Oil pump piston, some questions.
 

I bought the new oil pump piston (997-107-125-01) to replace the original piston (996-107-125-51) since this is recommended to help oil pressure at idle. I also bought a new spring but just noticed that the spring part number has not changed (996-107-127-53).
I wanted to compare the new and old pistons, so removed the piston from my torn down broken 3.2l engine. I understood that the old piston should just drop out, but that was not the case for me, I had to force it out from the top (which can be done with the pump removed) and once that it was I out I could feel that it was slightly mushroomed at the top, which I think is what made it stick in its bore.

http://986forum.com/forums/uploads02...1683051859.jpg

Old on the left new on the right.

Questions...
1) Is a new spring recommended because the original one may have lost some of its "spring"?
2) Are the old and new pistions identical except for the chamfer at the top of the new part? If so why could a resourceful person not just machine a slight chamfer on the old piston to get the same effect?
3) Is the mushrooming at the top the reason for the chamfer?
4) What exactly do the piston and spring do and how do they do it? My understanding is that it is a pressure relief valve.
Thanks in advance.

1. No, the new spring has a higher rate, which raises the oil pressure slightly at low RPM.
2. Yes, the chamfer is the trick, it stops the mushrooming and prevents the piston sticking in the bore.
3. See #3.
4. It is the pressure bypass system for the pump.

For around $20, you get everything new and do not need to machine anything...........

Thanks for the info. You make such a great contribution to this forum!

Because I am an ornery old fart I may just try experimenting with a machined piston and a washer added under the spring to see if I get the same result for free. It is easy to change the piston and spring and I have 3 engines I can experiment with.

Low rpm only?
So it won’t really help for track use?

It will help slightly at higher RPM's as well, but was created to address pressures so low at idle that the oil light flickered, and address stuck piston's; plus most track oil issues are related to oil control and/or starvation,

it's a pressure valve. if oil pressure exceeds a certain value (as determined by the spring) then oil pump will bypass back to sump.

you will see high pressure at start-up with cold oil. the stronger spring keeps the oil circulating a bit more before bypass which will help lubricate the chains.

also, the oil pump pressure is dependent on rpm - pump spins faster as rpms increases which results in increased oil pressure. so when flogging it around the track at high rpm expect that some oil gets bypassed to keep pressures within spec. something to consider when trying to determine where to plumb-in an external oil cooler or accusump.

also, when oil starts to build up in the heads when doing higher-g turns, pressures will increase and your pump will start to bypass just when you need it the most.

so, stronger spring is good.

Cool, thanks

I bought the new piston and spring, but did not use them in the 3.2 replacement engine that I was installing in my S. Instead I did as mentioned above, ground a chamfer into the old piston and placed 2 washers under the spring to increase tension. With that my oil pressure is a bit low on my after market oil pressure gauge, which I am sure is reporting the pressure a bit lower than it really is (oil pressure light never flickers). I replaced my reworked piston and the old spring/washers with the new piston and spring. After an initial test it would seem that the pressure at idle has increased a bit, but I will keep checking it.

yeah, adding washers (ie compressing the spring) doesn't change the rating of the spring. springs are rated at # / ". ie, it will take 40 lbs of force to compress a 40 #/" spring one inch. it will take another 40 lbs to compress it the next inch. ie, a total 80 lbs to compress two inches.

so, while the washers compressed your spring (and reduced amount of available travel) you didn't change the force required to compress the spring.

Hmm, I have always assumed that as a spring is compressed resistance to the compression increases, but I understand what you are saying. My relationship with springs will be changed forever.

My initial observation that the oil pressure at idle has increased a bit with the new spring seems to be true after a bit more running time. I will be changing that spring in all my engines and in my friend Tony's car.

The force you need to compress a spring is proportional to the pre-load on it. This means the more the spring is compressed the more force is required for the next increment of compression. This is how motorcycle shocks work when you make it stiffer for the extra weight of the passenger, you just add some pre-load on the spring. So, I think adding a washer pre-loads the valve and it will open at higher pressure.

i was going to type out a big long response but easier to just post this:

https://www.penskeshocks.com/blog/motorcycle-spring-preload-what-it-does-and-why-we-would-adjust-it

note the bit where they say "It’s important to point out that preload doesn’t affect spring stiffness."

i would note that you can get progressive springs that do change stiffness when compressed, but not in the application we are discussing (but perhaps on your motorcycle as it is more of a suspension thing).

