Fact/Fiction- cooling

Some owners aparently have cooling issues, especially on S1 models. (Got to love that single blade fan!). Fortunately, as of now, I do not. However, when it comes to cooling, I have heard, on some cars, that people have increased coolant flow by installing a larger water pump, or installing a smaller pump pulley to increase the pump speed, thus flow. But then, some say that will not work, as the coolant may flow so fast that it is not in the radiator long enough to cool, to lose its heat, and the engine overheats. Can water flow too fast through a radiator?

Well another question, does a smaller pump pulley increase flow or does it cause cavitation?

GREAT question. You do NOT want cavitation at the water pump as it will chew up the aluminum timing chain cover very quickly! In this case I would trust the Jag engineers and keep the water pump RPMs where they designed them. The newer pump impellers that are supposed to be more efficient are likely OK as the outer impeller diameter is the same as the stock impeller and that is where cavitation would occur (at the point of highest velocity).

Well Jaguar went with a larger water pump - I think it was with the S2. But then they probably knew what they were doing. Of course it isn’t just the pump that’s larger, the timing cover also had to change to accommodate the larger impeller.

A few random comments: Increasing flow will also increase speed - the coolant must spend enough time in the block to pick up heat. One of the purposes of the thermostat is to reduce flow for that purpose. If you eliminated the thermostat entirely you would probably decrease cooling capacity. When I was racing I’d take out the center section of the thermostat but leave the outer ring to slow flow. A few years ago there was an article published in the JCNA’s Journal by an Australian engineer reporting on experiments he did in restricting flow through an XK engine by reducing the size of the outlet ports (into the intake manifold) to the point where their total open area equalled the maximum open area in the thermostat (he also staggered the individual size of the outlets to increase flow to the rear of the head and block). He reported that these changes dramatically improved cooling.

So it’s a bit more complicated than just increasing flow. The later XJ’s had much bigger radiators than E Types.

Basically, no. But one must consider this as a two-edged sword.

First, the lesser point of why more flow is detrimental. Fluid (coolant) flowing through pipes generates heat from friction. So increasing flow actually adds to the heat that the radiator is trying to remove. Moreover, it takes work (energy) to operate the pump. That extra work is provided by the engine, which, because it is inefficient, must generate more heat of its own. So faster coolant flow heats the coolant not only directly by friction but indirectly because the coolant is cooling an engine that generates heat in proportion to work done propelling the coolant.

Now the major point. [Remember that the radiator is supposed to remove heat from the engine, not the coolant.]

At low flow rate, the coolant spends time in the radiator and lots of heat is removed from that particular batch of coolant–theoretically it can drop from engine temp down to the ambient as the coolant moves slowly through the radiator. Then, it enters the engine, with plenty of time to reheat, and the cycle repeats.

At high flow rate, the coolant spends little time in the radiator and its temperature hardly decreases at all before it’s back in the engine. Then, it’s quickly back to engine temperature and the cycle repeats.

So you might first actually think slow is better, but if you think more it would seem that the two are about the same. In the fast flow case, it’s almost as if the engine is just extended to the radiator–the same temp in both, just as if the radiator fins were cooling fins on an air-cooled engine. In other words, there is cooling in the sense that the engine would get hotter without the fins, but you just don’t see any cooling of the water between the radiator’s inlet and outlet.

But here’s the subtle difference…the amount of cooling is proportional to the temperature difference between radiator and ambient air. A cold day cools better than a hot one. So the hottest part of the radiator cools most–more heat is removed. If you reduce coolant flow so that the coolant temp decreases significantly from top to bottom, the bottom of the radiator cools less, and the engine runs hotter. Increase coolant flow, and the bottom of the radiator cools more too.

So the only difference is that higher coolant flow lets the entire radiator cool maximally, not just the top. The engine (and average radiator temp) goes down compared to slower coolant flow. BUT, there is a practical limit–higher flow (even with properly designed pump etc) itself generates more heat. There’s a happy compromise, indicated by a certain ideal temp decrease from radiator inlet to outlet. IMHO.

