Radiator Installation Design Guide
There are a number of basic rules to designing a radiator installation which, if followed, will considerably increase the chances of your installation working properly. These rules seem to be either unknown or ignored by a lot of people coming up with a custom radiator installation for their engine conversion. They are just as applicable to kit cars, hot rods, intercoolers, oil coolers, etc., - i.e. all 'cooling packs'.
Many of the worst examples often seem to be on radiator installations for water cooled engine conversions in previously air cooled Volkswagens. Air cooled VW's present their own problems for radiator installations, particularly the following:
- The lack of a factory fitted radiator location, or suitable place into which one can easily be installed. Radiators often seem to be installed in a 'minimum effort' kind of way, rather than one with any real engineering intent behind it.
- Many owners’ desire to keep the bodywork looking standard (i.e. air cooled) from the outside.
- Power increase is often one of the key reasons for doing an engine conversion. More power equals more heat to dissipate.
All of these result in the radiator installations in previously air cooled VW's tending to be in non ideal locations, which, in turn, means that everything possible should be done to improve their efficiency if they are to work effectively.
Basic Rules of Cooling Pack Installation Design:
- Heat exchanger size - that's size in terms of heat dissipation ability, not dimensions. Because many conversions will require the use of a radiator other than the one designed for the engine (for dimensional reasons, etc.), you should be looking to use a radiator capable of dissipating more heat then actually needed. It’s far better to install a radiator which is ‘too big’ rather than ‘too small’.
Without access to manufacturers information on how much heat a heat exchanger can dissipate, it is very difficult to tell which will be the most suitable. A good guide is core overall volume. Calculate the volume of the radiator core fitted the the engine you are using by the manufacturer. Multiply the length, width, and height of the radiator core (i.e. the bit with the fins - don't include the header tanks). Compare this measurement with that from other radiators which may be better suited to your installation. As long as the radiator is of the same type of construction (almost all modern ones are vacuum brazed aluminium), the core volume is a good guide to heat dissipation capacity.
If your radiator is going in a non ideal location (i.e. not at the front, with adequate air flow, etc.), you should look to get one with a larger core volume than the one designed for the engine. How much bigger depends on how well designed the rest of your installation is. For a well designed installation under a VW bus, 1.5 to 2 times the core volume would probably be a good start.
Increasing radiator core volume by increasing the the frontal area is considerably more effective than doing so by increasing the thickness. The thicker a radiator gets, the less effective the rear part of it gets. It's an inverse squared, or 'law of diminishing returns' effect, due to the air flowing through it progressively getting hotter, and therefore cooling less effectively, the further it flows through the radiator. Also the extra drag caused by the increased thickness will reduce the total air flow This means that increasing the volume of a radiator core will be considerably more effective if done by increasing the frontal area, as opposed the thickness. It's also why modern car radiators tend to be very thin, with a large frontal area. Recent Subaru ones are only 15mm thick.
- Ducting - a radiator which is blasted with air is quite resistant to the air actually flowing through it. Given the choice, the air would much rather just flow around the radiator. To make sure as much air goes through the radiator as possible, ducting is required. This is particularly important when the radiator is installed in a non ideal location. All the air which flows into the duct has to go through the radiator to get out. Ducting doesn't have to be in the form of a vent or scoop. It could just be a large closed panel with the radiator fitted (and fairly well sealed) into a hole. High pressure will build in front of the panel due to vehicle motion, and the lower pressure behind it will result in flow through the radiator. This method is typically used on the front mounted radiators of most modern cars. The radiator is typically mounted in the front crossmember / bulkhead, behind the bumper. In terms of the cooling system the bumper is just trim really, with plenty of holes in. The forward motion of the vehicle creates positive pressure in front of the bulkhead, so air flows through the radiator into the lower pressure engine compartment. This is why the electric cooling fans only ever cut in on production cars when stationary, or moving very slowly - the lack of forward motion of the car prevents air flowing through the radiator. This kind of 'flat panel' radiator duct is analogous to the 'infinite baffle' loudspeaker cabinet idea.
Radiator ducts work best when they are shaped to give a smooth transition between the inlet area and the radiator core area. The inlet should be smaller than the radiator core area - about 1/4 of the area works best. The increase in area along the duct promotes turbulent air flow, which is more efficient at removing heat from the radiator then laminar flow.
Most production car radiators are designed to be mounted vertically, and rarely have any ducting to direct cooling air to them. Those which are mounted at an angle to the available air flow such as the Lotus Elise or Toyota MR2 have ducts to redirect the air flow.
- Anti Recirculation - badly designed radiator installations allow air from the hot side of the radiator to flow back around to the cool side, and be heated further. This continues, and the system gets hotter and hotter. The radiator mounted in a large flat panel mentioned above prevents this recirculation of hot air. Recirculation is usually more of as problem at low speed, as the forward motion does not create enough air flow. Cooling packs for static applications, or slow moving vehicles have an enormous amount of effort put into making sure that the hot air can't get back to the cold side easily, but it's just as applicable to a road car stuck in traffic. For race cars, it's probably not too important a consideration, and for radiator installations in non ideal locations, the ducting required to get air to flow through the radiator will often also provide sufficient anti recirculation baffling.
- Coolant Air Temperature / Availability - although it sounds obvious, a radiator installation needs to have plenty of cold air flow available to it at all speeds. An unducted radiator mounted behind the engine will both have it’s flow heated and restricted by the engine. In terms of air flow direction at different speeds, you can learn a lot about flow direction by taping short pieces of wool all over the are you’re thinking of, and driving at different speeds. this is the technique which Porsche and Jaguar used in the 1950’s, before they had access to wind tunnels, etc.
If you want to see how much excess cooling capacity a radiator installation has, progressively block off more and more of it, driving it in between in the maximum ambient temperature you’re likely to see, and look out for temperature increases. If the temperature starts to rise when you have blocked off 1/3 of the radiator blocked off, you have 50% extra cooling capacity then needed.
Each of these points is discussed in more detail below:
- Heat Exchanger Size
Rad core size
Rad core type
coolant air temp
coolant air flow
mesh flow reduction
Rad can't be too big???
excess cooling capacity - card over ra