Thinking of tossing the FB25 crank in favor of a Billet one.... Going to toss it up on the FS forums

:thumbsup: Pressure for the sake of pressure isn't the goal. Pascals law says that in an enclosed system, an incompressible fluid will act upon all parts equally. If you increase flow you will increase pressure. If fluid viscosity is reduced, flow increases and pressure decreases. But if viscosity becomes thin or clearances become so wide such that flow is really high but pressure is very low then our engines fail.. So to some degree system pressure is indeed important to bearings. For the sake of clarity it's safe to say our engines need both flow and pressure.

Bearings do care about pressure but the pressure discussed is the overall system pressure since we cannot measure individual bearing pressure without knowing the system pressure plus the bearing size and clearances plus viscosity of the oil. The engineers calculated all that and determined that the ideal pressure is 73 PSI @ 6000 RPMS when 0w-20 oil is 176F.

Therefore measuring system pressure is a reasonably comprehensive way of monitoring the oiling system as a whole to achieve are targets set forth by the engineers.

The oiling system is a pressurized system not by accident and the oil between the bearings is a hydrodynamic cushion that is pressurized by 2 things: the front side (force pushing oil into the bearing or pump pressure) and the backside (the oils ability or resistance in leaving the bearing). If the oil flowing in and out of the bearing wasn't pressurized, it would easily be squeezed out (overcome) and contact would be made. That oil flow is very important as the oil needs to be evacuated and replaced on every rotation but the resistance of the system that keeps the fluid from leaving it when it needs to stay put, is pressure. This is why when engines wear down and their bearing clearances expand (backside pressure), oil pressure at any given viscosity decreases even though flow has increased due to less restriction or easier evacuation. Basically the increased clearances mean the oil can more easily escape the system (less pressure) and with less resistance to escape, meaning it flows better but yet the bearings are less protected. If the clearances grow the volume being pumped needs to increase. If you increase flow on the front side (pump side) you increase both flow and pressure.

Not taken into account is the Regulation of Pressure.
Generally OEM pumps are designed such that they can provide way more flow (or system pressure) than the system will ever need at any given viscosity and it's why they are regulated with a pressure relief valve on the pump. However the PRV is set to regulate the ideal pressure at the intended viscosity (oil at operating temp). Cars are "systems" and systems are a culmination of inputs or parameters...

When we change those parameters of system size (oil cooler), oil viscosity (extra heat) or bearing clearance (race build or worn engine), then the parameters of the regulator should also be changed. These things will affect the overall system pressure. Reducing the viscosity through heat allows the oil to flow through the bearings more easily (more flow) but the result is lower system pressure. Now keep in mind that the super thin oil is also escaping past he PRV more easily as well as it's a spring based plunger... think of a window being opened and closed, DO NOT think of the PRV as an on/off switch... it isn't! As the viscosity is thinned the "window" of the PRV doesn't need to open as much so PRV is effectively bleeding off pressure at 50psi instead of at 80psi. The oil in the system doesn't have to enact as much force on the PRV plunger to escape past it, and thus PRV doesn't work as effectively at maintaining proper system pressure. If that makes sense.

Thicker oil increases pressure but not flow
Raising the PRV spring pressure increases system pressure and flow.

Many people overcome this pressure drop by increasing the spring tension in the PRV which means it won't relieve pressure (start opening that "window") as easily and thus the pump will enact more flow on the system as a whole which increases system pressure until the PRV opens.

Anyone who has done this, however, needs to very conscious of their revs when the oil is cool because of the pressure increases caused by a stiffer PRV.



I'm really good at admitting I'm wrong, crazy or both. This is my understanding as this point in time. My mind is open to learning. Thank you.

The variable I don't have on this engine is what the system pressure would be, if the PRV was entirely closed, with an oil viscosity of @ 8 cSt. (that's the most probably cST of an oil at race temp).

I'm pretty sure the pump without any regulation is capable or producing >80psi of system pressure at that viscosity but I cannot confirm. This would be confirmed if shimming the PRV had any affect on increasing pressure. If it does then we can conclude that the pump itself is more than adequate.

new crankshaft ordered 8-12 weeks before delivery......

The CR you pick would depend on your cams and low end torque you're aiming for. The Mazda Skyactiv engines can run 87 partially because the headers are tuned but partially because the engine runs at lower volumetric efficiency (via abnormal cams or cam timing most likely) and thus has lower torque output. Note the mediocre torque output, despite having tuned headers, DI, etc.:
http://wardsauto.com/site-files/ward...teng-mazda.jpg

With stock cams and I/E piping on an FA20 going to a higher CR is probably not going to work. With high duration intake cams, there will be lower VE, lower torque, and effectively lower compression so you could go higher.

That said, 13-13.5:1 is probably a good place to stop regardless. Raising the compression ratio increases temperatures in the combustion chamber and the forces on bearings, so there's diminishing returns as you lose more heat to the heads and energy to friction. Higher would be good for fuel economy but wouldn't do much for power.

BMW is most probably doing straight up pwm control based on xyz strategy in the engine ECU which could be based on just coolant temp or engine speed/load or both. You could just use one of the cheapest arduino controllers to read coolant temps and engine speed-load (directly or can bus) and hook up some sort of bridge circuit with FETs to pwm control the pump (as opposed to voltage control). This would give you freedom from the low voltage variation on the sensor and give u freedom to choose any control strategy. Sorry if i was too pedantic.

Incorrect about the headers. The Sky Active can run lower octane because they are Atkinson Cycle engines. So it's the cam timing, but nothing that can be emulated here.
Also, compression ratio has nothing to do with cam timing. Cam timing changes volumetric efficiency, some like to call it dynamic compression ratio, but that's just not a real thing.

Semantics. There's no strict guideline that makes one engine Atkinson cycle or not. Some race engines have lower VE than a Prius engine at the same speed.

Cam timing changes VE, and thus cylinder temperatures. That's what I'm talking about. The headers absolutely have something to do with knock prevention as well as torque output. And it can be emulated here with different cams.

Thanks for the useless comment.

lets let this guy have his sweet na build thread and not ruin it with passive aggressive engine dynamics arguments

I should have said should instead of can. Apples to oranges. Let the man build his engine.

Are you going to install VTEC?

Isn't it mandatory?

It must kick in





yo.

In to see the results! I have been wanting to see a good N/A build :happyanim: