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Jubbie's Avatar
 
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Location: Friendswood, TX
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Pressure relief spring question

On the pressure relief spring at the base of the case, I see that before the SC's (including the early Turbos and Carrera 3.0s) the same spring as the side was used which was 901 107 531 00. This appears to be even after the pump bypass mod.

On the SC's and later they went to the 930 107 531 01 which required the spring guide.

What is different about these springs? Was the case machiened different in the SC's because of this spring?

The reason I am asking is when I did the rebuild on my 930/02, I used the new spring kit from our host which included the SC type base spring. I am now realizing that my oil pressure is high and am wondering if this is a possible cause or is it the C2 cam tower oil orifices. Warm, I am about 80 psi at 3000 rpm.
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74' 911S Targa + 930/02
76' 914 - Eternal Type IV project FOR SALE
Old 08-23-2005, 12:15 PM
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There is no difference in the case machining for the newer springs. I took out my tower oil line restrictors because the oil pressure was too high.

-Andy
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Old 08-23-2005, 06:30 PM
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High oil pressure?

Eagledriver,

I've got the updated tower restrictors to put in during my rebuild(in progress).Is there such thing as TOO MUCH oil pressure,if so,what would you consider too much.Seems,as long as everything is flowing correctly,you would also see more volume,which is desireable?No.Just curious,not doubting.

Chris
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85 Carrera 3.2(Back on the road after rebuild)
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Old 08-25-2005, 06:02 PM
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Chris,

I don't like to see oil pressure above 100 PSI on my engine. With the restrictors I was seeing over 100 psi even on a warm engine at high RPM. High oil pressure can cause leaks, blow out lines, burst oil coolers, etc. In addition the higher the oil pressure the more wear on the intermediate shaft gears and oil pump. You need to have a minimum of 10 PSI per 1000 RPM according to books I've read. I like one bar (15psi) per 1000 RPM on my engine. This is about 60 PSI at 4000 RPM.
Any thing above that is counter productive IMO.

My engine is in a race car also so higher oil pressure robs power that I need to win races.

-Andy
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Old 08-26-2005, 06:57 PM
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When I added the tower restrictors on my motors (stock SC, race 2.7), I didn't notice any dramatic oil pressure increases. Nor did I expect I would: oil pressure is (supposedly) set by the oil pressure setting spring and piston (the vertical one). And as a last resort, excessive pressure is supposed to be bled off by the relief piston - the horizontal one. So no matter how high you rev your engine, in theory the oil pressure will only get to a certain level, and will not rise after that. My SC typically sits at about 4 bar.

The pitch for the tower restricters was that there wasn't enough pump capacity to supply enough pressure everywhere. And that more oil than needed was going to the cams/valves. The restricters are said to redirect the oil the pump will produce, thus keeping pressure up in the main and rod bearings, squirters, and whatnot.

A guy can raise his oil pressure (or try to) by shimming up the vertical spring (you can even construct an externally adjustable system for moving the spring base up or down). But I don't know how you'd reduce pressure if it was too high and everything otherwise was stock. Do the several models of plug in which the spring sits have different dimensions? The base on which the spring sits is higher or lower? I suppose different sealing crush washers might have different installed thicknesses.

Or does really high pressure suggest the control piston is stuck in its bore?

My worry when installing the cam tower restrictors was increased cam/rocker wear, or less spring oil cooling. And that would not show up promptly on a dash gauge.

Walt Fricke
(wondering if by chance the broken outer valve springs on the race motor were related to this)
Old 08-29-2005, 11:29 AM
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Jubbie -- I noticed the same on my C3 rebuild. Equal length springs on the pistons. My oil pressure is high- but possibly not problematically so. I'd say I get up to about 7 bar at 6K.

