To blow-off or not to blow-off ?????????

[G@NDY]

I run one and still get the turbo stall?? go figure??:Tongue1

[benRX7]

great plug my man
 
colum

Yeah you cant miss these opertunities, lol
 
Blitzboy, I thought you would have known the difference, just checking

[Prof]

hmmm sounds like a description of a pop off valve, I thought a dump valve releases what ever pressure there was in the pipes on every gear change when on boost

Yeh - bad description wasn\'t it.
 
I meant to say when throttle plates are closed, and obviously the \'excess\' is whatever the spring in the dump valve allows - so a softer spring would allow more air to be dumped before closing up the valve again. Dump valve is operated by vacuum from the  manifold side of the throttle plate - whereas the other valve just lets off excess air.
 
Actually most dump valves will begin to leak at higher boost pressures in any case - hence the need for a harder spring !

[nikp]

I run one and still get the turbo stall?? go figure??:Tongue1

What one are you running? If it\'s not got the capacity to get rid of all the compressed air, the remainder of it will still stall the turbo.

[rico2k_uk]

just to remind people - turbo stall phrase isnt the classic stall of something stopping - its an aerodynamic term for the air that passes the fins of the turbine becoming more turbulant and pealing off the fins unevenly. the rotation spin of the blades isnt effected as much as you would expect. the sound of the flutter is the air becoming unstable, then stable, then unstable a little bit less, then stable, etc etc.

[G@NDY]

Ive got the box somewhere, i\'ll have a look. I think it was a FSE one. The descritption said that it can handle 30psi?? might need to change the spring, it did have another one with it.

[Chriszx]

it does hurt performance, basically becasue you are blasting all the compressed air out of the system into the atmosphere - takes some power to create it using the turbos!

But venting to atmosphere reduces turbo stall, in turn when you get back on the gas the turbo has to do less to spool back up to full boost. Just fitted an HKS copy to mine which not only sound excellent but has made the car even less laggy than it was (T78 single) :3gears-rh

[rico2k_uk]

no, its the other way around, they work harder to recompress all the air in the system, without the bov, the air is still compressed in teh system when needed so less work is needed..

doesnt matter,you get a big turbo, a little lag difference from bov or no bov wont be noticable!

[Chriszx]

no, its the other way around, they work harder to recompress all the air in the system, without the bov, the air is still compressed in teh system when needed so less work is needed..

doesnt matter,you get a big turbo, a little lag difference from bov or no bov wont be noticable!

But all that compressed air is heading back towards your turbo because your throttle is shut, forcing  pressure on the turbo to send that air back towards the inlet :Hammer . Anyway no blow off or dump valve is going to vent all the air in the system so so its not going to take long to re-compress.

[iaint]

oh no , not this old chestnust again :rollin
I vote no to a dump valve as it hurts performance, do a search on previous threads on the fors and against dump valves

On the other side of the coin not stakking the compressor means less spool time  between shifts.  May or may not be a problem depending on rev range, target boost and general laggyness of your setup though.

I quite like the noise of no dump valve - reminds me of WRC cars but I do notice the laggyness if I\'m not fizzing along at higher rpm.

Got any links to the previous discussions on this?

[slippers]

Heres a link for you

http://www.mazdarotaryclub.com/forums/showthread.php?t=19689&page=2&pp=15&highlight=bov

Scroll down to my post and read away!

Regards
Jon

[BlitzBoy]

oh god i cant be naffed talking about this anymore :rollin, there is a massive thread on Ausrotary. The turbo still rotates and does not stall when you snap shut the throttle plates, some air gets force back over the blades causing the flutter sound but does not stall the wheel. My own personal experiances has shown less lag between shifts without a BOV but hey thats just mine.
Ask you yourself this question why do WRC cars run without BOVs if it creates lag? and they do thousands of shifts per rally.

Since that old thread I changed my mind and decided not to run one

[rico2k_uk]

here is another good read, which a lot of skyline owners listened to:

http://www.skylineowners.com/forum/showthread.php?t=63478

[BlitzBoy]

Nice find Rico, quoting Nisfan excellant post


"so it does put more stress on the turbo?

