Accusump

304065 · 05-15-2007, 05:19 AM

I received a question via PM about accusump plumbing and thought I would share the results of my research and WHY I'm not going to use one.

First, plumbing. If you analyze the oil system diagram long enough, eventually you realize that the inlet to the pressure side of the oil pump is fed by the "S" hose from the oil tank. The pressure side pressurizes the oil, and if the thermostat is open, sends it through the engine cooler before sending it to the main oil gallery. At the flywheel end of the main oil gallery, there's an M18x1.5 threaded fitting into the case that's an adaptor for the oil pressure sender on the early cars, and the idiot light switch on the later ones. This is where you would plumb the accusump into the engine. If you hit the brakes and the oil sloshes forward in the tank and uncovers the port for the "s" hose, and a slug of air gets sucked into the pressure pump, the accumulator should make up the difference. I've heard comments that "there's not enough flow" but we're talking about a momentary interruption here.

I was going to come out of the case with a M18x1.5 male to 1/2" NPT pipe thread, then to a 1/2 NPT tee with one port containing a reducer fitting from 1/2" NPT to M10x1.0 for the oil pressure sender, then in the other port a 1/2 NPT to -8AN, then a piece of -8AN to the accusump, which has either a manual ball valve, remote manual ball valve, or electric valve, all in 1/2" NPT. So into the valve you would need a -8AN Male to 1/2" NPT male, then a 1/2" NPT male nipple into the accumulator. I was going to mount mine behind the passenger seat for weight distribution.

HOWEVER I have decided to NOT proceed with this installation because on an MFI engine, there's a longer banjo fitting into the pressure port. This banjo fitting is the oil supply for the MFI pump, and it's effectively a calibrated leak. We know from other threads that the flow through the MFI pump is very little, but I don't want to have 100 PSI oil pressure in the accumulator trying to escape through the MFI pump all the time. I realize this is what happens in normal operation when the engine pressure goes to 100 PSI, but I didn't want to monkey around with the pump's supply.

I know you can convert the pump to have it's own oil supply, but I talked to Gus and not only is this expensive to do, it requires that you remove the pump to change the oil. Removing the pump after every race isn't my idea of fun so I'm going to use an alternative solution to the classic oil starvation problem, which is either a FABCAR-modified Tank like Tom Butler has in his magnificent orange RSR recreation, or modify the fender well to use a Carrera tank.

Hope this helps somebody who's contemplating an accusump. For a carbed engine, I don't see why it would be a problem, you could plumb the oil pressure gauge into the other end of the oil gallery like it is on the later cars.

I had thought of going straight from the sump to the gallery and then then feeding the MFI pump with a line plumbed into the cam spraybar oil feed/tensioner, but I'd rather not experiment.

gestalt1 · 05-15-2007, 07:32 AM

good timing on your accusump install. i'm doing exactly the same install however i'm using an efi system insted of the mfi. the problem with the accusump is it will find the path of least resistance, not nessesarily the needed path. i was thinking of other areas to plumb in the accusump, is drilling and tapping a port into the oil breather cover an alternative?
-matt

304065 · 05-15-2007, 10:45 AM

Matt, nothing under the oil breather cover is in contact with an oil gallery. Perhaps I don't understand the question.

911pcars · 05-15-2007, 12:12 PM

John,
I don't have any direct experience with this, but wouldn't adding baffles in the oil tank help prevent oil from centrifuging or sloshing? I believe Chris Streit added some sheet metal baffles to his tank.

Sherwood

304065 · 05-15-2007, 12:44 PM

Sherwood, that is the FABCAR modification I refer to above. It adds baffles and additional capacity and cures the problem.

Grady gave us a nice road map for the ideal oil system in a 911 a few months back, similar to what the GT3 did. The oil tank does a lot of things, besides just storing the oil after it's returned from the scavenge stage (via the external oil cooler if the car has one). It acts as an air/oil separator for both the crankcase breather and the oil coming from the scavenge pump, which can be foamy if the pickup isn't fully covered all the time.

An air/oil separator would be a nice innovation to limit the amount of air entrained in the oil fed to the pump. Borrowing some of the design features from airplanes, where air/oil separators are particularly important to keep grease off the belly, you want as small an expansion area as possible so hot crankcase vapors do not rapidly cool down and condense the water, forming sludge. By keeping the expansion small, the oil gets pulled out of the breather air and yet the water vapor gets vented overboard.

A centrifugal separator for the oil from the scavenge stage might be nice as well. The scavenge line runs through the oil tank to the filter inlet, and after being filtered, the oil fills up the tank. This probably removes whatever air bubbles are in the oil. So it's the baffling around the pickup for the "s" hose to the pressure pump that's needed to keep the pressure pump from sucking air.

Anyway, there are lots of ideas that could be incorporated into a revised oil tank. I'm probably going to send my tank to FABCAR for the mods, however. It's a very proven design.