Keeping a rotary engine cool is one of the keys for making them reliable. We designed one of the largest radiators ever made for an FD RX-7, keep it cool with GC Cooling Fans, and worked with Hayden Automotive to cool the cars’ power steering.
After spending years looking at the cooling systems of FD RX-7s from around the world, ranging from street cars to track cars, wheel-to-wheel race cars, hill-climb cars, and time-attack cars, it became apparent that most RX-7 owners/builders do not have a strong understanding of the importance of ducting or cooling system design. We also came to the conclusion that the off-the-shelf radiator and oil cooling solutions would not cut it for our build. This is somewhat ironic because of the challenge and importance of keeping the temps of a rotary engine under control. Because of this, we decided to reach out to our motorsport contacts to design and manufacture a much larger and more efficient radiator for the FD RX-7.
The Billy Johnson Racing 56mm FD RX-7 Dual-Core, Dual-Pass, Cross-Flow Radiator
The 56mm Billy Johnson Racing (BJR) FD RX-7 Radiator is a cross-flow, dual-core, dual-pass heat exchanger designed to maximize cooling by fitting the largest radiator possible within the FD RX-7’s frame rails in a V-mount configuration using the 99+ front bumper and an RX-8 A/C condenser.
The BJR FD RX-7 Radiator is proudly Made In America and manufactured from raw materials in Michigan by a fin-and-tube plant with over 100 years of combined experience in the heat exchanger industry. With manufacturing tolerances of .002” for a precise tube fitment, and using a thick, .080” double-sided clad for the headers (unlike most companies that use a single-sided clad), these cores are some of the strongest in the industry, rated to handle over one million pressure cycles.
This cladding is an aluminum layer that is bonded to the core aluminum material that creates a fillet joint during the brazing process around both sides of the tube connection to the header, which doubles the strength of the joint and is where most radiator failures tend to occur. Having a manufacturer who builds their own fin and tube cores from scratch with an in-house CAB brazing furnace gave us the flexibility to specify the RX-7 core in any dimension with multiple tube widths, fin density, and design options.
By comparison, most radiators on the market are made by companies who buy pre-made cores from the same handful of overseas suppliers and then weld end-tanks on and put their name on it. There are far more radiator companies than there are fin and tube plant manufacturers (who actually make cores).
With an overall width of 27.5”, the BJR radiator is 2–inches wider than the typical Koyo radiator. To further maximize surface area and cooling, changing from the original vertical flow radiator to a cross-flow configuration, increased surface area by an impressive 20% over the commonly used Koyo radiator!
The core dimensions of the BJR radiator are 22.5” x 16” x 2.2”, which is ~40% more capacity than almost every radiator on the market for the RX-7. Additionally, the 56mm thick BJR radiator is a dual-core with two 1-inch tubes, which is 17% thicker than the more common 48mm single core/single tube radiators.
A cross-flow configuration was chosen for many reasons to improves efficiency. The RX-8 A/C condenser is also a crossflow heat exchanger, and the attached dryer tube overlaps the end tank and does not block airflow through the radiator like it does when used with a vertical flow radiator. We mounted the RX-8 AC condenser directly to the radiator with four brackets.
With a cross-flow radiator (right), airflow from the front bumper through the cooler is maximized without having a big, shiny bottom end tank of a vertical flow blocking a portion of the air entering the grille (left). The coolant path, packaging, and routing of the coolant lines is cleaner in a dual-pass, cross-flow radiator configuration for V-mount setups than vertical flow.
4 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).