Gen 3 Subaru STI

Project Gen 3 Subaru STI: Keeping it Cool with Koyo

By Mike Kojima

While awaiting the development of some ARK parts for our Project 3rd generation STI we decided to prep the cooling system in anticipation of the future installation of power parts.  Turbo cars run hot and when being track driven most have problems staying cool, especially on our Southern California tracks which are mostly located in the desert.

Koyo STI radiator
The Koyo radiator is high quality, tig welded, and all aluminum in construction.  The one piece construction can take way more pressure than the stock part. It is a high fin density double row design.

We turned to Koyo Cooling systems for one of their race radiators for our car.  The stock Subaru radiator is a single row heat exchanger with plastic tanks and an aluminum core.  The core is 16mm thick.

Koyo STI radiator vs Stock
 You can see the Koyo radiator's aluminum end tanks compared to the two piece crimped on plastic end tanks on the stock radiator.  You can also see the mounting pegs and the fan mounting tabs to fit the factory parts.

We replaced the stock radiator with Koyo part number VH13093.  The Koyo radiator is all aluminum in construction with tig welded end tanks.  It is a high fin density double row design 36mm thick, more than twice the thickness of the stock part.  The Koyo radiator had a ¼" NPT fitting near the inlet pipe for the mounting of a temperature sensor or a vent for a surge tank. The radiator was a direct bolt in and fit well with the factory fans exchanging perfectly.

Koyo STI Radiator vs Stock
The Koyo radiator is 36mm thick, more than twice the thickness of the stock part.  It has two rows to the stock single row for way more cooling area.

To test the radiator, we used an ARK MFD II to record the engine temperature at idle while warming up and during a loop we drove on the freeway to assess the cooling effectiveness.  We came up with the following data.

Read About our Reviews of the ARK MFD Here and Here!

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Der Bruce
Der Brucelink
Wednesday, October 27, 2010 7:44 AM
Excellent write up!

No matter how much the industry evolves some things are still the same. I remember my Dad upgrading radiators when I was a kid in the 80's. Whether hauling a big load, running down the 1320 or the twisties, bigger radiators and cooling equals longer engine life.

Now if the manufacturers learned to free horsepower and improve flow with e-fans, then how come no one is pushing for all accessory components to go electric? Electric fans, power steering, AC/heat. I keep hearing rumors that even Mazda is looking at a truly electric turbo? Less drag = more hp, more effeciency and more MPGs.

PS. Hate all the plastic incorporated into the latest models! Plastic fittings, manifolds, etc. :(>
Wednesday, October 27, 2010 9:02 PM
you should trim the upper radiator hose so that it is farther away from the alternator pulley. The motor can pitch forward far enough to cut the hose.
Friday, October 29, 2010 12:31 AM
I never get tired of saying good things about Koyo radiators. When I opened the box and saw the neat rows of welds, the really outstanding workmanship, I had to wonder how they could sell it even for $350-400, which while still a lot of money, I doubt breaks down into high $$$/hr. Fantastic product and results, hour sessions on hot days at the track running 9/10th's, never a problem.


Someone made a remark to me about didn't I regret the added weight in front with the coolant? Please, not when the car never overheats or gives me any problems, and maybe +4 lbs?
Wednesday, May 04, 2011 10:09 AM
Where has this project gone?
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