On a journal bearing turbo do you guys prefer the BOV open @ idle?
On a journal bearing turbo do you guys prefer the BOV open @ idle?
Almost all properly configured bypass valve shall hang open at idle. Every turbo car ever manufactured that I am aware of does this (OEM/stock).
The bypass should also recirculate back into the pre-compressor intake tract, just like every turbo car ever made.
Kinetic energy is imparted to air molecules. The compressor speeds the air up (mv^2/2) and sends it out- if you 'vent' this air to the atmosphere you have lost all of the energy that the turbo worked to add to that airflow. You are technically throwing away fuel, albeit a very very small amount (negligible)
however, if there is a re circulation tube, then the system may capture that kinetic energy and recycle it back to the intake tract thus less energy is required when it goes through 'again'. This speeds the compressor wheel all else equal, in other words it will reduce turbo lag and free up economy. The difference in economy may be negligible but the difference in compressor wheel speed can be monitored using a wheel speed sensor and is a tangible difference, depending on the compressor/housing config of course.
Last edited by kingtal0n; 12-18-2019 at 06:14 PM.
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Thanks talon but this is not a stock turbo setup. I've tried both springs that came with BOV. Supposed 14 and 22. The 14 is open at idle turbo spools quicker, the 22 is closed at idle the spool takes longer. Both seem to operate fine coming out of boost, throttle closed air is released and I get a whoosh. I like the way the 14 works but I'm wondering if I'd be leaving power on the table in the vacuum side of things having it close later.
I've thought able using the 14 and adding washers behind spring to have it close when I want. Maybe around 3000-3500 rpm
With the bypass shut, wheel speed can increase kinetic energy in the plumbing, however with the throttle closed at some critical wheel speed (w or angular velocity) the compressor goes into surge and begins oscillatory behavior, which means wheels speed becomes unpredictable at any point in time t.
The idea is to hold the bypass closed until critical w, then recirculate.
There is no difference between a stock turbo setup and a non-stock turbo setup (the engine and turbo doesn't know that it isn't 'stock') as far as whether you want it closed or open; all turbo work the same.
The biggest factor in 'non stock' setups is the large compressor wheel/turbine (large mass, slower to move, more lag in larger turbo) which means the bypass can stay shut longer while the turbo is colder (say, when it first starts on the engine and wheel speed is 0 or very small, the bypass can remain shut longer)
In other words, when the large mass wheel (large turbo) is moving slowly it needs 'help' so the bypass needs to remain shut,
however once the wheel is 'up to speed' due to being warmed up (1000*F+ turbine) and the engine returns to idle, there is so much kinetic energy in the plumbing (both sides) that the situation calls for the open bypass if one wishes to keep the compressor out of surge region, but only If the turbo is 'small' enough to do so near idle speeds (can it reach critical w)
lets use an example
say I put a 120lb/min turbo on a 1.8L engine
The engine's exhaust flow near idle is insufficient to bring such a large mass turbo up to critical w
So the bypass can remain shut at idle, whether the engine is hot or cold its fine, it won't oscillate.
However once you make boost (if it ever makes boost) and lift- you still want the bypass to open (to prevent the surge conditional) and return the engine to idle without bending the throttle shaft or slamming the compressor wheel around in the CHRA. So the bypass needs to be set 'loose' enough to do this job, as with all turbo setups, there should be no 'delay' in relief for the plumbing.
So this setup like all turbo configs still demands that the owner sets the bypass to properly open at the precise time (very responsive setup required) with no surge; but it is not sensitive to the oscillatory nature of closed bypass idle issues.
new example
50lb/min turbo on a 1.8L
This turbo is much smaller so it easily can reach critical w on the running engine near idle speeds.
This setup requires that we open the bypass near idle speeds and hold it open somehow (correct spring/vacuum config) once the wheel speed has reaches critical w.
We can tell w conditional has been met by using wheel speed sensor and holding the bypass closed until oscillatory behavior is reached; then write down the wheel speed where this is likely to occur on a fully warmed up engine (1000*F+ egt)
Then set the bypass to open before that threshold is reached.
What is happening? elaborate (I <3 turbo topic)
A turbo scavenges energy from the exhaust flow of an engine. The amount of energy per unit time is based on the temperature and flow rate of the engine, as well as other influences such as positioning/oil property/posture/gas fluid density and so forth (many influences) however for the sake of simplicity lets just look at temp and engine flow rate as if they are the only two important factors.
Hot exhaust gas contains more energy than cold, it takes up more space. 1400*F exhaust gas will move a turbo faster than the same mass of exhaust gas at just 500*F (typical idle EGT for cold engine)
Warmer engines typically have lower flow rates than cold engines. Warm engines and warm air molecules have more internal energy so they can use less fuel to 'stay moving', this is why auto manufacturers are pushing coolant temperatures higher and higher as the years go on.
So a warm engine needs less airflow to stay running, and a warm turbo makes more airflow rate. These two opposite sort of observations should lead us to the idea that a fully warmed up turbo could easily produce more airflow than a fully warmed up engine wants.
When the engine is at idle, throttle 0%, our goal is to get the turbo moving as fast as possible to get it ready. However if the turbo is moving fast, the flow rate is much higher than the engine's. So the airflow needs to go somewhere. If the bypass is kept closed during this state then the flow conditional is called 'surge' and the airflow moving backwards through the turbo causes it to slow down. This behavior repeats in oscillatory fashion as the wheel continues to speed up, surge, and slow down, wasting energy.
The fix is to open the bypass before the wheel speed reaches critical w and give the compressor flow rate a place to go (besides the engine)
If we dump to the atmosphere it wastes energy. This is undesirable.
If we recirculate then the energy is conserved, the air molecules move back into the compressor inlet with a fraction of their previous energy.
It takes energy to accelerate a given mass of air so the more energy we conserve the better our turbo response will be.
Talon thanks for the schooling, I understood the concept maybe not in educated terms, buy I had an idea of what was hapnin'/ I think I got it going today. I put in the weaker "14" spring in the blow off valve(sorry it is vented to atmosphere) but it IS on the turbo side of the intercooler. I ended up putting a large washer on top of the spring to help it close a little sooner. It works well, at idle the BOV is open no load on turbo,then about 2500 rpms I notice boost gauge start creeping up. when I lift the no skinny peddle I get a nice whoosh. Now the wastegate took a lot more fiddling to operate like I wanted
My basic advice is to keep it as closed as possible. And don't worry if it vents to atmosphere or recirculated.
Boost leaks are very bad so the best thing you can do once you "think" you have it figured out, is to pressure test the system.
Fill the plumbing with air compressor tool to 20~psi (whatever you run) to check for leaks. MAKE SURE the bypass doesn't leak at full boost pressure is the key point. Don't over look this.
The more you can keep it closed the faster the system will respond. Just make sure it flies open before the compressor notably surges is all, don't let it make that lovely noise, you know the one
I did some pressure tests here. First to the just the engine, then to the turbo compressor cover on a friends car for the video to show how it's done
https://www.youtube.com/watch?v=rYZmZqn3-x0
No, I do not like it open at idle. I don't see any reason for it. Dear god, now he is on here posting up books on this site too......
Just because you don't know how a turbo works doesn't make the OEM manufactures (every car ever made with a turbo from every manufacturer toyota/nissa/etc all use open bypass at cruise/idle to control wheel speed oscillation and reduce lag) wrongOriginally Posted by rpturbo
5.3/4l80e/turbo playthrough
https://www.corvetteforum.com/forums...ge-budget.html
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