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Old 06-07-2013, 01:33 AM   #1
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build boost early and not change the tune?

Most of the roots blowers out there have a vacuum pot that opens a valve under high vacuum conditions to equalize pressure and kill any boost. When you stomp on it and lose vacuum the valve shuts and you build boost. So here is the question.

What if you put a restriction in the vacuum line to the vacuum pot OR increased the spring tension to resist opening the valve, would boost build quicker and if so, would the tune that you have handle the boost coming on quicker (at lower RPMs)?

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Old 06-07-2013, 03:13 AM   #2
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Most likely no, roots blowers rely on engine speed to drive the rotors, and this compresses the air. If anything you'd run the risk of blowing your motor by building boost when you are off throttle. Toyota blew many of their 4.6 v8s when they supercharged them because they built boost under vacuum.
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Old 06-07-2013, 08:44 AM   #3
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It would slow down how the boost comes on. Magnacharger actually puts a little brass restricter in the bypass valve nipple. Removing it makes boost come on a bit quicker.
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Old 06-07-2013, 10:49 AM   #4
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Quote:
Originally Posted by 2SS45th View Post
Most likely no, roots blowers rely on engine speed to drive the rotors, and this compresses the air. If anything you'd run the risk of blowing your motor by building boost when you are off throttle. Toyota blew many of their 4.6 v8s when they supercharged them because they built boost under vacuum.
If you are off the throttle, that creates a high vacuum condition so still no boost. What I am asking is if you are spinning 2000 RPM, you are making some kind of boost (not much), but the vacuum pot is keeping the valve open so the engine never feels the boost. The idea is to restrict the vacuum so the valve stays closed longer which allows boost quicker.

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Old 06-07-2013, 10:53 AM   #5
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Quote:
Originally Posted by Alvin@PCMofNC View Post
It would slow down how the boost comes on. Magnacharger actually puts a little brass restricter in the bypass valve nipple. Removing it makes boost come on a bit quicker.
Maybe my understanding is backwards. Doesn't vacuum hold the bypass valve open? If so, how does removing the restricter, which I assume would allow more vacuum, allow boost to come on quicker? Your last statement is what I am after though. Not more boost, just to come on quicker.
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Old 06-07-2013, 10:57 AM   #6
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Removing the restrictor allows the vac to drop to the diaphragm faster upon WOT when the mani vac decreases (or in other words when absolute pressure rises), then it closes the bypass sooner. It will open quicker as well when vac increases.
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Old 06-07-2013, 11:23 AM   #7
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Also whats not being discussed here is that bypass valve is critical to relieving pressure that can damage the gaskets/seals and or rotor bearings and shafts from the longitudinal stresses fighting the internals if not relieved. That's what bot a wastegate and BOV do with a turbo or centri blower.
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Old 06-07-2013, 12:30 PM   #8
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I wasn't proposing it was a good idea to do that mod, just how it changed the response of the bypass. Slamming stuff around is never a good thing.
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