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javierb14 (Automotive) (OP)
18 Nov 02 0:33
does anyone have any experience with turbocharged cars? ive been debating with a few of my friends over the placement of a compressor bypass valve or blow off valve on the charge piping. some say it works best nearest the throttle body and others say it works best nearest the outlet of the compressor. correct me if im wrong, but i belive the valves basic function is to prevent compressor surge by preventing shockwaves. anyone have any good recommendations on websites and or books that discuss compressible flow?

thanks
javier
ivymike (Mechanical)
18 Nov 02 10:26
As far as I know, a blow-off valve is a simple pressure-relief device that bleeds off air to relieve pressure in the intake.  The only way it can affect surge is by keeping the intake pressure below the pressure at which surge might occur.  The valve doesn't do anything directly related to shockwaves, nor does it have the capability to respond to rapid pressure variations (depending on what you call rapid).


patprimmer (Publican)
18 Nov 02 11:15
As far as I know, the purpose of the blow off valve is to eliminate pressure spikes due to inertia in the compressor wheel. When the turbo is spinning hard after some full throttle operation, and you suddenly back off, the turbo still pumps air against a closed throttle plate, causing the pressure between the compressor and the throttle plate to rise rapidly, thereby potential damaging the blades on the compressor, and also acting as a brake on the turbo, causing some lag while the turbo spools up again when you get back on it.

Regards
pat

javierb14 (Automotive) (OP)
18 Nov 02 14:30
so where is the best placement for the valve? nearest the compressor or the throttle body? or does it make any difference at all?

javier
Custom Turbo Applications
http://www.full-race.com

NickB (Electrical)
18 Nov 02 19:22
Both placements have their advantages. The closer you get it to the compressor, the quicker it will vent the air from the turbo, and the turbo speed will drop less. Sometimes though, the pressure built up in the upper IC pipe and the intercooler will reverse, and flow backwards through the pipes to vent out the BOV.

Right up by the throttle body prevents any airflow reversion, but there is a pressure drop across the intercooler and the piping and flow will be slightly less. With a good setup, the pressure drop will be minimal.

Either location will prevent a pressure spike at the compressor from damaging the thrust bearing. If you have large pipes and IC with lots of boost, i'd suggest putting it up by the throttle body and investing in a good BOV. If the system is more reasonable, then i wouldn't worry about where you put it. The factory BOV from a 90-94 Eagle Talon/Mitsubishi Eclipse is a good BOV for most applications that aren't running tons of boost or airflow..
javierb14 (Automotive) (OP)
19 Nov 02 0:06
thanks for the info. keep it coming

javier
Custom Turbo Applications
http://www.full-race.com

duner (Automotive)
19 Nov 02 0:20
For my application I placed the BOV between the compressor and the intercooler. I didn't see any reason to vent already cooled air by placing it after the intercooler.  This way - any air that's drawn in thru the BOV will pass thru the IC also.

kimbo1 (Mechanical)
20 Nov 02 1:12
blow off valve should be nearest the throttle body, as air reversal occurs in the pipework between the valve and the closed throttle.  The less volume of air you are changing direction the better response you will get
AlexDring (Mechanical)
6 Mar 03 19:47
If it helps, I bielieve all the manufactures I have seen, have the blow off on the turbo.  In considering the design of the mechanism I would think it should be closest to the source of the pressure, thus quickly relieving pressure with the foot is taken off the gas, as in shifting.  I think all the aftermarket turbos for diesels have blowoffs on the turbos themselves.  Could be wrong though.
hemi (Automotive)
7 Mar 03 14:40
The theory and qualitative analysis of compressor bypass valves is pretty well covered in the preceding posts, so I won't go into that.  NickB obviously has some solid practical experience and I second his remarks.
I have tried a bypass valve in both locations in my intercooler equipped Merkur XR4Ti and it did a much better of mitigating compressor stall on throttle closing when I moved it from the compressor outlet to just in front of the throttle body.  All the OEM applications I've seen use this location also.  Something to consider is the temperature of air coming out of the compressor.  250-300°F is not unusual for engines runnig 15psi or more boost.  I have seen bypass valves fail due to overtemperature when located prior to the intercooler.

Geoff.

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