air bubbles coming out the oil galleys for the lifters. It's not foam by any stretch, but there are bubbles for certain.

Any and all direction is appreciated. Thanks for your time.

RE: Drysump pressure drop


I'd guess that your pressure pump inlet line is too restrictive, due to both the diameter and length, and is causing the pressure pump to cavitate.  A good rule of thumb for sizing your pressure pump inlet line is keeping the flow velocity below 6 ft/sec at the max pump speed.  The flow velocity can be approximated by this formula:

Velocity= .4085 x q / d^2

velocity is ft/sec, q is gal/min (flow), and d is inches (tube inner diameter).

I'd also recommend plumbing a swirl pot (de-aerator) into the scavenge return circuit, to help remove the air bubbles entrained in the scavenge oil.  Too much entrained air in the oil flow to the pressure pump inlet will also produce cavitation.

Hope that helps.

RE: Drysump pressure drop

So is the tank above the pump? Is the system scavenging back to the tank?
And what about suction side? Any airleaks?  

RE: Drysump pressure drop

So, turns out the tank was 8" below the pump. I'm in the process of moving the tank much closer to the engine, almost 5 feet closer.

When I was looking at the line filling the tank, I wasn't impressed with the volume of the oil coming out. There wasn't really enough flow to get the contents of the tank swirling - which meant really it was all bubbles.

So I'm keeping my fingers crossed that proximity will cure my problem. I'll stick with the -12 lines, and they will now be about 2' long each total.

The only remaining issue I may have is that third vacuum line. The pump stage is different than the other two, and according to the pump maker this is on purpose for what's mainly a vacuum line. Anyway, that line comes off the back of the head to its pump section - but the inlet is almost never covered by oil so I'm certain it's sucking a lot of air.

I'm going to add a third outlet on the pan, and if the problem still isn't fixed I'll move my third scavenge to the pan.

I'm also going to figure out how to pressure test the lines and make sure there's no leaks anywhere.

Thanks for all the guidance.

RE: Drysump pressure drop

All the scavanging or return stages of a dry sump pump will be sucking air. After all, you have one stage pumping oil into the engine through restrictions in the bearings and lifters and such while you have 3 or 4 or 5 stages all sucking the oil back out. If you don't "suck" more than you feed the engine will fill with oil. Your return system or tank needs to de-aerate the oil.


RE: Drysump pressure drop

Supply line and tank height definitely seem like the issues. It's fairly common to run -16 lines for longer supply and return lines on dry sump systems with proper tank height so you had everything working against you on the supply side.

RE: Drysump pressure drop

Thanks everyone.

In order to move the tank to the passenger footwell, I had to find a suitable place for the ECU and move it, along with moving pretty much the entire electrical system (fuse housings relays, etc).

I've also found a nice used Huffaker sump pan so I'll be swapping that out and looking at the bearings while I'm at it.

I'll post the outcome next week.

RE: Drysump pressure drop

Hello,i am not sure if you fixed your issue but being that i race in 750ci plus nitrous class and do all my own work i have alot of experiance with many things dealing with dry sumps and also vacum pumps....

My Tank is 10quarts and i run 1 -12 to the pressure side and I have 4 scavange so its a 5 stage...I also run a Star vacum pump that will pull 25 inches of vacum easialy if i dont regulate it....but I also do testing for a dry  pump dealer and on the dyno with no vacum pump hooked up we pulled 16 inches from the 4th stage in the back of the motor in the lifer valley...Now with the sepreate vacum pump its like all of the motors in are class and in Pro Stock we pull from the valve cover.....

Several things can happen that alot of people are unaware of and one of them is if you dont add spiral spring to the inside of your braided suction lines they can collaps and i dont care who braided line it is...add it anyway..its a pain to do but if you dont your asking for trouble..

Next thing is if your suction for the vacum side is high and you dont have the proper drain and air venting the vacum in the top of the engine will hold all the oil to the top side....by the time you get it apart its drained back...

