GSpeed C7 Z06 Cooling Development
 

Over the past two months or so, we've been working with Operations's 2015 Z06 Z07 to solve the overheating problems that plague these cars. This thread will detail our development efforts as we narrow in on a solution.

Since we're located at the Motorsports Ranch in Cresson, TX, we've got the ideal thermal testing environment literally in our backyard. For those of you not familiar, OEMs come here on a regular basis in the late summer to stress test cooling systems on track in the 100°+ heat. Our goal for this car is to run a full HPDE session at Circuit of the Americas without overheating. For us (and many other racers) that means more than just not going into limp mode. The car will run with water temps of 235° or more, but we don't consider that acceptable performance for a track car.

As detailed in this thread here (https://www.corvetteforum.com/forums...r-results.html) the first step was a DeWitt radiator and oil cooler. While it made a difference, it was nowhere near enough to solve the problem.

To give a brief history, the car ran at COTA in Sept. 2015 in 100° weather. Water temps were north of 260° and oil temps were almost 300°. The car lasted 6 laps before it went into limp mode and came back in. That was with the DeWitt Gen1 radiator.

In Feb. 2016 in 70° weather, the car ran almost identical temps as the previous summer, but was able to go an extra two laps before coming in.

So while the DeWitt radiator is a step in the right direction, it's clearly not enough for a track car.

The first step in this phase was to install "cheek mounted" heat exchangers for the supercharger.

http://gspeed.com/wp-content/uploads...i8jcuv4f4y.jpg

This involved cutting up the bumper, so for our trials we bought a new bumper so as not to ruin Operations' until we had a final solution.

https://cimg0.ibsrv.net/gimg/www.cor...f97a2d8831.jpg


We knew we were facing a significant challenge. At the risk of oversimplifying the problem, this car simply cannot dissipate enough heat for the power it makes. "X" amount of air entering through the front can only dissipate "Y" amount of heat, regardless of heat exchanger efficiencies, packaging, etc. That fundamental theory is why we chose the LG Motorsports kit.

To attempt to understand the behaviors of the myriad heat exchangers in this car, we set up an Aim MXL2 with eight temperature sensors throughout the engine bay to log various fluid and air temperatures.

https://cimg3.ibsrv.net/gimg/www.cor...c95ad43e6d.jpg


We noticed a while back that the C7 Stingray has the same radiator (down to the GM part number) as the Z06. Doesn't really make sense, given the extra 200hp of the Z06, and is most likely part of the problem. So to increase the overall cooling capacity of the system, we added a second auxiliary radiator in front of the main radiator. This radiator measures 18" x 8.5". To monitor the performance of this radiator, we added water temp sensors at the engine inlet and outlet, as well as between the two radiators. We also are monitoring air temperatures in front of all the radiators, between the auxiliary radiator and main radiator, and behind the main radiator. With the two water temperature sensors monitoring the blower coolant temps, we filled up the 8 analog channels we had on the MXL2. It's not as thorough as an OEM's vast array of thermocouples, but we also have a small fraction of their R&D budget.

The eight sensors and extra heat exchangers were arranged like so:

https://cimg2.ibsrv.net/gimg/www.cor...29baca0838.jpg


We also fabricated and added a much larger reservoir above the intercooler pump to give it as much of a chance of success as possible, given the increased load. We're already worried about getting heat out of the system fast enough, we don't want to be fighting a cavitating pump. You can also see the two intercooler temperature sensor blocks in the lines just below the reservoir.

https://cimg0.ibsrv.net/gimg/www.cor...72bab96eb5.jpg


Once we got all of this installed, we went testing. We drove one session Thursday afternoon (6/30), but we were fighting the active handling and Ediff the whole time. We were never able to run a full lap "at speed," but we got some very useful data nevertheless. Here's what we learned.

https://cimg7.ibsrv.net/gimg/www.cor...9e050db3dc.jpg


Here's the engine coolant temps throughout the engine bay. The red line is coolant coming out of the engine, purple is between the two radiators, and blue is returning to the engine. You can kind of tell the temperature drop isn't split evenly between the two, especially considering the larger size of the main radiator. More on that in a sec.

https://cimg9.ibsrv.net/gimg/www.cor...7d22ff18d4.jpg


Here's blower coolant temps. Same color scheme, red is coming out of the engine, blue is returning.

What was really interesting, though, was the air temps.

https://cimg4.ibsrv.net/gimg/www.cor...d0052369d3.jpg


Blue is air coming in through the grill, purple is between the two radiators, and red is out the back of the main radiator.

