ZR1 Track Day update (road course) - didn't go well :(

[Vette Guy]

Here's a short clip of what I had to do to catch-up and pass another Challenge Car running a built engine with 100HP over my stocker.... it's all in the late / hard braking and cornering. Straight braking, no wobble, no spin.

https://www.flickr.com/photos/215612...7676290388995/

[cbaclawski]

Originally Posted by Vette Guy

Here's my $.02.... Don't simply replace anything - you need to find the problem and then see what you need to replace. Not sure a faulty master cylinder is the problem, and simply replacing it may give you a false sense of security for next time.

I think our J55 calipers are fantastic. They are nicely matched with the rears, and as Russ indicated, with the DRM bias spring make for one heck of a balanced braking application. I do race (88 Corvette Challenge) with the exact calipers and master cylinder (different brake booster) and am a threshold braker - meaning I typically go full throttle to full brake every lap, and I've competed the car hard on race tracks for the past few years. https://www.youtube.com/watch?v=hy6DG7In0sc&t=671s
Each test/practice/qual/race is 25-30 min. While I don't make a whole lot of power, I consistently push and catch up with faster cars in the turns. If you'd ride with me, you'd be amazed at what your stock brake system can do - it surprises me too! I've never once boiled my brakes, and a well balanced bias will stop you in a very short distance.

You started off with a faulty system if the brake pedal was going to the floor before you began bleeding the system... also, not getting a good bleed is another indication there is a big problem. My guess is that you have junk in your calipers and or master that will only be removed with a good cleaning. During the rebuild process, you should find the "ah ha" problem.

Prior to racing, I rebuilt the system - HIGHLY RECOMMEND the DRM pistons - I attribute my ability to keep the brake fluid from boiling to these pistons - my more expensive calipers on another race car do not have aluminum pistons - they transfer heat from the pads to the fluid quickly; the DRM stainless pistons will withstand much more heat.

A great time to rebuild and not replace your master cylinder is during the installation of the DRM bias spring. I found junk in mine that was blocking a passage for the rear brakes, meaning that I would have had too much front brake, and likely spun the car under any sort of hard braking. (in my case, I found a cracked spring withing the master cylinder, which then required I replace the master). Also, if there is any junk in your calipers, you'll find out when you switch out pistons. The rebuild kit for our J55s can be found at any Autozone, O'Rileys, or NAPA. I burn my dust shields almost every race, and replace them at the end of the season.

Lastly, use at least Super Dot 4 fluid (500 dry boiling point minimum). I recommend Pentosin, but started using Amsoil Dominator Dot 4 in the race cars - 580 dry boiling point, and it's not super expensive as is Motul.

Your existing brake system is likely more than you will ever need for a track day - and what was used during the Corvette Challenge and in later pro-racing series. Switching up to Wilwoods, etc., will require some time to find the proper bias. Don't get back on track without finding the exact cause of your problem.

To give you an idea what I run.... DBA 4000 series rotors (Club Spec with temperature indicating paint), Carbotech XP-12/10s, DRM pistons and bias spring, and >500 dry boiling point fluid. I don't recommend the same Carbotech models for street tires, rather you may like the XP-8's - which are grippier than the HP plus, but still streetable.

Regards,
Mark

Thanks much for the advice!


Much of what you discussed is pretty far above my level of mechanical ability. Replacing the master seemed like a quick and relatively cheap starting point. I did ask the owner/mechanic at the race shop I typically use for track car maintenance if there was a way to test the master to see if it was the problem, his response was "Not really, and it's cheap and easy enough to do, just throw the part at it". Also, when the brakes DID "work" I didn't notice anything to indicate a major un balance either front to rear or left to right. Perhaps naively, I took that to mean that it must be the master, as I thought it highly unlikely that I would have the exact same level of problem at all 4 corners at the same time. I already ordered the new master and might as well install it I guess...

I am running ultra high temp fluid, It was done at the shop and I don't remember the brand, but it is true racing fluid. I was planning on flushing the entire system with motul 660, which I have had good luck with and never boiled in my other cars, when I installed the new master. I also have replacement slotted rotors. Again, I don't know what brand as I didn't install them and forgot what I was told, but they are supposedly suitable for track use...


I *THINK* I'll be able to tell by pedal feel and how the flush goes if everything is working properly. If I have any doubt whatsoever, I'll read up on rebuilding the calipers, the pistons you mentioned, and whatever the hell a DRM spring is...


I don't plan on racing this car, and typically run 20 minute, occasionally 30 minute sessions with plenty of cool down in between. I agree with you that the stock calipers, with good pads and fluids should be easily up to that task if functioning properly.


