In general I also trust the engineers what they originally intended and how they designed it. But I'm also open to technical meaningful improvements to make things even better. And that's exactly what Porsche did for decades with these engines...
Here is a discussion started back in 2006 in the engine rebuild forum here and continued last year, with some interesting postings there, with statements among others from Henry Schmidt...
https://forums.pelicanparts.com/911-engine-rebuilding-forum/310463-cam-oil-line-restrictor-adapter-3.html
Also our host,
Wayne Dempsey wrote in his book "How to Rebuild and Modify Porsche 911 Engines 1965-1989" in chapter 4 in section ""Performance upgrades" some things about the restrictors.
Summarized:
Initially the oil lines to the chain tensioners were connected with the well-known 90110536100. In 1991, the intermediate piece with the smaller passage was installed for the first time on the 964 Turbo, part number 90110536101 / 90110536102 (recognizable from the outside by a groove of the hexagon)
Reason: Foaming engine oil in the crankcase (due to the backflow from the camshaft housings)!!!
Apparently the PCA first came up with the idea of installing these new 90110536102 in older engines: The new spacers reduce the amount of oil that is pumped to the camshafts and from there flows unused back into the crankcase
This also reduces the amount of oil in the crankcase itself! Reason: This prevents the oil from foaming there due to the rotating crankshaft, with the effect that the oil pump also sucks in less air (foam contains air!), thus ensuring more constant filling of the oil tank and thus a stable oil circuit
Welcome side effects:
-Higher oil pressure throughout the engine
-More stable oil film in the bearings
-Higher oil pressure in the chain tensioners
-Better piston crown cooling
All in all: only advantages!
And so I replaced last summer the original two 90110536100 on my 1981 SC by two 90110536102 with reduced diameter holes (less than 3mm/~1/10 inch). The old ones are more than 5mms/0.2inch iirc.
BTW: I had some trouble with the threads - I couldn't screw them in by hand because they blocked after half turn and they're made of steel and you screw them in an aluminium case... so I had to recut them to make them fit and then I could wrench them in easily by hand ...
Preliminary remark: I never saw the oil warn light when engine was hot, even on high summer temperatures! But when the engine was hot on a hot summer day the pressure on idle was pretty low...
My experiences in short: I can report significantly higher oil pressures, not only when engine is cold, but also when engine is hot!
Before installation the engine reached ~4bars with 20W50 mineral oil when running cold, but when warm far less than 1bar when running hot on idle on hot summer days as said. Oil preussre climbed little less than 1bar per 1000rpm as expected up to a little more than 3bars also on hot summer days...this made my concern... I also know my engine is old...
After installation of them I experienced a far better oil pressure when engine is warm, especially on high ambient summer temperatures above 30°C/86°F it keeps roughly 1 bar on idle and increases the pressure exemplary as designed with 1bar per 1000rpm up to more than 4bar on high revs as well. So I'm pretty happy with it now. Oil still the same 20W50, it wasn't replaced...
Also I can report that the engine indeed now runs cooler which I lead back to a better piston cooling due to spraying more oil especially on lower rpms to the their bottom and therefore the whole engine runs cooler now: The temperature gauge now climbs a little over 80°C / a bit over the white box of the oil temperature gauge, no matter if I cruise or wait at the traffic lights, but no more than 9 o'clock position on an inspired drive, before it climbed above 9 o'clock up to 10...
So there is no need for that much of oil on the camshafts as originally designed to be. There will still plenty of oil afterwards!
What I also expericed: The oil tank doesn't drain that much oil than before on WOT....the fluctuations are far less than before. A complete drain of it (in the gauge) I've never seen since...
I cannot recommend enough the usage of the new oil restrictors on all aircooled 911's with hydraulic chain tensioners, especially on high mileages (as mine is), because this ensures the oil to make it's job of lubricating and cooling where it is needed to be. Tuned and race engines are different, no question. But here the named pelican thread above also dicusses drilled restrictors to enhance the new ones a bit to get a good compromise for these engines as well.
Thomas