I never looked at the springs closely enough to determine if they were heavier gauge such that you still get the same amount of restriction, and therefore maximum oil pressure. I suspect the cam restriction lines allow the system to approach-- but not exceed this maximum.
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Old 08-29-2005, 02:55 PM
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Walt,

My impression is that I only "hit" the pressure relief when the engine is cold. The pressure will climb to 130 or so and then drop about 50 psi suddenly then climb back toward 120 or so with a cold engine as the rpm increases. When the engine is hot it shows no sudden drop just a slow climb to 90 psi or so as the revs increase. Based on this I assume (you know what that stands for) the pressure relief isn't playing a part in my hot oil pressure. With the pressure restrictors on the cam lines I noticed I was getting over 100 psi on the track even with a 200 degree oil temp.

-Andy
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Old 08-29-2005, 04:48 PM
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One thing to look out for with oil pressure readings is the match of the sender to the gauge. 911s started out (when a pressure gauge was added) with a 10 bar/150 psi gauge and a sender to match. Sometime in the SC era (I think the SC's and 3.2s inherently operate at a slightly lower oil pressure, but I could be wrong there) this was changed to a 5 bar/75 psi gauge. I think the gauges are the same except for the faceplates (the same resistance puts the needle in the same position), and the senders (which are marked) had different operating characteristics, with their resistance dropping down to the 100% of gauge resistance at a lower pressure for the later senders. So a 5 bar sender with a 10 bar gauge gives readings 2x high.

Another thing which could affect readings would be the spring used. The '77 and later (oil modification) cases (and I think that the pre '77 930 cases also had this since the case I have does, but I don't know this for sure) use the same left side spring. The older cases used the same spring for the right side, vertical, spring. But after the oil mod, with the different piston (Wayne's 911 book has a good explanation of this, and Anderson covers it as well), the right side spring is longer (hence the guide tube inside), and appears to be of thinner wire. Both mean it has a lower spring rate. Using the short spring in the right side position is going to affect oil pressure. Using the wrong (old) pistons is also going to cause problems.

Sudden jumps or changes in pressure suggest something sticking, or perhaps this is just what one sees when the secondary or real relief valve opens to prevent overpressures.

I think these oil pistons run in a steel tube which is inserted into or cast into the case. I don't know what effect that might have on their sticking versus case and oil temperatures. One might think this would prevent differential expansion.

Walt Fricke
Old 08-30-2005, 12:17 PM
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Hello Walt
these are good news.. do you have pics of the oil pressure circuit ?
I've noticed a low oil pressure before starting my engine rebuilt (2.7RS factory), around 2 bar all the time, i dn't really know how these pistons work, what you think about it (rod bearings clearance ok...)
the engine was working well, and strong
thanks
Philippe
Old 08-30-2005, 01:56 PM
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This is the '73 - '89 oil circuit
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Old 08-30-2005, 02:24 PM
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thanks Bill
I'm pretty sure my problems come from the springs, now
i'll check when i get my case back from machining..
Old 08-30-2005, 02:40 PM
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On 993s the pressure spring is 89mm and the relief valve is 70mm

Have you seen this thread?
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Old 08-30-2005, 03:12 PM
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Bill
What is your source for the spring length on 993? I just looked up the springs on Pet and they all use the 930 107 531 01 spring from SC to 964 to 993. Curiously there is a spacer with a 964 number shown under the spring in 964 and 993. This would appear to functionally stiffen the spring and would raise pressure like the cam tower restrictors but the spring number appears to be the same.
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Old 08-30-2005, 06:22 PM
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Quote:
What is your source for the spring length on 993?
993 shop manual, yes, the pressure spring has a spacer and guide not used on the relief side.
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Old 08-30-2005, 07:37 PM
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the spec for Carreras is
main opens @~6.2 &#177 0.8bar/88.2 &#177 11.3psi
relief opens @~8bar/113.7psi
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Old 08-31-2005, 09:02 AM
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Found my problem. Walt gets the prize. The 930/02 I put in had a 0-5 bar sender. I have got a 140 psi (10 bar) gauge. Not sure why the C3 engine had a 5 bar sender (shows a 10 bar sender in PET). That must be a PO story.

I did go back to the spring set up that I orginaly had in the C3 (both equal lenght) and there was little change.

Got my 10 bar sender from my 2.7 engine and all is better now.

THANKS GUYS!!!
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74' 911S Targa + 930/02
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Old 09-01-2005, 08:25 PM
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