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Old 05-03-2008, 06:08 AM   #1
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Crankcase vacuum question.

Hi, I'm freshly registered and have a question which I haven't seen discussed in my short time lurking.I"m mostly a drag race oriented guy,but still like to save money/gas in the daily driver. In drag race engines,belt driven pumps are often used to keep the crankcase in a partial vacuum to increase power.Usually anywhere from 5 to 15 inches of Hg.The benefits are said to be due to better piston ring seal,and the reduced windage in the engine allowing oil to drain back more quickly.I have considered doing the same thing to see if it would increase m.p.g., but I would not use a mechanical pump. Instead I plan to use the p.c.v system already on the engine and put a "bleed off" valve of some sort on the fresh air inlet to the crankcase. This would allow the p.c.v. system to function as it was intended, but pull the crankcase into a partial vacuum before allowing air in. I already did a preliminary check and I can pull 6" in the engine at idle.I thought I would plug the inlet,stick my vacuum gague in the dipstick tube, and drive at hiway speed and load to see how high the vacuum would get.After finding out what the maximum vacuum was,I could then make/buy a bleed off valve just under this vacuum level to allow air in. Has anyone tried this and what were the results? I would think that these same benefits which increase horsepower would also increase m.p.g. It would cost almost nothing and be easily reversed if desired.Thanks for your thoughts in advance.
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Old 05-03-2008, 06:35 AM   #2
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I wonder if the best way to do this would be to get a PCV valve from a smaller motor and put it inline with the air breather hose... then it's pulling through against that resistance the other side and pulls more vacuum all the time. Of course finding the design specs of PCV valves is difficult, you will have to guess whether you're sticking in a ~10cfm one or a ~2cfm one.
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Old 05-05-2008, 05:21 AM   #3
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I bought a cheap pressure regulator and will test this sometime this summer...having the reg set at maybe 5" hg (using a vac gauge) where the PCV flow goes into the engine from the air filter ...then thru a Condensator.

If I don't use a regulator...I get 12" hg...too much.

Guy who tested this got a 10% gain and was using an oversized PCV valve and no PCV jar.

Only issue might be oil starvation of the seals? A dragster goes for a few seconds vs full time driving.

Do any track racers use crankcase vacuum?
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Old 05-05-2008, 03:38 PM   #4
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10%!!!! Sounds too good to be true,but if it is true ,I could live with it for the costs involved. I am going to have to remove my throttle body and modify it to regulate the inlet air. The p.c.v. system gets it's air through a passage just upstream of the throttle blade. This passage makes a 90deg. turn,goes into the intake,and to the crankcase from there.G.M. was nice enough to press a metal cup plug into the other end of this(90deg.) hole so it can be accessed from outside,and the original opening can be plugged.I figure any generic, small, hardware store valve should work for a restrictor as long as it can flow enough to hold the vacuum where I want it. Once it's set it shouldn't require too much attention.
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Old 05-05-2008, 04:00 PM   #5
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What about using a modified BOV?

Have one end connect to the crankcase (in place of breather) and the other to your intake, set the spring in there so at anything below 5" in the crankcase and the valve opens to raise the pressure in there and anything above it's closed, to drop pressure (kinda like intake pressure vs manifold vacuum but it's atmospheric vs crankcase) and then you won't need a PCV, just a straight hose to the intake manifold. It would also self regulate for different altitudes.
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Old 05-06-2008, 05:05 AM   #6
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Quote:
Originally Posted by ZugyNA View Post
Do any track racers use crankcase vacuum?
We've used the exhaust emissions valve to develop vacuum on decel on the crankcase with good results.

But that was deemed illegal under the rules as if the engine started to pump oil out of the breather, it was going into the exhaust rather than air box or a separate can.

I didn't have any dyno results, but I will admit that the engine seemed more free. I've looked at this with some curiosity. I don't know anyone using it, but it might be something to look at.
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Old 05-06-2008, 11:06 AM   #7
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On my twin turbo 3000gt i run about -19 to -20 vacuum on the boost gauge at idle. Under normal load and no boost its -8 to -10. On our forum for the GT's it is recommended to keep the pcv valve set up. Under hi boost (14 to 19 psi) i do get some oil blow by and i have a ketch can for this purpose. I am using a SSQ Blow off valve by HKS. It can be set up to be external or internal. We keep the pcv set up because if we don't out ECU will not work at its best.
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Old 05-06-2008, 06:25 PM   #8
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Quote:
Originally Posted by dkjones96 View Post
What about using a modified BOV?

Have one end connect to the crankcase (in place of breather) and the other to your intake, set the spring in there so at anything below 5" in the crankcase and the valve opens to raise the pressure in there and anything above it's closed, to drop pressure (kinda like intake pressure vs manifold vacuum but it's atmospheric vs crankcase) and then you won't need a PCV, just a straight hose to the intake manifold. It would also self regulate for different altitudes.
If I understand what you're suggesting correctly, this would cause the engine to idle very fast upon start up. All of the air in the crankcase would be pulled into the engine,like a vacuum leak,until the preset vacuum level was reached.Don't know how long it would take for the r.p.m. to "settle down" or how fast it would run until then.
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Old 05-06-2008, 06:30 PM   #9
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On my GT it takes about a minute or two to drop from 1200 to 700 rpm. Its a hard beast to warm up.
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Old 05-06-2008, 06:31 PM   #10
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Originally Posted by 1cheap1 View Post
On my twin turbo 3000gt i run about -19 to -20 vacuum on the boost gauge at idle. Under normal load and no boost its -8 to -10. On our forum for the GT's it is recommended to keep the pcv valve set up. Under hi boost (14 to 19 psi) i do get some oil blow by and i have a ketch can for this purpose. I am using a SSQ Blow off valve by HKS. It can be set up to be external or internal. We keep the pcv set up because if we don't out ECU will not work at its best.
Under full throttle ,or boost, there wouldn't be any vacuum in the crankcase at all because there wouldn't be any manifold vacuum.The blow by past the rings may be great enough to actually create a positive pressure in there. Of course,if you're running at 0" of vac. or in boost, gas mileage probably isn't the top priority at that instant anyway.
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