What size radiator do I need for my car?

Lots of people; including Westfield; use the VW Polo MK2 1043cc (63.6ci) radiator (1983 – 1990), but is it big enough for a V8 and is there anything else I could do to improve cooling?

The Polo radiator fits inside a Westfield nose-cone almost perfectly. see link here. They can be bought new from under £35.
It seemed the first choice for many builders, be it bike engine or car. This should have given me some confidence, but I always feel happier to do a little research and a few sums.
The standard Polo radiator is made mostly of plastic and there must be something else out there?

Unfortunately, after trawling dozens of radiator websites, I found nothing of similar size.

VW Polo Radiator dimensions

I found the above dimensions of the Polo Radiator on the web and I wanted to test some sums I had from years back on a really tatty photocopy. It looks like it’s from a 1960’s / 1970’s American car magazine.

Equation for Calculating Car Radiator Size

Start with 2 cubic inches of core for every cubic inch of engine. Increase or decrease that value by the following factors:

ADD

  • 0.1 for a vertical flow radiator core
  • 0.1 for an in-line engine
  • 0.1 for a small trailer towing
  • 0.1 for a 2 row radiator
  • 0.1 for double evaporators
  • 0.2 for outside temperatures of 105°F (40.5°C)
  • 0.2 for a medium trailer towing
  • 0.2 for a small engine fitted to a heavy car
  • 0.2 for a radiator fan with diameter less than 90% of smallest dimension
  • 0.3 for air conditioning
  • 0.3 for no fan shroud
  • 0.3 for an antique car with small engine compartment
  • 0.4 for large trailer towing
  • 0.6 for a diesel engine

SUBTRACT

  • 0.1 for remote transmission cooler (not within radiator)
  • 0.1 for standard in-line transmission
  • 0.1 for a single row radiator
  • 0.1 for a V6 / V8 engine
  • 0.2 for a spacious pickup truck engine compartment
  • 0.2 for outside temperatures less than 90°F (32.2°C)
  • 0.2 for a full fan shroud
  • 02 for a horizontal flow radiator core
  • 0.3 for a large engine in a small car

VW Polo

I wanted to see if the equation matched what VW had fitted to their Polo.

Firstly, I calculated the cubic inches of the Polo Radiator.

38.4cm (15.1″) x 32.2cm(12.6″) x 3.3cm(1.3″) = 249ci

Then I used the above equation:

  • add 0.2 for a medium trailer towing
  • add 0.2 for outside temperatures of 105°F (40.5°C)
  • add 0.3 for no fan shroud
  • add 0.3 for air conditioning (not fitted in UK but possibly abroad)
  • subtract 0.2 for a horizontal flow radiator.
  • subtract 0.1 for a single row radiator

2.0 +0.2+0.2+0.3+0.3-0.2-0.1 = 2.7

Final equation : 2.7 x 63.65ci = 171ci
NB. The Polo radiator seems a little large (131%), possible because it is made of modern plastics not more heat conductive metal.

302 Mustang

I then looked at the published results the article had for a 302ci Mustang. The production Mustang is fitted with a 648ci radiator.

  • add 0.3 for air conditioning
  • add 0.2 for outside temperatures over 105°F (40.5°C)
  • subtract 0.2 for a horizontal flow radiator

2.0 +0.3+0.2-0.2 = 2.3

Final equation : 2.3 x 302 = 695ci
NB. The Mustang radiator seems a little small (8%).

XR4x4 Sierra

Finally, I also checked the sums with an XR4x4 Ford Sierra 170ci (2792cc). The production Sierra has a 600x368x33mm 445ci (7286cc) radiator.

  • add 0.4 for large trailer towing
  • add 0.3 for air conditioning
  • add 0.2 for outside temperatures over 105°F (40.5°C)
  • add 0.2 for a radiator fan with diameter less than 90% of smallest dimension
  • add 0.1 for an in-line engine
  • subtract 0.2 for a horizontal flow radiator
  • subtract 0.1 for single row radiator
  • subtract 0.1 for a V6/V8 engine

2.0 +0.4+0.3+0.2+0.2+0.1-0.2-0.1-0.1 = 2.6

Final equation : 2.6 x 170 = 442ci
NB. The equations seems very close to target (less than 1%)

Conclusions on equation

I concluded that either VW were being very cautious and the Mustang might be prone to over-heating or the equation could be somewhat inaccurate. Is a large trailer, big for the size of car or just a massive trailer?
The equation does however point out most of the factors that need to be considered when picking a radiator and for some cars (possibly mid sized e.g. Xr4x4) it may well be a bit more accurate.