Aside: There are vendors of muli-pass and the like radiators who advertise large temp drops from inlet to outlet. These radiators have more resistance to flow, so they slow the coolant, thus giving the larger drops. But if you look at their data, the drop is greater, but the inlet temp is up. The engine is running hotter, but the water in the radiator is cooler. You want to cool the engine, not the water that happens to be in the radiator.


Hi Gents,
The radiator tanks and side panels are wide enough to accept a larger 4 core. I had one installed soon after I got my Series 1 and have not had any over heating problems since.

Generally, S1 cooling is adequate while the car is moving along and only becomes a potential problem in slow moving traffic. Jaguar addressed perceived S1 cooling issues in the S1.5, subsequently in the S2 - same air intake and water pump and essentially the same radiator - by increasing air flow through the matrix via twin cooling fans, but even these were not optimal. Replace them with high cfm aftermarket fans mounted to the existing shroud and you should have no cooling issues. I certainly don’t, even with stock fans, but summer temperatures here in Niagara rarely reach into the 90s. I do find engine temperature will begin to creep up in stop-and-go traffic but I think that’s my original otter switch, which works when it wants to. I have a otter switch bypass set up that allows me to flip the fans on when things get slowed down.

If you “trust” JAGUAR Engineers, then can you explain why we have so many cooling problems that my Ford or Commodore don’t have?

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Yes Terry, done cooling mods to many engines, both 6 and V12 with great success.

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slightly, but not completely off topic but can you elaborate on advantages of more capacity (bigger rad) compared to more/better air flow? I have had improvements on other vehicles by simply allowing more air in and insuring it goes threw, not around the rad. better fan and shroud helps too but not always an option. obviously any would help.

I trust them on the water pump sizing and impeller speed (centrifugal pump impeller design was pretty well defined by 1960), not so much for overall cooling system design for use outside the UK. With all the supposed cooling issues with these cars (mostly due to improper maintenance) I’ve never heard anyone say “we need a faster water pump” with authority and the analysis to back it up.

Doug, then maybe you can explain why the V12 E water pump runs at a much lower speed than the same engine in the Saloons.

As Robert explained so well, heat transfer increases with temperature differential. If the water “stays in the radiator” a long time, it cools to ambient temperature and no heat transfer occurs. If it’s moving quickly, then it will be hotter as it exits, so more heat will transfer along the run. Faster is better. Don’t worry, a properly designed radiator has enough spare capacity so that you won’t ever be forcing super hot water back into the engine. Using a restrictor or gutted thermostat to “slow flow” actually has exactly the opposite effect. The restrictor creates a venturii, and well, look that up yourself. The restriction does increase local pressures in the head a tiny bit, which may have some marginal benefit.

The series 1’s suffered from poor air flow. My theory is that the electric fan was another example of a simple, elegant, inexpensive and obvious solution being utterly wrong. What I think happened is that they measured the airflow of a mechanical fan at idle, and replicated that flow with the electric fan. Conveniently, the windshield wiper motor they chose happened to draw about 8A, which was all they could spare with that little generator. The other desperately stupid idea is that “the fan doesn’t matter once the car is moving.” Unfortunately, it just worked, but was never enough for hot climates. So inadequate testing and observation confirmed their engineering biases. What’s the problem? SU’s tend to develop leaky spindles as they age, and it gets tough to hold low idle speeds. But the fan doesn’t care, it runs at a fixed speed. Eventually, problems with carburetors become problems with cooling because unlike a pump fan, airflow doesn’t increase with engine RPM. At low road speeds, it’s even worse. The long bonnet, small opening and messy plenum mean there’s minimal natural airflow until the car really gets moving. So the engine is working hard while the little fan isn’t getting much help from forward motion. It wasn’t that the concept was bad, but the execution was inadequate.

