An interesting read from SPD Tuning>>> Blow Off Valves>>>
02-05-2004, 02:35 AM
#1
An interesting read from SPD Tuning>>> Blow Off Valves>>>
Real blow-off valves are used in high boost (20+ psi) to prevent the momentary rise in boost pressure of up to 50 psi from blowing out intercooler side tanks or popping off hoses when the throttle is shut. But at normal boost levels (under 18 psi) we can use the rush of over pressure to prevent our turbocharger from stalling all the time when we lift the throttle. This helps the turbo make better throttle response and boost quicker in real world conditions.
Enter the $35 by-pass (or priority valve) concept first developed on the AUDI rally cars and first introduced in the Audi Quattro Turbo and Porsche 944T. The very first Subaru EJ20 in 1989 also had this type valve. Your WRX has one also and we want you to keep it on your motor, unless there comes a reason to change it out.
The compressor by-pass valve is connected from the turbocharger compressor high pressure side (intercooler) back through to the low pressure side (the intake section) just ahead of the turbo compressor. When you close the throttle to shift gears this valve opens. It makes a loop of air back into the inlet with the boost spike of air pressure in the intercooler. Everyone has probably heard a blow-off valve go 'whoosh'. Well, we want that 'whoosh' to go right back to work keeping the turbo compressor wheel from slowing down. Instead of spending money on an aftermarket blow-off valve just to make noise, we leave the factory priority valve in place. Why?
The loop air flow effectively puts the turbocharger into a no load condition and the wheel keeps spinning. Under this very low load condition the high pressure air in the intercooler blows into the intake via the by-pass valve and pin-wheels compressor impeller. When the throttle is opened again, the turbine wheel is still spinning at a relatively high rpm and will go right back to work putting out boost.
Without this by-pass valve feature the compressor/turbine wheels tend to stall between shifts and it takes longer to build up boost again after each shift. This is also true when the throttle is lifted in a corner for any reason. When you get back in to the gas, the by-pass valve actually is slow in closing and lets the turbine free wheel for an eye blink, bringing the boost on much quicker. Why any driver would spend money and give this throttle response advantage in away for the sake of noise of a blow-off valve is almost funny to think about. Yet, many shops will sell you a blow-off valve with a straight face. But, now you know. This is a classic case of, 'If it ain't broke, don't fix it".
Enter the $35 by-pass (or priority valve) concept first developed on the AUDI rally cars and first introduced in the Audi Quattro Turbo and Porsche 944T. The very first Subaru EJ20 in 1989 also had this type valve. Your WRX has one also and we want you to keep it on your motor, unless there comes a reason to change it out.
The compressor by-pass valve is connected from the turbocharger compressor high pressure side (intercooler) back through to the low pressure side (the intake section) just ahead of the turbo compressor. When you close the throttle to shift gears this valve opens. It makes a loop of air back into the inlet with the boost spike of air pressure in the intercooler. Everyone has probably heard a blow-off valve go 'whoosh'. Well, we want that 'whoosh' to go right back to work keeping the turbo compressor wheel from slowing down. Instead of spending money on an aftermarket blow-off valve just to make noise, we leave the factory priority valve in place. Why?
The loop air flow effectively puts the turbocharger into a no load condition and the wheel keeps spinning. Under this very low load condition the high pressure air in the intercooler blows into the intake via the by-pass valve and pin-wheels compressor impeller. When the throttle is opened again, the turbine wheel is still spinning at a relatively high rpm and will go right back to work putting out boost.
Without this by-pass valve feature the compressor/turbine wheels tend to stall between shifts and it takes longer to build up boost again after each shift. This is also true when the throttle is lifted in a corner for any reason. When you get back in to the gas, the by-pass valve actually is slow in closing and lets the turbine free wheel for an eye blink, bringing the boost on much quicker. Why any driver would spend money and give this throttle response advantage in away for the sake of noise of a blow-off valve is almost funny to think about. Yet, many shops will sell you a blow-off valve with a straight face. But, now you know. This is a classic case of, 'If it ain't broke, don't fix it".
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