I know there is much more science to it these days, with:

  • aero-dynamic modelling,
  • vehicle speed,
  • high efficiency fans,
  • turbos,
  • average engine revs,
  • mass air flow,
  • radiator material (steel, aluminium, brass, plastic)
  • construction (e.g. tube diameter)
  • and many more factors

I guess this is how they did it in the past?! Plus, I’m guessing when the equation was written in the 1960’s, town centre traffic was flowing freely, not stationary for hours on end like today.

The equation did point out that when fitting a radiator to a Haynes Roadster or indeed any Kit Car, there are probably going to be more factors to consider than with an upgrade to a production car.

One very obvious thing it highlights is, a full fan shroud has a huge effect on cooling and should be high on the list of cooling requirements.

Just for kicks, I thought I’d use the above factors and do some extra sums………………..

What cc Engine could a standard (33mm) Polo Radiator cope with?

I’ve used a standard Haynes Roadster configuration.

  • Electric Fan (12″)
  • No trailer
  • Temperature under 105°F (40.5°C)
  • add 0.3 No Fan shroud
  • add 0.1 For in-line engine
  • subtract 0.3 For large engine in small car
  • subtract 0.2 Horizontal flow radiator
  • subtract 0.1 Single Row Radiator
  • subtract 0.1 For standard transmission

249ci ÷ 1.7 = 146ci (2400cc) Engine

I just about have room (I think) to make a fan shroud. What difference would that make?

249ci ÷ 1.2 = 207ci (3400cc) Engine

NB. The above sums assume the plastic construction is as good as the metal counterparts the equation was designed for (which is unlikely). Personally, I would knock 30% off of these figures to be safe. (1680cc & 2380cc)

What cc Engine could an uprated 50mm Polo Radiator cope with?

38.4cm (15.1″) x 32.2cm(12.6″) x 5.0cm(2.0″) = 377ci (6182cc)

  • Electric Fan (12″)
  • No trailer
  • Temperature under 105°F (40.5°C)
  • add 0.1 2 Row Radiator
  • add 0.1 For in-line engine
  • subtract 0.3 For large engine in small car
  • subtract 0.2 Horizontal flow radiator
  • subtract 0.1 For standard transmission
  • subtract 0.1 For a V6/V8 Engine

No fan shroud:

  • add 0.3 No Fan shroud

377 ÷ 1.8 = 209ci (3425cc) Engine

With full fan shroud:

  • subtract 0.2 for full fan shroud

377 ÷ 1.3 = 290ci (4752cc) Engine

What cc Engine could a custom made 70mm Polo Radiator cope with?

38.4cm (15.1″) x 32.2cm(12.6″) x 7.0cm(2.75″) = 528ci (8655cc)

  • Electric Fan (12″)
  • No trailer
  • Temperature under 105°F (40.5°C)
  • add 0.1 3 Row Radiator
  • add 0.1 For in-line engine
  • subtract 0.3 For small engine in light car
  • subtract 0.2 Horizontal flow radiator
  • subtract 0.1 For standard transmission
  • subtract 0.1 For a V6/V8 Engine

No fan shroud:

  • add 0.3 No Fan shroud

528ci ÷ 1.8 = 293ci (4801cc) Engine

With full fan shroud:

  • subtract 0.2 for full fan shroud

528ci ÷ 1.3 = 406ci (6653cc) Engine

For My Car

Obviously, that old photocopy might be a load of old rubbish. But the author definitely knew more about the subject than me, so for now, I’ll trust him.

I’ll use an uprated Polo radiator from Fleabay, Aluminium if I can spare the extra £. I’ll also be making a fan shroud and fitting the best fan I can find.
The standard uprated units are under £200 whereas the Aluminium ones are £300.

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