Re: [T3] oil temp (and pressure)

["Shad Laws" <shad@lnengineering.com>]

<x-charset iso-8859-1>Hello-

> I was going to post this when things calmed a bit but Shad has 
probably
> already delt with this so:
> 
> With the new Notch engine the hot oil pressure is about 5lbs lower 
than it
> should be...
> well, when compared with a stock engine.  Im running full flow on the 
Notch
> and I was told by a mechanic locally here that there have been issues 
with
> pressure drops over the Fram oil filters.

The mechanic is correct, but that doesn't directly reflect the oil 
pressure problem.  That's like me saying: "What's the temperature like 
outside today?" and you saying: "Well, there's a couple clouds in the 
sky."


> Are fram too restrictive?  is there a better brand to use that is 
less?
> what should I expect in pressure reduction?

The primary sucky point of Fram is the filtering itself.  Get a K&N, 
Purolator, Mann, Wix, etc. filter - they are good quality.


> Lastly... why is there an issue with oil filter pressure problems 
with full
> flow?  

Again, it's the temp vs. clouds problem...

First, let's look at your oil system after the cooler, pump, filter, 
etc.  Pretend that you put a fitting into the galley and you pump 
whatever you feel like from somewhere else.  For given rpm and given 
load and given oil temp, you can determine the flow rate needed to make 
a certain pressure.  More flow = more pressure.  If the bearing 
clearances are too big (or they're too old), the oil is too hot, the 
load is too high, or the rpm is too low, the galley pressure will be 
less.  (Yes, it is LOW rpm that kills correct lubrication, not high 
rpm, so no hard lugging with hot oil :-).

Now, put the filter and cooler back in.  What you'll find is that the 
cooler and filter create a pressure drop.  For the same galley 
pressure, you'll get the same flow, but the pressure BEFORE the cooler 
and filter is higher.  This is normal.

Now, put back in the oil pump.  Idealized, it is a constant flow 
device - more rpm = more flow.  But, in reality, it is not a constant 
flow device.  If it has to pump to a higher pressure, it will leak back 
some oil.  This is okay - they all do.  While it is possible to make a 
nearly perfect gear pump (like in 5000psi hydraulic aviation systems), 
they aren't needed in our cars due to complexity.  So, we have this 
problem.  Now, with thin oil and a needed certain pressure, the oil 
pump can't keep up at the rpm you are at, so the flow and pressure drop.

Swapping out the filter *might* help a *little* bit.  However, unless 
the filter was totally clogged, it won't do *that* much.  Get a new 
pump (perhaps a bigger one), blueprint it, and watch the pressure jump 
back up to the happy range.

Next question: why do we need X oil pressure for Y rpm?  Ah, complex 
answer... I have to leave soon, but if still interested, I'll respond 
in a week or two when I get back to IL.  Remind me.

Take care,
Shad Laws
LN Engineering
http://www.LNengineering.com

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