^thank you^



so basically /thread now lol

 

To understand what the computer is doing for the no-lift shifting it's important to understand what happens on a normal shift. When you're at wide-open throttle (WOT) under boost and approaching redline the turbo may be spinning between 120k to 150k rpms. When you get ready to upshift to the next gear you'd normally left off the throttle as you're engaging the clutch. When you get off the throttle, even for a moment, the throttle blade closes off but the turbo is still spinning and moving air into the intake tract. This hits the now closed throttle body and like a pressure wave reversion it stacks up and heads back towards the compressor outlet of the turbo.

If there wasn't a way to relieve this pressure it would hit the still spinning compressor wheel (which may still be spinning well north of 100k rpms) and the shock wave would cause it to near instantly stop spinning. Depending on a number of factors in the design of a particular turbo that can be extremely hard on the shaft/bearings and can even eventually fatigue the compressor wheel fins. Luckily all modern turbocharged vehicles have some type of blow-off valve. As its name implies, this is a valve that is situated in the intake tract somewhere between the turbo's compressor outlet and the throttle body.

When you suddenly let off the throttle the valve opens and allows the pressure to vent so it doesn't stack up and get forced back into the turbo. The blow-off valves are normally actuated by a mechanical vacuum control when the car goes from boost to vacuum (such as what happens when you get out of the throttle) or is controlled via a solenoid by the computer. The type of blow-off valve used used on the LNF Ecotec Turbo is a surge or bypass (recirc) valve that when open recirculates the built up pressure from the compressor outlet side back to the inlet side.

This helps prevent compressor surge which is what would happen if you didn't have a relief valve and that pressure slammed back into the spinning compressor and suddenly stopped it from spinning (which would normally make a noticeable "laughing" noise; there's also another type of surge relating to a compressor wheel flow map when the boost pressure spikes too high but the turbo isn't moving enough air mass and the wheel stalls, but that's for another discussion). When the blow-off/bypass valve opens it releases and softens the pressure spike in the intake tract caused by the closed throttle. With the throttle closed and the engine not producing exhaust gas since it isn't burning a large fuel/air mixture, there isn't exhaust energy to drive the turbine (which is directly connected to the compressor wheel) so there's no way to keep it producing boost with the throttle closed and the blow-off/bypass valve open so it dramatically slows down.

When you complete the shift and go to wide-open throttle again the engine has to start producing exhaust gas to drive the turbine wheel which is connected to and spins up the compressor wheel in order to build up the boost pressure again. That takes a little bit of time. As the throttle was closed when the shift was initiated and the bypass valve was opened the wastegate would also normally be opened by many engine management systems. The wastegate is a small flapper valve that sits on the exhaust side of the turbocharger and when opened it bypasses the exhaust gas around the turbine wheel and is used to control the max speed the turbine (and therefore compressor wheel) spins in order to regulate boost. So you get back on the throttle, the computer tells the bypass valve to close back up and it also ensures the wastegate actuator (WGA) vacuum canister closes off the wategate flapper valve to ensure all exhaust gas if forced through the turbine wheel. When the turbo fully spools up and the maximum boost level the computer wants to see is reached it uses a boost control solenoid to control the WGA canister to crack the wastegate flapper open to bypass exhaust gas around the turbo and maintain or lower the boost level (since that will provide less energy to drive/spin the turbine/compressor and slow it down).

That's what would happen on a normal shift. The no-lift shift feature starts out with you going WOT and accelerating in a gear. Right before you reach redline you're ready to upshift but instead of letting up on the throttle as you're engaging the clutch you instead keep the gas pedal to the floor. The computer keeps the electronic-controlled throttle blade in the throttle body open so the pressure doesn't stack up in the intake tract and slow the turbo down. It should also not actuate the computer-controlled bypass valve which remains shut and doesn't vent any of that pressure from the intake tract.

According to the technical brief on posted on the GMPD web site, the throttle is kept open and the ignition timing and fueling is altered when the computer determines you're in the window for a no-lift shift (which is right at the verge of the rev limiter at ~6300 rpm). I have to get an update for my scan tool to log high-speed, manufacturer specific OBD-II data to verify it, but what's probably happening is that ignition timing (i.e. when the spark plug fires) is retarded and fuel injector pulse-width (i.e. how long the injector fires/opens) is reduced. The popping noise is more than likely caused by that lowered timing and the computer still firing the injectors occasionally to keep the engine spinning at the rev limit. Go over the three-tenths of a second limit for the no-lift and it should revert back to a normal soft-touch style rev limit and shut down fueling and ignition (and may open the bypass valve and wastegate).

So the "pop" is just fuel burning and the ignition firing a few times during the no-lift shift window of operation. I do have a few tech questions that haven't been covered in any GMPD documentation that I'm trying to get answers on. A couple of the magazines mentioned that the cam timing is altered but the tech presentation mentions it is ignition timing. I don't think there's any type of "anti-lag" setup that dumps a large amount of fuel to provide exhaust energy during the no-lift shift but there is probably a small injection that helps keep the engine spinning and leads to the pop. I also suspect that while it helps the turbo get back into boost faster (it will still slow down a little during a no-lift shift due to air flowing through the open throttle and through the engine but not anywhere near as much as fully getting out of the throttle) that when the clutch is released after a no-lift shift there may be a slight torque management being applied involving fueling and timing to get back into power but not shock the driveline like what would happen if you tried to "WOT shift" a normal turbocharged stick shift car (which would probably also be opening the surge/blow-off valve and not hold boost as well as the Cobalt's implementation).


