Anyone tried this crank evac idea yet?
 

Have you noticed on some dragsters they've run lines from the collector area of the header to the valve cover breather? This uses venturi effect from the exhaust to provide a low pressure (suction) to the breathers on the valve covers. Kind of works like a crankcase evacuation pump only no moving parts. My friend installed this on his 59 Chevy and from what we can gather it does provide some vacuum any where from just above idle to about midrange then as the revs go higher it kind of gives out. Has anyone tried this on their mustang yet? I'd like to know if you noticed any positive results. Most likely this will be used on street/strip cars with the emphasis on strip.

i believe that would be simply berouli's principal causeing the suction. the venturi is whats in your carbeurator, and it is simply a constriction in the intake tract like this: )( that speeds up the air lowering the pressure (berouli's principal), therefore bringing the gas from the bowl in to equalize the pressure. since the exhaust gas is already flowing, you are simply employing berouli's principal, as no venturi is actually used.

the terms venturi and bernouli are often interchanged, so i don't not know whether venturi refers to the constriction, or to the act of equalizing the pressure (pulling a substance from a resivoir using the flow of a fluid) but i know that bernoulis principal simply states that as a fluid increases in speed, it must decrease in pressure to maintain a constant amount of energy along the flow.....so maybe this is venturi....

I put this set-up on my strip car and it seems to draw good vacuum at high rpms.I can't say I noticed much E.T.improvement,but it did cure my soppy,oily breather cap problems.

Been giving the idea some thought on my fuelie 5.0?

I figure the best place to plug the line in is down stream of the oxyegen sensor? This might not be the most efficient location since the speed of exhaust gases are probably slower, but ahead of the oxy sensor with stock electronics might throw things off.

the idea is intruiging though, since you can suck alot crankcase pressure out (improved ring seal, reduced pressure for the engine to work against, ect)

Yea but isn't this usually done on long tube headers and the hose mounted on the end where the exhaust is coming out?

And also would this not be lower pressure at that point and if you put it further back in the exhaust I would think it would have the opposite affect because the exhaust is building backpressure?

I don't know if what I said makes much since but it doesn’t seem like it would work unless it is further back in the exhaust but I could be wrong.

Someone please explain how that would work.


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James Cox
nochevy@hotmail.com
1991 Mustang LX
12.565 @109.38mph 1.764 60ft

It definately works. I welded some on my friend's Chevelle. Before it would puke oil everywhere after a full pass, now it's as clean as can be. Good investment for non smog cars.

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92 GT, 4:10 gears, centerforce clutch, 6# powerdyne blower, bbk h-pipe, flowmasters

Thanks guys. Just to clear up some questions for some. The venturi effect is generated by placing the tube or port into the moving exhaust stream. Even though you're thinking that the exhaust is at a greater pressure than the surrounding air by placing the tube into the exhaust stream causes a low pressure. Bernouli's principle is used for carbs and in making aircraft fly. It basically states that a mass of air will increase in pressure if it's velocity is decreased or it's pressure will decrease if the velocity is increased. The venturi in your carb sticks out in the air stream and causes the air to move around it at that point a little faster so that resulting low pressure is what pulls your fuel in.

WOW you guys are sure getting scientific!!
But for us laymen the answer is this type of crankcase evac system works well on long tube headers and needs a one way valve at the point of exit at the headers, this agrangement is much harder on short tubes but in theory should work. the exit tube should be in the collector just before the exit flange to take advantage of the flow from all cyls. I have ran this system for over 20 years on all sorts of race cars with good results and have seen horsepower increases on the dyno in real time. I do run a vac. pump on one of my combos now but see no real difference in performance numbers over a good functional valve cover to collector system (not counting mega RPM mega HP motors). I do not recommend putting this system ahead of a O2 sensor.
Just my o2's worth. oh yeah I hate having to go to a dictonary to look up a word.

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93GT 398" 14:1,1080 Barry Grant Carb, nitrous fogger, c-4, Street Heats, 2" RCI long tubes, more.
79 notch 306, 5speed, carb, small plate nitrous.
88 turbo coupe 5 speed stock
91GT conv. 5.0 5speed, 3.73, and the normal bolt on stuff, just added to herd a sweet 86,SVO,5speed,306,msd,650dp,bbk,autometer,ect.
91 f-150 4x4 tow truck