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Old 05-15-2006, 02:34 PM   #1
RemlapaN
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Member#: 58251
Join Date: Mar 2004
Chapter/Region: Tri-State
Location: Northeast PA
Vehicle:
2004 Sti
GT35R 530whp @ 27psi

Default Silly question about oil pressure + boost

As of lately (maybe it has been doing it all along and i never noticed), I have noticed that when under heavy boost (more than 9psi), when i let off the throttle to shift or something, i see my oil pressure drop to about 30psi, then go straight back up to 60-80. It seems like as soon as the BPV opens, the oil pressure drops, then the system normalizes and it goes back to normal.

So my question, is... am i being paranoid, or does this always happen?

Car has stock turbo / stock IC. Only mods are a DP and intake (and hydra ems)
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Old 05-15-2006, 11:52 PM   #2
Ians06
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Oil pressure follows engine RPM's. If you let the rpm's go down your oil pressure should follow. As it goes up, oil pressure will go up. Hope that helps. If thats not the case, elaborate more on your symptoms.
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Old 05-16-2006, 09:08 AM   #3
RemlapaN
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2004 Sti
GT35R 530whp @ 27psi

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Sorry if my post was misleading, i am aware of how the oil pump works meaning from idle to 3000rpms your pressure will increase.

What I am referring to, is say you are at 4000rpm in 4th gear, and step on the throttle hard enough to get over 10psi, then cut the throttle (as if you were going to shift, but not letting the clutch in). My oil pressure will drop sharply, then go back up - this is without the engine RPM changing, only boost pressure changing. The more boost from the turbo when i cut the throttle, the more the oil pressure drops.

My fear is that perhaps i am getting a lot of blowby, which is pressurizing the crankcase, and causing it to depressurize when the BPV opens, causing the oil pressure to drop for a second. However i am still on the stock turbo, only pushing 270whp, and there is only 32k miles on the car.

So is this normal? (/me hopes so)
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Old 05-16-2006, 09:54 AM   #4
WRX 2002
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Rally Blue

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IMO it would be hard for the blowby to make that much difference in presure in your crank case even if something was terribly wrong in your engine. The crank case is vented to the inlet tube so realling cannot hold presure and the change in oil presure your talking about is 30-50psi with only 10psi or so boost presure. If the crank was presurized to that amount your seals would have exploded.
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Old 05-16-2006, 10:27 AM   #5
2phless
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Where is your oil pressure gauge plumbed to? Oil pressure and crankcase pressure aren't measured from the same areas of the motor. I'm sure you understand, and this may sound dumb but I'm just making sure it's clear. Oil pressure is moving the oil through the channels in the motor, to the turbo, to the AVCS plumbing etc... Crankcase pressure is just the free-air around the oil... kinda like the power steering pump has pressure on the oil in the lines, but not in the reservoir.

Are you consuming oil (more than normal)?

Do you have a catch can? If so, is it collecting more oil than normal? If not, check your TMIC plumbing for lots of oil.

Serious ring blow-by (or cracked lands) will cause extra pressure under boost, and will increase the oil in those places. But I don't recall that having much effect on the gauge reading until enough oil blew by that the level was scary low. The dip to 10 psi might just be the mechanics of the gauge setup. Fluid dynamics... something odd like that.

I can't really wrap my head around a cause off hand, or if it is even a bad thing you are seeing. My oil pressure is around 90 when the motor is cold at idle or under a load. Then warm it's 75 under load, 20-25 at idle.

Fuel pressure follows load... you aren't looking at that gauge are you? I'm sure not. I'll see what mine looks like (it isn't a gauge I look at while gunning it / shifting / whatever) but my setup is a lot further from stock than yours. But I'm still using a recirculating BPV and the stock manner of crankcase ventillation, so it should react about the same.
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Old 05-16-2006, 02:21 PM   #6
CasopoliS
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I think what you are seeing is ok. My friends car did the same thing. I DO think oil pressure response is affected by where you place the sender for the gauge. I am hooking mine up soon to the galley plug on the top of engine.... I will let you know if I get the same results.
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Old 05-16-2006, 02:47 PM   #7
RemlapaN
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2004 Sti
GT35R 530whp @ 27psi

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My oil pressure gauge is installed in a sandwich adapter on the oil filter, mainly because i was lazy and didnt feel like removing the galley plug when i was installing the oil temperature sender.

As far as oil consumption, it depends, but I am probably going through 1 quart every 2000 miles or so.

The oil pressure is completely normal all other times, over 80psi when cold, 60-80 when driving, and around 15-20 at idle.

The thing that is driving me nuts, is if this is normal, why would it have taken me 25,000 miles to notice it...

I currently do not have a catch can installed, nor have I ever had one. Last time i pulled off the TMIC was about 6 months ago, and it did have oil in it, which is to be expected.

I guess the safest thing to do at this point would be pull the plugs and do a compression test, and see if things are where they belong. Probably a good thing to do a compression test before i go ahead with the sz49 or 55 plans i am working on anyway
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