Creating Crankcase vacuum
 

This is just a thought. My knowledge of this is limited. But a post by Grady made the rusty wheels and cogs turn inside my noggin.

Could one plumb from the breather to a sealed catch can to the vacuum fittings on weber carbs to create vacuum in the crank case? Perhaps there's not near enough cfm pulling from those ports, but I'm just thinking out loud here....

You could but you'd likely be drawing oil and unburnt gas fumes into the intake at the same time, and most likely not create enough vacuum to derive the expected benefits. If you source engine vacuum from the carbs, that would create a vacuum leak and upset normal operation.

Maybe create vacuum from an outside source, e.g., a properly-sized vacuum pump, electric or belt-driven.

Sherwood

The breather is just above the crankshaft and it pucks an unbelievable amount of oil. I had to change my oiling system so the breather could puck directly back into the oil tank. See below. That is a -16AN hose.

My next engine I plan to run a 993 oil pump. The larger scavange stage of the pump will help pull a vacuum, I hope.

http://forums.pelicanparts.com/uploa...1345151943.jpg

You'll need a GT-3 pump and the Cup crank seals to even come close. :)

Well, the factory did this. On my old '66 there is a hose from the breather to the oil tank, then a hose from the filler neck to the aircleaner. So there is a continuous flow.

However, Dr. Ing. Weiner is right-- it takes a very big oil pump indeed, as well as a check valve, to actually get negative pressure in the case.

Yeah I thought about this being a basic function of stock but was wondering if there's a way of tweaking it a little.
Thanks everyone for the answers.

Steve - I have the GT3 pump in my short stroke 2.8. Which is still in the testing phase, so I could make seal modifications without having to pull the motor.

I hadn't planned on negative crankcase pressure, but it seems worth trying. More of concern is whether, without the right seals, I will be sucking race track dust into the motor with this pump. My race motors with the turbo pump have not had issues like this.

1) Will the GT3 nose bearing seal fit in the old #8 bearing?
2) I guess the GT3 flywheel seal won't fit my early turbo/carrera 3.0 case with the 6 bolt crank. I'd have go look elsewhere for a seal? Hawgryder mentioned a vendor, but I don't know if this is the kind of thing which can be retrofitted easily.

Walt

Has anyone measured crankcase pressure on the stock engine?

It would be useful to know as it would then be possible to do a rough calibration of windage versus crankcase pressure.

Crankcase pressure as a result of blowby, windage and RPM will vary for each engine. However, there might a ballpark figure to use for starters.

Realize that testing the case pressure means plugging all external breathing paths to measure the gauge pressure. If not monitored correctly, excessive pressure will seek the easiest path to vent to atmosphere and create an oil leak, maybe temporary, maybe not.

Sherwood

Walt,

Porsche Motorsports used a system on the GT-3 RSR engines that helped them make over 500 BHP using the GT-3R pump, special low tension crank seals and a one-way check valve. It may not fit the 6-bolt crank, however I've not checked that for myself to confirm.

Bear in mind that these engines so configured do use some oil and thats no issue with a race engine that receives constant maintenance.

GT-3 RSR engines turn over 9K so this was part of the efforts to control windage (along with case mods).

Sherwood,

There should be a 'typical' or optimum value and the suction side of the pump should help to create some pressure drop.

The bigger the scavenge section the better it should be and it would be interesting just to fit a pressure trasnducer and see the result.

I'm unsure how a large suction section in the oil pump wouldl create the needed vacuum in the air cavity above the oil level. Don't think the crankcase/remote tank oil flows into the pressurized system at a rate high enough to produce appreciable neg. pressure (vacuum). But I could be wrong.

Sherwood

1) - ah, I can quit worrying about whether my GT3 pump will produce negative pressure without me having installed the right seals - because I don't have a check valve.

2) Seems to me that if the scavange side is larger than the pressure side, and you have a check valve or otherwise prevent air from entering from the atmosphere, then you will pull more, including air above the amount of oil which enters the cavity, out than comes in. The pump may not be super efficient as an air pump, since it is an unsealed gear pump not built to the tolerances optimum for air molecules. But the oil will act as a bit of a seal? And even if inefficient, it ought to work. Quite a bit of air gets entrained in the oil pumped out, does it not, under ordinary conditions.

Iwould agree that scavenge pumps are not the greatest air pumps and that the vacuum level is not too high. I just though it was worth having a few numbers and some targets.


I built a test rig, a few years ago, to measure the aeration of oil in a scavenge system. The efficiency of a de-aeration system is imortant to try to reduce oil tank volumes as settling times would normally need to be 1-2 minutes.

The oil out of a scavenge pump looks a bit like Guinness when first poured into a glass and can be as high as 60% air in a high rpm engine. (Circa 20 000rpm)

Measuring air volumes is tricky and we used irradiated oil - (not the Air-X system that has more recently been developed).

Porsche Motorsports as well as other experienced race engine builders have done extensive R&D on this subject using a variety of vacuum settings, low-tension rings, check valves, etc to determine optimal crankcase pressures (+/-) for maximum BHP.

Naturally, this assumes that all the normal windage mods have been performed on the case, cylinders, and crankshaft.

These engines need very little vacuum for peak performance and the combination of breather check valve, proper seals, low-tension rings, and GT-3R oil pump (with scavenge mods in the heads) works very well.

While the idea of using manifold vacuum to lower the crankcase pressure, it will only work at part throttle. At WOT there is no vacuum in the intake manifold, so no lowering of crankcase pressure. Part throttle lowering of crankcase pressure is useless.

-Andy

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.

Andy - doesn't the pressure in the intake system, including at least the lower part of the manifold, as well as the combustion chamber, drop below atmospheric during the intake cycle? Not as far at WOT as at closed or part throttle, but still below? Otherwise what pushes the charge air in?

Yes the pressure must drop some, but it is very "noisy" It fluctuates wildly as the air flows in and stops and the valve opens and closes. If you measure the manifold pressure (airplanes often measure this) you would see an average very close to atmospheric pressure. In fact if you had a very good intake system you would like to see above atmospheric pressure at the intake valve. That's what intake tuning is all about.

-Andy

Vacuum
 

On a well sealed up cylinder package and a GT3 pump with no check valves we see the oil film moving toward the engine on the dyno in the tygon breather hose from the wall mounted oil tank, we have clamped that off for a quick pull and its worth 6 to 7 hp just like that, the vacuum helps stabilize the ring package as well. On engines that have a large to fair amount of breather activity due to poor ring seal or cyls. that can't stay round the breather looks like a -16 scavenge out line all the way up the hose to the tank. The oil temps can't be controlled with all that heat blowing by and several GT3 pumps wouldn't help. The drag race crowd uses belt driven vac pumps to generate that on wet sump engines and have witnessed enough vacuum to overcome the ability of the internal oil pump to function and the oil pressure goes away. One customer could not figure out why he kept loosing engines at 3/4 track until he had it on the dyno and seen what his pressure did, disabled the vac pump and the pressure was normal. He ended up moving his pump outside the block and it was fine.
Mike Bruns

Also the GT3 has two extra scavenge pumps on the camshaft housings to help vacuum.

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.

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?

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.

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

scavenge press.
 

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

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.