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Discussion and questions related to the course Boost Control
Im runging now two turbos setup with one 3 port mac solenoid connected to external wastegates. Im thinking of using two mac solenoids for each turbo and mounting them as close as posible to turbo and wastagate. Could this system work faster and solve overboost problem after gear change?
Latent speed can be present on a quick gear change because turbo speed is higher at high engine speed, and like anything in motion, it takes time to slow down.
Another possible cause is ECU strategy on gear change. Ignition cut with reduced ignition timing can result in significant turbine drive pressure in the exhaust manifold. If that's how you have things set up, you may want to try a different combination of ignition cut, timing retard, and fueling during cut.
everything works perfect with lower boost, like 1bar. Thats why im thinking its some sort of restrictor in wastegate control. If we make 1.4bars of boost we get spikes of boost after gear change. wastegates are mounted 45 degrees to the exhaust flow, so it should be okey with it.
Are you flat shifting or lifting off during the gear change? Where is the pressure spike being measured? Do you have a blow off valve in place?
Vytenis at higher boost levels you have much higher exhaust flow and exhaust drive pressure, so the behavior will be significantly different and does often require adjustments to flat shift strategy.
I may be missing something here, and I'm not a "turbo'" guy, but...
As Mike said, the basic problem is that at the top of the previous gear the turbine/impellor assembly is spinning at it's maximum speed, and it has rotational inertia (momentum) that to re reduced - the pressure spike is because it was pumping the air required for the boost and air demand of the engine at the top of the gear and when the engine was in the next gear the airflow demand was less, and the turbine was supplying an excess, and so the pressure increased.
If you're looking at just using waste-gates to control the "over-boost" you're not going to succeed because, while you're able to reduce the drive pressure on the turbine, you have no control over the inertia.
What you need to do is actually bleed off some of the pressure/volume from the intake side, and this is done by using a "blow-off" valve - AKA "dump valve" or "bypass valve" - in it's simplest form, it's a spring loaded valve referenced to open when the plenum pressure exceeds a specified pressure, but I expect you can also operate it electronically from the ECU.
Again, not my thing, so consider it a discussion topic that may help.
Some ECU strategies do allow for a timed reduction in WGDC during shift to help the turbo slow down quicker by reducing drive pressure, and I have used it with success on my R35 GTR which has a rather aggressive boost ramping with increasing RPM to avoid making over 1100 ft lbs of TQ, but if you have a boost curve that doesn't aggressively ramp up with RPM this usually isn't required as long as drive energy on the turbine is reduced sufficiently during the shift, which comes down to the flat shift strategy I mentioned.
Thank you for great response. I understand that there is a lot more than adding one more booost solenoid, and we are working on that too. But my question was could the system with two mac solenoids work faster. Actually we are finishing installing second solenoid and short plumbing of WG. and i can let everyone know if we will find benefits of it.