jreifler

Second weekend of track duty with my 7.2, and I don’t know how else to describe the behavior I’m noticing other than to say that the motor doesn’t seem to slow down as quickly as the car does ? ? Is that a thing?

In 15 years of tracking other cars (GT4, 987 race car, old 911, BMWs, etc.), I’ve never really encountered what I’m trying to describe. Even on slicks with serious brake setups. It’s as though the revs don’t fall fast enough to match the physical slowing of the car. Downshifting sooner or later seems to have no affect on the dynamic. It just feels like the flywheel and revs are fighting to keep up with the braking.

Is this a thing? Would a LWFW help if it is ? It’s unsettling, because it’s interfering with my normal brake points, resulting in much earlier braking to net smoothness.

Insight greatly appreciated as always.

ChrisF

Strange.

Want to make sure I understand. If you heel/toe and go hard on the brakes, you're saying that the revs are hanging? Have you tried braking hard with clutch in and see if the revs are hanging as you brake independently?

IIRC, this car is somewhat new to you correct? Was it possibly tuned? Not sure what else it could be but possibly mechanical issue as I never had the behavior you're describing in my 997.2 GT3. GT2...a little but more from boost overrun but that's definitely not an issue for you

Elliotw44

Some more details would help, as I am not following.

jreifler

Yes, exactly. Like today at Barber, there are a couple 110-120 to 45 corners, and every time, as I heel-toe from 4th to 3rd (and occasionally 2nd to play with gearing), the brakes seem to overcome the revs, which lag behind.

I have not tried to just clutch in and brake. That’s a good idea. But I’m not over-revving on heel-toe. It’s after the blip, and fully into the in-gear / brake phase where I notice it.

jreifler

Further - Not new to me anymore, but only my 2nd time on track with it. It has a Sharkwerks tune and exhaust, but the drivetrain is otherwise stock.

Got some Garmin footage today, but don’t have my laptop with me. Will upload when I can to see if what I’m describing is noticeable on camera.

Elliotw44

interesting... when I am hard on the brakes doing a 4->3. Im in 4th until most/all of my braking is done and then heel-toe to get into 3rd. Based on your previous posts you have much more experience than I so my technique could be non-optimal. But I haven't noticed this using the technique I've used. I don't think there is a track I normally drive where I need to come down from 110+ to 45. But this is all fresh in my mind as I just did two days at Area 27.

jreifler

This video shows the 2 sequences I’m talking about. @:30 and :50 approximately in the video. That’s in my GT4, but those are the turns. I’ll post the GT3 version for comparison when I can.

Elliotw44

at the 30 second mark I do hear two quick shifts(I'm guessing 4->3 then 3->2). I slowed down the video, and you are in 3rd for less than a second. If I were driving, I'd go hard on the brakes and then 4->2(again could be non-optimal)

Wonderdan

I’m having a hard time visualizing… do you have the tach by chance in the gt3 vid?

I will say without a doubt my heal toe is easier w/LWFW, I found I kinda would end up over reaching my foot in my GT3 because it felt almost all or nothing

Actually I have a little of that habit still and my blips are a bit over zealous now

8Lug

Assuming that what you feel is actually happening, is there any explanation other than a slipping clutch? There is no other way an engine could hold an RPM higher than what’s normal for any given gear and wheel speed when the clutch has the engine and trans mechanically connected. It’s strange that you would only notice it on downshifts though, but they may be more abrupt than your acceleration so that could be the explanation.

jreifler

No videos showing tach, unfortunately, and it's not something I've been able to pay attention to on track. I'll probably try to recreate on backroads now that I'm aware of the issue, and will take note of what it's doing.

That hadn't occurred to me, but it makes perfect sense. I've not noticed any slipping on acceleration.

Uploaded a couple laps here:
Here, in the same braking zones as the prior video (and on the braking zone for the first straight), you can hear what I'm describing. A strained / slow engine speed deceleration under heavy braking that transitions into "normal" engine speed deceleration immediately after heavy braking and into trail braking. Forgive the inelegant 2d gear blip in the first lap . . .

jreifler

You're right. In that clip, I do go 4-3-2, but the fast way around that corner is to stay in 3rd. With the tall gearing in the GT4 I was always stuck between wanting to leave it in 3rd for smoothness and momentum, and dropping down to 2nd to power out. So the *right* way to take that turn in most cars is just to leave in 3rd, and I often would, but also often played with 2nd for the torque.

jreifler

My mechanic also raised the possibility that throttle could be sticking ever so slightly in the transition to heavy braking . . .

Spyerx

How's your diff?

The diff has a massive impact on braking and steadiness on these cars. If it's stock or not upgraded with GT parts this is the first thing I'd consider.

I'm not sure I see anything odd in the videos from my experience.

If your throttle is sticking open or you are pressing throttle in braking your data will show that easily (Throttle position and throttle body position)

jreifler

That's the consensus. Alex at Sharkwerks and my mechanic both pegged diff as soon as they saw the video. It's the initial threshold braking application where the car is making that odd / laggy decel noise that's the issue. And although I've already done RSS suspension bits all around and proper alignment, the rear is definitely still sketchy under heavy braking, and it tends to want to "walk" on hard throttle power out turns.

Edit - Given I don't really know the car on track yet, I'm also still running with TC on, which Alex said will further exacerbate the issue when the diff isn't doing its job . . .

I inspected the throttle and it's not sticking. So, ordered GT internals to beef up the diff, which was on my list anyway, but I figured I could track for a season first without the upgrade. Not so much.