Not really. (Here is the medium version, I\'ll leave out the heavy stuff)


Please remember that boost is a consequence of the engine not being able to flow as much air as the turbo, therefore the pressure rises. As it rises the engine can take in more air, as the engine is volume dependant not air mass dependant.
Higher pressure means more mass per cc of air, and that is how the engine gets to take more air fuel into the cylinders, when \'charged\'.
With this in mind, you will see that the turbo is always \'blowing\' against a backpressure, so nothing new there. It was built to handle this operation.

Here\'s a little scenario......

Lets imagine that the turbo was not connected to the inlet of the engine, only the exhaust side was connected, and the engine kept running at high enough revs to develop full boost. The air being pumped by the compressor is used to blow leaves off the lawn or something, and we have a small nozzle on the garden blower. We also have a boost controller to maintain say 2 bar in the blower.

If we were to block off our garden blower (compressor outlet), what would happen?

Airflow would come to a halt in the compressor housing, boost would remain constant although the turbines wastegate would open slightly to stop pressure rising any further. But, it would still need to pump reasonably hard just to maintain our set 2bar pressure.
(remember in this scenerio the engine is not slowing as it is not connected to the compressor, and is still running at the same revs as earlier)

Right, the airflow has now stopped in the compressor outlet. This is where surge comes in.
Surge is basically stall, this is an aerodynamic term. If the pumped medium were water, it would be called cavitation. Similar kind os thing.

Because the way the compressor works - air gets picked up by the blades and deposited at a higher speed the other side of them. It can no longer maintain this smooth operation because with the pipe blocked, the air the other side is not going anywhere. The air pressure builds up on the blades putting more load on them, and suddenly the normal laminar airflow breaks away from the blade in turbulant flow.
This is called stall.
Same happens on aircraft wings when the angle of attack becomes too great, or the wings are heavily loaded, like when a plane does a high G turn.
In this stalled condition, instead of the airflow running smoothly over the blades, it breaks away from the surface in a tumbling motion.

Lets get back to our compressor. During this stalled state (remember we are still powering the turbine in this little scenario, so no loss of speed as would occur on a closed loop feed back into the engine set up). What would happen if we kept like this?
When the compressor goes into surge (aerodynamically stalled) it no longer \'pumps\'. Because there is still 2 bar of pressure on the outlet side, and we are no longer pumping, the airflow reverses and spills back out the compressor. This happens until the pressure is low enough for flow to start up, and the stall or surge stops and the compressor is happilly pumping again. If the pipe remains blocked, it will just carry on riding through these surge / flow cycles.
Is this bad for the Turbo?

If you maintained a turbo in this continuous surge /flow cycle, there is a buildup of pressure on the blades, which would put the bearing mechanism under more load than normal, just before stall occurs. When stall occurs, there is turbulant buffeting around the blades, which would cause vibration. This vibration would be again be carried by the bearing system.
If we were to measure these additional loads, it would be a reasonalby small percentage of what the turbo sees day in day out.

Right, lets forget about the leaf blower above and get back to a normal car engine.
Unless you have a horribly mismatched turbo on your car, this constant surge thing never happens. (It can happen, especially on big twin turbo RB\'s when you get this chuffing sound, that is surge, but lets tal;k about normal sized turbos)
What does happen is this throttle off effect, when changing gears, or just when slowing down (foot off accel pedal).
Lets say the turbo is producing again 2 bar, foot flat. Turbine happily spinning at 120K revs.
We close the throttle flap to hook up the next gear. Immediately the exhaust gas energy drops massively, causing the turbine to slow down. We would have some inertial energy in the turbine that would keep it speed decay down a bit (opposite of turbo lag).
On the compressor side though, we would go into the same surge/ flow cycle as mentioned above.
Although this time the turbine is continuously slowing. The first surge would see the peak additional forces, and each surge cycle thereafter would be less and less, pretty soon even lower than the turbo would see in normal boosting operation.

Now we are back to our original question, is it bad for the turbo to not have a BOV?

Well, considering that even if you did have one, it would not react fast enough to eliminate the first surge cycle, remember this is the big daddy one.

It possibly would start to work by the second or third surge, but by that time the additional forces are likely to be lower than what would be normal under normal boost conditions. So how much help can that be.

In reality depending on the volume of air between compressor and throttle valve, the effect could be reasonalby good on a fast acting BOV, but these are the same ones that people avoid or tone down due to engine running issues.

If you really wanted to look after your turbo, you would fit an ECU with flat shift capability, and change gear with your foot flat on the throttle."

[iaint]

Great find BB.  Thanks for that!