Creating Crankcase vacuum

[AlfonsoR]

Originally Posted by Walt Fricke

Steve

I thought the benefit of low tension rings was reduced parasitic drag on the cylinder walls. I have assumed, perhaps wrongly, that such rings would allow a bit more oil to enter the combustion chamber past them. But how would they help decrease crankcase pressures? By allowing combustion chamber vacuum to suck some of the air back out of the case? That couldn't be right.

Walt, I believe he meant that the system works well in terms of performance. From what I have read, you need some vacuum to run low tension oil rings, otherwise without the vacuum, you will probably have oil control issues.

[AlfonsoR]

Originally Posted by MBruns

.... the vacuum helps stabilize the ring package ...
Mike Bruns

Mike what do you think is going on here? I guess the pressure wave from the power stroke possibly vibrate the top wring wildly or perhaps loads it unevenly? So the vacuum in the crankcase helps suck some of the excess pressure away or helps stabilize it somehow so that the ring gets more uniform loading? I'm just drawing at straws here, but you always get the ring groove at the top of the cylinder, not at the bottom, so I'm trying to connect the dots, I'm guessing that the pressure wave from the power stroke pushes the top compression ring fairly hard against the wall on its initial travel down.

Mike or Steve or Henry.... I have heard of some folks who build race engines don't install a second compression ring. Have you any of you guys ever tried this on a Porsche engine?

[chris_seven]

I think 2 ring pistons, as a means of reducing friction have become quite common and are said to yield between 3-4 HP per cylinder at 8500rpm. CP and Omega make a number of different designs.

I think the top ring design is totally different to a standard top ring.

It is a recomandation that 2 ring pistons need to be run with dry sump engines with a vacuum pressure in the crankcase to help with sealing but I can't find any numbers.

I am not sure you can just take a ring out of a 3 ring piston but I could be wrong.

[marlinaness]

sorry to break in, but you guys might know an answer here. Does anybody have a guess on the oil pressure on the scavenger side of the pump. Can we assume it is low as the only impediment is the oil filter? Or can it go higher e.g. greater than 20 psi?
thanks
marlin

[MBruns]

Many years back we had a race customer that pulled the 3/8" threads from his Oberg screen filter on a cold morning start up, which was first in line to protect his oil cooler before the front tank. I put a fitting in the -16 line with a gage before the Oberg and started it up and found that it pegged the 300 # gage at a fast idle to a short blip, and that was on about a 35 deg. day with 40wt engine oil. so with out thermostats and cold weather being the scavenge side is not protected with a bypass it could go to the moon. I have seen coolers split wide open on race cars with frost on the tents and its no wonder.

Mike Bruns

[Walt Fricke]

Marlin - like you, I supposed scavange side pressure was low. And it might be pretty low if the oil was just dumped back into the tank. But in most case it can go to oil radiators, and I have got to believe there is a fair amount of resistance to flow there. Which is why the 911's external oil thermostat also has a pressure bypass built into it, just in case the temperature part does not work I suppose. Which aftermarket thermostats lack.

A racer once told me he put a gauge into that line, and recorded rather high pressures. I'd have to see if I could find the e-mail, though it wasn't the 300psi cooler splitting pressures Mike recorded. At least as high as engine oil pressure as I recall.

I've not had trouble with my race car's setup, which has a filter before the two oil coolers, and another one before the front mounted sump. But I have an oil tank heater, and try to be careful to let the car idle a while on cold track mornings.