To make things a little more easy to interpret, here's the water temp differential across all three. Calculated by subtracting outlet temp from inlet temp:

https://cimg4.ibsrv.net/gimg/www.cor...b2ed8df26f.jpg


And air temp differentials across the main and auxiliary radiators:

https://cimg1.ibsrv.net/gimg/www.cor...1bc9214152.jpg


Looking at those graphs, it's pretty clear the auxiliary radiator is hurting airflow through the main radiator. Although the overall cooling performance has increased, the total lack of air temp change across the main radiator tells us it's not doing near enough.

What's happening is the hot (235°) coolant is hitting the cool (95°) air coming in across the auxiliary radiator. When that happens, the air is heated up to about 180°, and the coolant is cooled to about 180°. The end result is zero heat transfer across the main radiator.

So for our next test day, we need to rethink how we're running coolant through the auxiliary radiator. It seems like the extra capacity is helping, but there's a lot of improvement left.

Jake

So for our next testing configuration, we decided to reverse the routing of the main coolant circuit. We would be reducing the heat transfer across the auxiliary radiator, but increasing it across the larger main radiator. In theory, that would net us more total cooling. Here's a diagram of how it was routed:

https://cimg2.ibsrv.net/gimg/www.cor...37719c1ed5.jpg


We went out and ran another session, and here's what we found:

https://cimg5.ibsrv.net/gimg/www.cor...aa8d5c7630.jpg


There's already a much more visible gap between the three coolant temperatures. That's a good sign that the main radiator is working harder now.

https://cimg7.ibsrv.net/gimg/www.cor...20bb49d7de.jpg


We found this odd anomaly in the blower temps, though. For some reason, blower inlet temperature measurements are spiking for around 4 minutes. We're not sure exactly what's happening since we're not logging mass flow rate or pump amperage, but we've got some things we're going to investigate. Blower temperatures overall seem fine, though.

https://cimg1.ibsrv.net/gimg/www.cor...deab377752.jpg


Airflow temperatures are spread much more evenly, and interestingly enough, the temperature leaving the main radiator is significantly higher than yesterday. We're now seeing temps well over 200° on the backside, when we were barely crossing 180°.

https://cimg6.ibsrv.net/gimg/www.cor...4c0a9d5e81.jpg


https://cimg9.ibsrv.net/gimg/www.cor...297cc5c784.jpg


Temperature deltas now look much more like what we'd expect to see. Simple temperature gradients aren't enough to totally characterize a heat exchanger, but in the absence of mass flow rate, etc. we're definitely more confident that the main radiator is working more efficiently than in the previous layout.

We ran the PDR this time, and here's what we got:

Max Water Temp- 250°F
Max Oil Temp- 302°F
Max Trans Oil Temp- 210°F

We attempted to log some ECU parameters with HP Tuners' VCM Scanner since the onboard PDR won't share anything more than a few specs, but the laptop came unplugged from the scanner during the session, so we didn't get anything.

Next, we'll try to re-arrange the heat exchangers to get a little more airflow across the important bits. We're making strides towards a solution, but there's several steps here. Step 1 is getting blower temps down. LG Motorsports already solved that with the big cheek coolers. Step 2 is now getting engine coolant temps down to keep it out of limp mode. Steps 3 & 4 will be doing the same with engine oil temps and transmission fluid temps.

Jake

For Saturday's test runs, we laid the auxiliary radiator down horizontally in front of the main radiator.

https://cimg7.ibsrv.net/gimg/www.cor...3aa30135dd.jpg


In this picture, you can see the radiator laying down, along with the super-high-tech cardboard vanes. :D

Here's a schematic of how we routed everything:

https://cimg0.ibsrv.net/gimg/www.cor...d1803f6fcc.jpg


First session of the day was at 10:30 on Saturday morning. We weren't able to get the second air temp sensor wired up in time, so we don't have temperatures off the back of the auxiliary radiator, but here's what we saw for water temps:

https://cimg5.ibsrv.net/gimg/www.cor...43001137f2.jpg


Our intercooler fluid gremlin is still present, but otherwise the blower coolant temps seem to be happy around 120°F.

https://cimg7.ibsrv.net/gimg/www.cor...07d5c916bd.jpg


Airflow temps are looking good, you can see ambient temps were about 90°F at the time.

