gbyron you awake? CFM of 928 blow by gasses
08-08-2004, 12:36 PM
#46
Developer
I am copying a post I made into the "2 and 6 bearing failure" thread as it applys here too.
Just want to suggest.... before you go and spend $400 and more on a belt-driven vacuum pump to scavange your oil pan, that the older solution - a
venturi welded into your header collector does one hell of a job. Here is the info and the pictures:
=============================================
I have one or two things that we have tried and found succesful to help
anybody racing a 16v 928:
Try this exercise: remove the oil filler cap and separator screen from your 16v 928 and have an associate start the motor and take it up to about 2K with several 4k blips. Hold your hand over the inlet port of the circular oil filler chamber....
We found the amount of solid oil droplets (not so much a mist) thrown up into that chamber to be amazing. And this was AFTER the installation of a DEVEK louvered oil baffle in the crankcase vent. When we were installing that louvered oil baffle, I remember looking straight down and seeing a connecting rod and crankshaft counterweight directly below. That meant to me, as well as all the oil "misting" problems you guys have all correctly written about - oil was just plain being SLUNG up there by that counterweight and rod! I love Porsche too - but I gotta say - that location to take the vent off of the crankcase is a really poor one. Anyway - that's what we have to deal with.
Two things that I have not seen mentioned in this thread that have helped our race car:
1) we found that we would "pump" more oil up into the oil separator/vent under hard braking rather than hard cornering. We first thought it was oil coming up the drain hose from the front of the motor so we added a one-way check valve to that thing. Did not help. So we went the other way, and we increased the size of the hose that drains out of the vent can down and into the top of the oil pan. This helped quite a bit. We think that, on hard braking, the forward crank counterweight can get submerged in oil and a tremendous amount slung up into the separator chamber - apparantly more than can drain back into the crankcase at one time. I do not think the bigger hose is passing more oil (as the inlets and outlets are the same) but that the bigger hose is functioning as an "accumulator" to hold the oil until I am off the brakes, on the gas, and it can drain back into the pan.
2) the second thing that has helped in a big way is to vacuum the crankcase to lower the pressure. The early hot rodders discovered a long time ago that, when they could draw a vacuum - or a partial vacuum - in the crankcase - the rings would seal tighter and they would gain HP. In fact, positive crankcase pressure starts to lift the rings off the cylinder walls and even more blow-by will result! This is now so common among drag racers that they mount an external belt-driven pump onto the motor just to pull a vacuum in the crankcase.
But the early soultion - the low-budget solution - that still works VERY well
was to weld a hose nipple into your headers at an angle so that the passing gasses would draw a vacuum on the hose. The other end goes up to the crankcase and you are all set. Simple, elegant, very effective. What I like best about it is that the higher the rev's, the more vacuum it draws - perfect!
In the attached picture below you can see both ends of this little system. In the picture on the left it starts at the top of my oil catch tank, and leads into a #10 AN braided steel hose. That hose runs under the car and attaches to the nipple welded into the collector. Done. That little round thing that looks like a vacuum cannister right at the exhaust nipple is a one-way valve so a backfire cannot go backwards up into my catch tank.
I found the oil nipple and backfire valve at Summit Racing, and the #10 stainless braided hose I got from Pegasus. Under $50 for the whole thing.
When we were diagnosing blow-by and crankcase pressure in 2001 (our first year with the supercharger on the 928) we installed a vacuum pressure/boost gauge on the crankcase and mounted it in the cockpit. Under WOT and full boost (9 PSI) at 6,000 RPM, we would spike just briefly as much as 2.5 psi in the crankcase before it would go down again. We were worried about blowing gaskets.
After the installation of this exhaust vacuum system, the crankcase ventilation has been much better - but not yet where I want it. But, the vacuum system drawing air out of the crankcase is not at fault - I have two cylinder walls with scratches in them and the blowby caused by this is the culprit. I am building a new motor to cure the blowby problem at the source. Then I expect this vacuum system to be able to keeopp up no problem.
Just want to suggest.... before you go and spend $400 and more on a belt-driven vacuum pump to scavange your oil pan, that the older solution - a
venturi welded into your header collector does one hell of a job. Here is the info and the pictures:
=============================================
I have one or two things that we have tried and found succesful to help
anybody racing a 16v 928:
Try this exercise: remove the oil filler cap and separator screen from your 16v 928 and have an associate start the motor and take it up to about 2K with several 4k blips. Hold your hand over the inlet port of the circular oil filler chamber....
We found the amount of solid oil droplets (not so much a mist) thrown up into that chamber to be amazing. And this was AFTER the installation of a DEVEK louvered oil baffle in the crankcase vent. When we were installing that louvered oil baffle, I remember looking straight down and seeing a connecting rod and crankshaft counterweight directly below. That meant to me, as well as all the oil "misting" problems you guys have all correctly written about - oil was just plain being SLUNG up there by that counterweight and rod! I love Porsche too - but I gotta say - that location to take the vent off of the crankcase is a really poor one. Anyway - that's what we have to deal with.
