lower pressure when hot

[svandamme]

hi guys ,

this has me stumped

i had my engine ready to rebuild, but at the last moment , i decided to have it rebuilt by a shop specialized in early 911's
and mostly raced and rallyed 911's


now , Waynes book and Bruce Anderson both say that if you machine the bypass modification
then you need the newer plunger and longer spring
or there will be low oil pressure problems

the rebuilder said , nah , just the plunger , not the longer spring


now my rebuild is breakin in nicely , and my oil pressure cold is fine, just like before the rebuild

as soon as the engine is getting hotter, when it gets near 100 degrees, i notice a drop in oil pressure

my pressure gage is a 0-2-4-6-8-10 one

at idle , 1000 rpm , cold it reads 2+
hot , it hoveres smack in the middle of 0 and 1, with the oil warning light almost coming on ( slight flicker )

revving the engine , cold it goes up to 8
hot , it barely makes 6

before the rebuild , it went higher, even when hot

i have no front oil cooler, and i'm running 5w40 shell ultra


would this be a problem with the relief spring? should i put in the longer one ?

or would this be a problem with the oil weight? 100 degrees is smack in the middle of my gauge (150 is max ) so i kind of doubt that the oil could cause this at this temp...

any suggestions?

[rw7810]

Those levels actually sound right on. Same as mine on a year old build. Common for pressure to go low at hot idles. A 6 is 6 bar or about 90 psi.

[svandamme]

it still worries me , the idle pressure is really low, and before the rebuild , it did never go that low , not even when it was hotter than i've gotten it after the rebuild, and even when going near redline, it does not go above 6.

we drove an 86 Carrera today , that one has the front oil cooler , and we drove it till it was plenty warm (it was hotoutside as well ), and that one still goes to the 8 mark ( there's no markings on that gauge, but the same mark where mine would have an 8. it never goes below the 2 mark , even when idling... and that is how mine behaved before the rebuild and bypass modification... am i beeing paranoid about this??

[johnsjmc]

First the oil pressure relief spring will likely only open at higher revs,so it isn,t likely the source of low pressure at idle it could explain a lower than normal high speed pressure reading. If you have the same weight oil in the engine before and after a rebuild and pressure is slightly lower at idle. It could be a result of regrinding the crank and fitting the bearings a little looser than before. I would discuss it with the shop that rebuilt it and see if they think it could indicate a problem.

[Nitrometano]

1bar equals to 15 psi.

[89red911]

I found my oil pressure to be lower hen i used thiner oil. you could try to put in some thicker oil like 20/50 thats what im using in one of my cars and it works great i have also found that some oils thin out more than others when hot. just a thought

[svandamme]

i'll also try cam oil feed restrictors

[Eagledriver]

The 86 Carrera's I had both had senders that were 100 PSI max. The car you have has 150 PSI max. The needle will be higher on the Carrera guage than on your car.

-Andy

[svandamme]

installed the restrictors today

when hot now, the pressure indicator stays a at a more confidence inspiring level now... so i'm a happy camper