In summary the no-lift shift feature is keeping the throttle open, altering fueling/timing to hold the engine at redline, isn't opening the bypass valve, and the computer may be applying torque management after the shift is completed to ensure that while boost and power is ready faster it isn't a harsh driveline shock. The turbo will still slow down and boost will drop a little during a no-lift shift (since there isn't a load on the engine because it doesn't take much fuel burnt to spin the engine but it does take a lot to move the car which will produce a lot more exhaust gas energy that can drive the turbo), but the boost drop is nowhere near what it would be during a conventional shift. And again remember that no-lift shifting is providing better transient boost response after a shift is completed but you still need to use the clutch pedal and shift normally but quickly (that means no "power shifting" where you shift without using the clutch and that you need to shift normally but quicker than normal but not so fast that you jam or grind a gear). You also need to start the no-lift shift by engaging the clutch and shifting right as you're hitting redline and not at something like 5500 or 5800 rpm.

 

i tried my no lift shift out last night its so sickkk. floored it at 3k in first shifted took a quick peak at the boost and it stayed at 15 INCREDIBLE. chirped second and barely chirped 3rd but i heard the tires. my head was in the back of the seat the whole time except for when i shifted its a kick ass feature.

 

This information about the needlessness of a turbo timer and the explicit and detailed explanation of how the No-Lift Shift feature works needs to be stickied. It's a great read, and answers a lot of questions.

 

God this thread is old! who brought this aging beast back?

 

Someone using the search button.

 

He sounds so angry I used the search button...

 

any one know what happens when you get no pops from the exhaust on a no lift shift this only happens when doing a 2-3 shift ill start the shift around 5800 and complete it dont lift dont hit the rev limiter but sometimes no pop.

 

You won't get the pop if you physically complete the shift fast enough that the engine controller doesn't have to keep the throttle open and bump the rev limiter (which is where the pop comes from). One thing to keep in mind is there's a good chance that the factory tach has a little lag/ inaccuracey to it like most factory tachs. If you start the shift right before a gauge indicated redline, you might in reality already be bumping the limiter by the time you're starting the shift which would cause the pop. You don't want to start the shift and push the clutch in too early and have the engine jump up hundreds of rpm's quickly and slam into the limiter, but perhaps on your car an indicated 5800 rpm is really slightly higher and just perfect for the no-lift shift.

 

I'm actually happy someone did use the search button, good on ya!


Do you have the Launch Control button setup when you NLS?

 

The no-lift feature is active in all four stability/traction modes. It works the same whether you're in Competitive Mode (which does have to be on for Launch Control) or the other modes. I still have some videos I'll try to get posted up testing all the modes.

 

Blackbird is one badass!!!!Thanks dude!

 

Somewhat related, since no-lift is always active and at the drag strip Launch Control usually allows too much wheel spin (killing your sixty foot times) there is another benefit to using the Competitive Mode. Like the ETC Off mode, GMPD stated in their tech presentation that the electronic/brake-activated limited slip function is only available when traction control is not. Although with good traction at the drag strip and only going straight, not having the optional hardware limited slip diff isn't as important as if you auto-x or road race, the electronic limited slip feature may still be beneficial. (That and traction control also kills your launches.)

 

A little word to the wise. You dont have to be in CM to NLS, but its recommended. For example in my case, I did it with SC on and ended up actually throwing a DIC message that read ECS Inactive so I took it into the dealer, and the MT said that it came from NLSing in SC mode. But then again I was going from 1st to 2nd as well, so it could have snatched it. Either way, whats it going to hurt to press a button twice, or hold it down for 10 seconds?

 

redlines at 6.2k stock,pop the no lift at 6k

 

I pop at 5k, is that to early?

 

You NLS @ 5k?????

 

^^^^^Yea, is that to early?

 

yes, its set to hold the rpms at like 6K i believe

 

thats not true, iv never been in comp mode and it worked all the time, just make sure ur at wot and ur rpms dont have to be at readline, but they should be pretty high. make sure u shift fast. Trust me once u do it right, u'll know, its fricken awesome. u'll get pushed into ur seat nicely.

 

launch control holds at 5k.while driving, nls at 6k, its about timing,if its not timed right then u will prollymiss a gear.btw fck my space bar, it sux

launch control holds at 5k.while driving, nls at 6k, its about timing,if its not timed right then u will prollymiss a gear.btw fck my space bar, it sux



and btw chris, nls works either way.just a fyi

 

no wonder, somtimes i don't get a pop and all I hear is a squel, god good know what happens.

 

I usually NLS about 5800rpm... still get the "pop"

 

u get the pop nomatter what.we turbo and spit unburnt gas rofl

 

I don't understand why so many people have trouble with the no lift shift. So many people say you have .3 seconds to do the "NLS" and if that was true no one could do it, if you could you don't need the computers help. You have at least 1 full sec.