That is correct, although RB also admits cold air intakes are best for top end power. My original point was that on the run at 120 KPH (70 MPH) ambient 25 C or so, I measured the OEM intake temp at 47 C. Basically 20 to 25 above ambient. At slower speeds it goes up to about 60 C or higher.
The AJ6 plus torque mods, which I have and are very effective on my car, add (as well as other things) a second set of intakes with trunking from below the rad area just after the undertray trailing edge. These are meant to add some cold air as well as improve breathing. In fact I have recently discovered they actually add hotter air at speed than the under bonnet OEM intakes see! So I am going to shift the extra intakes out into the cold ambient air somehow.
Greg

 

This is completely at odds with the research TWR did. TWR added cold air intakes over the radiator. I don't understand how hot air can be good for economy when cold air makes more power you need less throttle to make the same power as hot air??????

 

My suggestion is to use a water manometer and test for the highest pressure area in the front. I will be doing this but have too many other projects to get done first.

I have seen people take cold air intakes from the inner guard, which is a low pressure area and will cause a vacuum.

 

TWR was concerned with full throttle power. When the HE was designed Jaguar was concerned about fuel economy in urban use and up to motorway speed, which in reality is only about 15-20% throttle

At low throttle, low rev situations ingesting hot air ensures the fuel is completely vapourized in the intake manifold and well mixed with the air. Due to the pressure drop across the throttle plate at small openings the air cools, and it cools again when the fuel is injected and evaporates. Hot air helps ensure there are no puddles of fuel that remain liquid and are not burned.

High load/high rev situations are different, there is much more turbulence and higher air speeds in the inlet tracts, that alone provides better air/fuel mixing. The cooling effect of pressure drop across the throttle plate is gone too, so the air will be inherently warmer that under the same conditions at idle.

TWR and Jaguar had different design objectives.

 

TWR did all the R&D for Jaguar on the 6.0L road car engine this is what I am referring to, NOT the race car.

 

Warrjon
JB is correct, I believe. For the V12 anyway, at part throttle, fuel is used more efficiently in the engine if the engine is inducing warmish air. This promotes better fuel evaporation, so fewer droplets condensing out on the intake runners, and fewer droplets in the cylinders and this better quality vapour to burn. Hence there is a part throttle fuel efficiency price to be paid for cold air intake better BHP at WOT.
(Whether, once a V12 installed in an XJS gets up to temperature, a cold air feed would end up with the air inside the airbox being reasonably warm, is another matter!) jaguar's own cool air intake effort on the facelift V12s, maybe indicates at slower speeds such cold air does get warmed up enough!
While it is true that TWR basically showed Jaguar that a 6 litre version could be achieved quite cheaply and could make a decent amount more power, it is also true to say that in developing their own pre-Jaguar 6 litre TWR engine conversion offering to customers, TWR were not in the least concerned about part throttle fuel efficiency, nor were their potential customers; they wanted maximum BHP!
This part throttle efficiency point is Roger Bywater's expressed view in his book "Engine technology for the modern world", which incidentally, is a rattling good read for car engine fans (mechanical or electric, ho ho).
Greg

 

Warren, your test results on cold air intake placements should be very interesting reading. Looking forward to it. Lowering overall temp is my priority over power, HP, etc weather it be inside the engine or under hood temp's. Of course I don't want to give up performance either since my 5.3 temp's are very good, even in Florida. With that said I do open the hood & put a box fan in front after use when possible of course.
Lawrence

 

I 100% agree it is far from obvious where cold air actually is, let alone its pressure, as my own temp measurements of my own "cold" air intakes showed all too clearly.
What I am going to do to solve this for my supplementary intakes is to use intake temps as a proxy. I am going to rig up various gash cold air intake positions until I find one that really does produce ambient air at the airbox across the sped range.
Greg

 

Greg,

Do you have a link to the device you're using that you could share?

 

No bother. You need two of these, as the ones they come with are not long enough to reach into the cabin:
5m Wire K Type Thermocouple Sensor Probe Test Temperature | eBay


and one of these:
Digital Thermometer 2 K-Type Temperature Thermocouple Sensor Probe | eBay


greg

 

It is a fascinating book, that must be where I read about cold or hot air intakes.

 

Padre, i lived in daytona for 30yrs,Smoky Yunick was a close friend, many tales of racing under the radar! RIP

 

No kidding!!

