Finding blow off valve pressure

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03-15-2007, 06:03 PM

#11

Slo_crx1

3.0 BAR

Join Date: Aug 2003

Location: NE-PA

Posts: 4,666

Re: Finding blow off valve pressure

Quote:

Originally Posted by juce

ok so im dumb, but answer me this

Exhaust pressure of say 10psi, makes for an intake manifold pressure of how much?

You may be confused. There is pressure on the exhaust side...backpressure. It's usually measured to determine how far out of efficiency your exhaust wheel/ A/R size is compared to engine pressure. The lower the backpressure number the better, but if it gets too low you end up sacrificing boost threshold time.

03-15-2007, 09:29 PM

#12

Atticus

1.5 BAR

Join Date: Oct 2006

Posts: 1,147

Re: Finding blow off valve pressure

Quote:

Originally Posted by slo_crx1

what do you mean by boost threshold time
i thought that its best to have the least amount of back pressure
hence ppl having straight pipe or open down pipe

03-15-2007, 11:49 PM

#13

ifly87

3.0 BAR

Join Date: Dec 2005

Posts: 2,081

Re: Finding blow off valve pressure

Quote:

Originally Posted by juce

ok so im dumb, but answer me this

Exhaust pressure of say 10psi, makes for an intake manifold pressure of how much?

You are going to need to sit down and look at a turbo setup to see how it works, if you are going to school for mechanical engeneering you probably learn best that way. You seem very confused and it is normal when first trying to take in all this "turbo guff". Once you understand it you'll be like "ooooh yeeaah"

03-16-2007, 11:02 PM

#14

Slo_crx1

3.0 BAR

Join Date: Aug 2003

Location: NE-PA

Posts: 4,666

Re: Finding blow off valve pressure

Quote:

Originally Posted by Atticus

what do you mean by boost threshold time
i thought that its best to have the least amount of back pressure
hence ppl having straight pipe or open down pipe

It usually is best to have as little backpressure as possible. But consider this...say a .48 a/r exhaust side will produce 30 psi of back pressure at 12psi of boost. Then you measure a .63 a/r exhaust at the same boost pressure and say you only get 20 psi of back pressure. Seems like the .63 a/r would make more sense right? Depends on the setup and what you want from it. Backpressure spools turbos fast, so that .48 housing might spool at 2800 rpm, but that .63 won't hit boost threshold until around 3500rpm. Downside is the .48 housing will be making too much backpressure at higher rpms, and will limit the power of the turbo. You get what I'm saying? The point of highest backpressure is going to be at the exhaust housing itself...all the other stuff like too-small exhausts are just restrictions.

03-17-2007, 10:55 AM

#15

juce

0.5 BAR

Thread Starter

Join Date: Mar 2006

Posts: 56

Re: Finding blow off valve pressure

Quote:

Originally Posted by E-b0la

He said it best. The wastegate just controls how much exhaust flows through the turbine. More exhaust = faster spinning of turbo = more boost. Now the wastegate is controlled by boost pressure on the compressor side. when it sees say, 7lbs of boost the actuator opens the wastegate some to let aditional exhaust bypass the turbine, this hold the boost pressure at 7lbs.

The blowoff valve will only go off when the intake manifold pressure drops (vacuum), and the throttle plate closes infront of your boost pressure. That is why a vacuum line goes to the BOV, it opens in vacuum when the boost in the charge pipe pushes it open.

Help any?

Now I see.. the waste gate is affected by boost pressure (pressure in charge piping). So boost pressure is strictly the pressure going into the manifold, and as long as your using the right spring, its correlation with exhaust pressure is irrelevant.