I am willing to go to a full race pad if necessary, I'm unlikely to drive this car much, if at all, on the street. With the Racetech seats and 6 pt harnesses installed, it's not exactly easy to get in and out of, let alone comfortable to drive in traffic... (I'm about 6'3" 275, so that doesn't help)


Do you do anything for Brake cooling, i.e ducting? Just eyeballing the setup, it doesn't look like there is much airflow in there...

Thanks again, Curt



Edit - just saw the video - AWESOME!!! Everybody talks about horsepower, but it's brakes and tires that make you fast! - on a circuit at least...

[Vette Guy]

Curt,

You can still disassemble your old MC and see if it is filled with goop - brake fluid does go bad, and with moisture and time, can clog passages. If not getting full brake pressure to the back, I think you could easily spin the car. Also understand that many of the problems you will encounter on the track, you may never see on the street.

A real race shop can test brake pressure to each caliper.... (https://www.amazon.com/Allstar-Perfo...pressure+gauge) else just throw parts at it and let you tell them if it works or not on the track. Was the bleeding problem with one or both rear calipers? May indicate a problem in the MC bias mechanism. If only one caliper had this problem, there may be another issue. I've taken apart calipers and found gunk that looked like coffee grounds clogging the fluid path.

Also, brake cooling ducts are important - if you look at the photos, they are on the sides of the air intake (under the driving lights) - as setup by Chevrolet for the Corvette Challenge Series. 3" high-temp hose that directs air to the rotor, but does not go directly to the rotor. The importance of having a good slotted rotor is that it acts like a fan to uniformly cool the rotor surface. Should you purchase the DRM pistons, I don't believe you will have any brake fluid boiling issues, even without ducts.

Hope this helps!

Mark

[cbaclawski]

Originally Posted by Vette Guy

Curt,

You can still disassemble your old MC and see if it is filled with goop - brake fluid does go bad, and with moisture and time, can clog passages. If not getting full brake pressure to the back, I think you could easily spin the car. Also understand that many of the problems you will encounter on the track, you may never see on the street.

A real race shop can test brake pressure to each caliper.... (https://www.amazon.com/Allstar-Perfo...pressure+gauge) else just throw parts at it and let you tell them if it works or not on the track. Was the bleeding problem with one or both rear calipers? May indicate a problem in the MC bias mechanism. If only one caliper had this problem, there may be another issue. I've taken apart calipers and found gunk that looked like coffee grounds clogging the fluid path.

Also, brake cooling ducts are important - if you look at the photos, they are on the sides of the air intake (under the driving lights) - as setup by Chevrolet for the Corvette Challenge Series. 3" high-temp hose that directs air to the rotor, but does not go directly to the rotor. The importance of having a good slotted rotor is that it acts like a fan to uniformly cool the rotor surface. Should you purchase the DRM pistons, I don't believe you will have any brake fluid boiling issues, even without ducts.

Hope this helps!

Mark

Honestly, the bleed was suspect at all 4 corners, I wasn't the one pumping the pedal, I was at the track and didn't know the person in the car, but he was complaining that the pedal was going to the floor *sometimes* before I even loosened the bleeder screw. A few times when I opened the valve very little fluid came up the tube, other times it was much more and seemed normal. This is part of the reason I assumed it was the master, maybe a broken seal in there...


Despite this, in my infinite wisdom, I took the car out anyway. In hindsight I can't believe I was that stupid. (stupidity + inexperience = crash)


Thanks for all your help - I wish you were nearby and I could get you to have a look!

Curt

[GOLDCYLON]

Originally Posted by Vette Guy

Here's a short clip of what I had to do to catch-up and pass another Challenge Car running a built engine with 100HP over my stocker.... it's all in the late / hard braking and cornering. Straight braking, no wobble, no spin.

https://www.flickr.com/photos/215612...7676290388995/

Nice vid Mark

[Vette Guy]

Thanks! And here’s one from when things go wrong.....

https://youtu.be/gw60JxU-zYQ


Sent from my iPhone using ZR-1 Net Registry

[randy ransome]

I'm waiting on pins and needles to hear this story. Glad you're alright.

