The idea behind using a Honda Civic radiator and a Ford Probe intercooler is to mount them side-by-side in place of the original Miata radiator so that the intercooler doesn't block any airflow to the radiator. It will not only maximize airflow, but also only require a few feet of intercooler piping with only a few bends, resulting in an intercooled setup without the lag of conventional around-the-radiator piping. There are some drawbacks though. First is the physical size of the radiators. The Honda radiator is about half the size of the stock Miata radiator. You will also need to consider the coolant volume lost by swapping the radiators. Sure it may sound easy, but I can assure you it's not. A mounting bracket needed to be made for both the radiator and intercooler, then after that you need to pipe the radiator inlet and outlet as well as the intercooler inlet and outlet. It took a lot of work experimenting with different setups, and I ultimately decided against it do to the possible overheating. I didn't want to risk it with Hawaii weather. For other folks who don't see 90+ temps, this may be a possibility.
Here are the two main components of the setup. A 92-95 Honda Civic radiator and a Ford Probe intercooler. The radiator is an aluminum core that I was able to get for cheap with the fan, and used only for mock-up purposes. If this build were to ultimately follow through, an all aluminum dual-core radiator will be required to help compensate for the loss in volume and cooling efficiency. This intercooler could also be found in the Mazda MX6, and is a popular upgrade for the 323 GTX crowd. I can't remember the exact size, but the inlet and outlets are an awkward 1.75''.
The first step would be to remove the stock radiator, which involves removing the undertray, draining the coolant, disconnecting the inlet and outlet hoses, and unbolting the 2 fans from it. You can see it here sitting in front of my house with the two fans next to it.
Armed with my trusty riveter and some 1'' angle aluminum from my local Home Depot, I set forth to fill that empty void that the stock radiator left. I started off by making a frame that bolted to the sheet metal panels near where the stock radiator brackets fit. Some drilling was required to make holes for the 2 bolts for each side of the frame. Once the frame was bolted in, I could start devising a lower mount for the radiator and intercooler. I'd recommend boxing the frame to prevent the horizontal section from twisting. Other than that, it looked pretty stable.
The Civic radiator has two 0.5'' diameter pegs on the bottom left and right so that it would sit in the mount using only the two pegs as guides. The top of the radiator has only one peg in the very middle. The plan was to sandwich the radiator using two lower brackets and an upper bracket and use some rubber spacers as vibration dampers. I got to work and cut two smaller pieces of angle aluminum to drill and rivet to the lower frame. The radiator sat nice and comfortably in the two tabs. The lower mount for the intercooler is a different story. The intercooler has a flange for the upper mount while having a wierd metal web thing on the bottom that I had to somehow work with. I ended up
cutting off some of the bottom of the flange so it would mount at the same height as the radiator. Because of the fairly strong upper mount flange, I decided to only put 1 bolt at the bottom.
Here's the couple attached to the lower mount. They look good right there. There's no upper mounts yet, that's the next step. Notice the placement of the radiator inlet as well as the intercooler inlet and outlet. The radiator inlet is near the same spot as the stock radiator, but a little bit smaller, so piping it will be rather difficult. The radiator outlet, well, I won't get to that, I didn't get that far until I decided to go with the under-the-radiator setup, this setup was not for me, not with the high Hawaii temperatures. The intercooler inlet and outlet will most likely need to be cut down for clearance issues. Piping that isn't going to be a walk in the park, but don't get me wrong, a few (I've heard of two, and seen pictures of only one) have this setup working on their car.