[QUOTE=The Radium King;657185]

note the bit where they say "It’s important to point out that preload doesn’t affect spring stiffness."

[/QUOTE

Thanks for the article, it got me thinking. It also says this: The more preload, the harder the suspension is.

But how can the suspension be harder if the same force is required to further compress the spring?

I think they mean by stiffness is the spring constant, which does not change. However, when you pre-load the spring, you increase the potential energy in it, which competes against you when you further compress the spring.

Take a look at this forc vs. displacement relatainship on a linear spring:
http://986forum.com/forums/uploads02...1699564547.jpg

It's from here:
https://www.sciencebuddies.org/science-fair-projects/references/linear-nonlinear-springs-tutorial

The more force the more displacement, which also means to me that if you displace your spring from 0.2 to 0.3 requires more force than if you compress it from 0.1 to 0.2. Similarly, if you take a valve spring for instance, and you compress it 10%, it is easier than if you compress it from 10% to 20%.

Progressive springs are the same, just this equation above is supralinear.

That's my understanding. I'm not a physisist though :confused:.

with preload the spring is pushing back with the same amount of force that caused the compression due to preloading. once overcome the spring has the original spring rate.

say a 10 lb / inch spring is preload two inches. you have to apply 20 lbs of force before the spring starts to deflect. but once the preload is overcome it still requires just 10 lbs to deflect the spring each inch. in the end, a total of 3 inches spring length change (2" preload, 1" deflection) is achieved with 30 lbs force (10 lbs / inch). 4 inches with 40 lbs, etc.

in the case of the oil piston and spring with washers you've preloaded the spring, but in the end the same amount of force is required to deflect the spring enough for the pump to bypass.

Yes, as you are saying, ONCE you overcome the pre-load. But that's not negligible, and the pre-load is adding up as you are compressing he spring further..

If the same amount of additional applied force caused the same amount of compression each time, the chassis of the bus would sit on the wheels after 50 people boarded.

Anyways, we might not agree on the springs, but shimming the relief valve is definitely a thing, many engine builders do that to increase the maximum oil pressure. I'm not sure if it makes sense in a Boxster though. I added an extra shim to the valve in my Yamaha and measured an increase of 10 psi in the max pressure.

They even sell relief valves in adjustable versions, where the pre-load is adjusted by a set screw:
https://www.7ent.com/products/adjustable-oil-pressure-relief-valve-kad1011005.html

I found a video of an adjustable relief valve in operation, it's not a car oil pump, but I believe it's the same principle, around 2 mins:
https://www.youtube.com/watch?v=4eXDK7OGcRs

Adding preload shifts the red line to the left. Depending on how much you add the equation goes from y=20/.2+0 to y=20/.2+10 or something.
Just make sure you aren’t getting close to coil bind which you probably aren’t, if it’s only a washer or 2

i'll give. i was thinking overall spring length and not travel. quick math:

4" spring. has to travel 2" to bypass at 50 psi (made-up numbers).

so 2" at 50 psi = 25 psi / inch spring.

shim say 0.1" . that is equivalent to 2.5 psi

so, 2.5 psi to get spring moving, followed by 25 psi / inch spring rate.

has to move 2" = 50 psi.

will now bypass at 52.5 psi.

I appreciate the discussion... now for my real world observations. A new spring has increased the oil pressure at idle compared to an old spring with 2 washers under it. The next test would be to try more washers under an old spring to see it that would bring the pressure up. Someone else can try that as I have already bought new springs for all my engines. I am using my old pistons with a chamfer "machined" into them. Machining was by using a belt sander.

it really depends on how much the piston has to travel before bypass i think.

if you look at my math previous, if the piston only had to travel 1" then it would be 25 + 2.5 psi or a 10% increase in pressure.

if two inches then a 5% increase. if three inches then 3.3% increase, etc.

a hard look at the inside of the oil pump cover shows that the piston travels quite a ways before bypass (as opposed to those by other companies that bypass immediately upon deflection).

but yeah, most of my thinking around preload had been in a suspension situation where the preload was against the static weight of the car (ie, adjusting a coilover for ride height) vs against a fixed surface. in the suspension situation the opposing preload force is already overcome by the weight of the car (unless your springs are so stiff that they don't deflect when the car is sitting on them) and the springs are operating in their linear range.