wastgate chatter

[RAW]

"There is a common myth in the automotive world about so called "wastegate chatter", a noise created on lifting off the throttle in a turbocharged car. The sound is commonly described as a chipmunk or a rattlesnake and is often stated incorrectly as being a result of the turbo\'s wastegate closing.
 
The noise is in fact the air compressed by the turbo passing back through the compressor housing of the turbo after having hit the now closed throttle plate.
 
A blow-off valve (BOV) will usually eliminate this noise, but in some cases, i.e. where it doesn\'t open fast enough or is setup to only react to high boost, some chatter will remain.
 
The chatter noise is very noticeable on WRC Rally cars, where no BOV is allowed."

thank you! this is an interesting discussion, but at last someone has answered my question as to why i have both!

[CraigN]

the chattering noise is turbo stall

think of a propeller when it spins one direction air flows one way due to the orientation of the blades on said propellor. If it were to spin the other way the air would flow in the opposite direction.

When the throttle closes on a turbocharged car the air has no were to go apart from backwards through the turbo.

The turbo is spinning one way and the air is trying to go the wrong way this is what stalls the turbo and makes the chatter noise.

[CraigN]

bah i was too slow Ross allready posted it.

[88GT-R]

i have seen a snapped shaft on a gt rollerbearing turbo

on a cossie running over 2 bar, but was told the shafts on them are weaker

Bang on.... was waiting for a comment like this. Running the standard turbos with no BOV will result in severe shaft damage!! No blades snapping, or nuts shearing and no squirrels having kids! The REW turbo pack uses 4mm shafts. These are tiny, and will snap if there is no charge relief...espeacially if run high enough to produce in the region of a bar of boost.
Now when you go single, the shafts are alot bigger. For example a To4e is 7mm in diameter, and is a fair bit more robust than the twins. Move onto the larger Mitsubishi and Garrett turbos, and you will find these sporting 8-12mm shafts. the inherant strength of the thicker shafts lends them to be more suited to running without any charge relief, hence the shouts of \'its been fine on  my setup!!\'

Peace,
John

[rx7veilside]

got to say i wouldnt touch a car thats been running with no dump valve but each to there own

[kev-yorks]

Right just to add a bit more to this one this is a section taken from a garret turbos tech paper.
Blow-Off (Bypass) Valves
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.

[rx mad]

The reason why he was banned is there is a big fight about AI on the forum and other members on there, trust me he knows his sh*t
Tony in the 80\'s Formula One Turbos did not use BOVs, Rally Cars do not use BOVs.
Ask yourself why, the main reason why is the "drop" in performance, the repressurizing of the intercooler and pipes is the main reason as the BOV releases all the pressure that the turbo has built up, so the turbo has to work harder to repressurize therefore time is lost in building up the pressure again so this will cost you time in the quarter mile and Track.
There has been no real eveidence I have seen over the years on all these forums that has shown damage due to not running a BOV, plus a turbo keeps spinning even without a BOV there has been no real eveidence of turbo stall.
If there was Stall then this would hurt performance and Rally cars would run them.

greer sports a respectable tuner backs this theory aswell i dont want a big rammy started i just wanted to no how to get it thats all

[Ricco_04]

If you are concerned with the effect if has on the drag strip then my mate used to change gear so fast that the dump valve never had time to fully open before he was on the power again.  The dump valve effectively welded itself shut! :Laugh

Of course I am no use to the argument which method is best! :King

[TonyC]

Bang on.... was waiting for a comment like this. Running the standard turbos with no BOV will result in severe shaft damage!! No blades snapping, or nuts shearing and no squirrels having kids! The REW turbo pack uses 4mm shafts. These are tiny, and will snap if there is no charge relief...espeacially if run high enough to produce in the region of a bar of boost.
Now when you go single, the shafts are alot bigger. For example a To4e is 7mm in diameter, and is a fair bit more robust than the twins. Move onto the larger Mitsubishi and Garrett turbos, and you will find these sporting 8-12mm shafts. the inherant strength of the thicker shafts lends them to be more suited to running without any charge relief, hence the shouts of \'its been fine on  my setup!!\'

Peace,
John

Now that\'s the kind of think I like. And makes perfect sense regarding the different experiences of people. Different turbo\'s different sizes, different strengths.
You can easily see how tuners who run larger, stronger turbos can say \'i never have a problem without a bov\', and you can understand why manufacturs fitting smaller cheaper mass market turbos have to do something to protect the compressor.
There seems no doubt that without venting it does stall the compressor, and it can have a negative effect on turbo longevity - but the overal \'wear\' is going to dependent on the turbo size and quality.

Interesting as well regarding the experience on the strip with and without bov - I\'d cerainly like to see a few runs with and without on the same day and same car.

[rx mad]

damn it ive started world war 3:Hammer :chat

[TonyC]

damn it ive started world war 3:Hammer :chat

Nah, you\'e started a decent debate.:Thumbsup!

[efiniste]

Bang on.... was waiting for a comment like this. Running the standard turbos with no BOV will result in severe shaft damage!! No blades snapping, or nuts shearing and no squirrels having kids! The REW turbo pack uses 4mm shafts. These are tiny, and will snap if there is no charge relief...espeacially if run high enough to produce in the region of a bar of boost.
Now when you go single, the shafts are alot bigger. For example a To4e is 7mm in diameter, and is a fair bit more robust than the twins. Move onto the larger Mitsubishi and Garrett turbos, and you will find these sporting 8-12mm shafts. the inherant strength of the thicker shafts lends them to be more suited to running without any charge relief, hence the shouts of \'its been fine on  my setup!!\'

Peace,
John

I know Rice Racing runs his stock twins without BOV or charge relief at stupid amounts of boost (using WI) and has done for a long time with no problems.

[Prof]

It must be possible to \'tune\' the BOV to release only at and seal at certain pressures.  It could then be setup to only let go when there is a large amount of pressure in the system that \'might\' cause problems, but would effectively not open at all at when only light boost and throttle plate closure occurs.
 
I presume the bigger turbo\'s may also have different shaped compressor blades that are designed to reduce the effects of charge backwash - much like in the modern turbofan engines on airplanes which have to take bird strike in their stride.
 
I\'ll be keeping my BOV as I only have a small Turbo, but that has the advantage that it respools pdq anyway.
 
Cheers.

[88GT-R]

Please correct me...but am 100% positive that Rice WAS running BNR stage 3\'s with no BOV. Now looking along the lines of Garratt TO4Z

[kev-yorks]

It must be possible to \'tune\' the BOV to release only at and seal at certain pressures. It could then be setup to only let go when there is a large amount of pressure in the system that \'might\' cause problems, but would effectively not open at all at when only light boost and throttle plate closure occurs.
 
I presume the bigger turbo\'s may also have different shaped compressor blades that are designed to reduce the effects of charge backwash - much like in the modern turbofan engines on airplanes which have to take bird strike in their stride.
 
I\'ll be keeping my BOV as I only have a small Turbo, but that has the advantage that it respools pdq anyway.
 
Cheers.

The upmarket BOV\'s are tunable for pressure Prof also the some of the bigger turbo\'s are now coming with ported shroud compressor housings which may help.
Here\'s a pic of the ported shroud housing instead of stalling the air bypasses via the ports in the shroud four of them around the compressor wheel and back into the compressor to keep everything spinning.
Kevin