Hello all,
I have a '89 Firebird, 305, with a P1SC. My question is what exactly is that bypass valve used for. The valve is open under normal conditions which is forceing heated air back into the supercharger which will increase the air temp even more. What I did was cut the hose close to the supercharger and plug it, (so no unfiltered air goes into the supercharger, next I removed the hose from the vlave and just let it open to the air. I have some data from , the inatake air temp, (withthe bypass hooked up) I had a temp of 120 Deg, with hose pluged, the intake air temp I got was 109 Deg. I did this same day, and drove the car the same way both times. Any comments on this modifiaction will be greatly appreciated.

Brad
San Diego

 

I have a '89 Firebird, 305, with a P1SC. My question is what exactly is that bypass valve used for.

It is used to vent air that is produced by the compressor that the engine does not use. If you didn't have it, the tract from the blower to TB would always be under positive pressure, which is not necessarily a good thing for the compressor as it imparts a load on it at ALL times. With the valve open under normal conditions, it routes all the unused air elsewhere. I say elsewhere because there is a choice:

1) ATI has it recirculating back to the blower inlet, or
2) Vent it to atmosphere, which is what you've done. There's nothing wrong with doing this.

Advantage for #1: No noise -- that whooshing sound.
Disadvantage for #1: It recirculates hot air back to the inlet, thereby greatly accelerating the temperature of the compressor outlet air.

Advantage for #2: Keeps the compressor inlet air as cool as possible.
Disadvantage for #2: That whooshing sound.


....intake air temp, (withthe bypass hooked up) I had a temp of 120 Deg, with hose pluged, the intake air temp I got was 109 Deg.

I have three air temperature gauges:

1) In my custom airbox,
2) At the blower outlet (before the intercoolers), &
3) After the intercoolers.

Not only is the air introduced into the engine cooler, but air compressor air outlet is 15-20 degrees cooler too by venting to atmosphere.

If you cannot stand that constant wooshing sound by venting to atmosphere, there is another alternative that helps keep the compressor discharge air cooler. Move the surge valve from before the IC to after. The air temp here is much cooler.

I have found that temperature is proprotional to impeller speed. The faster you turn it, the hotter the air gets. I have my D1SC pullied for maximum speed and living in southern Arizona, I feel it's imperative to keep the blower as cool as possible. I use a pair of surge valves. The first is connected AFTER one of my intercoolers (cooler air) and it routes air back to the inlet side. The second is connected BEFORE the other intercooler (hotter air) and it vents to atmosphere. Although both valves function off of engine vacuum automatically, my second valve can also be manually opened by flipping a switch.