Why V-Mount?
FD RX-7 has relatively small openings in the bumper to cool the radiator, intercooler, oil coolers, and feed the turbos with fresh air. Originally designed to cool 255hp, as you increase the power output of an FD, it quickly becomes more difficult to keep the temperatures in check when limited by a small amount of airflow. The 1999+ facelift increased the inlet size to the radiator to ~150 sq-in (25” x 6”) and oil coolers to ~120 sq-in (10” x 6”), but these openings are still pretty small compared to most performance cars.
When it comes to upgrading the intercooler of RX-7s, there are three options:
SMIC (Stock Mount Intercooler)
SMIC (Stock Mount Intercooler) is usually the easiest upgrade option because the radiator configuration is not touched. SMIC intercoolers have short charge pipes, share the inlet air from the front bumper opening with the radiator like the stock configuration, and allow the turbos to receive relatively fresh air from in front of the intercooler, which are positives. Quality SMICs have proper ducting (like this one) that takes the air from the narrow factory gap above the radiator and forces the air through the intercooler with ducting that is sealed to the face of the cooler.
The negatives of SMICs are the dimensions of the intercooler tend to be the smallest of the 3 options and have the most limited airflow to the intercooler, which limits the power potential. The biggest negative of the SMIC is that they are more prone to heat soak and overall cooling efficiency because the radiator ‘exhaust’ heat is directed directly into the SMIC, which need to cool the charge air to temperatures much lower than the coolant temp. The Pettit Racing Cool Charge III kit is probably the nicest SMIC on the market and would be my go-to choice for the majority of FD owners.
FMIC (Front Mount Intercoolers)
FMIC (Front Mount Intercoolers) are often the choice of big power builds for the street or drag racing because there is more room in the front bumper to fit large intercoolers. Placing the intercooler at the front of the car gives it the best airflow and cooling to make the most power. Prioritizing lower charge temps for 8-15 second pulls is worth the tradeoff for street cars and drag racing where sustaining cooler water temps for long periods of time is not as important. FMICs typically do not require any changes of the radiator and only require small cutting and clearancing of the front bumper support to fit the charge pipes through them, although some setups re-mount the radiator to be more vertical and feature zero ducting which often lead to water temp overheating issues.
However, there are some pretty big negatives of FMICs. The biggest issue is that they block airflow (and heat up the air) to the A/C condenser, radiator, and turbo. This often leads to higher water temps and overheating during continuous full-throttle conditions in standing mile competitions, hill climbs, and 20–30-minute sessions on road courses/road racing. There is a reason that not many track cars use FMICs on road courses. FMICs also typically have reduced air conditioning performance and overheating issues at very low speeds such as stop and go traffic where the only airflow in the system is created by the radiator fans. This makes them less than ideal for ‘street’ cars where the performance of A/C is important.
FMICs also require more fabrication and routing of long charge pipes, which can affect transient throttle response. The turbos often suck in superheated air exiting the radiator since it is difficult to feed the turbos ambient air because most of the inlet air is blocked by the intercooler, which is not good for power.
5 comments
Fantastic update and all the custom work on the cooling system is so cool to read about. I am patiently waiting to see this thing finally unleash some HP!
Technically, you should have a ‘blade’ that bifurcates upper, and lower flows at the start of the duct. If you modeled this in CFD, there would be obvious turbulence that could easily be avoided.
Cheers.
Was about to comment the same thing.
Also, the air out of the IC needs to go somewhere… I don’t remember what the hood looks like for this project, but a proper vent with gurney flap and ducting would do wonders at speed.
Can’t wait to read the next articles.
It’s not that crucial to bifurcate the ducting. Many morern OEM cars don’t separate the inlet ducting of a common duct/plenum that feeds multiple heat exchangers because airflow is not laminar in the ducting and the important aspect here is creating a pressure differential across the heat exchangers. Proper ducting that seals the air from the front bumper to the heat exchanger creates a high pressure area on the front side of heat exchangers in the V-mount setup (or “L”-mount in many OEM applications) when using a common plenum (or air box, or radiator ducting).
I am currently in my FD build and would love to have one of these radiators, could y’all please guide me in the right direction to purchase one if they are available. I left a message on the BJR site to no avail. HELP…..someone??