I know as i fought this issues for a month with are new engine 4 years ago and all the names couldnt help me till I did my own diagnoses....I found all these things I have mentioned to be a issue.

Several things need to be done...you only need the 2 in the pan...Top Fuel cars only have 1 or 2 pick up in the pan...

Add air vents if possable near drain..now we add them in the lifter gally in a V-8...also I run drian lines from my heads at the back and lowest point down to my oil pan...-6

If you check all and open your oil drains backs which many engine builders dont do or even check as some aftermarket head gaskets can block or restricked as well as silcone but all motors can be worked over with time as far as oil return and windage....

Last of course is to know your pressure relife in the dry sump pump is ok as well as how you check the level when the engine is running...you want the tank above the pump but once primed at idle it wont lose pressure unless it losses fluid....airation of fluid is the major cause of most oil related engine failures next to lack there of also....

I hope this helps you some and others.....Listen no one knows your motor or car better then you do so always check it yourself and do as much of it as you can yourself....


RE: Drysump pressure drop

You are almost certainly sucking air on the pressure supply side or getting cavitation of the pump.

Poor tank design or to tight a bend in the feed line to the pressure pump are the normal culprits followed by an air leak on the suction side of the pressure tank.

As has been pointed out by others, the system is designed to return mostly air to the tank.

Under no load, I would run it down to 4 or 5 psi for a few seconds before I shut it down. It really only needs full pressure when under load.

Have you checked the oil level in the tank as it loses pressure.

If you are using SS braided (aircraft) line, have you checked to see if the liner is collapsing on the suction side of the pressure pump.

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RE: Drysump pressure drop

Pat,as i stated if its droping off after a few minutes then its sucking air for sure,the reason for this are many as i stated above but i am not familar with his system....

All suction lines made of anything other then hard tubing should have spiral widing coil added to them..like I said it a pain but needed for sure and vacum will hold oil to the top side also not allowing the oil to return to the pan and of course back to the tank...

To see if its a low oil all is needed while at idle and it drops off open your tank and look...if its low then the oil is stagnate somewhere for sure....if there is as he stated a vacum suction line at the top of the motor and there where not enough areas of drain and no vents then oil will not drain back to the pan.....

All the best

RE: Drysump pressure drop

Morning folks.

Moving the tank from the trunk up to the passenger footwell made a big difference - but did not solve the problem. After say 10 minutes on track the pressure starts dropping again - much slower than before but still drops to very low (under 10psi at idle).

I have made three changes: first I got rid of my old pump - it's an old TDC pump made in the 70s. While I did have it rebuilt, I have no confidence in it. I've switched to a nice new Aviaid pump, with larger pressure section and equal sized scavenge sections.

Second, I upgraded from a -12 to a -16 pressure supply hose (tank to pressure section) in hopes of alleviating any cavitation in that area. It's a nearly straight run, 26" long, with only a rise of about 4" during its length. There's a 45 on the tank end and a 90 on the pressure section end - I hope that'll cure any issues there.

Finally, I noticed the oil filter mount had a very small hole putting oil through the filter. So I upgraded to a canton system with -12 O-Ring fittings in hopes that pressure out of the pump would have an easier time getting to the motor. After the filter, the lines do go down to -10 through the cooler and into the engine; I'm not sure how much this change will help but I figured I'd remove what was pretty clearly a restriction.

Anyway I'm almost back together - ran out of -10 hose last night from the head to the first scavenge section or I would have an answer for you today.

I will let it run and if the pressure starts getting lower I'll look at the oil level.

I did check the bearings and all is well after the pressure was so low for my last race.

Finally, the miata head has (at least) 4 drain holes from the head down into the block - I would be surprised if oil were unable to get back to the sump but at this point I suspect everything.

Will post on Sunday evening when I have it running again - or after Dyno time monday.

Thanks again for all the guidance.

RE: Drysump pressure drop

Did you put spirl core inside the suction lines?This is key and your still making lines so do it right and this helps remove a possable problem now and down the road because those lines will weaken over time as far sucking and restricting flow....

