DDMWorks Coolant Re-route

DDMWorks Slingshot Coolant Re-route

Backround -
The Slingshot has been a lot of fun to develop product for and since introducing the first supercharger kit for the Slingshot, we continue to strive for the most reliable forced induction setups possible. While development on the supercharger kit was progressing we learned a lot about engine failures that had been caused by aftermarket turbo kits installed on Slingshots. Unfortunately, there was not a lot of discussion about these engine failures on the forums, but it was something that we wanted to learn about to make sure that we would not see the same failures with the supercharger kits. After working hard to find information for several months, we were able to track down many pictures of engine failures and started to see a pattern to some of the failures. We just want to say thank you to those people that sent us pictures of their engine failures as a lot of this knowledge would not have been possible without their help.
Although the Slingshot is new, the engine used in it has been around for a long time and we have been working with the 2.4L Ecotec since 2007 in the Pontiac Solstice and Saturn Sky. In those applications we typically do not see the engine failure rate, at the power levels that engines had failed on the Slingshot. This failure rate had us intrigued and we wanted to see what we could figure out as to why there was a difference between the Solstice/Sky 2.4L and the Slingshot failure rate at similar horsepower levels. After looking over some of the pictures, talking to several ownders and physically inspecting actual engines that failed, we came up with 2 main reasons that appear to be causing the majority of the failures.

The first failure type we have seen appears to be caused by high engine load at low RPM. This kind of condition is usually caused by some of the turbo kits creating high boost early in the RPM range. This high load at low RPM, if not controlled properly, especially in the higher gears (4th or 5th gear) can cause some serious damage from detonation. When in the higher gears, this high load situation exists for a longer period of time since engine acceleration in those gears is relatively slow. Typically this type of damage has shown up in cylinders 1 or 4, what we have seen is broken connecting rods, severely damaged blocks, etc. Although the damage may be found in cylinder 2 and/or 3 also, most of the pictures show it in cylinder 1 or 4, with cylinder #4 being the most common. There are reasons for this type of damage to cylinders 1 and 4, but I will not get into that on this thread.
The second failure type that we have seen is the failure of cylinder 2 and/or 3 from high/excessive heat build-up. To understand this failure we first need to understand some of the things that happen when a turbo or supercharger is hooked onto the engine and how a cooling system works.

The Problem -
The goal of installing a turbo or supercharger on the engine is to increase the mass of air entering the engine, then with proper fueling and ignition timing, the cylinder temperature and pressure is increased, which creates higher torque on the crank and increases horsepower. Gasoline engines though are not that efficient and only roughly 1/3 of the heat energy from combustion is converted to mechanical energy for power, 1/3 is converted to sound and exhaust heat and 1/3 is put into the cooling system as heat. So as we increase the power, we also increase the load on the cooling system also. That extra heat needs to be transported away by the cooling system efficiently and expelled by the radiator to the atmosphere to keep the engine from overheating. With a turbo, higher than stock exhaust backpressure also keeps more heat in the cylinders and head, making it even more important to keep the cooling system as efficient as possible.
First, lets look at the way that the cooling system is ran on the Slingshot. There is a lower hose coming from the bottom of the radiator that comes into the back of the engine block and once the thermostat is open, coolant flows into the water pump and then the engine. That coolant enters the block and then is pushed up through passages into the cylinder head. After getting up into the cylinder head the coolant exits out the top and back to the upper radiator where it releases heat to the atmosphere and the cycle starts again.

Within the engine cooling system though, the coolant is not a constant temperature. As the coolant travels in from the back of the engine it starts to increase in temperature. As it works its way up from the bottom of the block into the cylinder head, its temperature increases and is the hottest when it is just exiting the cylinder head. This means that not as much heat is taken away from the cylinder head and upper cylinders as is taken away in the lower engine block by the coolant. Typically, the hottest surfaces are those adjacent to the combustion chamber at the top of the block, exhaust valve area and the spark plug seats in the cylinder head.
When the coolant comes in contact with these hot metal surfaces of the engine, if the coolant temp gets hot enough, it will boil and change to steam. Then because of the pressure in the cooling system, the gas bubbles that are created will be pushed from the localized boiling spot and carry with it the heat. If everything is working correctly, as the steam bubbles move away, coolant replaces them and the cycle keeps repeating until the localized heat is decreased. There are several different important phase changes though that the coolant can go through at these hot spots -

Convection phase -
This is the most common phase and where most people drive, most of the time. Very low load situations like idle and cruise and light throttle, no boiling occurs at all, and the movement of heat into the coolant is by free convection or by forced convection caused by the water pump pressure. In the cooling system, the majority of the heat transferred from the cylinder bores to the coolant occurs by natural and forced convection currents where the heat flow in the metal is relatively low.

Nucleate boiling phase - https://youtu.be/LSR-n2kDlVI
As engine loads and speeds increase, the rate of heat flow through the engine and cylinder head increases until steam bubbles are formed in certain regions and on the surface of the water jacket. These areas are typically around the exhaust valve area, spark plug seat area and top of the cylinder liner. Nucleate boiling involves the nucleation and growth of the vapor bubbles that originate at the sites of the higher temperatures. In this phase, large numbers of bubbles form on the hot surfaces and travel through the bulk of the coolant, later condensing as they move to lower temperature regions. This condition is seen on short and longer full throttle runs.

