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Understanding Oil Pressure Readings

Old 02-17-2011, 10:37 AM
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Understanding Oil Pressure Readings

So with the S2's, we have a port on the side of the engine where we can directly plug in an oil PSI sender. I have the prosport halo (same as premium) sender connected as well as a gauge. Here are the behaviors I am experiencing

Cold Startup : idles at about 60psi
After 1 tach light goes off : down to about 25psi
After all tach lights off : up to about 30-50psi (i will have to check this one again)
Driving : from 60-130psi
WOT Redline : upwards of 140-150psi

My question is, is my 25psi normal during the startup phase? Not saying anythign is wrong, just curious and trying to learn a bit more about oil pressures and their affect.
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Old 02-17-2011, 11:04 AM
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I have a S1 but with the OP mod from Mazmart and a conversion to a single large oil cooler but that sounds about right for me. A lot of people talk about oil weight effecting oil pressures but honestly I have not seen that, mine seem to be the same using 0W-40 and 5W-40 as it was when the factory 5W-20 and 0W-30 was in the engine.
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Old 02-17-2011, 11:11 AM
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Try 20-50w or straight 40w and you will see a difference.
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Old 02-17-2011, 11:43 AM
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Yea I figured that different oils and different weights would give different numbers, just didn't know if I was in the right ballpark. I am actually running RP5/30 which actually gave me more solid numbers than when i ran Pennz.YB 5/30 which had my startup and WOT pressures a tad lower.
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Old 02-17-2011, 02:40 PM
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I can't comment on RX8 pressures, as I don't have a gauge, but my 350Z has a gauge on it, and the behavior is very similar.

Stone cold oil is very thick, so the pump can generate very high pressures at the top of the oil system (where the senders are), as there's a big resistance to flow through engine.

As the oil warms, it thins, and the pressures drop, idle always being very low compared to driving around.

What I've not seen is idle pressure come up as the engine continues to warm (as you noted when the final tach light is off). But there might be a valve in the system that opens (or closes) and that changes the flow through the engine (total speculation there).

Here are the pressures on my 350Z (from memory, so maybe a bit fuzzy).

cold idle: 60-90 psi
cold driving: 100+ psi
warm idle: 20-30 psi
warm driving: 60 psi
warm redline: ~90 psi
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Old 02-17-2011, 02:42 PM
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Originally Posted by paimon.soror View Post
Yea I figured that different oils and different weights would give different numbers, just didn't know if I was in the right ballpark. I am actually running RP5/30 which actually gave me more solid numbers than when i ran Pennz.YB 5/30 which had my startup and WOT pressures a tad lower.
That implies that when cold the Penn.YB 5/30 is thinner than the RP 5/30 is. So it might be a better choice for minimizing cold-start wear.

The pumps are positive displacement, and the pressure read is the back-pressure from forcing the oil through the filter and the engine oil passages.
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Old 02-21-2011, 06:47 PM
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paimon.soror, you talk in your post about a "...port on the side of the engine where we can directly plug in an oil PSI sender." I have a S2 and would like to find out more about this so I could get a true oil pressure gauge and also a oil temp gauge.

Can you point me to some more info?

Thanks!
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Old 02-21-2011, 08:18 PM
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Originally Posted by Surtur View Post
paimon.soror, you talk in your post about a "...port on the side of the engine where we can directly plug in an oil PSI sender." I have a S2 and would like to find out more about this so I could get a true oil pressure gauge and also a oil temp gauge.

Can you point me to some more info?

Thanks!

As far as the oil pressure port. If you look on the driver side (LHD) right above the upper rear spark plug, you will see a hex screw. I don't remember the size, maybe 8? Anyway, you can go ahead and remove that (its a tight spot) and it will give you the perfect spot to plug in the Prosport premium sender. I dont know if the standard sender will fit down there since I never tried. You can remove that plug without draining the oil.

As far as oil temp goes ... there really is no reliable way of getting that on the S2's without doing some things that may void the warranty (my first thought was to get hte reading from the feed in to the OMP, but Mazda would have a $hit fit if i did. Instead i got a converter so that I could convert the size of my temp sender to the size of hte oil drain plug. Gives more of an "average pan temp" but it's better than nothing.

I will take a pic tomorrow if that will help you locate the plug.
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