Why we got rid of our Radiator cap

For the last 10 years, we have always had issues keeping our Pikes Peak Subaru WRX cool.  We have tried a number of things to increase the cooling capacity of the car, and we have had some success, but the car always seemed to find a way to overheat.

Then we went to Super Lap Battle at Circuit of the Americas this year, and we couldn't even get a lap in before the car would overheat and push coolant out of the overflow.  We spent that weekend trying to troubleshoot the issue, and found a lot of odd issues (we have one video out on this all ready, and we are going to dive into the final results of the engine teardown here soon).  But what came from all of that is that a big part of our problem was likely to have been our radiator caps.

And as we looked into the differences between the 1-way and 2-way radiator caps, we also found that a down-flow radiator (which all turbo Subaru's have had here in the states) are also prone to cooling issues on track as well.

Everything is starting to fall into place and it is starting to look like simpler is better, and the old ways are better than the new ones.

So we have made some changes to our cooling system, and this video is a discussion about why we have removed one of the caps.

And here is a link to the Proessional Awesome article on cooling mentioned in the video.  It is well worth a read:

Professional Awesome Cooling Article

Thanks for watching and Stay Tuned!

Here is a summary of the key points we discuss in this Video:

The Issue with Subaru's Dual-Cap System [01:00]
Previous Discoveries: Turbocharged Subarus use a dual-cap cooling system. Using two 2-way caps (often the case when upgrading to aftermarket caps) causes severe issues with the system's ability to pull coolant back in and properly burp air out.

The Downflow Radiator Problem: Subarus (along with Evos and Hondas) use a "downflow" radiator design. In a downflow radiator, the hot coolant enters the top tank and flows downward through the core. Because the top tank is where the coolant enters from the engine, it is an inherently high-pressure zone.

Premature Opening: Because the top tank is a high-pressure zone, having a radiator cap located there makes the cap highly prone to opening prematurely under heavy load. In the Mitsubishi Evo community, this is a well-known issue, and their solution is to remove the cap from the radiator entirely and add an expansion tank.

The Single-Cap Solution [04:08]
Subaru's Built-In Advantage: Unlike other platforms that have to build an expansion tank system from scratch, turbocharged Subarus already have an expansion tank (located near the intake manifold) designed into the factory system.

Going Back to the 2002 WRX Design: We decided to remove the cap on the radiator completely and rely solely on the single cap on the expansion tank. This is not a radical new idea—in fact, it is exactly how the 2002-2003 WRX cooling system was designed from the factory before Subaru added the second cap to prevent the plastic end tanks from separating.

Benefits for Race Cars: By moving to a single-cap system, it becomes much easier to run a high-pressure cap (to raise the boiling point of the coolant/water) without dealing with the complexity and premature opening issues caused by having a second cap in a high-pressure zone on the radiator.

Implementation Details [07:06]
Don't Just Weld it Shut: When deleting the radiator cap on a modern aftermarket radiator, you shouldn't just weld the neck completely flat. The neck is the highest point on the radiator itself, and if you weld it completely shut, a pocket of air will get permanently trapped inside the top tank.

The Fix: Instead of welding it entirely shut, we welded an AN fitting (with a screw-on cap) in place of the radiator cap neck. This allows them to open the fitting to bleed air out while filling the system, but when tightened down, it creates a bulletproof seal that will not act as a pressure relief valve while out on the track.