Now that we have outlined what the plan is for the RX-7 radiator and intercooler, it's time to get to work and start designing the intercooler. We used cardboard, plywood, a 3D printer, liberal amounts of duct tape, and anything else we had lying around to do the initial mock-ups.
Once the general layout was known, it was time to select an intercooler core. For this we chose to go with a bar and plate unit from Garrett. Yes, the industry's most renowned turbocharger company also makes some of the best flowing and most thermally efficient intercooler cores in the business. All Garrett cores are hand made using bar and plate construction from aerospace grade aluminum alloy.
We started the core selection process by checking out Garrett's intercooler size chart on there website. On this chart there is advertised horsepower rating, overall dimensions, and weight for each core Garrett offers. Since space was the limiting factor in this build, we went with the biggest possible “frontal area” core we could fit to maximize heat rejection. When considering that end tanks would add a minimum of 3″ to each side of the core and that we had to fit a 4″ intake on one side of the intercooler, we ended up choosing a core that was 18″ wide and 12.1″ tall. The thickness choice was a bit more complicated and we'll get into that in a second, but first let's take a look at the core.
Back to core thickness which, as stated earlier, isn't always as straight forward as reading a horsepower rating chart and selecting a core based purely on horsepower requirements. Some people believe that a thicker intercooler core will cool better than a thinner core with the same cold-flow area, but I challenge those to go to Garrett's website (after finishing reading this article of course) and comparing two cores with the same width and height, but different thicknesses (say 3.5″ and 4.5″ thick). What you will find is that the two cores are rated for nearly the same horsepower. You might ask, how could this be and why would anyone choose the thicker, heavier core over the thinner one if both meet the horsepower requirements? The answer is pressure drop.
Pressure drop is the difference in pressure between the inlet and outlet of the intercooler. It is the easiest and most reliable way to tell if an intercooler is too small (in terms of hot-flow area). The reason that the two cores in the example above have similar horsepower ratings is because adding thickness to a core only helps thermal efficiency to a point, then suffers the wrath that is the law of diminishing returns. The real benefit of a thicker core is that it does not restrict air flow as much as a thinner core which in turn means that the turbo doesn't have to work as hard to produce the desired manifold pressure.
Again we'll get back to the intercooler physics discussion, but first let's take a look at the differences between a “budget” bar and plate intercooler and the Garrett unit and get a better idea of what we are talking about. The pictures below mostly speak for themselves, but it is a bit startling how much better the Garrett core looks.
Okay, now let's finish up the intercooler physics discussion by taking a real world example of why pressure drop should be minimized. If we take a turbo car running at the track with a 90F turbo inlet air temperature and 1 bar of boost (manifold gauge pressure). Let's assume that there is 1psi of pressure drop across the intercooler and 1psi of pressure drop through the rest of the system. Let's also assume that the turbo is operating with a compressor efficiency of 72%. This would result in a turbo compressor outlet temperature of ~273F (yes, that hot). Now let's take all the same numbers except increase the intercooler pressure drop to 4psi (a very undersized intercooler) and assume compressor efficiency didn't decrease (unlikely). This would result in a compressor outlet temperature of ~298F. So the best case scenario is that the undersized intercooler has to compensate for an extra 25F compared to an intercooler that is sized properly. In short, it's good to remember that there are other things that need to be considered when choosing an intercooler core beyond heat transfer alone.
Now this intercooler core is undoubtedly the focal point of this portion of the project, but there are many components that are required to utilize a core like our Garrett unit. We need end tanks, tubing, couplings, and clamps. We also need to mount the damn thing and do a ton of fabrication to make it happen.