Engine Question
02-21-2009, 01:22 PM
#33
Three Wheelin'
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Fortunately I don't have a cat 
But yes I agree with you on that. Both turbo and n/a cars need a controlled closing of the butterfly to avoid that puff of rich exhaust that occurs when the throttle closes suddenly. N/a cars use a dash pot that slows down the butterfly. Our turbo cars use the diaphragm valve that dumps high pressure air from the i/c into the engine behind the butterfly. Both are good at killing fast shifting and in our case, turbo "freewheel speed".
In my case I didn't disconnect the vacuum source from the diaphragm valve so that it would still operate and just dump air to the atmosphere. This works good for low boost as the blowoff valve won't open until high boost is achieved but the relatively high pressure air still is relieved from the intake between the butterfly and the compressor.
I really like this setup as it allows for fast shifting and lots of boost on tap ( boost now starts at 5 psi at initial acceleration instead of zero as the turbo hasn't lost so much speed) and overall makes the car feel a lot faster.
But yes I agree with you on that. Both turbo and n/a cars need a controlled closing of the butterfly to avoid that puff of rich exhaust that occurs when the throttle closes suddenly. N/a cars use a dash pot that slows down the butterfly. Our turbo cars use the diaphragm valve that dumps high pressure air from the i/c into the engine behind the butterfly. Both are good at killing fast shifting and in our case, turbo "freewheel speed".
In my case I didn't disconnect the vacuum source from the diaphragm valve so that it would still operate and just dump air to the atmosphere. This works good for low boost as the blowoff valve won't open until high boost is achieved but the relatively high pressure air still is relieved from the intake between the butterfly and the compressor.
I really like this setup as it allows for fast shifting and lots of boost on tap ( boost now starts at 5 psi at initial acceleration instead of zero as the turbo hasn't lost so much speed) and overall makes the car feel a lot faster.
So if you didn't dump the lower boosted air on decel from the intercooler, wouldn't you still have that charged air ready to go through the butterfly as soon as you hit the throttle again?
I am having trouble seeinghow venting to the atmosphere vs. the intake manifold affects the ability for the turbo to spool up. Either way the charged air in the intercooler is being moved out of the system. I would think this clears the way for the turbo to spool.
Pardon my ignorance...I just like to really understand things before I do them. I did plenty of research on the decel valve but not alot of performance related info.
Cheers!
RT
02-21-2009, 01:42 PM
#34
Three Wheelin'
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Ok. After staring at the car some more I think I am getting it.
Basically when you snap the throttle closed, it is like the air hits a brick wall which would increase the "back pressure" on the turbo outlet, thereby causing it to slow down faster since you are dropping RPMs and not forcing the turbo to spool.
It is kind of like a baby diverter/blow off valve.
Now for dumping to atmosphere vs. intake manifold vs. where the diverter valve reinjects...I think we have all read that you don't want to use a BOV to dump to atmosphere since the air has already passed the meter plate. I would think that this is no different.
So why not dump this air to the same spot as the diverter valve?
Regardless of where you dump it, you are relieving the pre throttle plate pressure. I don't see where you are getting gains for the turbo spool in any configuration as long as the decel valve is working properly.
The only thing I can think of is that your manifold is in vaccum and once you dump enough air from theintercooler, the decel valve will close, possibly before you have bled off enough intercooler pressure. By dumping it to atmosphere, you might be able to bleed off more pressure? In this case, it seems it would make sense to dump it to the diverter valve insertion point.
Thought of one more thing...if you do not dump the air to the intake, it will stay in vacuum. When you open the throttle plate again, would that vacuum be beneficial to bring air in quicker/easier? Am I getting warmer?
RT
Basically when you snap the throttle closed, it is like the air hits a brick wall which would increase the "back pressure" on the turbo outlet, thereby causing it to slow down faster since you are dropping RPMs and not forcing the turbo to spool.
It is kind of like a baby diverter/blow off valve.
Now for dumping to atmosphere vs. intake manifold vs. where the diverter valve reinjects...I think we have all read that you don't want to use a BOV to dump to atmosphere since the air has already passed the meter plate. I would think that this is no different.
So why not dump this air to the same spot as the diverter valve?
Regardless of where you dump it, you are relieving the pre throttle plate pressure. I don't see where you are getting gains for the turbo spool in any configuration as long as the decel valve is working properly.
The only thing I can think of is that your manifold is in vaccum and once you dump enough air from theintercooler, the decel valve will close, possibly before you have bled off enough intercooler pressure. By dumping it to atmosphere, you might be able to bleed off more pressure? In this case, it seems it would make sense to dump it to the diverter valve insertion point.
Thought of one more thing...if you do not dump the air to the intake, it will stay in vacuum. When you open the throttle plate again, would that vacuum be beneficial to bring air in quicker/easier? Am I getting warmer?
RT
02-21-2009, 04:59 PM
#35
Rennlist Member
The gain.
It's like this: you accelerate, you get off the gas to shift, the revs "hang" for a second and then drop sloooowly. In an effort to keep from destroying your clutch, you begin to count "one-thousand-one, one-thousand-two...." At long last the revs have dropped to 500 rpm or so and it's time to shift. Problem is the turbo has dropped speed from 120k rpm to some number which requires more energy (and time) to get back into the boost region.
I'm using a Forge hybrid BOV in conjunction with having the diverter valve vent to atmosphere. When I'm driving at part throttle (not standing on it at all) I still like the revs to drop fast so I can shift right away. There's not enough vacuum to open the BOV at part throttle so the diverter valve opens in that instance. No performance gain there; consistent behavior is the benefit.
Many worry about air past the metering plate. When it's past the metering plate and on it's way to wherever, why would it matter? If that metered air is vented to atmosphere when the throttle shuts, the worst thing that could happen is you get a puff of rich exhaust, that's it. Meanwhile, during that event, there is no more flow over the metering plate so it closes and fuel delivery drops to idle speed delivery, which is what is desired while shifting anyway.
Try it and see if you like the results. It's easy enough to go back if you are uncomfortable with the setup.
It's like this: you accelerate, you get off the gas to shift, the revs "hang" for a second and then drop sloooowly. In an effort to keep from destroying your clutch, you begin to count "one-thousand-one, one-thousand-two...." At long last the revs have dropped to 500 rpm or so and it's time to shift. Problem is the turbo has dropped speed from 120k rpm to some number which requires more energy (and time) to get back into the boost region.
I'm using a Forge hybrid BOV in conjunction with having the diverter valve vent to atmosphere. When I'm driving at part throttle (not standing on it at all) I still like the revs to drop fast so I can shift right away. There's not enough vacuum to open the BOV at part throttle so the diverter valve opens in that instance. No performance gain there; consistent behavior is the benefit.
Many worry about air past the metering plate. When it's past the metering plate and on it's way to wherever, why would it matter? If that metered air is vented to atmosphere when the throttle shuts, the worst thing that could happen is you get a puff of rich exhaust, that's it. Meanwhile, during that event, there is no more flow over the metering plate so it closes and fuel delivery drops to idle speed delivery, which is what is desired while shifting anyway.
Try it and see if you like the results. It's easy enough to go back if you are uncomfortable with the setup.
02-21-2009, 05:06 PM
#36
Rennlist Member
You could even plug the port where your BOV goes into the intercooler, turn the BOV around and let it vent to atmosphere.
