home register FAQ memberlist calendar

Go Back   Turbobricks Forums > Mechanical > performance & modifications

Reply
 
Thread Tools Display Modes
Old 01-02-2019, 10:30 PM   #1
TonyS9
Board Member
 
Join Date: May 2012
Location: Holywood
Default dump/recirc valve mythbusting

I need some back up on another forum and hoping that people here have the smarts. Seems this crap is on you tube aswell. People don't seem to understand physics.

I need to dispell 2 myths about recirc valves, using a 940 turbo as an example.

1. "A recirc valve recirculates the unwanted air so that the measured air stays the same and doesn't confuse the ECU"
This is nonsense. in fact the opposite is true. The standard valve (on a TD04 say) dumps the air out the air filter in order to decompress the inlet, partly because the air reading would be then invalid (although its probably done like that because it has to dump for other reasons), but also because the throttle body and idle valve is not designed to control the air at pressure for idle. Without dump (and this is easy to test by disconnecting the control pipe), the engine won't idle for 4 or 5 seconds after boost because the mixture is too weak, it just dies.
The ECU cannot deal with stored air, or air it measured some time ago. It can only measure air now, and assumes that is what is going into the engine.

2. "a dump valve dumps to the atmostphere, whereas a recirc valve recirculates"
A recirc valve just means it can stay open during idle, and has no remaining pressure.

Tip - because the air is reversing out it tends to pressurise the crank case via the big breather and makes redblock turbos a bit oily. In high boost it literally spurts out of every seal during dump. A one valve helps keep your engine clean (as used by most diesels).
TonyS9 is offline   Reply With Quote
Old 01-02-2019, 11:09 PM   #2
2turbotoys
Salaminizer
 
2turbotoys's Avatar
 
Join Date: Nov 2007
Location: Reading pa
Default

The problem with a "dump valve" is that it can be slightly open at idle, causing an unmetered air leak. That is a problem with a mass air flow controlled car, causing weird idle and/or stalling. If you use a MAP sensor it's not a problem. A recirculating valve doesn't matter if it's slightly open because its metered air.
I used a dump valve with no problems on a 740T automatic, for some reason it compensated for the vacuum leak. On my 740T manual car it wouldn't idle until I ran a hose to recirculate the valve.
Neither of these scenarios should pressurize the crankcase. I'm not sure what reason you have for saying that. Many diesels don't have bypass or blowoff valves, maybe the newest ones do, but none of our diesel engine powered stuff at work has them. None of them even have wastegates to control boost.
There are a few things wrong with your description of a recirculating valve, but the name is a clue, it recirculates boost during a shift, thats why Porsche and Ferrari to name a couple, have always used them on race cars since the '70s. There is remaining pressure.
I have pressurized my crankcase before, and it had everything to do with worn rings, pistons slapping around, etc.
Not trying to argue, but this stuff is pretty well documented. Read the old school book "Maximum Boost" by Corky Bell for clarification on some of this. Its old tech but these valves havent changed much except for dual ports.
__________________
1979 242 B21 on MS, waiting on L33
1991 740t RIP at 320K, dam rust...
2006 V50 T5 AWD Eurosport Tuning Stage 1 kit, downpipe, intercooler
2turbotoys is online now   Reply With Quote
Old 01-02-2019, 11:32 PM   #3
swedefiend
...
 
swedefiend's Avatar
 
Join Date: Jul 2004
Default

Quote:
Originally Posted by TonyS9 View Post
Tip - because the air is reversing out it tends to pressurise the crank case via the big breather and makes redblock turbos a bit oily.
Ahem...

This is a well documented problem with Volvo engines. Not just the red block (and it has nothing to do with the compressor bypass).

The "oil separator box" on these cars is frequently neglected to the point of obstruction.

Last edited by swedefiend; 01-02-2019 at 11:38 PM..
swedefiend is offline   Reply With Quote
Old 01-03-2019, 12:02 AM   #4
linuxman51
BRANDSCHUTZVORSCHRIFTEN!
 
linuxman51's Avatar
 
Join Date: Dec 2002
Location: mont, AL
Default

well to really get a grasp on what's going on, you need to understand how the control systems work on, say, a 940 turbo (since td04 was mentioned).. And where various components are located.

