Question About Anti-surge And Bovs.....

[rich81]

Hi guys,

Been doing a little research into turbo options for an RB30 and the GT3582 can be had with anti-surge cut into the intake shroud, which I'm assuming is the ports they drill around the shroud to allow air to escape out of the compressor housing back to the intake to alleviate surge etc, e.g. if the turbo is making more boost than the engine can flow (for whatever reason, e.g. throttle closing etc), any excess pressure that would normally cause a surge escapes through the shroud, is that right?

BOV's as far as I'm aware are designed to relieve pressure between the turbo and the throttle plate when the plate snaps shut, preventing the compressor working against a charge of compressed air that ain't going anywhere.

Now, my question is..... does having an anti-surge shroud effectively negate the need for a recirc/blow-off valve, since the air will have somewhere to escape when the throttle snaps shut?

Is anti-surge even worth it? I heard it can lead to *slightly* delayed pressure rise etc (without affecting spool time directly) but the benefits are that you won't kill your turbo (or engine) if you encounter surge conditions.

Can anyone shed some light on this? I'm interested in knowing more about surge in general, what can cause it in a setup and whether the preventatives measures are worth the extra cost and tuning consequences (if any) etc......

Cheers guys.

[R31Nismoid]

Anti surge is designed for the ramp up onto boost - stopping the compressor hitting the surge area on throttle coming up in the lower (left hand side) of the turbos comp map.

It won't have the same effect off throttle where the BOV comes into play and certainly wont pass the volumes in terms of air a BOV will.

You will still get the typical flutter/surge if you come off throttle with a anti-surge housing as that is not the cause they were designed to stop

[rich81]

Anti surge is designed for the ramp up onto boost - stopping the compressor hitting the surge area on throttle coming up in the lower (left hand side) of the turbos comp map.

It won't have the same effect off throttle where the BOV comes into play and certainly wont pass the volumes in terms of air a BOV will.

You will still get the typical flutter/surge if you come off throttle with a anti-surge housing as that is not the cause they were designed to stop

Ah ok, so you mean anti-surge is designed to prevent the turbo surging in conditions where it's capable of *increasing* the flow rate faster than what the engine can flow at a given RPM?

I've been reading compressor maps for the GT35, GT40 etc.... doing my head in trying to work out the best selection for a nice responsive RB30 that'll still make decent power... I know a lot of people have said the GT35 is a good match, but I'm keen to figure out what figures it can do *within* the 76-79% efficiency envelope. I'm sure there's a lot of people out there who have slapped a big turbo to their car and wound up the boost without considering compressor maps etc.....

In terms of using flow rate/pressure ratio to determine what HP a turbo will support, how do you apply that logic to calculating engine HP? Is the HP figure for a turbo (e.g. 40lbs/min equates to 330hp etc) just added on to the N/A equivalent horsepower of an engine?