Project [Rotary] FD RX-7: Part 6 – V-Mount Cooling System (Overview and Intercooler)

Project [Rotary] FD RX-7: Part 6 – V-Mount Cooling System (Overview and Intercooler)

by M-P Spierer

The cooling system in the FD RX-7 is widely regarded as its most glaring weakness. This is partially due to the design of the rotary engine itself and partially due to issues specific to the design of this generation RX-7. We can't fix the flaws inherent to the rotary engine, but we can do something about those external to the engine. Most notable of these is the fact that many of the engine's cooling components are constructed out of plastic, the intercooler is wildly undersized, and the orientation of the radiator does not allow for proper airflow to the intercooler. In the next two installments of this project we tackle these issues by implementing a 100% custom v-mount cooling solution.

The first question you might be asking yourself is, “What is a V-mount cooling system?” Before we answer that question though, let's take a look at the stock cooling system configuration and its flaws. Unlike many turbo cars where the radiator and intercooler sit vertically one after another behind the front bumper, the stock FD RX-7's setup is different (I know, what's new?). In the RX-7 the radiator is angled forward while the intercooler is located above it and is fed by a small duct. The intercooler is also a tiny thing that consists of a 4.5×10.5×2.5″ tube/fin core fed by a pair of plastic end tanks.

 

A graphical representation of the stock cooling system orientation. Notice that the duct for the intercooler gets very little priority compared to the radiator, which is for all intents and purposes unducted.

Because the stock intercooler is so small, it really needs to be upgraded if any RX-7 (even a stock one) is going to see any track use. I have personally felt the power sapping effects of super high intake temps due to the stock intercooler within only a couple of laps at a short track. The main issue with upgrading the intercooler in the stock location is that there is barely enough room for one that is significantly larger and not enough room for the ducting that is required to feed such an intercooler. The best option is to flip the radiator so that the intercooler and radiator make a “V” shape…hence the name V-mount.

 

A V-mount radiator and intercooler orientation is fundamentally similar to the stock orientation, but allows for much better ducting of air to a larger intercooler without sacrificing flow to the radiator. Proper duct work is extremely important for a V-mount cooling system to function properly. 

Now, if you remember back to Part 1 of this project, there was a HKS v-mount intercooler system that was already installed on the car. Clearly this is not the first time we have had this thought. Originally the intent was to keep this system in place and only improve it by ducting it better. However, as time went on and the build progressed deeper into the world of uncompromised one-off custom fabricated components, we began thinking about how the cooling system could be improved even further and decided that a new ground up solution was in order. 

 

The old HKS V-mount intercooler setup that was previously installed on this project was nice, but there are several things about this design that we thought could be improved. Increase intercooler efficiency and reduce pressure drop by moving from a tube/fin to bar/plate core design, increase radiator cooling performance by changing the radiator angle and increasing cooling area, and increase both radiator and intercooler performance by implementing a true set of sealed ducts.
A significant amount of time was spent developing the RX-7's cooling system and nothing says this more than the fact that we created an entire 3D CAD model of it along side the cardboard mock-ups. Physical prototypes are still completely necessary when determining space claim for new components such as these, but there is something reassuring about having a CAD model to play with as well. It helps to further validate the design before wasting real dollars on scrapping parts because they didn't work exactly as intended.

2 comments

    1. Hope…yes. How much? Not a lot for something soon. Long story short, I moved 2 years ago and the car was in a storage unit in a different state until 2 months ago. Finally have it with me, but currently have very little time to work. The project is still alive, just no where near the top of the priority list at the moment. Keep checking back and you’ll see something eventually :).

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