The idea behind re-circulation is likely multiple, but at the core it deals specifically with the mass air flow sensor. If you dump a bunch of already metered air (vs stalling the intake flow at the sensor temporarily) to say atmosphere, the ecu has to be able to deal with that in light of it seeing
1) airflow through the compressor and thus through the maf while the turbo spins down
2) missing air that was already accounted for previously in the form of load range on the fuel map, i.e. fuel volume

In this case, it has but one recourse to "adjust" for that missing air: o2 feedback. which, btw, is slow. so while you are "training" the ecu (i.e. skewing and grossly abusing it's long term fuel trims) to cope, you can expect it to run like hot garbage, stall on lift, etc.

The same thing happens if you have a sufficiently large boost leak (i.e. blow an intercooler pipe off)... the car will not run, or it will run extremely poorly. Think of it like that.

Now the recirc function may or may not blow air backwards out the maf (remember, the compressor wheel is still spinning and still sucking air in), I would think actually that reversion through the maf would be an absolute worse case scenario seeing that the maf doesn't really know "direction" it just simply sees mass movement via cooling of the hot wire; instead it would stop drawing air through the maf (or drawing significantly less) until things settled down a second or two later.

As for creating positive crankcase pressure, doubtful, but even if it disrupted the vacuum in the pcv system for a second, that would not be enough time to
a) create a serious pressure spike inside the crankcase
and
b) cause or increase oil leaks.

Generally, operating under boost will lead to greater pressure in the crankcase than a very brief (if even existent) pressure spike in the compressor inlet plumbing. Keep in mind the volume needed to pressurize to cause reversion back into the crankcase: the intake plumbing, the long relatively small hose going over the engine, the breather box, the air box, not to mention the air filter.

As to why you would want to vent the pressure, it's a bigger deal on manual cars and in higher boost scenarios, but the short of it is transient response and load on the turbocharger itself. the pressure wave can and does stall the compressor wheel, and in certain applications could potentially lead to premature turbo failure. I had a 67mm years ago that kicked the compressor wheel off the shaft (it was not a reverse-threaded turbine shaft, so a sufficient push backwards on the wheel can and did cause the nut to back off) after the blow off valve stuck shut. The other more likely reason esp for volvo, is the noise. Your average person doesn't particularly want to hear things chuffing around under the hood for any reason
__________________
"They bum rushed them in their own crib, they drank all their beer, they partied with their ladies and they left with the trophy"

Now with in-house Dyno tuning!

Megasquirt Tuning!
Plug and play LH 2.4 Megasquirt, now with stealth mode!
linuxman51 is offline   Reply With Quote
Old 01-03-2019, 03:21 AM   #5
swedefiend
...
 
swedefiend's Avatar
 
Join Date: Jul 2004
Default

Quote:
Originally Posted by linuxman51 View Post
As to why you would want to vent the pressure, it's a bigger deal on manual cars and in higher boost scenarios, but the short of it is transient response and load on the turbocharger itself. the pressure wave can and does stall the compressor wheel, and in certain applications could potentially lead to premature turbo failure. I had a 67mm years ago that kicked the compressor wheel off the shaft (it was not a reverse-threaded turbine shaft, so a sufficient push backwards on the wheel can and did cause the nut to back off) after the blow off valve stuck shut.
You know that I never got around to playing with big turbos.

But, I was initially having problems with 10+ PSI downshifts and "chirping". I was told this was due to compressor surge and warned it could cause premature failure.

So, I added a BOV and just dumped the pressure to atmosphere when shifting.

Relevant info:
9.1:1 SCR b230
Td05-12b
MSnS (speed density)
Manual transmission.
swedefiend is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump


All times are GMT -4. The time now is 11:50 PM.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2019, vBulletin Solutions Inc.