Wednesday, May 16, 2018

Keeping it Cool - Radiator Upgrade for the CMS STi

As part of the last round of upgrades to our very own 2005 STi, we needed to address the stock radiator and fans. While still doing its job just fine, it was 13 years old and essentially a ticking time bomb. The factory Subaru radiators use plastic end tanks mated to the aluminum core, and over time the plastic gets brittle and cracks, especially when subjected to high heat.  Well, our car is "high heat", and in order to ensure our STi was ready for a week of hard driving in the mountains, the stock radiator needed to go.  We decided to put a new CSF aluminum radiator in it's place.  Why?  Read on to find out!

Our STi has done very well over the years in a mixed bag of uses - autocross, track days, and street driving.  It has also been used to run through the NC mountains once or twice a year over the last decade or so.  It has evolved over time and has received a lot of upgrades to coincide with it's intended use.  The last couple of years has seen it be more and more focused towards a twisty road/mountain weapon. While track days and autocross are fun (and still on the list of activities), focusing a car for either of those disciplines usually means compromising it for just about everything else. Instead, modding the car towards a fun street driven vehicle allows it to be streetable, but still be competitive enough for autocross use, or up to the task of a track event. It was with this plan in mind that the recent round of upgrades was undertaken, in preparation for a week-long trip to the Appalachians.

As previously mentioned the stock radiator uses plastic end-tanks and a single aluminum core. Which works fine for most stock STis that see street use.  Up the horsepower, and the associated heat however, and the stock radiator can quickly become overwhelmed.  This is especially true when subjecting the car to punishment at a track day event or repeated runs through the mountains.  The extra heat load can be too much and the cars coolant temps will start to rise, this can actually cause the radiator to fail, or it cause other components in the engine bay to become compromised. Somehow our stock radiator had survived the last 13 years of abuse, and it was still adequately cooling the car under most circumstances.  However, we didn't want to chance the stock unit holding up a whole lot longer, and a busted radiator in the backwoods of North Carolina was not on our list of "fun things to do next week". It was time to swap it out for something with more cooling capacity and better durability.

We opted for an all aluminum "racing radiator" model from CSF Cooling.  It's an all aluminum welded design, so no plastic end-tanks to split, and it's double the size and cooling capacity of the factory radiator. This model radiator utilizes a 2-row 42mm core that features CSF’s exclusive B-tube technology, as well as an ultra-efficient 6.5mm multi-louvered fin for maximum surface area contact and heat dissipation - So what does all of that mean exactly?  Well according to CSF - "Unlike a regular oval shape “O” type radiator tube, CSF uses a specially engineered tube in a shape of a “B”. These “B-tubes” are carefully formed and then brazed over the seam to seal. CSF is able to use thinner and lighter aluminum material (better cooling efficiency) because this design is actually stronger than normal “O” shape tubes that are welded. The design (inlet in the middle of tube that is seam brazed) increases the heat transfer surface area of the tubes by approximately 15% over regular tubes. You get the efficiency of 2 smaller tubes vs. 1 large tube within the same space criteria. With “B-tubes” you are able to get “dual liquid laminar flow.”  Basically, you get more cooling within the same space/envelope as a standard tube design.  Cool!

It comes packaged well!

We received our radiator and immediately unboxed it to check it out. It comes equipped with an all-aluminum racing style drain plug for quick and reliable fluid changes, a CSF 1.4 Bar (20 psi) radiator cap, and new hardware to mount a fan shroud. The radiator is hand-polished, is very well presented, and is also well packaged (often overlooked lately).  We set about getting it ready to drop into our STi.  First step was to drain the coolant and remove our stock radiator.  Next up we removed the factory fans and then bolted them to the CSF radiator.  Everything lined up perfectly and the new hardware CSF sent was a nice touch.  Installing the new radiator and fans back into the car to check for fitment revealed what we figured - we had very little clearance between our header and the factory plastic fan shroud.  This is common, as the CSF radiator is about twice as thick as the factory radiator, and pushes the fan shroud back towards the engine.  We should note that it all fit well and it could be run this way, and there would be plenty of clearance with a factory exhaust manifold. We wanted to have some extra clearance though, as we have seen plastic fan shrouds melt from being in close proximity to a very hot header (we are running a Tomei UEL setup). So the whole assembly came out and we set to work with a solution.

Factory fans bolt right up with zero issues.

Our solution was to do away with the bulky OEM fan shroud assemblies and install an aluminum fan shroud with high-flow SPAL fans.  The aluminum shroud and the SPAL fans will result in a more compact package, which provides extra clearance around the header tubes. Aluminum also won't melt under the same conditions the plastic one would.  Another advantage is that the SPAL fans we chose are very high-flow units, and they move a ton of air compared to the factory units.  Win-win on this one.  The aluminum fan shroud again bolted right up the CSF radiator with no issues.  On top of that we installed the SPAL fans, and then wired them in using OEM connectors, along with our own relays. The SPAL fans draw a lot more current than the factory fans, so to play it safe and not blow fuses all the time, we wire the fans in with relays mounted to the frame rail.  

SPAL fans being mounted to our aluminum fan shroud.

The CSF radiator/shroud/SPAL fan combo slid into place with minimal fuss, and cleared the header as planned! The factory overflow bottle bolts up to the radiator shroud and although we have some pretty tight clearance between the bottle and the timing cover, it works. With tight clearance in some spots a few zip ties and some rubber trim keeps things happy and not rubbing together. We installed our old Perrin radiator shroud in place and filled the system with coolant. The cap CSF sends is about the same pressure rating as the factory cap that goes onto the radiator itself, so its safe to use the factory cap on the coolant reservoir (pressures will match OEM specs). The radiator looks good in the car, the polished aluminum adding a nice contrast to the drab engine bay (it's dirty, we know). With everything together we burped the coolant system and then took the car for a drive, no issues!

Fast forward a couple weeks and we covered about 2,500 miles during our road trip up to NC and back.  5 out of the 8 days were hard driving, up and down mountain passes, high rpm, and lots of boost.  We had zero issues with our new cooling setup, and we saw lower average temps.  We also had comfort in knowing that not only did we have the extra cooling capacity of the CSF radiator, but we also did not have a ticking time bomb of an old factory radiator up front!

Dragon Touge Runs!

Circuit Motorsports is a performance auto shop located in Orlando Florida.  We specialize in performance upgrades, ECU tuning, and fabrication.  If you are interested in performance upgrades or a full build consultation contact us today!

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