https://cimg9.ibsrv.net/gimg/www.cor...2ffae76af3.jpg


Water temperature differential shows a pretty even split.

https://cimg0.ibsrv.net/gimg/www.cor...6737c009b5.jpg


Since we only had two air temp sensors on Saturday, we didn't get air temp differential across the aux radiator, but the main radiator is looking pretty strong, with over 100°F air temp change.

https://cimg5.ibsrv.net/gimg/www.cor...66e8945d2a.jpg


Here are the stats we got off the PDR:

Laps Completed: 6
Best Laptime: 1:21.8
Max Water Temp: 232°F
Max Oil Temp: 302°F
Max Trans Temp: 237°F

So water temperatures are now looking pretty good. There's a slight discrepancy between the PDR numbers and our Aim datalogger's numbers, which we'll be looking into further. Oil temperatures, though, are pretty concerning. So for the next session, we rigged up a second oil cooler in the front bumper just inboard of the left brake duct.

Here's the temperatures we saw:

https://cimg2.ibsrv.net/gimg/www.cor...0c20ef19cb.jpg


https://cimg8.ibsrv.net/gimg/www.cor...0302369e2d.jpg


https://cimg4.ibsrv.net/gimg/www.cor...bfede8c06f.jpg


You can see the air temps are much higher (100°F ambient) so everything is pretty thoroughly heat soaked.

https://cimg8.ibsrv.net/gimg/www.cor...f27e2e74e5.jpg


https://cimg2.ibsrv.net/gimg/www.cor...0baaf6b283.jpg


What's somewhat interesting is that as the ambient temperatures climb, the main radiator begins to bear more of the load. The intercooler is still pulling plenty of heat out of the blower, and the engine is pretty happy, overall.

From the PDR:

Laps Completed: 12
Best Laptime: 1:22.4
Max Water Temp: 237°F
Max Oil Temp: 302°F
Max Trans Temp: 275°F

We ran 12 laps at an aggressive pace in 100°+ weather, which is a massive improvement over stock. Water temps of 235°ish aren't great, but we'll accept it enough to focus on other problems. Like trans and oil temps. We're a little surprised our extra oil cooler didn't help anything, so we're going to revisit that this week as well. The trans cooler is underneath the auxiliary radiator, so it's no surprise it's not doing much good under there when everything gets hot.

At the end of the day, we put Operations in the car (our test driver had been at the wheel for previous tests) to log some data and enjoy the car.

https://cimg4.ibsrv.net/gimg/www.cor...1d73ff5afc.jpg


https://cimg7.ibsrv.net/gimg/www.cor...430b69787a.jpg


https://cimg8.ibsrv.net/gimg/www.cor...69a4ad1fd2.jpg


Overall, things are pretty happy for a supercharged car running in 100° weather. Engine coolant temps reach a pretty happy equilibrium at around 220°.

https://cimg1.ibsrv.net/gimg/www.cor...7433434bb3.jpg


https://cimg6.ibsrv.net/gimg/www.cor...a7f84a94ec.jpg


Temperature gradients look pretty similar to what we saw earlier in the day.

PDR Data:

Laps Completed: 6
Best Laptime: 1:23.8
Max Water Temp: 230°F
Max Oil Temp: 297°F
Max Trans Temp: 275°F

It's pretty clear transmission temps build up at anything over 6-8 laps. That's our next hurdle, getting trans and oil temps back down to reasonable/survivable track car levels.

Jake

We had focused these days on coolant temp, with an eye on oil temp. We will continue this week improving coolant with a focus on oil temps and trans temps now that the car will run long enough to heat up the other systems.

Louis

Wow, this testing is like a dream come true. I feel like I'm getting access to a paid subscription.

Need to soak more in before comments.

The second radiator for the engine is sandwiched with the main correct? Not like GM'second solution?

Because this is an A8 car, we could not mount the aux radiator like the GM setup. But wait, there's more on that to come ;)

We're validating everything, and a statement with out fact based data is just an opinion. We have data here. And a lot of it. No opinions needed.

Louis

Sub'd

Very interested in this piece. Am I interpreting it correctly that the aux radiator brings down coolant temps only 7 to 8 degrees on average? Since it heated up the air and blocked the flow a bit, by the time the coolant makes it's way through the main radiator the aux radiator lowered the overall coolant temps by much less than 7 to 8 degrees correct?

