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Old 10-13-2018, 04:45 PM   #1  
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Default oil pressure fluctuation at high rpm?

my car has had this issue for a while.
at high rpm the oil pressure gauge will fluctuate and drop 1-2 bar. this only happens at high rpm (6k+)
car is running a older boundary engineering oil pump assembly. it has a mocal oil cooler thermostat adapter plate and dual oil coolers up front.
the car has had this issue for a long time, and it does not eat bearings. no metallic particles in the oil and its been running 400+ whp for years.

Wires from the oil pressure gauge have been checked/replaced and it otherwise works fine. im not sure how to go about fixing this, but i would be more comfortable if it didnt have this issue

any ideas or experiences ?
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Old 10-14-2018, 01:50 AM   #2  
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Quote:
Originally Posted by hf-mx5t View Post
my car has had this issue for a while.
at high rpm the oil pressure gauge will fluctuate and drop 1-2 bar. this only happens at high rpm (6k+)
car is running a older boundary engineering oil pump assembly. it has a mocal oil cooler thermostat adapter plate and dual oil coolers up front.
the car has had this issue for a long time, and it does not eat bearings. no metallic particles in the oil and its been running 400+ whp for years.

Wires from the oil pressure gauge have been checked/replaced and it otherwise works fine. im not sure how to go about fixing this, but i would be more comfortable if it didnt have this issue

any ideas or experiences ?
There are some vibration harmonics that mess with the internals of he stock electric sender that mounts to the block. A fully mechanical aftermarket Gage won't read those fluctuations. Remote mounting the stock sender also seem to make it go away as well. Some spec Miata companies sell kits that run a line from the block so you can mount the sender to the chassis and isolate it from the vibration of the engine block at high rpm.



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Old 10-14-2018, 09:13 PM   #3  
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Pump is cavitating. Melling has some graphs on this.

Did a few data logs showing this with the Miata pump, LS3 pump and LS7 drysump.
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Old Yesterday, 02:13 AM   #4  
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is this a common problem with the old na8 boundary engineering pumps? would it be a good idea to get a new pump or would this still be the issue on those?
this car has been running for many years on the same pump, so if i could replace the pump and get peace of mind and see a stable oil pressure i sure would do that
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Old Yesterday, 03:57 PM   #5  
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Spoke with a couple of oil pump manufactures at the PRI show and its just a normal thing with oil pumps driven at crank RPM. Miata-GM LS ext.

Does not seem to be much of a problem on the Mazda motor as there are a zillion of them being hammered to death in many forms of racing.

As long as flow/pressure does not fall below threshold your motor needs? Remember your pump makes flow. Restriction & viscosity make pressure. Cavitation-foam reduces viscosity quickly.

One engineer did comment it could be a factor in stock cast/powdered pump gears giving out.

Can be mitigated a bit with oils that foam/cavitate less like Amsoil, Joe Gibbs ext. as well as optimizing hot on track viscosity. Its all a balancing act.
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Old Yesterday, 04:44 PM   #6  
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Is this on the stock gauge or an aftermarket gauge? Because the stock gauge tends to go wonky at high rpm sometimes. I assuming you have an aftermarket gauge but just thought I would ask.
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Old Yesterday, 04:56 PM   #7  
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this was on the stock gauge, in the process of connecting up a new pressure sender to my mini dashlogger
I THINK that my car actually has oil pressure.. since it has not blown apart even after a long time and running well over 400whp. but ill have to doublecheck. If i still get the same reading on a different gauge, what could cause this ? not enough flow from the na8 oilpump(i dont have VVT so it should be enough) ?
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Old Yesterday, 05:06 PM   #8  
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Quote:
Originally Posted by hf-mx5t View Post
this was on the stock gauge, in the process of connecting up a new pressure sender to my mini dashlogger
I THINK that my car actually has oil pressure.. since it has not blown apart even after a long time and running well over 400whp. but ill have to doublecheck. If i still get the same reading on a different gauge, what could cause this ? not enough flow from the na8 oilpump(i dont have VVT so it should be enough) ?
My guess is the stock gauge is off, get an aftermarket gauge on it and I bet it shows normal.
I would think that pump would cavitate at very high rpms, are you revving past 7400 rpm? If you are maybe the stock pump is having trouble keeping up at the higher rpm.
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Old Yesterday, 05:44 PM   #9  
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im not revving really high, i have the limiter at 7600 but i see this fluctuation well before that.
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Old Yesterday, 11:09 PM   #10  
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My stock oil pump gets pushed past 9000 and the is no oil pressure fluctuations in my logs or on my aftermarket gauge. Good pressure (ie more than 50PSI) is fairly critical at high revs otherwise the bearings will fail rapidly. Given that your bearings are still alive at 400whp I'm guessing it's just the stock gauge failing rather than an actual pressure dip.

Also as a side topic, oil pumps in wet sumps tend not to cavitate unless they are sucking air like a dry sump pump. Oil by itself won't boil to a vapor as quickly as water so you actually need air bubbles trapped in the oil that will expand to cause cavitation. I also run a sump baffle because under very high braking G's I was seeing an oil pressure dip. That means the pickup was sucking air partially due to the oil moving forwards in the sump under brakes and also that more oil was being trapped up in the head due to the higher revs. It's not normally a big problem in a turbo miata because you guys aren't high in the revs when the pressure dips happen, but when you are money shifting to 9000 rpm the big end bearing load is at it's highest load right when that air is being pushed through the bearings. I only noticed the dips once I was running aero and slicks and then a few laps later I did a big end. Since the rebuild and installing a sump baffle I'm now pushing more G's under brakes and I'm not seeing the same dips in oil pressure.
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