Also please dont think of this as a wet sum pump...these thngs move a massive amount of oil to the top side of the motor ....much more then you think and what happens is over a period of time you can put 60 to 70% of your oil reserve at the top side and not no it....then those 4 noles become coverd and the top end is like a bowl of oil....the oil cant drain if all hole are coverd....DONT OVER LOOK THIS...

and alot of times you can still have pressure and not oil and in your racing unless you have a data recorder for everything like I do you wount see it..i actualy put flow meter on the system and that led me to the drain backs on the heads and in the lifter galleys putting 4 vent tubes down the middle of the valley ...

When you get time and you can pull the head off do your self a favor and add return line from the head to the pan externaly....whats it going to hurt...?   

RE: Drysump pressure drop

Correct, Miata = MX5. BP16 engine if i'm correct.

Here in Australia they are called MX5's too, but in japan, miata.

I had a very quick skim through the thread so excuse me if I missed this point but,

I havn't seen anybody mention oil grade? To my understanding, the oil has a higher viscosity when cold and as the oil warms up, the viscosity goes down. No surprise. Is it also not true that, an oil that is more viscous will result in an increase of oil pressure due to the lack of flow, the less viscous the fluid, the flow increases, thus the pressure drops. So this leads me to my next question;

What oil are you using?

RE: Drysump pressure drop

While oil pressure certainly varies with temp, it should not vary 60 psi, sorry. I'm using Mobil 1 10-30 nothing fancy. Many other racers are using zero weight with no issues.


RE: Drysump pressure drop

Oil pressure dropping of alarmingly with temperature is often a result of a worn out or wide clearance oil pump.

There are still a number of questions unanswered.

If the scavenge pick up is drawing to much air up into the tappet cover, it would still return that oil to the tank via the top scavenge point.

If to much oil is going upstairs, an oil restriction at the head to block oil gallery junction could help.

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RE: Drysump pressure drop

>>Oil pressure dropping of alarmingly with temperature is often a result of a worn out or wide clearance oil pump.

>>first I got rid of my old pump - it's an old TDC pump made in the 70s. While I did have it rebuilt, I have no confidence in it. I've switched to a nice new Aviaid pump, with larger pressure section and equal sized scavenge sections.

Sounds like we agree.

I'll post results on Monday.

RE: Drysump pressure drop

Racer - Won't a scavenge line from the back of the head (valve cover area) take care of the oil in the head?

I think everyone agrees that the most likely causes are either a collapsing pump feed line or oil aeration.

RE: Drysump pressure drop

Lionel - I have two scavenge lines fron the pan, and one coming from the head.

There is a tiny restrictor fitting in the head to keep most of the oil going to the block - there's just not that much oil in the head.

From what I've witnessed, there's almost no oil going down the scavenge line from the head - most drips down through the head into the crankcase and returns through the two pan lines... but that's just me watching it during cranking and using a drill to run the pump while the engine is stopped.

I'm gambling on aeration or crappy pump - thus the new one.


RE: Drysump pressure drop

It could be aeration.
It could be a collapsed liner in the lines.
It could be not enough oil.
It could be a poor designed tank, resulting in the above.
It could be a stuck or leaky pressure relief valve.
It could be excessive oil clearance in the bearings

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RE: Drysump pressure drop

Yes you could add a scavange line to the head but since we add it to the valley area more for help with vacum and its sucking ait and oil....the biggest problem is the vacum created with the vacum pumps also at idle..this scavange line is -10...i have no issues...I was giving all things to look at as i have been there done thst and have 100' of feet of line to prove it and alot of hours of work finding it.....

Pat if you have a suction to the top side of the of the area of return and it gets coverd with oil that said vacum will hold the oil from returning to the pan properly and the pump will over fill the top side of a the motor...

Having vents that stand up above any pooling of oil will help along with adding returns...Also I alwayas start and warm the engine with are vacum pump line lossend from the valve cover...I have also moved it to the liffer galley but this pump is a monster...it will pull 25 inches on a 750 inch nitrous motor going down track...I try to okeep the pump regualted at 15 to 18 as even with added piston squirters and coted wrist pins the little HP made from added vacum isnt worth on a nitrous motor....I also use the vacum reading from the pass to help tell me the seal of the engine from rings to gaskets....

