IcemanG17

Andy
ill address this one. A full “street” 928 oil system is nearly closed due to smog regulations. So if the oil system suffers some ring blow by in addition to normal pressure from moving pistons it pressurizes the oiling system. That pressure has to go somewhere, in typical street 928 this pressurized oil vapor gets injected into the intake. Which effectively lowers the octane of fuel and can increase knock and oil consumption among other issues

Think of squeezing a balloon....squeeze one end the other gets bigger...the pressure must go somewhere and always takes the path of least resistance.

Now think of moving pistons....5L or 6.4L in your case.....in order to move they must overcome at least 14.7 psi of typical air pressure....which no wetsump 928 ever sees.....always more..the higher the engine speed the higher the pressure no way around it. Now as the oiling system pressurizes it takes more HP just to run.

A dry sump adds an external vacuum which removes that pressure, even a slight vacuum in oiling system has huge gains ...so back to balloon....let’s say a typical 928 “balloon” is 16 psi of pressure at X rpm. Add dry sump and now it’s 14.0 psi..so less hp is lost because pistons move easier in block. Easier to squeeze balloon. Not to mention the performance gain in race conditions.

I recall studies done years ago on 6.4L strokers like yours that estimated dry sump vacuum could gain as much as 5+% hp gain at high rpm

ptuomov

To me, the primary reason for pulling a vacuum in the 928 crankcase wouldn’t be increasing power but increasing reliability. The piston pumping pulses get less problematic with lower crankcase gas density. They don’t have the energy to move oil as much.

vze2jshn

Interesting stuff guys. But you may be swimming against the current since the 928 engine ain't a racing motor.

worf928

Oh? Why's that?

skpyle

There are a number of 928 racers who would dispute that...

dr bob

Hell, people race riding lawn tractors with "hot-rodded" Harbor Freight engines. Racing engine means it fits in a car that's raced, right?

Was it intended to be run at full throttle all afternoon? Umm, yeah. The advertising offered the car as a reasonable alternative to commuter airlines in Germany. Stop every 200km for fuel and oil refill. Why do you think the fuel tank filler neck has the "oil OK?" on the flap?

So keeping the oil in the crankcase is the challenge, separate from the racing challenge.

BC

These discussions are always around avoiding the obvious. Dry Sump is the solution. Its been mentioned for decades. If no true drysump, then scavenging from where the oil goes and putting it back to where it is supposed to be.

ptuomov

Out of curiosity, what are the ring widths in the new and old pistons?

andy-gts

So , is there a product that is not a dry sump that still pulls vacuum, my engine is a closed system as designed by Greg, it has the suction off the heads and pumped back to the oil pan, if a little vacuum can make the engine more efficient I am all for making things better....

is there a device, I know this has been kicked around before and I remember someone mentioning a centrifugal device that sucks slight vacuum that is used in mercedes engines....

ptuomov

If you can fit a vacuum pump under the hood, pulling a modest vacuum in the 928 crankcase is always a good idea. To do that with an external vacuum pump, you need to pick up the gas thru some sort of baffling and preferably from the block chimney. You also need a pressure regulator inlet in the valve cover, and some place to vent the gas (and oil mist).

However, since you already have the stock pressure pump and a scavenge pump mounted and running, one would think that a fuller solution might be converting the whole engine to a dry sump with an external reservoir etc.? Just thinking out loud.

andy-gts

I guess it would be best to check the crank case pressure first and see if it high or not......dipstick tube here I come