Toysrus

Took the new car to an autocross and HPDE day over the weekend and ran into an unexpected braking problem:

IT was a 1.5 mile airfield course so fairly long and fast for your typical autocross but experience what can only be described as a loss in vacuum assist during a quick transition from accelerator to brake pedal. oddly it didn't happen consistently but fairly frequently in the same corner and then after the stop box (85mph in 2nd)

The following HPDE day, at the track, the brakes were flawless.

Any suggestions?

fleming23

So basically a really hard brake pedal with what feels like no power assist?

If so, I've been experiencing this quite a bit lately in autocross but also road course. It feels nearly identical to "ICE" mode and you cannot get the car slowed. I've nearly crashed a couple times now with mine and hit a number of cones unnecessarily when I go to jump on the brake only to experience a high hard pedal. It really sucks and I have not found a solution yet.

Toysrus

This is exactly the issue. I was really concerned heading to the track however it never happened. A friend of mine drove it and thought it might be rpm related. Does the LT4 have a variable cam? What are the potential causes for a vacuum leak at high rpm?

Toysrus

GAve it some thought overnight and remembered I left foot brake on the autocross course but don't on the track. Could it possibly be the quicker application of the brake happens when there is no vacuum?

fleming23

What was suggested to me is that the supercharged motor is possibly using up, or not building quickly enough, the available vacuum. If you left foot brake, you are only compounding the issue. This is apparently why when you make quick transitions from WOT to hard brake, you have limited vacuum. It was suggested that I add an additional vacuum reservoir, but I haven't thought too much on the idea yet as I don't want to just hack something together.

Note, I do not left foot brake with this car and am having the same issue. Not every time, not every corner but often the same corner (if it is going to happen anyway.)

An additional point - I never experienced this with the stock Carbon Ceramics on the car but is possibly due to learning the limits of the car and not pushing as hard as I do now. I did a number of autocross and track days on the CCBs without a single issue. I then swapped to iron Girodisc rotors and Raybestos pads and began to experience the issue during autocross. I assumed the pads were too aggressive and causing problems with the ABS, actually assumed this was an ICE mode problem. I recently swapped to AP Racing's Radi-cal kit both front & rear along with Ferodo DS1.11 pads and continued to experience the problem, both on the road course and auto-x.

I'll be at the track again this weekend (auto-x and road course at NCM) and am going to try some PFC-01 pads but if the issue is vacuum related, I'm not sure my experience will be much different. Hopefully the course is long enough to allow some transition time between getting off the throttle to stabbing the brake pedal.

JRitt@essex

I asked one of our engineers to look at this thread, and this is what they said:

"It does sound like running out of vacuum. Left foot braking would accentuate such a problem because the brake would be applied when pressure in the manifold is highest. Supercharging works against the system keeping the intake pressurized most of the time when you need just the opposite for the brakes.

This is easy to determine using an old school method. Stick a vacuum gauge on the dash and run a hose to the vacuum reservoir. That will clearly tell you if there is a problem maintaining sufficient vacuum. Otherwise, one is guessing. If good vacuum is present when the brakes go away, then it will be ice mode. But, if you cannot maintain sufficient vacuum, then you need to fix the leak, or . . .

Some pressurized manifold cars are using an electric vacuum pump since they cannot depend reliably on the intake manifold for vacuum."

fleming23

The C7 has an electric vacuum pump tee'd into the brake booster at some point. I am not sure if it also ties into the motor or if the brake booster circuit is closed as I don't have a diagram to reference. It would only make sense if the entire system was somehow interconnected, otherwise the vacuum issue would not happen right off heavy throttle, which seems to be the case every time for me.

Toysrus

Does the LT4 have a variable cam? IT seemed only to happen after heavy throttle application and when on the high cam (if that makes sense). I am fairly certain now it is a vacuum related problem.

fleming23

I wonder if it would show anything if I datalogged the map sensor in HP Tuners.... May need to try that this weekend if I begin experiencing the problem.


Edit - I've gone back and watched the few videos from the C7 PDR where I've recorded the hard pedal (auto-x and road course). Each and every time I have the hard pedal issue is after immediately coming off WOT.

naschmitz

Can you log the individual wheel speed sensors? If so you can tell if Ice Mode activated or not. Check out the wheel speed sensors I logged on a C6 in Ice Mode here: https://www.corvetteforum.com/forums...m-solo-dl.html Note how once Ice Mode initiates the front wheels spin freely but the rear wheels spin 30% slower -- all the brake bias goes to the rear so you can steer on ice. Pedal is hard because there is no ABS action.

Ice Mode triggers when three or four wheels lock simultaneously -- in one of your videos you brake as the road slopes away and the car is light -- perfect conditions for initiating a momentary lock up at all four corners.

fleming23

As an update - I ran the National Corvette Museum event with USCA this weekend and did not have a single issue...

I did swap to a different pad compound front/rear so maybe that was a contributing factor, although if vacuum related it should not have mattered. I did get the car tuned by Vengeance Racing before the event as well but would not think the tune would have impacted anything.

It took a while to build confidence that the brakes were not going to go rock hard and leave me crashing but eventually I was running pretty well at full tilt and it felt great. Leaves me scratching my head a bit but I'll continue to use the PFC pads (I've used these in the past on a different combination and really liked them) and see if I have problems in the future at any events.

bill12s

fleming23

https://www.corvetteforum.com/forums...ing-modes.html

X25

So you're saying all these issues were just ICE mode? I'd doubt it, especially since it happens on autocross sessions, not road course track, suggesting loss of vacuum.

EDIT: I see that there's a vacuum pump:
https://parts.campchevrolet.com/oem-...Y4LWdhcw%3D%3D

As such, I presume and hope it'd not be a vacuum issue : )

QKSLVRZ

That pump should make all of the vacuum it could use, and while I don't believe it was in my vette, I've had abs give exactly those symptoms, hard pedal, no slowing.

Oh, my pickup also did this below about 7mph, and it was a wheel sensor issue.