RE: Drysump pressure drop

One more thing to add here thats been stated, I run an all aluminum motor with a billet block and main caps...in one minute and this is with heated and primed oil to 100 to 120 degrees are clerances X2 in the mains...

As long as i have 15 to 20 at idle and it has 70 going thru the lights i am fine...as long as there are no drops and spikes durring the run....pressure robds power and 10lbs per 1000 XRPm is fine....volume is whats needed more then pressure....I run alotg of oil to the top side of are motors because of the valve springs pressure of 400 to 425 at closed and well over 1200 open so are push rod cups and rockers needs as well as springs oil...I run a 9/16 taperd exhust push rod and if i could it would be 5/8....anyway going off track here of the original post sorry...

RE: Drysump pressure drop


This guy is running a Mazda inline 4.

It has no valley.

It has big oil returns.

It is DOHC so the need for lots of oil overhead is not there.

Two suction points in the sump will draw more volume than the one in the top, admittedly most of the oil will also be in the bottom and using that capacity to a greater extent.

I see no problem with that aspect. It is much more likely the positioning of fittings or the routing of lines so they bend and collapse under suction, or the air is still trapped in the oil as it enters the pressure pump pick up.

If it is air it is likely tank size/design at fault.

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RE: Drysump pressure drop

Pat I would agree,the suction and collapse of lines is something always over looked and the lack of adding the spiral core as its a pain is the butt.....tank design is something since he did say it was a old system he used at the start so it could be a poor tank also......

I guess we will se what happens when he finishes his lines....

Sorry for being stuck in the large V-8 world but the suction side is something I use to over look years ago and learned the hard way....even high end engine builders dont add the spiral core...learned that from Johnsons pumps...smart man....

RE: Drysump pressure drop

Did you fabricate your own oil lines, or did you farm them out? If you did, (and I mean no disrespect here) did you use the proper assembly tools? With the mandril that enters the hose liner, as the fitting is assembled onto the hose?
(I'm assuming 'real' aeroquip style hoses.)

I have seen hoses that when screwing the fitting onto the hose,(without the mandril) the fitting cuts the liner, and raises a sliver of it up inside the hose, obstructing it.

In service, especially in a scavenge situation, the flow can get up between the liner layers, causing it to bulge out, cutting flow even more.

In the days of my youth, I was called upon to evaluate high oil consumption on a turbocharged aircraft. It was determined the oil was departing through the turbo exhaust. An expensive turbocharge change was accomplished. No help. The scavenge hose was inspected, by blowing air through it, which showed ok, but one of the old moss-backs suggested removing the fitting & inspecting the hose itself, which had indeed suffered a lining separation.


RE: Drysump pressure drop

Just a little I thought I might add...

I've been using braided SS hoses and AN fittings for many years.  I've assembled them with the "proper" tool and "by hand" in the field when necessary.  All I can say is that if you use skill and caution, barring a few punctures and loss of blood, proper connections are possible. Always double ck the job, either way it's done.

Collapsing AN -012 scavenge line?  "You betcha"! (sorry, I couldn't help myself...too much cable news).  On our current scavenge lines we use a 3/4" industrial item specifically designed for high temp suction/vacuum use in the oil industry.  We had them made up by the local "Hose Man".  A better option for the purpose, albeit not cheap.


RE: Drysump pressure drop

Folks, my problem appears to be solved.

After running 15 minutes, at 180* oil temp, I have 42psi of oil pressure at idle (about 2K).

Thanks John at AVIAID for supplying a great pump and being so helpful.

I guess I won't really know whether the pump was the fix, or if one of the other two changes I made was the real fix... I guess no one knows for certain.

If I run into any issues at miller this weekend, I'll consider adding the springs - if anyone has a suggestion as to where to buy such a thing I'm all ears.

...and thanks to everyone for their suggestions and patience.

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