Adding an extra row to a radiator can help a bit, but the problem is that more rows means more airflow restriction. The itty bitty lawnmower fan doesn’t work well in the best of circumstances, but at higher static pressures it just mocks you. So adding rows isn’t a great idea unless you also do something about airflow. That said, a shiny new core will always work better than a 50 year old core.

Some of the compromises in the S1’s cooling system design arose from the choice of thermostat. Although wax thermostats were already in wide use by 1961, Jaguar stuck an obsolete bellows thermostat in the S1 3.8. I’m pretty sure that the thermostat was located at right angles to flow in an attempt to reduce lifting, perhaps allowing a more powerful pump. The pressure sensitivity of the bellows thermostats undoubtedly contributed to the car’s poor reputation for cooling. Subsequent replacement with waxstats likely accounts for the redesigned pumps of later engines.

As for strategic flow modifications, this is a very old idea for long block engines. Chrysler discovered in the 1930’s that adding an external pipe from the pump outlet to the rear of the block dramatically improved cooling for their straight 8’s. And in the Mercedes world, it’s been popular to replace the block drain with a hose fitting and run a line back from the thermostat housing. Probably a lot easier than modifying the head passages, although the XK architecture may not offer a good place to tap the pump.

Speeding the pump increases both flow AND the tendency to cavitate. A smaller pulley increases flow at low RPM’s but may well result in cavitation at the red line.


The cooling system on my S1 4.2 is all stock with the exception of a Coolcat multi bade fan and motor. No overheating, ever, in any conditions I have encountered including 105F ambients. (Driver overheating was another matter).

I should note that engine was rebuilt about 15k mile ago, so still no internal corrosion or silting to speak of.

Terry, according to thermodynamics, the larger the temperature difference between the two materials, the better the heat transfer. Thus cooler water within the block, which could be accomplished with faster flow would result in better cooling within the block. However, it would also reduce the temperature entering the radiator and reduce the temperature difference there. Since air water transfer is less efficient, you would rather have higher temps enter the radiator, thus arguing for slower overall flowrate

The more/better air flow issue is IMO virtually identical to the coolant flow issue.

More coolant flow transfers, via convection, more engine heat to the radiator so that ideally the engine and all of the radiator fins are isothermal (max possible radiator temp). That maximizes the temperature gradient between all of the fins and the ambient air.

Similarly, the job of air flow is to maintain this maximum gradient by assuring that all of the air presented to the fins (and to subsequently convected away) is at the minimum possible temperature–that of the ambient. Insufficient flow through all or some of the fins means that the air in contact with them heats up while passing through the radiator. So the temperature gradient at the rear of the radiator is less than at the front. As with coolant flow, using very big fans to pump the air through will consume power and generate heat, so there is a sweet spot for a given radiator.

The radiator capacity issue is above my pay grade. The idea here is to transfer (via conduction) coolant heat into the fins. Obviously, making the radiator bigger in square inches of fin area is generally good. Number of cores, use of aluminum, multi-flow, micro fin, turbulence, etc. are engineering concepts rather than simple physics. I think I understand the use of aluminum but none of the other stuff. IMHO.


Twenty to 30 years ago you could go to an auto wrecker and buy an electric fan off a GM car. They came with a self shrouded fan, and you could get them in various fan diameters, one of which was a perfect fit in a Ser 1 fiberglass shroud. They had small but very powerful motors that permitted you to make a simple bracket to bolt them to the picture frame bracket on the Ser 1 cars. They worked better than any aftermarket fan I used - but were single wire, permanent magnet motors which necessitated isolating them from the electrical circuits in the car, They ended any issues with overheating I ever had.

No. That’s an old wives’ tale.


My experience backs this up. I put in a brand new brass radiator that has more rows than stock and it did not help much. Next I switched from the stock fans to the Coolcat fans and it solved my hot running problems in Houston.

68 E-type FHC