My favorite story is when they limited the size of the gas tank, Smokey just created an 11-foot 2-inch fuel line that held 5-gallons of gas. Voila! If it's not in the rules, it's not cheatin'.

I hope to hear some Smokey stories from you sometime.

Padre

 

Hi Greg,

I still don't buy it. And lest face it Jaguar were not interested in Fuel economy as much as meeting US emission regulations. Baywater can say what he wants he is selling products so his testing will be biased towards these, IMHO.

I have seen no conclusive back to back tests of COLD vs WARM air at part throttle that has the warm air fuel consumption better than cold air. At best the results are within the Uncertainty of the test method.

A well textured intake manifold and good squish and swirl (all of which the V12 has) are far more effective than warm air. Might have been more important in a pre-HE.

The throttle at part opening is too large an orifice and the pressure differential too low to provide any cooling effect.

 

Not so! Ever wondered why throttle bodies or intake manifolds on many cars have coolant going through them? If cold air is for power, why would any manufacturer heat the manifold?

It's to prevent icing and the throttle plate freezing shut. I've experienced that on an E Type, it was a cold day, only about 1C and raining. My throttle froze while sitting at a traffic light, took a good shove on the pedal to brake the ice and get the linkage to move.

I have a 1966 S Type with a 3.8 that I have converted to throttle body EFI, so there is a fuel injector sitting on top of the carb body in place of the piston and dome of the SU, but the throttle plate is as original. The engine bays on those small saloon gets very hot ( I have measured air intake temperatures of 85C), and on an idling engine fully warmed up the throttle bodies are cool to the touch.

It takes heat to evaporate fuel, that heat comes from the air. It's exactly the same principal as air conditioning, liquid changes state to a vapour and that pulls heat from the surrounding air, making it cold. It's basic thermodynamics. Ingesting hot air helps that evaporation process, ensuring the fuel completely turns to vapour and is mixed with the air. Liquid fuel doesn't burn, so it's important for both economy and emissions that it turn to vapour.

 

JB we will have to agree to disagree on this one.

 

Really fascinating discussion! And quite gentlemanly too.

I think you're both right on this one. The pressure differential in the throttle body has only a small effect on the intake charge temperature. And the v12 air pump pulling 1C air through an alluminum tube will transfer a lot of heat from the alluminum.

 

While I don't imagine that inadequate fuel vaporization or fuel condensation is a frequent issue for many XJS owners. Finns may disagree.

I'm in the pro cool air intakes camp but more philosophically than religiously. A temperature controlled flap valve to breath from behind the radiator on cold mornings and from a cold source when it's warmer is a tried and true simple solution that offers both.

 

A very interesting discussion. I hope to provide some further data on intake temps as soon as the bits of 38mm ID tube and hose I have ordered arrive.
I have measured that OEM intake temps at ambient 25C or so are plus about 20/22C C (ie 47 C) at speeds above 70 KPH


I am pretty sure that the supplementary intakes I have running from just below the front bottom of the rad to the airbox do not significantly heat the air, as the air temp I have measured at the airbox intake from these tubes is the same as I measured at the intake mouths themselves at speed. My problem was that these intake mouths were drawing hot air from behind the rad at speed, not ambient air from in front of it.


My next measurement will be to place the mouths right out in the airstream (ie ensuring ambient air only is entering the supplementary intake tube mouth) and measure the airbox supplementary intake temps then. I have already measured that if they are out in the airstream only ambient air gets into them.


I drove the car at ambient 18 degrees two days ago, and there was significantly more grunt and liveliness. The V12 LOVES cooler air, that is for sure. So I do believe (marginal fuel economy gains aside) that reducing intake temps does have a really noticeable effect on power. We shall see, and if it does, my next question after "where to site the supplementary intakes both safely and in the airstream" will be "do I do something about the OEM intake trumpets"...
Greg

 

It would be interesting if you could fit the 6.0 intake system and see if there is a measurable difference in fuel economy.

I don't have a direct comparison, as I used to have a Series III V12 sedan and I would typically average 21-22 mpg (imperial gallons) on the highway, and I have a 94 XJS convertible that will usually get 23-24 mpg on the highway. Not a like for like, as one is a 5.3 with the 3 speed and the other is a 6.0 with the 4 speed and overdrive/lockup.

I do notice the highway fuel economy is noticeably worse with the top down on the convertible than top up.

I was actually surprised the difference in fuel economy between the Series III and XJS wasn't greater, I would have though the transmission would have made a significant difference.