[Vette Guy]

Thanks Randy! Built a motor for the Challenge Car (replacement Challenge engine #129) and went through bearings at Fontana, and then Indy with Brett after having it again rebuilt. Had the ‘96 WC GT car for a few years, only had it at the Gathering one year. Decided to bring it out to finish the year - tore it completely apart - everything except motor (LT1). Ran test day, practice, qual, and what you see is the first lap of the first race. Steel crank let go and blew out the front of the block. No warning. Fortunately it occurred right as I shifted. I didn’t immediately realize what happened, and did not immediately egress - my window net new and super tight, and once I realized there was fire, I had a hard time releasing it. (Will definitely practice egress!). Oil caught fire on the headers and burned the heat shielding in the tunnel as well as the rubber lower boot on the ZF (still looking for a replacement lower boot). Didn’t pull the fire suppression system, as I wanted to attempt to get it with the handheld first and avoid filling the car with AFF. The 5lb bottle bought time for the safety worked to get another to me, while awaiting firefighters to arrive.
Have engines for both cars currently with the race engine builder. He’s one of the few certified to build the sealed engines for the Trans Am series... (we in vintage are like the opening act for the TA series).
While not a ZR-1, I’ve learned a ton over the last 3 race seasons. Best of all, everything carries over to our cars. The Challenge Car is more ‘street stock’ and the WC GT is a whole different animal. Best advice for making a ZR-1 track ready is not to change anything as it is already very capable, rather make sure everything is operating correctly and bushings are fresh. It was developed on the track and provided to you with a track ready tuned suspension and braking system. Yes, it can be improved for the track, but you need to understand the impact of each change and ensure front and back are in sync. Same with brakes. There’s also a limit to how far you will want to go.... fast track cars make terrible street cars, and in road racing the engine is about 1/2 of the equation for having a fast car - chassis is the other.
Biggest bang for the buck I can recommend for the track is changing your alignment to provide negative camber, which will keep more of your tire surface on the track throughout the range of motion.
Currently planning to have one of the race cars at the Gathering this year, and may try to run it on the MSP for our track day. I’ll be leaving for a race immediately after the Gathering.


Sent from my iPhone using ZR-1 Net Registry

[cbaclawski]

Originally Posted by Vette Guy

Thanks Randy! Built a motor for the Challenge Car (replacement Challenge engine #129) and went through bearings at Fontana, and then Indy with Brett after having it again rebuilt. Had the ‘96 WC GT car for a few years, only had it at the Gathering one year. Decided to bring it out to finish the year - tore it completely apart - everything except motor (LT1). Ran test day, practice, qual, and what you see is the first lap of the first race. Steel crank let go and blew out the front of the block. No warning. Fortunately it occurred right as I shifted. I didn’t immediately realize what happened, and did not immediately egress - my window net new and super tight, and once I realized there was fire, I had a hard time releasing it. (Will definitely practice egress!). Oil caught fire on the headers and burned the heat shielding in the tunnel as well as the rubber lower boot on the ZF (still looking for a replacement lower boot). Didn’t pull the fire suppression system, as I wanted to attempt to get it with the handheld first and avoid filling the car with AFF. The 5lb bottle bought time for the safety worked to get another to me, while awaiting firefighters to arrive.
Have engines for both cars currently with the race engine builder. He’s one of the few certified to build the sealed engines for the Trans Am series... (we in vintage are like the opening act for the TA series).
While not a ZR-1, I’ve learned a ton over the last 3 race seasons. Best of all, everything carries over to our cars. The Challenge Car is more ‘street stock’ and the WC GT is a whole different animal. Best advice for making a ZR-1 track ready is not to change anything as it is already very capable, rather make sure everything is operating correctly and bushings are fresh. It was developed on the track and provided to you with a track ready tuned suspension and braking system. Yes, it can be improved for the track, but you need to understand the impact of each change and ensure front and back are in sync. Same with brakes. There’s also a limit to how far you will want to go.... fast track cars make terrible street cars, and in road racing the engine is about 1/2 of the equation for having a fast car - chassis is the other.
Biggest bang for the buck I can recommend for the track is changing your alignment to provide negative camber, which will keep more of your tire surface on the track throughout the range of motion.
Currently planning to have one of the race cars at the Gathering this year, and may try to run it on the MSP for our track day. I’ll be leaving for a race immediately after the Gathering.


Sent from my iPhone using ZR-1 Net Registry

My expert opinion - It was probably the Master cylinder

[randy ransome]

Wow Mark, I know you're having the time of your life racing in the Challenge Series. That video gives a good perspective of the process. I saw all the smoke and kept thinking when is Mark going to decided it's time to get out of there. It looked like you were being patience, cruising around the track then making the pass. Any idea what made the crank give up?

I drive my Z every now and then, not as often since getting my 67, and every time I do I'm still amazed at it's ability being a street car.

I was at the gathering the year you brought your Challenge Car, Nice!
Hope to make it back to the Gathering soon.

Good Luck and Have Fun Racing this coming year.