My radiator is tilted at 55 degrees which he indicated was fine. He had a rule-of-thumb angle at which he said you needed to be very careful and do CFD and wind tunnel testing to ensure good mass air flow. I don't recall the angle, but I remember thinking I've only seen those types of angles on race cars.
The following equation is helpful:
I can't figure out how to enter Greek letters, so this will need to suffice:
- A: Area of radiator core
- DT: Delta T, the temperature difference between the coolant and the air flowing through the radiator
- K: Radiator heat conduction coefficient
- M: mass flow rate of air through the radiator
- Q: The amount of rejected heat
The outlet increase vs. the inlet is driven by how much the radiator, condenser, etc. increase the temperature of the air (i.e., DT). If you know that number, it's simple to calculate (with another simple formula) how much the air has expanded and what the optional exit size would be. However, that will only be optimal at one DT and engine and ambient temp are anything but constant. So long as the angle of the radiator doesn't have a big effect on mass airflow, the angle doesn't change the ratio (it will affect drag and perhaps downforce).
Before I engaged the aero guy, I bought an expensive custom radiator from C&R. It's slightly larger than the stock one (bigger A) and it has a thick high-end core so its K is significantly higher. So everything should be great right? Well, when he got the pressure drop across the core data below from C&R for the thick core + condenser + fan shroud and did a CFD analysis, he didn't like the mass airflow number (M). He suggested a thinner core, but I'm not going to do that unless I have a problem. C&R is top notch and I took their recommendation not to mention that CFD isn't a perfect science -- there's a reason engineers still use expensive wind tunnels! He spent some time designing a molded "flick" for the leading edge of the outlet (does the same thing as a Gurney Flap, but looks better), optimizing the location of the outlet, etc. Time will tell, but I'm pretty sure I'll be OK with the thick core.
55mm core
Inlet | Inlet Pressure | Pressure | Mean Core | Ambient | Air |
Temperature | At Core Face | Drop Across Core | Face Velocity | Mass Flow Rate | Density |
(°C) | (mbar) | (Pa) | (m/s) | (kg/s) | (kg/m³) |
35.0 | 1.10 | 110 | 2.00 | 0.473 | 1.15 |
35.0 | 2.89 | 289 | 4.00 | 0.945 | 1.15 |
35.0 | 5.35 | 535 | 6.00 | 1.418 | 1.15 |
35.0 | 8.45 | 845 | 8.00 | 1.891 | 1.15 |
35.0 | 12.22 | 1222 | 10.00 | 2.364 | 1.15 |
42mm core
Inlet | Inlet Pressure | Pressure | Mean Core | Ambient | Air |
Temperature | At Core Face | Drop Across Core | Face Velocity | Mass Flow Rate | Density |
(°C) | (mbar) | (Pa) | (m/s) | (kg/s) | (kg/m³) |
35.0 | 0.91 | 91 | 2.00 | 0.473 | 1.15 |
35.0 | 2.39 | 239 | 4.00 | 0.945 | 1.15 |
35.0 | 4.41 | 441 | 6.00 | 1.418 | 1.15 |
35.0 | 6.97 | 697 | 8.00 | 1.891 | 1.15 |
35.0 | 10.07 | 1007 | 10.00 | 2.364 | 1.15 |
So to answer your question regarding core thickness... if all things are equal, a thicker core will increase K and decrease M. You'd only want the thicker core if the increase in K was greater than the decrease in M (i.e., Q is larger) and you're OK with the increased drag. Since Q is larger the air will expand more and you'd ideally have a slightly larger outlet.
The F1 guys spend the time and money to test and determine what all of the numbers are. C&R sent data above which helped with the mass air flow modeling, but they were unable to provide radiator heat conduction coefficient (K), You'd probably need to test your exact radiator to get that number. I couldn't find anyone at Vintage Air that even understood the same questions regarding their condenser. Worst yet, I wasn't able to provide any data on how much heat the engine generates which means that I couldn't specify the problem I was trying to optimize. This means that most of us are going to have to rely on rule-of-thumb calculations. My aero guy did his thesis on F1 radiator ducts and then spent 18 years doing aero design and wind tunnel testing for McLaren, Honda and Mercedes. He had a list of questions that I couldn't answer so despite spending the money on CFD analysis there is still a fair amount of rule-of-thumb.
Interesting a stock SL-C has a radiator core that's 45.7% smaller than the inlet which is pretty close to the 40% rule of thumb. My custom radiator is 41.7%, but that was luck. At the time I ordered it I didn't know about the 40% rule-of-thumb and was just trying to maximize A. The stock radiator "outlet" is a disaster, but there are a lot of street SL-C's with the stock setup and no issues. Net-net you can look at similar cars and see what's working for them.