need help

[clhuff]

hello i have 99 eclipse gst...if i get a catback exhaust do i need a blowoff valve....and how much would the psi be increased from the exhaust

[spyderturbo007]

I think you are a little confused. Let's see if I can get you on the right track. A catback will do nothing for boost pressure. All the cat back does is increase the exhaust flow. This will translate into a higher flow rate at the same pressure. For example, if you are pushing 26 lbs/min air at 15psi, adding a cat back might bump that up to 28 lbs/min at 15psi. The pressure won't go up, just the flow.

As for the BOV, that's a totally different thing. The 2g BOV tends to leak at pressures above 12psi or so (really depends on the valve). I would suggest upgrading to a 1g BOV. Mine holds a steady 20psi with no problems. The BOV will run you about $40 and you will need a flange if you stick with the stock UICP. I think the flanges are about $10.

If you are going with a 1g BOV, I would suggest an UICP upgrade. Mine was about $90 from RRE and came flanged for the 1g BOV. Here is a write-up I did on the UICP:

http://www.dsmtuners.com/forums/showthread.php?t=196116

The BOV is there to protect the turbo. When you are making boost, that pressure is feed along the intake track and into the engine. At this point the throttle plate is open. When you take your foot off the gas, the throttle plate closes and all that pressure begins to "back up" in the intake track. Without a BOV, this pressure would back up to the turbo and abruptly slow down the compressor wheel. This is not a good idea when the turbo spins at upwards of 100,000 rpm.

Anyway, what we need to do is vent that pressure when the throttle plate closes. This is the job of the BOV. When the throttle plate closes, the BOV opens and vents all of the pressure away from the spinning compressor wheel. Basically, it's a turbo safety device. That's why you hear some people call it a CBV, or compressor bypass valve.

Ok, my fingers hurt......