Annex: Cooling System Capacity Calculation#
This annex documents the simplified calculations used to verify that the installed intercooler, radiator and cooling fans are appropriately sized for the current engine output.
The calculations are intentionally approximate. Their purpose is not to establish absolute component limits, but to provide a conservative engineering sanity check based on the actual dimensions and installation of the cooling system.
No manufacturer horsepower ratings are used.
Assumptions#
| Parameter | Value used | Rationale |
|---|---|---|
| Engine output | 273 hp / 204 kW | Current measured engine output used as the design point for the calculation. |
| Intercooler dimensions | 550 × 180 × 65 mm | Physical dimensions of the installed aluminium intercooler. |
| Radiator dimensions | 625 × 348 × 40 mm | Physical dimensions of the installed aluminium radiator. |
| Cooling fans | 2 × 12" SPAL | Installed fan configuration. |
| Fan free-air rating | 1,221 CFM each | Manufacturer airflow figure for each fan under minimal restriction. This value is not used directly as the installed airflow. |
| Estimated real installed fan airflow | ~1,800 CFM combined | The two fans are rated at 2,442 CFM combined in free air, but the intercooler, A/C condenser, radiator, shroud and bodywork all create resistance. A lower installed value is therefore used as a realistic working estimate. |
| Coolant heat load | ~65% of engine brake power | A simplified estimate for the fraction of engine mechanical output that the cooling system may need to reject as heat under sustained high load. It is intentionally conservative and is used as an engineering approximation rather than an exact engine heat balance. |
| Intercooler charge-air velocity | ~20–25 m/s | Used as a reasonable average internal flow velocity for a road-car air-to-air intercooler. The aim is to avoid assuming unrealistically high flow speeds that would imply excessive pressure drop. |
| Charge-air density | ~2.0–2.2 kg/m³ | Approximate density for compressed intake air under boost. The exact value depends on boost pressure and temperature, so a range is used rather than a single figure. |
| Effective intercooler flow | Below geometric maximum | The nominal cross-section cannot be treated as completely open because internal bars, fins and passages reduce the effective flow area and create pressure losses. |
| Airflow-to-power relationship | ~1 kg/s ≈ 1,200 hp | Used as a simplified conversion between intake-air mass flow and approximate petrol-engine power. It is a broad rule of thumb for estimating capacity rather than a precise engine model. |
| Radiator heat-rejection range | ~150–180 kW | Estimated from the radiator’s frontal area, 40 mm core thickness, aluminium construction and expected automotive airflow. The range is deliberately broad because detailed core data and measured airflow are unavailable. |
| Intercooler estimated capacity | ~350 hp | Chosen from the lower part of the airflow-based theoretical range to retain a conservative margin and avoid treating the upper theoretical value as a realistic limit. |
| Radiator estimated capacity | ~300–320 hp | Deliberately lower than the theoretical equivalent of 150–180 kW because the radiator sits behind the A/C condenser and its lower section also receives air that has already passed through the intercooler. |
Note: These values are not intended to model the cooling system with laboratory accuracy. Where exact data is unavailable, conservative assumptions are used so that the final capacity estimates remain realistic rather than optimistic.
The heat exchangers are arranged so that the intercooler sits at the front and occupies approximately the lower half of the cooling package:
| Section | Airflow path |
|---|---|
| Upper | Ambient air → A/C condenser → Radiator |
| Lower | Ambient air → Intercooler → A/C condenser → Radiator |
The intercooler receives ambient air directly, while only the lower portion of the radiator receives air pre-heated by the intercooler.
Intercooler capacity#
The piston/core geometry and airflow assumptions above give the following chain of results:
| Step | Calculation | Result |
|---|---|---|
| Core volume | \(0.550 \times 0.180 \times 0.065\) | 6.44 litres |
| Charge-air flow section | 117 cm² | 0.0117 m² |
| Volumetric flow at 20 m/s | \(0.0117 \times 20\) | 0.234 m³/s |
| Volumetric flow at 25 m/s | \(0.0117 \times 25\) | 0.293 m³/s |
| Theoretical air mass flow (density 2.0–2.2 kg/m³) | \(\dot V \times \rho\) | 0.47–0.64 kg/s |
| Usable range (allowing for internal core restriction) | conservative derating | 0.28–0.38 kg/s |
| Theoretical power range (1 kg/s ≈ 1,200 hp) | — | 336–456 hp |
| Estimated intercooler capacity (conservative, lower end of range) | — | ≈350 hp |
| Estimated intercooler margin | \((350-273)/273\times100\) | ≈28% |
Radiator capacity#
| Step | Calculation | Result |
|---|---|---|
| Frontal area | \(0.625\times0.348\) | 0.218 m² |
| Core volume | \(0.625\times0.348\times0.040\) | 8.7 litres |
| Current engine power | 273 hp | ≈204 kW |
| Required coolant heat rejection (~65% of brake power) | \(204\times0.65\) | ≈133 kW |
| Estimated radiator heat-rejection capability | high-quality 40 mm aluminium core | 150–180 kW |
| Equivalent engine power at 150 kW | \(150/0.65\) | ≈310 hp |
| Equivalent engine power at 180 kW (favourable conditions, not a practical limit) | \(180/0.65\) | ≈371 hp |
| Estimated radiator capacity (derated: sits behind A/C condenser and intercooler) | — | ≈300–320 hp |
| Estimated radiator margin | — | ≈10–17% |
Fan airflow#
| Step | Calculation | Result |
|---|---|---|
| Combined free-air rating | \(2\times1{,}221\) | 2,442 CFM |
| Estimated real installed airflow (accounting for restriction) | conservative derating | ~1,800 CFM |
| Converted to SI units | — | ≈0.85 m³/s |
| Radiator face velocity | \(0.85/0.218\) | ≈3.9 m/s |
The aluminium shroud covers almost the full radiator surface, so this airflow is distributed across most of the core rather than only through the circular fan areas.
Summary#
| Component | Current demand | Conservative estimated capacity | Margin |
|---|---|---|---|
| Intercooler | 273 hp | ~350 hp | ~28% |
| Radiator | 273 hp | ~300–320 hp | ~10–17% |
| Radiator thermal load | ~133 kW | ~150–180 kW | ~13–35% |
| Installed fan airflow | — | ~1,800 CFM | — |
| Radiator face velocity | — | ~3.9 m/s | — |
Conclusion#
The calculations indicate that both the intercooler and radiator are suitably sized for the current 273 hp engine output.
The intercooler has the larger capacity margin, while the radiator operates closer to its conservative estimated limit.
The radiator is helped by the actual physical arrangement of the cooling package: the intercooler occupies only the lower section, so the upper part of the radiator receives air through the A/C condenser without first passing through the intercooler.
At low road speeds, the twin SPAL fans and full-width shroud provide substantial airflow. At higher speeds, cooling performance increasingly depends on the aerodynamic flow through the front of the car.
These values are approximate engineering estimates and should not be interpreted as hard component limits.