The issue of VDC being bad for a forced induction set up has been discussed in some other threads here, but I wanted to highlight the following in its own thread.

The folks at Avalon Racing, who did some of my Vortech SC install a coupe of weeks ago, sternly warned me not to drive aggressively with VDC left on. They told me that of dozens of Zs and Gs into which they installed superchargers only one car's engine blew up. They believe this was due to VDC for various reasons surrounding the engine failure.

I researched this issue in the 2003 G35 service manual and came up with the following on page BRC-8:

TCS Function AFS001KY

The wheel spin of the drive wheels is detected by the VDC/TCS/ABS control unit from the wheel speed
signals from the four wheels, so if wheel spin occurs, the rear wheel right and left brake fluid pressure
control and engine fuel cut are conducted while the throttle value is restricted to reduce the engine torque
and decrease the amount of wheel spin. In addition, the degree the throttle is opened is controlled to
achieve the optimum engine torque.

Depending on road circumstances, the driver may have a sluggish feel. This is normal, because the optimum
traction has the highest priority under TCS operation.

TCS may be activated any time the vehicle suddenly accelerates, suddenly downshifts, or is driven on a
road with a varying surface friction coefficient.

During TCS operation, it informs a driver of system operation by flashing SLIP indicator lamp.


A further search through the manual shed no light on exactly how fuel is cut during VDC operation but I'm almost certain that during wheel slip/skid-induced yaw the throttle position sensor is highjacked by VDC and ignores the driver's foot. The engine is drive-by-wire---an electronic, virtual linkage to the accelerator pedal. This enables VDC to immediately cut engine speed and torque to control skidding, just as if the driver was lifting off the accel pedal. Once skidding stops, the highjakcing is immediately terminated and the driver's foot takes over again.

So how does this damage the engine? My theory (presuming manual transmission):

Let's say you're at WOT under boost in third gear going up a hill and wheel slip occurs while VDC is on. Immediately, the car becomes sluggish and unresponsive to your pedal input as above. However, although VDC "lifts off" the pedal, it doesn't downshift for you! Suddenly, with traction regained, you're now going slower with drastically lower engine RPM, still going up hill with your foot flat to the floor. You will now be LUGGING THE ENGINE WITH A WIDE OPEN THROTTLE BODY and some increased intake pressure from the blower. This is a perfect set up for detonation, bent rods, etc.

 

I believe it to be a different scenario. The throttle plate closes under skid forcing air to be jammed in the plumbing. Bad for the s/c.

 

Yes--bad for the SC for sure, even with a blow off valve--but, not causative of detonation or engine failure. Lugging, however, could definitely cause this. The two worst things you can do to the internals of an engine are lug it and over-rev it. Over-revving is prevented by the rev limiter. But, lugging is CAUSED by VDC! At least, this is my theory--possibly incorrect, but I doubt it.

 

I'm am ever more glad I got a Base model. With the addition of the Nismo LSD it performs just as well as any other minus the anti-fun computers///

 

I think the concern is due to cars that have cable throttles. The only available option to control engine torque in these cars is to muck with timing and fuel. Tuners have just gotten used to that being a problem in general.

It only makes sense that a "throttle by wire" car would use the method described in the documentation you pasted.

The only other scenario I can think of is that it might be bad to go from full throttle to part throttle under certain situations. However, the drop in boost caused by throttle closure will lag behind the actual throttle closing, so I think the engine should run plenty rich until things stabilize.

I don't know how well VDC works with a engine signficantly more powerful than stock. I'm sure VDC has an expected torque output and assumes that in some of its calculations.

--
Jeff

 

In the two cases when the car went into VDC my natural reflex was to take my foot off the peddle. Keeping your foot to the floor when suddenly and unexpectedly loosing power is not natural..

 

My concern is not about VDC closing the throttle to reduce torque and control skid, it's when VDC releases control of the throttle and now I'm going up a hill at a slower speed with low engine RPMs, my pedal to the metal and a boosted engine, still in the same gear as when VDC came on. Lugging is never a good thing,

 

I see your point, but insanely gunning an engine up a hill with 400 HP under you is not natural either. I think most of us with FI cars are not "natural" in our driving habits--at least some of the time.

 

Every time that I have set off VDC, torque has come back gradually. Granted, I don't have FI. But I think there is a gradual reintroduction of torque via throttle position.

Even so, for your scenario to cause a problem you would have to lose a lot of vehicle speed from VDC kicking in before you hit the lugging zone of RPMs. I have found that I don't really lose speed, I just lose acceleration. Maybe if the hill was real steep....

I guess it's possible that VDC could start pulling fuel before the throttle plate adjusts giving a few brief moments of a lean condition. Even so, I think this could be tuned for. Especially if you have MAP (don't cut fuel until boost pressure drops).


--
Jeff

 

The problem is with the engine taking over your throttle BUT not downshifting for you (if you're driving a 6MT). I definitely lugged up a hill when VDC went on because of an icy road surface. I was moving slowly in second gear and as I lost speed with my foot pressing harder on the pedal and being confused why I had just lost all power, I could hear the engine lugging. Now, I'm just extrapolating to a higher speed situation where wheel slip is caused by boost, the engine power gets cut by VDC, speed backs off along with the engine RPMs, and I'm in the same gear as before boggin down with my foot still to the floor. Then, the throttle plate reopens with the blower going, momentarily lean, and lugging with the pedal floored.

I wouldn't want to try to prove this WON'T happen by testing it in various scenarios under boost. It's just easier to turn off VDC unless I really want it on, need it on, and I'm going to be driving conservatively.

Basically, this is just another reasonable theory as to why engines may blow up while accelerating under boost and the fix for it is simple, immediate and totally free of charge (unlike buying shielded crank angle cables, knock sensor systems, etc.)--just turn off the VDC button. If safety is a concern (such as during a storm or on a crappy road), then we shouldn't be hammering on the engine anyway. In that situation, turning VDC back on is appropriate and probably safe for the engine.

As an ER doctor, the last thing I would want is people disabling a potentially life-saving technology. But, I feel it's just too risky to drive like a maniac and hope that VDC will save you without killing your engine at the same time.

 

As the son of an ER doctor, I would think he would say cost of engine vs. cost of funeral.

 

A simple cremation is far cheaper than a new engine.

 

Haha, yeah I guess so.