Oil pressure dip under brakes

[madjak]

My oiling setup: Stock sump (no baffle) -> Stock oil pump (no pressure relief valve) -> Sandwich plate -> Peterson external oil pressure valve -> oil cooler -> sandwich plate - > engine

On the track I see a constant 60PSI which is what my external pressure valve is set to except for a short dip to approx 30PSI around 1 second after I first apply the brakes. I'll sometimes get a second dip another second or so later. I'm assuming this is the oil slopping around in the sump, moving forwards, back then forwards again. I might be rev related but I doubt it, given that it drops lower at these points than when I'm putting around the track at 3000 rpm. I'll need to check the logs to be certain though.

Here is a video that clearly shows it. Oil gauge is on the A-pillar


I've read several threads about the issue and most people aren't concerned, however I don't think it's that great, given I'm often blipping the revs up to 8000 as I'm dropping through the gears probably right when this low oil pressure is there. Not only that but if the oil pressure is dropping, it must mean the oil pickup is getting air which means the bubbles will be going through the entire system, the engine and the bearings.

I'm surprised this isn't a bigger topic given that I'm not even running big aero. I'm not sure of the g's I'm pulling under brakes as I haven't hit the logs yet, though I'm close to lockup on all 4 wheels when it occurs the worst.

What's the best solution? Does baffling the sump fix it?

I've posted this up on Mt.net also to see if the race guys there run baffles / altered pickups.

[plohl]

I just bought a maruha sump baffle direct from japan. It was around $150. A mate much faster then I was having the same issue and fitted one - drop in op went away.

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[beavis]

I have heard of this and i have seen threads on the US sites about it.
Accusump was a solution. Sump baffling also.

http://www.miataturbo.net/race-prep-75/ ... 235/page2/
http://www.miataturbo.net/race-prep-75/ ... pan-49230/

[Magpie]

For me the oil pressure 'dip' happens on lift off from high revs, just before hitting the brakes hard. I have mentioned this to Plus on the first few occasions that it happened and there was no concerns. However I only get this issue once and from memory not one warning then another.

We run very similar oil setups. The only difference is that in my build the main bearing support plate was upgraded. An Accusump was going to be fitted in the build but when it was finished there was no oil pressure concerns so it was never fully plumbed in.

I'll post some data of my when I get home.

The dash is set up with an oil pressure warning at below 25 psi and the revs below 3,999 and below 40 psi when revs above 4,000.

[madjak]

I run a main bearing support plate too thought it's only been modified to fit the sump, not other mods. I was thinking about getting a boundary Engineering oil pump just for insurance... but they are pretty pricey and I don't think it would solve this issue at all.

I think I'll baffle the sump and see if it's better. I can't really see the benefit of the accusump because it won't stop the air being picked up, it will just keep the pressure. Having hair bubbles go through the bearings is generally considered a very very bad thing in any other race engine scenario.

I'm also half considering a dry sump. I'd keep the stock sump - run two 10AN pickups, one rear drivers side, one front passenger side. I've read of the e-production guys also doing an oil pickup in the head as they can fill with oil lowering the sump level. 3 scavenge stages also pulls a better vacuum. My head already as the oil drains cleaned out and enlarged a little I believe so I'm not sure if it's needed. I'd remove the stock oil pump, run a 4 stage pump, 3 scavenger and one pressure so that it gets a good vacuum.

Comparing the two:

4 Stage dry sump pump ($800)
Swirl tank ($300)
Fittings etc ($200)
Custom mounting / belt etc ($150)
$1500

Boundary Engineering oil pump ($450)
Accusump ($350)
sump baffle ($150)
$950

Probably a bit serious but if it gives me 5 more HP why not.

[Magpie]

Can you up the sampling rate on your data to see exactly how long and when it is happening?

Dry sump on a MX5, not too many of these around. As a bonus you could lower the centre of gravity on the car as well!

[madjak]

Can you up the sampling rate on your data to see exactly how long and when it is happening?

Dry sump on a MX5, not too many of these around. As a bonus you could lower the centre of gravity on the car as well!

It's strange, because the posts on MT.net show that it's a very common problem, but most of them ignore it. Maybe the racers are running dry sumps or at least a swirl tank or something.

I doubt the engine could be lowered much... the steering rack is the high point and that sits right under the lowest point of the sump which just clears the MBSP. Might get 15mm. But it's hardly worth having to modify the engine supports etc for that. I'd probably do it cheap, buy a 2nd hand 3-4 stage pump, plumb it into the existing sump, add a 6 liter swirl tank and use my existing tank as a breather. I can fab up all the mounting points easy enough.

I think I'll definitely just install a sump baffle first. See and document how much it helps. If I end up with a dry sump pump on the stock pan I'd still need the baffle anyway.

