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Extreme 280zxt page

Blowoff Valve

The picture is of a HKS valve on the right and a GREDDY valve on the left. The blowoff valve is important in a high performance application as it will stop turbo back spin and keep the turbo spinning for quicker spool up. The GREDDY unit, although it looks trick would be used for a lower Hp application as the larger HKS valve. The valve must be able to effectively bleed off the air in the intake under boost when the throttle is let off. A good rule of thumb is adjust the valve at -10psi manifold pressure.
The blow off valve can be setup to exhaust into the atomsphere, or recirculated. I have set up my system recirculated. When it is exhausted into atomsphere, you create a slight rich/lean condition in the engine. The MAF takes readings of the air flow into the engine, and adjusts the fuel mixture accordingly. When that metered air is exhausted, the fuel is not and causes a slight rich condition. The recirculated set up will not have this condition as the air is put directly into the turbo again past the MAF and again is utilized. Opinions are varied as to which is best. I went the recirculated route, besides it gave the perfect reason to use some more of that shinny steel braided line(makes the car go even faster you know)

A note on the HKS blowoff valve: The standard spring in the HKS valve shown on my page has a spring pressure of about 15 psi to actuate the valve. There is some adjustment with the adjuster, however the adjustment is limited.
I set my unit for 15 psi to open, idling, or let off the throttle right. The car would buck when I let off the throttle under normal driving conditions and I had a good idea it was the blowoff valve. I spoke to a few people and they informed me they set the unit up to open at 10 psi and that seemed to work well for them. I modified my spring in my valve so that it opened at 10 psi and the bucking prolem went away when I let off the throttle.
I run my system recirculated, so there is really no need to worry about the valve opening under boost as the boost pressure and the vacuum portion of the valve then becomes pressurized and also helps to keep the valve closed under boost conditions.
The final outcome of the modification resulted in a much smoother transition from open to closed throttle reactions. The car has no further problems with bucking when I let up on the throttle, and the reaction time is very fast.