Turbo Oil Scavenge

[jrickert]

Is anyone willing to give up what pump they are using as a turbo oil scavenge pump. The lightest solution i have found so far is this one (http://www.scavengepump.com/)
Has anyone had success with just hooking up PCV between the throttle and restrictor? We did a test today to see how the turbo reacted to suction on the return today using an external 120vac pump. It seems to solve the problem but i would really like to avoid adding a scavenge pump to the car.

[Kirk Feldkamp]

Search the forum quickly before you burst into flames! This question has been discussed at great length in the past.

-Kirk

[jrickert]

Yes, It has been discussed but never to any useful conclusion. I think its an issue worth opening again. This thread is more specifically about scavenge pumps.

[VFR750R]

Lingenfelter and other performance shops turn to electric scavenge pumps on turbo Corvettes since the turbos are always mounted low. Call them and ask, I bet they'll tell you.

The other option is a mechanically driven pump, like a single stage oil pump(homemade or aftermarket) or vacuum pump like sold by moroso. I'd also look at belt driven fuel pumps as they are basically single stage oil pumps, and just about any size would be big enough to scavenge the turbo.

[Kirk Feldkamp]

We used a Holley Red fuel pump because it has all-metal guts. There aren't a lot of pumps out there that I'd be certain that would survive the oil temps. You really don't NEED a scavenge pump if you do the rest of the system 'correctly'.

If my memory serves me (and it usually doesn't), the Lingenfelter kits use the Mocal pump... but it's WAY too big for reasonable FSAE use. I think the STS kits also use a relatively big pump as well.

-Kirk

[Denisk31]

After exhausting my efforts of searching the forums and various websites, I thought I would join in on this thread about turbo oil scavenge pumps.

Currently, we are running our turbo with a gravity feed. However, I want to mount the turbo lower and therefore utilise an extra stage on our dry sump pump to scavenge the oil away from the turbo. The only worry I have is that at high engine speeds, the scavenge pump will create too much vacuum and therefore suck the oil through the seals. For this reason I have been looking at the Holley Red fuel pumps that Kirk recommended (I also saw this on a few different posts).

My question is if anyone has any experience using the dry sump pump to scavenge the oil from the turbo? If so, do you experience any problems or do you use a vacuum release valve (I currently cannot find a suitable product) to relieve the pressure.

/Denis

[Pete M]

Why is it bad for a scavenge pump to suck a vacuum on the turbo?

[Denisk31]

My initial thoughts were that if too much of a vacuum was created, then you could starve the turbo of oil. Do you think this is a possibility?

[Kirk Feldkamp]

No, you will NOT damage the turbo. In general, the more vacuum you put in the center housing, the better the seals will work. Think about what vacuum means as far as the direction of flow.

If you already have a dry sump, then you're already much better off than using any sort of electric pump. Look at how low Cornell gets their turbo... it's a great example of what you can do if you have proper oil scavenging.

-Kirk

[Denisk31]

Thanks for your help Kirk. Unfortunately I have only a rough schematic of the inside of the central housing, therefore I am struggling to understand that the more vacuum generated, the better it is for the operation of the seals in the turbo. The way I am thinking about it is that the more vacuum pressure you have in the central housing, the more chance there is for the scavenge pump to suck at the oil supply and therefore starve the central housing of oil? This is obviously largely dependent on the mass flow generated by the scavenge pump. Currently we are utilising a Pace 2-stage pump, which I want to upgrade to a 3-stage pump, with one of the stages designated to the turbo oil scavenge. If the scavenge pump does not receive the correct amount of oil, it is likely to create quite a large vacuum (at high engine speeds) and this is what I am worried about. I apologise if I am completely ignorant of your previous explanation, but I would really appreciate your help in explaining this topic in further detail

Thanks
Denis

[VFR750R]

By increasing the vacuum you are effectively increasing the pressure ratio across the bearings (equivalent to more oil pressure). You will not starve the bearings of oil, because oil is incompressible, for the oil to pull out of the bearings it must be replaced with...more oil. It's like sucking coke through a straw, if you suck harder...you get more coke, not less. In this analogy, the straw is the oil line feeding oil to the turbo.


Start thinking in terms of absolute pressures and not pressure and vacuum. The oil doesn't see vacuum like you and I do. It sees a 'lower' pressure then where it's coming from.

Say you pull a perfect vacuum on the housing. abs press = 0. Your oil pressure differential is now 90psi instead of 75psi like on your gauge. You're bearings now see MORE oil flow.

And seals are non contact so they don't care how much oil they see. But with vacuum in the housing, now there is a positive pressure differential from the compressor housing to the bearing housing so flow is from compressor to bearing housing vs the other way which causes oil to leak into the compressor and then engine.

[Denisk31]

Thanks VFR750R, that explained it perfectly. I was not thinking about the oil been incompressible. It totally makes sense if you consider the system as a whole, therefore the oil pressure supplied to the turbo must increase to balance the pressure and mass flow requirements of the scavenge pump.

I am still a bit worried about it but I am convinced enough to go for it! The only thing I am currently worried about is the diameter of the hose connecting the turbo oil outlet to the scavenge pump. I am worried that if I make this the recommended 1/2" (for the gravity drain system), then it will suck more air than oil, considering the hose is practically horizontal. Again, if you think about it the way you explained, then it should be okay, but it is just a slight worry. I want to ensure I have exhausted all possibilities before manufacture...it is one of the most critical aspects of the turbo system so needs to be right.

Thanks again,
Denis

[VFR750R]

Line size will have little to do with the amount of air the scavenge pump will pull. The air pulled will be directly related to air 'leak' past the seals which is proportional to the square root of the pressure differential you create with your scavenge pump. Meaning...if you pull 10" vac and get x cfm of air leak, at 20" you will have sq. rt. 2 * x cfm.

The final steady state vacuum your pump pulls will be an equilibrium between leakage of the seals, oil flow rate in, internal leakage of the pump, and volume pumping rate of your pump. Line size will just determine speed of air/oil mix in line, not how much.
Additionally, this air will be very small in quanitity compared to air leaked past main seals and piston rings in your motor, so...i wouldn't worry about that air.