i have never played with turbo's until now so i have a few questions, my understanding is that a internal waste gate operates on boost from a hose connected to the inlet manifold moves the actuator arm opening a flap and depending on spring tension in the actuator it will bleed off unneeded exhaust gases by bypassing the turbo, and a blow off valve uses a piston system to bleed of unwanted boost except it will only bleed when coming off boost or closing the throttle quickly, is that correct ? so is it the actuator spring that sets how much boost??and also is a blow off valve needed ??

Correct, the actuator controls the boost levels.

A blow off valve is not needed but some think that not having one can damage your turbo.

There must be something to this since manufacturers go to the trouble and expense of fitting them from factory.

Noise and emissions.

Nizpro remove the blow off valve on their stage 2 and above Xr6t's. You would think they know a thing or 2 about turbos.

stops the hectic dosing all the VL turbo boys are so fond of.. might lengthen the life off your turbo as well.

The emissions thing is that once you snap the throttle shut when on boost. When the ISC opens more air enters the plenum than is required because the entire inlet tract pre throttle body is pressurised. In my brothers car you can see the dial on the boost gauge dance lke a fairy whe he snaps the throttle shut. Bear in mind his pressure feed for the gauge is taken from the plenum so it is assumed that it is reading correct.

The reason those supersonic/meagasonic/pooftersonic blow off valves are illegal are are mainly for noise (yes the EPA seem to think noise is a major form of polution).

But there is another reason, particularly on airflow meter equipped cars. If the airflow meter is before the turbo then what happens when you snap the throttle shut and the blow off valve opens? There is still a s**t of air getting sucked throough the airflow meter (even though almost all of it will get dumped out through the blow off valve).
The airflow meter seems to thnk that all the air that passes through it actually makes it into the engine so it tells the ECU to set the mixtures to full rich and this my fiends is what the EPA really really hates about these things (plus the pooftersonic noise outputs).

here is some info i dugg up from following Garrett

The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.

Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract to atmosphere to relieve the pressure; helping to eliminate the phenomenon of surge.

Cheers xcabbi, I had previously read the theory of plum back POVs for airmeter cars.

Dansedgli - usrely by stage 2 Nizpro have increased the boost a fiar bit? So surely this is dangerous to have all this preasure just sitting in the piping?

Simon from nizpro has put a long post about blow off valves in the nizpro forums. Take a read then ask him. Im not the expert.

I would run one just to be safe and will do when my GT51R goes on.

That condition is not surge when you snap the throttle shut. Some people call it flutter, others call it reversion and others still reffer to it as stalling the compressor.

Surge is when you fit a turbo that is waaaayyy too big for your engine. Then as its coming onto boost the engine cant swallow the air fast enough so it practically spews it back up the inlet tract and back out through the turbo. The turbo then stalls with the result a temporary loos of boost pressure. The turbo then try's to build some boost again and the engine swallows then spews again.
This is surge and when it happens it is so violent that engines have been known to shake hard enough to smash engine mounts and cooler pipes to pieces and damage internals.

Just remember when selecting a turbo for your car, stay to the right of the surge line.

refering to the post above.
funny that i could stall the stock turbo on a exa????

Stall isn't when it physically stops spinning. Stall is when the pressure in the compressor housing is greater than the pressure in the turbine housing, thus the turbo will not make any usefull boost in that short time. Kind of like when a plane stalls. The engine doesn't just stop spinning. It still spins, just that gravity is greater than lift.

well x-cabbi that info about surge when you snap the throttle shut come straight from the tech pages on the garrett web site and considering thay are one of the world leaders in developing and manufacturing turbo's i think thay might know a little more about turbo's than a cab driver, lols.


http://www.turbobygarrett.com/turbobyga ... enter.html

I'm more than just a s**t ex cab driver.

Has anyone got the corky bell book "Maximum Boost"? I wonder what his definition of surge is. Many turbo experts consider that book to be the turbo bible which still applies today even though it was written many moons ago.