Unstable film boiling phase -
Under severe engine loads and speed, the vapor bubbles start to become so large and numerous that the liquid has difficulty in flowing back to the hot metal surface. When this point is reached the hot surface of the water jacket suddenly becomes insulated by the steam bubbles, which join together to form a thin film. This film insulates the water jacket from the coolant and the temperature of the metal surface increases dramatically. This rapid local increase in temperature though will not be shown on a temperature gauge though, since the coolant is also being insulated from picking up heat from the metal, the coolant temperature indicated on a gauge will not increase. Since the heat can not get out of the cylinders as well, everything in the combustion chamber starts to get hotter. As the temperature in the cylinders increase, several things start to happen. The pistons start to increase in temperature, that heat soaks into the piston and is transferred to the compression rings. As the rings get hotter, they expand and the ends of the compression rings eventually touch. Once the rings touch, if the heat continues, there is damage to the pistons typically causing a piece of the piston to come off and subsequent failure of the piston. This kind of piston failure is one of the types of failures that we have seen in numerous pictures sent to us here.
There is actually a very important part of the coolant system that we did not mention earlier though, it is designed to help eliminate this problem, the steam line system. The steam line system is made of small coolant hoses that comes off of the front of the engine at the highest point in the system and on the 2015-2016 also comes off the top of the radiator on the passenger side. The job of the steam lines is to help relieve any steam production and keep it from becoming a problem. The steam lines route the steam out of the engine and into the surge tank located on the firewall on the Slingshot. When the steam gets into the surge tank it can rapidly expand, decrease in temperature and condense back to coolant and then return to the system. When this system is working properly, it helps prevent steam buildup in the engine. However, in the Slingshot there is an issue in the way that the stock steam lines are ran that prevent this system from working optimally.

Looking at the way that the coolant system is ran in the Slingshot, if you look at smaller steam line at the front of the cylinder head, you will see in the stock configuration, that stock steam line comes out and then has to travel downward along the front of the engine. This steam line travels downward about 2" vertically, it makes the turn and travels upward toward the surge tank at the firewall. Now, compare the Slingshot steam line routing to steam lines in any other factory GM applications and there is an important difference. The steam line on other Ecotec applications always travels uphill the entire way to the surge tank. Steam does not like to travel downward as it is always trying to find the highest point in the system, so the stock Slingshot steam line does not allow optimal flow of steam out of the engine.

Testing and Solution -

After looking at it, we thought the downward travel of the steam line might be a possible source of a decrease of flow of steam out of the cylinder head contributing to some of these engine failures we have seen pictures of, so we set up a simple experiment. We cut the stock steam line on our engine and installed a section of clear tubing and ran the engine at 2500RPM. This was not a high load, but we figured that if we could see any difference here, it would be even worse under heavier loads. What we found even at these lower load conditions was the steam line in the stock location was full of air and did not allow coolant to flow freely. We then simply changed the steam line to our coolant re-route and under the same conditions it was very easy to see difference. With the coolant re-route, the coolant could be seen easily traveling through the tube, along with small bubbles that would have been trapped in the stock configuration.
The coolant re-route is a simple change of the steam line from going along the front of the engine and traveling downward, to a line that travels along the side of engine toward the firewall and directly into the surge tank.
We are recommending to customers that are looking for another way to increase reliability on forced induction engines to take a look at the coolant re-route. The kit is relatively inexpensive, and is easy to install. All of our surge tanks already have a provision on them to install the coolant re-route for an even simpler installation. It also can be done without our kit with parts that are available at most autoparts stores.
We know that there are going to be plenty of people that have not/will not have a problem with the stock steam line configuration even on a turbo, etc. A lot of this will depend on how the vehicle is driven, boost levels, ambient conditions, etc. Also, this discussion is applicable only really for forced induction engines and will not really be applicable to naturally aspirated engines since those engines will not see the higher loads and temperatures of the turbo and supercharger engines.

However after a lot of testing and documenting the differences, we do feel that this is something that can help prevent some of the engine failures that we have seen pictures of here. Hopefully the coolant re-route will allow everyone to enjoy their Slingshot even more with higher reliability and higher horsepower levels. This coolant re-route does not mean Polaris did a bad job designing the coolant system on the Slingshot. A naturally aspirated setup as designed and delivered by Polaris would most likely never see any issues because of the lower heat produced in the stock configuration.

I know that was a lot to read, and if you have any questions, please let me know.
Dave

If you would like to purchase a kit to do the coolant re-route we have a kit that allows anyone with any year Slingshot do the install available here -

DDMWorks Slingshot Coolant Re-route

Quote from funinthesun

Just one question. Does running the Evans waterless coolant help in any way, or will this problem still exist?

The evans waterless coolant does help with this also.

Regular coolant boils at around 265F when under pressure in the coiling system, Evans has a boiling point of 375F which under most conditions should keep it in its liquid state.

Quote from Pascal

Dave, question?
Do you still run the steam line from the top of radaitor back to surge tank? In your pic with new Hose on driver side I don't see the other hose.

On the 2015-2016 models that have that line from the radiator you do keep it. The 2016.5 and newer models do not have that line, so you just run the single steam line from the cylinder head to the surge tank.

Quote from Pascal

Ok thanks Dave, would their be any advantage or disadvantage to plugging it?

Since Polaris is not running the line from the radiator to the surge tank on the 2016.5 models, I would think you would be ok without that line in place. That line is there to get any air/steam out of the radiator, but since the upper radiator hose should allow any substantial build up to go up to the steam line, I would think you should be fine without it and is probably the reason that Polaris stopped it on the 2016.5 models.