Two things that I have not seen mentioned in this thread that have helped our race car:
1) we found that we would "pump" more oil up into the oil separator/vent under hard braking rather than hard cornering. We first thought it was oil coming up the drain hose from the front of the motor so we added a one-way check valve to that thing. Did not help. So we went the other way, and we increased the size of the hose that drains out of the vent can down and into the top of the oil pan. This helped quite a bit. We think that, on hard braking, the forward crank counterweight can get submerged in oil and a tremendous amount slung up into the separator chamber - apparantly more than can drain back into the crankcase at one time. I do not think the bigger hose is passing more oil (as the inlets and outlets are the same) but that the bigger hose is functioning as an "accumulator" to hold the oil until I am off the brakes, on the gas, and it can drain back into the pan.
2) the second thing that has helped in a big way is to vacuum the crankcase to lower the pressure. The early hot rodders discovered a long time ago that, when they could draw a vacuum - or a partial vacuum - in the crankcase - the rings would seal tighter and they would gain HP. In fact, positive crankcase pressure starts to lift the rings off the cylinder walls and even more blow-by will result! This is now so common among drag racers that they mount an external belt-driven pump onto the motor just to pull a vacuum in the crankcase.
But the early soultion - the low-budget solution - that still works VERY well
was to weld a hose nipple into your headers at an angle so that the passing gasses would draw a vacuum on the hose. The other end goes up to the crankcase and you are all set. Simple, elegant, very effective. What I like best about it is that the higher the rev's, the more vacuum it draws - perfect!
In the attached picture below you can see both ends of this little system. In the picture on the left it starts at the top of my oil catch tank, and leads into a #10 AN braided steel hose. That hose runs under the car and attaches to the nipple welded into the collector. Done. That little round thing that looks like a vacuum cannister right at the exhaust nipple is a one-way valve so a backfire cannot go backwards up into my catch tank.
I found the oil nipple and backfire valve at Summit Racing, and the #10 stainless braided hose I got from Pegasus. Under $50 for the whole thing.
When we were diagnosing blow-by and crankcase pressure in 2001 (our first year with the supercharger on the 928) we installed a vacuum pressure/boost gauge on the crankcase and mounted it in the cockpit. Under WOT and full boost (9 PSI) at 6,000 RPM, we would spike just briefly as much as 2.5 psi in the crankcase before it would go down again. We were worried about blowing gaskets.
After the installation of this exhaust vacuum system, the crankcase ventilation has been much better - but not yet where I want it. But, the vacuum system drawing air out of the crankcase is not at fault - I have two cylinder walls with scratches in them and the blowby caused by this is the culprit. I am building a new motor to cure the blowby problem at the source. Then I expect this vacuum system to be able to keeopp up no problem.
08-08-2004, 03:25 PM
#47
Inventor
Rennlist Member
Rennlist Member
I put my exhaust crank evac in basically the same spot, and it works well. On the dyno with only a K&N breather on the oil fill cap, I would puke oil everywhere from my leaky seals, now, no leaks.
If it were closer to the engine, I think it would pull more vacuum with the hotter and higher speed exhaust gases (and/or perhaps the smaller tube size). In my case, I had to put the pilot tube in after the oxygen sensor and muffler. (see here)
I want to remove the oil tower assembly and cap the hole. So one of my next projects is to put banjo fittings on the 16V valve covers (in place of the screw caps, 2 each side), run the vapors through an external separator mounted where the fan used to be, then out to the exhaust.
...
Here's an inexpensive belt driven pump I found (no affiliation), it has Teflon wipers and carbon fiber vanes:
"Sportsman” Vacuum Pump (5” deep by 5” diameter). Tests at 21” hg and 22 CFM at 3,500 pump rpm. Inlet and exhaust on back of pump. - $179.00
“Super-PRO” Vacuum Pump 3-Vane (6” deep by 5” diameter). Tested at 25” hg and 35 CFM at 3,500 pump rpm. Inlet and exhaust on side of pump. - $259.00
click pic for 'Technical Notes'
Note that the pulley is extra, but you can use a proper sized pulley from most any airpump from the junkyard.
All belt driven vacuum pumps need oil mist/droplets to lubricate them, so the separator has to be after the pump. For the street you might have to put in a drip oiler before the pump.
If it were closer to the engine, I think it would pull more vacuum with the hotter and higher speed exhaust gases (and/or perhaps the smaller tube size). In my case, I had to put the pilot tube in after the oxygen sensor and muffler. (see here)
I want to remove the oil tower assembly and cap the hole. So one of my next projects is to put banjo fittings on the 16V valve covers (in place of the screw caps, 2 each side), run the vapors through an external separator mounted where the fan used to be, then out to the exhaust.
...
Here's an inexpensive belt driven pump I found (no affiliation), it has Teflon wipers and carbon fiber vanes:
"Sportsman” Vacuum Pump (5” deep by 5” diameter). Tests at 21” hg and 22 CFM at 3,500 pump rpm. Inlet and exhaust on back of pump. - $179.00
“Super-PRO” Vacuum Pump 3-Vane (6” deep by 5” diameter). Tested at 25” hg and 35 CFM at 3,500 pump rpm. Inlet and exhaust on side of pump. - $259.00
click pic for 'Technical Notes'
Note that the pulley is extra, but you can use a proper sized pulley from most any airpump from the junkyard.
All belt driven vacuum pumps need oil mist/droplets to lubricate them, so the separator has to be after the pump. For the street you might have to put in a drip oiler before the pump.
