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Just finished a Hipo rebuild on my c900t cv. Spent $8000 in parts alone. My labor is obviously free. I'd like to Respond to some oil breather discussion which took place a short time ago.
I use a custom Garrett turbo with Cosworth compressor and impeller. No seals, so while idling, the oil sump builds up enough pressure to create a differential across the turbo shaft bearings. This forces oil into the intake and creates a smelly, oily exhaust after idling for a few minutes. Otherwise it's clean during regular driving.
I did away with the stock system because I thought it was ineffective as a proper pcv system. This was not one of saab's best. Instead, I blocked up the hole in the intake tube, and added a crank breather to the PCV hose at a remote location in the engine bay. I still ran vacuum to the nipple on the black barb, but this time I used a smaller check valve to limit the vacuum flow. Then I added a BMW brake booster check valve (1/2" diameter barbs on either end) to the hose going to the remotely located atmospheric breather.
What has this done?
Well, fix my oiling problem. The small check valve provides enough vacuum at idle to cause a pressure drop in the crank case at idle. Now, the differential across the turbo bearings favors the intake. The vacuum is kept to a minimum as the bearing clearances in the turbo are just large enough to allow a leak path of air.
During boost, the vacuum check valve closes and all crank pressure is diverted through to the atmospheric breather. The large BMW check valve on the breather hose opens and crank pressure is equalized to atmosphere. Once vacuum hits again, the large check valve closes and the vacuum line sucks through the crankcase once again. If I didn't have the check valve, the vacuum line would simply be sucking direct from the breather line and not the crankcase (path of least resistance).
The key is to have some vacuum in the crankcase, but not a lot. Just enough to provide a pressure differential across the bearings. Since the turbo oil drain is tied right into the sump cavity via a large steel tube, all is well.
posted by 136.2.1...
, Mon, 4 Nov 2002 22:43:46
, Mon, 4 Nov 2002 16:54:47
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