I ask this because in my regression analysis I get that GM's second radiator lowered temps by only 4*. Of course I have mine horizontally so there is less airflow going through it. I wouldn't get 7 to 8* coolant reduction where I have it mounted. Ron Fellows claimed 14* improvement. I think that's a pipe dream since both our results show it only does less than half of that.

You guys are using the GM aux core correct?

We are using another coolant core, bigger than the GM core, and twice as thick.

Post # 3 will explain everything, and you'll be pleasantly surprised, IMO.

Louis

:lurk:

Great stuff you guys. Thanks for doing this.

Alright, post #3 is up with Saturday's three sessions of data. Punchline is water temps are down to "normal" Corvette levels (although still not acceptable), but trans and oil temps are high. Transmission can stay cool for 3-4 laps at a time, but once we started running 12 laps at once, it would cross 300°F.

Tomorrow we're going to wire the MXL2 up to the OBD port since the HP Tuners software wasn't as much help as we thought. That'll give us more information on engine oil temp during each lap, as well as IATs, etc.

Jake

Subscribed for the "finality"

Out of curiosity, what are you guys gonna try to get oil and trans temps down?

Our target temps are:

Water 220 or below
Oil 245 or below
Trans 240 or below

We have the next revision complete, hope to have the AIM setup and a session in before lunch tomorrow.


Louis

Is this the same track you did 1:18 on stock with Super Sports Lou?

Edited to ask Lou directly lol

Also, it would be great to have the PDR info as well. I want to know whether this track is more like Thill 2 mile or the 3 mile. I overheat on one in 10 minutes but can't break 250* coolant on the shorter one. Since you guys don't have comparisons with a stock car, it's hard to tell what is what. The only solution is to understand the track environment.

Really enjoying the process and progress. Where does more hood vents come into play? Cars like the Viper have options for some really aggressive hood cutouts. While I'm sure it won't solve the problem alone and I have no clue about the aero consequences, improved exit ventilation of the confined engine bay seems like it could be beneficial as well.

It seems there aren't that many places for air to exit the engine compartment looking from below. You can try to force more air through the front fascia, but if air can't exit you're not going to get much more forced in.

Ok, I'll go back to being a physician now :).


https://cimg1.ibsrv.net/gimg/www.cor...60d93dee23.jpg

Nice work.


Curious to know how you calibrated all your temperature sensors, boiling water at 212F? How close do they all read to each other, as you are measuring really small temperature deltas. A 4 degree delta is very small in terms of the overall scale and needs a really tight calibration setup. As you mentioned, you have a difference between PDR data and data from your AIM logger, and in one graph f post #3 it looks like you have coolant temp below 225 F yet PDR max is 237?

Yep, you're right there's not much room for air evacuation under there. We're not optimistic running extra hood vents would help, mainly since we ran this car without a hood at all and didn't notice an improvement. The cheek-mounted intercoolers exhaust into the wheel wells, btw.

These sensors weren't calibrated individually on this car, no. We've used these sensors (Bosch NTC) and logger before in other setups with good results. It may be a little cavalier from a professional intrumentation standpoint, but we felt we could trust the sensors enough to get started with some big-picture numbers. Like, a reading of 230 is not necessarily that different than a reading of 235, but definitely significant over a reading of 250 degrees. Like you said, that takes a pretty precise calibration that we may not have due to sensor manufacturing variances, pullup resistor variances, etc. etc. Things it sounds like you're already aware of. :D

That being said, calibrating in boiling water is definitely on our to-do list for this week. Not only for the Bosch/AIM sensors, but the OEM water temp sensor as well. We'll be posting those results in this build thread as we go. Hope that answered your question.

Jake

We have the PDR info, and no problem sharing it. This project has taken 2 engineers, 2 fabricators, and a technician several weeks to organize, plan, and execute. This post is the culmination of weeks of hard work from our whole team.

The track is a bull ring, honestly. We see high temps here, we don't see elsewhere. That's ok, because Jake mentioned this track is used by teams and OEMs to test on. The Viper ACR has the street car lap record here, to give you an idea, posted and executed by SRT.

We removed the hood completely, and it did nothing for temperature. There is something to be had with a proper duct, we think. If we can solve it with out a hood, it would be most beneficial for the end user, as this cost can easily spiral out of control.



We noticed this too. We ordered an Ecu temp sensor, and pigtail to test. Our sensor is a known Bosch sensor, and reads correct at ambient. Hopefully we have time to boil both sensors tomorrow, as there is a 15-22* difference between the two, and quite frankly, we won't leave any stone unturned.