[sailaholic]

I've noticed it in my NA as well. Cruise along a clear straight road at 60/80 and stand on the brakes. Needle dips and recovers.

My plan was just to add the baffle if/when the block is ever out of the car.


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[StillIC]

I have the Boundary pump and the Maruah baffle and I still see the drop under brakes, directly after lifting off the throttle. Only lasts for a blip. My thinking is that it may be related to the intertia of the oil up high in the engine, trying to 'pull' against a slowing pump, noting the pressure sender is on the block. I have no evidence or support for this hypothesis.

But I don't think the issue is related to surge, as it happens instantaneously with relation to off throttle/on brake. And I don't think it is related to an inadequate pump. As I write, perhaps it is the inertia of the needle on the gauge dropping so quickly from a high pressure reading that it overshoots its actual lower pressure reading to show almost no pressure???

[madjak]

I don't think it's the needle inertia at all, I'm seeing it my ECU logs and that has no gauge. It is most certainly a blip, in that it's dropping low then back up again in a very short time. All my ECU logs show a dip spike, with no examples lasting a longer length of time.

I think you're first idea is smack on and makes more sense than my first guess which is the oil slopping in the sump. There is a large mass of oil held in the oil cooler, AN lines, block and head etc, this is flowing at a fast rate and then all of a sudden this slows to half the rate at the pump. The oil mass will continue moving at the original speed and cause a low pressure dip at the point I'm measuring it, ie right after the oil pressure relief valve. Just like water hammer in a pipe. Given that oil can't compact very well, I think that actually means that the oil flow and pressure is still dropping in the block and head. It almost needs a damper in the oil system which is basically what the Accusump is doing.

Interesting! I'll have another think about it all.

EDIT: If it is oil inertia then it's easy to engineer a test. I can rev the engine up whilst stationary, hold it at revs for a few seconds, then drop the revs. If the oil dips we know it's not surge.

[Magpie]

This is a little messy and the data rate could be a little higher. This was coming into T3 on the National at QR.

The legend is at the top so I will let people make their own analysis (I did have a big response typed out but I deleted it).


Oil Pressure by Eipeip, on Flickr

[madjak]

So if everything was nice, the oil pressure would follow the RPM trace, but be capped at a set pressure, almost cutting off all the peaks. But what you can see instead is that as soon as you back off, there is a 1 second dip in oil pressure, the same as I'm getting, and then another a few seconds later... each time dropping down to 30PSI. It doesn't really match up well against a high G-force

This could be the oil inertia pulling the oil through the engine and creating a low pressure 'hammer'. But looking at the data, the dips also correspond a little to where the higher G-loading is too.

Really I think oil surge is highly unlikely given that this would be very common in our engines and there would be far more motors with dead bearings if the pickup was getting air. So it must be something else. Who here is pulling the most G's under brakes in a NA / NB?

One other thought. Could it be voltage related? So the alternator might be dropping voltage as the revs drop, causing the sensor signal to drop. I'll have to check it my logs.

[Magpie]

If it was a voltage issue you would see it across all the sensors. I'll graph the voltage next or I could send you the raw data? Looking at the earlier data it would appear that the oil pressure dips are linked to oil temperature. That is early in the morning the dips were not as dramatic as they were by session 4.

The lowish oil pressure is not happening at high revs, this is a good thing. If the oil pressure was dropping below 40 psi and revs above 4,000 I would be concerned.

[madjak]

Looking at my logs its definately a voltage drop at that point. It drops from 14 to 10 then back again. Maybe its my brake lights or the alternator dropping charge or something.

[greenMachine]

That seems like a big voltage drop, how long does it last?

Re the Accusump, it should not have air in the oil. It empties (feeds into the oil line) when pressure is low, accepts oil when pressure is high. If you open the valve on starting, it empties then, and when the engine is running, the oil pressure forces oil into the holding chamber. It stays there until the pressure in the chamber (set with a bladder) is greater than the pressure at the accusump outlet (where you have joined the device to your oil lines), when the bladder expands forcing oil out and balancing the pressures. When the oil pressure recovers, the oil is forced back into the holding chamber. You have to remember that you have a few litres of oil in there when doing oil changes though, or ...

If you remain concerned about air in the chamber, mine has a bleed valve, and I have installed mine with a diverter (like the ones on your downpipes) to trap any air before entering the chamber.

I think you risk over-thinking this (understandable in light of your recent experience). Baffle the sump, run an accusump, in that order. If that doesn't fix the dip, then look at a dry sump. Also buys you time to accumulate the parts for a dry sump, if you want to go down that path anyway (and the accusump can be sold off at that time).