Mike Murphy

I was on the PCA website, under the 964/993/996/C2/C4/GT3 Engine tech articles and came across the following Q & A:
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QUESTION:
Joel, with a warm engine on a 2003 C4S (185 degree coolant temp & after 30 miles) I have the following oil pressure indications:

@ idle 900 rpm 2.1 bar
@ 2000 rpm 3.6 bar
@ 3000 rpm 4.8 bar
@ 4000 rpm 4.8 bar
@ 5000 rpm 4.8 bar

What do you think? I am scheduled to bring it to the dealer on Wed. It is still under warranty. To me, 4.8 bar, when the owner manual states 3.5 is normal, means 4.8 is not normal (37% difference).

ANSWER:
We also need to know what the throttle position is at each rpm. When you have the gas pedal down, the pressure will (and should) be higher than when you release it.

3.5 vs 4.8 is probably OK. What oil are you using? If you are using Mobil1 0-40, or Castrol Syntec 5-50, as you should be, then it does sound high. But if you are running 15-50 or something like that, that might explain it. If you don't know, and if the oil is dark, I would have it changed while you are in on Wednesday.

Joel Reiser - PCA WebSite - 4/25/2006
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So, all you engineers out there, does it make sense that throttle position affects oil pressure? I can only guess as to the following reasons, but I have never seen oil pressue change upon throttle position:

1.) Does Increased throttle position assumes increased engine load, which creates more pressure on the bearings and thus "pushes back" on the oil, creating more pressue?

2.) Does increased throttle probably means more power, which generates heat, causing the oil to expand in the oil channels within the engine, causing more pressue?

3.) Is there a pressue-relief valve tied to the throttle that creates higher pressure when the engine is under load?

From a physics perpective, I was under the impression that you really don't need "oil pressure," per se, in an engine. It's not the pressure that keep the bearing floating. What you need is oil between the bearing surfaces, and to keep the oil there, requires pressure. Also, oil pressue creates oil movement, which is only neccessary because it takes heat and dirt away from the bearings and out of the engine. But if heat and dirt did not exist, and you could find a way to isolate the bearing so that oil couldn't escape, you wouldn't need any oil pressure at all in the engine.

Comments, answers??

Edward

It's not as much a discussion of "throttle position" as much as the effect of increased throttle position: increased RPMs and load. As you will readily recognize, even at moderate throttle, the oil pressure will rise with RPMs, even though the throttle has not changed position. The incresed RPMs, and it's increased load on critical mating surfaces such as bearings, cylinder walls, and the like, will attempt to "squeeze" oil out (or tighten surface tolerances which can prevent oil from getting into those critical gaps in appropriate quantity). Thus, the components with critical tolerances need increased oil pressure to counteract the pressure to change those tolerancess as load on these components increase. Of course, too much pressure is bad: the goal is to achieve as good a balance as you can under dynamic conditions ...ahhh, the challenge of every mech engineer.

Edward

84_Carrera

Rule of thumb on american muscle V8s is 10psi pressure / 1k rpms.

~14.5 PSI = 1.0 BAR.

So, with a typical hot motor, if you're at 3k rpms, you're generally looking for at least 30 psi of oil pressure. You're producing 4.8 Bar at 3k (hot), you're making almost 70 psi of oil pressure, which is pretty much what you'd want through redline.

Now, that's a generalization on motors, I'd look to those far more knowledgeable on the boxer engine for specifics.

Mike Murphy

I agree with you, but the thread gave us RPMs already, and the responding tech is still asking for throttle position. So the question really is about why throttle position matters, and I believe it makes no difference whatsoever what the throttle position is on a car to effect oil pressure. I can see the "squeezing" that you mention to have an effect, but have you or anyone else actually noticed this? In other words, hold the brake while driving while increasing your throttle. Do you see an increase in oil pressure? I do not as long as the revs remain constant.

psalt

This is nonsense, there is no relationship between gauge pressure and throttle position. The actual pressure in the bearings is higher by a factor of 50-100 times the gauge reading the oil galley pressure. The galley pressure is regulated by a relief valve. More likely error in the electric gauge/sender, temperature, the relative weights of the oils or a combination of all. in any event, 70 psi is nothing to worry about, but if you are ****, a test with a know good mechanical gauge with the specified oil at the specified temperture would be the correct response.

Mike Murphy

Thank you, I thought so.

Edward

Sorry if I was unclear. I meant to say that there is no positive correlation between throttle position and oil pressure. I did say that as rpms rise there will be a corresponding increase in oil pressure, but I guess I muddled it up in my verbose reply. Ooops

Edward