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 Post subject: Evac System Question?
 Post Posted: Fri Sep 23, 2011 1:45 pm 
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OK so we've all seen these Evac Systems with their hoses run to the anti-backfire valves on the nipples in the collector but has anyone besides me ever wondered if those anti-backfire valves are actually needed? I mean I somewhat understand the concept of pressure running up the hose but with a short collector is this really a problem since that 4" collector opening is much larger than the 5/8" hose running to the valve cover?

So I'm wondering if I really need to run them and if I don't, wouldn't it be logical to assume that even more vacuum could be pulled if there's less restriction in that line? Of course this is also related to the volume of blow by gas in the crankcase.

Just saying......................

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 Post Posted: Fri Sep 23, 2011 8:19 pm 
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Never really thought about it, but I like the idea of the valves there. I get some popping out the exhaust on decel and the thought of blowing off the valve covers just isn't a good one to me............But you do make a good point..........

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 Post Posted: Sat Sep 24, 2011 10:35 am 
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I run exhaust on my truck and mine went bad... They were open.. The exhaust was blowing into the valve cover... working backwards. I am sure the back pressure of the exhaust is why it was going in there... Not sure. Maybe it is time for a Vacuum pump? Over the winter...

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 Post Posted: Sat Sep 24, 2011 2:45 pm 
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us7race wrote:
I run exhaust on my truck and mine went bad... They were open.. The exhaust was blowing into the valve cover... working backwards. I am sure the back pressure of the exhaust is why it was going in there... Not sure. Maybe it is time for a Vacuum pump? Over the winter...


If you run mufflers you're suppose to put the evac tubes in on the other side of those. On my Camaro I had mufflers and the evac tubes were in the 45* turn downs past the mufflers and they worked fine.

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 Post Posted: Sun Sep 25, 2011 9:25 pm 
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We have been dynoing an engine with draft tubes in the collectors in front of the mufflers. Without mufflers it would produce about 2" of vacuum on this engine. With mufflers on it would only produce about .2" of vacuum. It has anti backfire valves on. I'm guessing without the anti backfire valves it would produce no vacuum or pressure at lower rpms and some vacuum at higher rpms. We haven't had time to test it with and without. As rpm went up the time between ex pulses would be less so the anti back fire valve may not be needed. It wouldn't be hard to test on the dyno or in the car. On this engine we will use draft tubes with anti back fire valves in front of the mufflers because it still showed some vacuum with mufflers on and we have to run mufflers.

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