Howdy all. I'm preparing for going turbo and building an engine with a 94 block, looking into everything necessary, and I found this article that gave me pause:

Assembly By Keegan Engineering, Pt. II - 1997 Mazda MX-5 Miata Long-Term Road Test

Specifically, at the end of the article...

The stock oil boss (directly behind the red fitting), however, remains untapped. Keegan does not like how it would bleed oil pressure directly from the main oil galley at the rearmost main bearing. There's no sense depriving the heavily-loaded mains from any oil when there's a more benign source for the turbo's oil on the passenger side of the block -- at the VVT oil supply.

I searched and couldn't find any discussion of this issue. My 94 block has these ports tapped, and would be nice and clean/simple to use. For a high HP (350ish) turbo setup, is there any evidence that this oil port is a less desirable solution than sourcing oil from the other side of the block, or is this a 'theoretical' problem? Keegan seems to know his stuff... but so do many on here, and many(most? all?) that have that port available use it. Will running a line around to the oil pressure sender port make any appreciable difference in rear main bearing wear? Does the size of the factory block port restrict the flow sufficiently, or must the line be sized to intentionally limit the pressure bleed off?

 

I would absolutely use the port.

 

Do you know why Mazda put those ports there in the first place?

 

To bleed pressure from the rearmost main bearing, right?

 

Yeah, i know... just asked because OEM solutions are designed for OEM power levels... i.e. a dinky turbo. I wouldn't have asked if it weren't coming from a respected Miata engine builder.

 

BPDs were plenty capable of making that sort of power in qualifying trim.

 

That was the stock oil feed location on the 323GTX - but maybe there was a restricter in that line/port to limit the amount of oil?

Question: How much oil do turbos need? Is there a difference in oil requirements between a sleeve bearing turbo vs a roller/needle bearing turbo?

 

Turbos, in general, require very little oil.

Sleeve-bearing turbos tend to self-limit their oil consumption, since the bearing clearances are quite small. Just as with the main bearings in the engine, sleeve-bearing turbos require a fairly high oil pressure in order to keep the shaft "floating" properly. You can run a line directly from the oil supply hole on the back of the block to a sleeve-bearing turbo and everything will be just fine with no need for any flow-limiting devices.

Ball-bearing turbochargers require very little oil pressure, and very little flow. They do require an external flow-limiter, as they will tend to pass huge amounts of oil if allowed to. Some designs incorporate a restrictive orifice in the oil inlet port itself, some have a drop-in device, and some require an external limiter such as this:





There's a reasonably good primer article on turbo bearings here: AutoSpeed - Turbo Bearing Tech - Part One


At one point, I seem to recall some company experimenting with permanently-lubricated bearings (eg, like a greased wheel-bearing) as part of an attempt to convince the world of the inherent superiority of rear-mounted turbo systems. Not sure if anything ever came of that.




Also, if anyone is still concerned about robbing the #4 main of oil, don't be. The port on the side of the block has its own passage drilled all the way back to the main oil galley, separate from the feed to the #4 main bearing. And the main galley is so large that the concept of depriving it of oil through a turbocharger bearing is like trying to measure the amount by which you can cause the level of the Mississippi river to fall by sucking water out of it through a drinking straw.

(edit: the struck portion of the preceding is incorrect. That port does hit the rear main bearing feed. None the less, the amount of oil taken by a turbo is miniscule as compared to the supply available at that location, and there are enough factory-turbocharged 323GTXes and '04-'05 MSMs running around drawing their turbo oil from this point that we can state without question that this is a suitable location to lubricate a turbo from.

Here is an image of the oil supply to a MSM engine (note that the turbo an manifold have been replaced with FM-supplied parts):

 

These turbo's joe? I think their real benefit would be vertical mounting in the engine bay on subarus.

 

That's the one.

I find the idea that some bearing grease and a couple of O-rings are capable of lubricating a turbocharger to be fascinating. I really don't know whether I believe it or not. I am curious about the comment that "Since the bearing system is easily removed as an assembly from the bearing housing, the bearings can be re-greased at appropriate intervals," and I wonder what an "appropriate interval" is considered to be. For a street car, I could probably deal with having to remove and rebuild the turbo once every couple of years. Not so much once a month.

 

"MNIVET," the LSx turbo black NB Miata runs one of those in a rear mount configuration. Seems to make gobs and gobs of power, hasn't been any issues with it.

 

I wouldn't consider 750whp or whatever he ran before he blew it up gobs of power for an LS1, but its still pretty legit in a v8 miata where there's no room anywhere else to mount one of those bad boys

 

Yeah, double stock LS1 power to the wheels is pretty weaksauce.

Personally, i want to see 1500whp in my Miata.

 

Well played.

Actually, something occurs to me: Presupposing that the grease is the *only* maintenance item (eg: the O-rings are fine), why not design the thing with a Zerk fitting on one end and a removable plug on the other?

 

Yeah yeah, you know what I mean fgt.

Built and boosted LS1's are making stupid power.