XJR Normal IAT value?

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irishjag
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XJR Normal IAT value?

Postby irishjag » Mon Jan 08, 2018 10:46 pm

I had my code reader plugged into the car and was looking at the live data stream information for the engine.
The value of the 'iat' which I assume to be 'inlet air temperature' was reading 31 deg C rising to 45 deg C after a few minutes of the engine at idle. As the outside air temp here in Wexford and in my garage is about 3 deg C this evening this seems wrong or is the iat something different? If this is the IAT as measured as part of the MAF downstream of the air filter then the readings look incorrect.
Any comments welcome
All the best
Chris
2003 XJR
1992 XJS

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Re: XJR Normal IAT value?

Postby J44EAG » Tue Jan 09, 2018 12:58 am

I`ll try to clarify that, Chris.

There are two IAT sensors working on the car at the same time. One is at the MAF sensor and the other below the inter-cooler bricks as I recall, screwed into the off side cylinder head in one of the head inlet tracts.

The inlet air charge passes through the trunking, along the expansion plenum, rockets through the throttle body, does a 90 degree turn under the cowhorn pressure manifold and enters the supercharger at the back end. Until it hits the charger lobes, a partial vac depression is in force. Once into the charger lobes a pump action occurs and the air then becomes slightly pressurised. It exits the charger, goes up the cowhorn pressure manifold does a 180 degree downward turn before passing through the air to "water" (coolant with antifreeze) inter-cooler heat exchangers and then goes down the cylinder head inlet tracts to the inlet valves. A rise in air pressure within a confined vessel will be equal and undiminished in all directions. It will also be accompanied by a rise in temperature. In the case of the engine at working temperature, heat soak will also cause a rise in air temperature.

At full chat and wide open throttle, air pressure and also air temperature from the supercharger obviously increases dramatically. Temps at the cowhorn manifold can reach anything up to 160 degrees. Passing that heated air through the air to water inter-cooler bricks should get temps back down to a more acceptable maximum of around 90 degrees. Inter-cooling is essential because if 160 degree air were to be fed to the cylinder head inlets, valves and combustion chambers, spontaneous and uncontrolled detonation (pinking) would occur and melt holes in pistons and around the valves. To prevent that occurrence, inputs from the IAT, MAf, MAP, CHT, fuel pressure, crank position and anti-knock (anti detonation) sensors feed to the power train control module. That calculates in milli-seconds the correct ignition timing and fueling required to keep an engine within acceptable parameters. To get a grasp of what occurs when air is pressurised, recall what occurs when you inflate a bike tyre with a bicycle pump. You pump with the right hand perhaps whilst holding the end of the pump in your left. What happens and what do you feel in that left hand? HEAT!! That is due to the compression of the air in the bike pump cylinder and its unavoidable increase in temperatures.

It is important to realise that the EXTERNAL air temperature of the day has a huge influence on the eventual temperature of the air coming out of the super charger. On a cold day, the IAT that you see on your diagnostic kit will be proportionally lower than you would find on a scorching hot day. You might have noticed that your XJR goes like stink on a frosty morning when the ambient air is below zero? Diversely, on a hot day, if you have sat idling at traffic lights for a few minutes, the car comes off the line like an old lorry with the hand brake on. Plob, plob, plob, plob.....After a quarter of a mile or so, performance begins to return due to the inter-cooler coolant heat being pumped through the inter-cooler bricks and dissipated by the ambient external air passing through the inter-cooler radiator mounted in front of the main engine coolant radiator.

In my opinion, the temperatures you are seeing via the diagnostic tool are quite acceptable and normal for this current winter weather cold snap....try it again on a hotter day for your interest.

A few more important points. The supercharger coolant is pumped around the system circuit by a small electric Hella pump. It operates immediately the ignition is switched on and whenever the car is in use. It is mounted at the front of the radiator immediately below the off side main beam headlight. The bumper cover has to be removed to get at it. The pump is adequate for a standard car and works reasonably well with the standard sized inter-cooler radiator...except perhaps on the hottest of days when the standard Hella pump and heat exchanger radiator can become inefficient and go flat on you as described at the traffic light scenario. In such an instance, IAT readings are likely to rise and the ecu then retards ignition timing and over fuels slightly to take the heat out of the air charge and protect the engine from being thrashed in an unstable situation.

Being aware of these summer issues caused me to fit a bigger Bosch high flow coolant pump in place of the rather wimpy Hella unit. This large pump makes a significant impact on the way the car performs even around town but more importantly in hot summer conditions. The pump gets the coolant around the system faster and allows the inter-cooler radiator heat dispersal potential to be maximised. If you were to fit either a 6% or 10% smaller supercharger pulley, then to get the best from them, an ecu uprate and the big Bosch pump would help keep coolant and IAT temps under better control.

IAT can also be lowered by fitting water/methanol injection which further helps....thats another story!

PTJS1... Paul and I have explored many of these IAT conundrums together over the past couple of years. This culminated in our attendance at the Double Twelve event at Brooklands last year. Air temperatures of the day were high but undaunted we entered the XJR for three full throttle, methanol aided blasts up Test Hill. This is ten seconds of abject terror on a hill climb in a trench with a very rapid stop at the top. The XJR was certainly the fastest car up the hill that day.... burning rubber and laying a damp alcohol smelling cloud of steam on the start line.

But this year Paul and I can achieve even faster times up the hill by lowering inlet temps further with the aid of a box of Wickes Plumbers Pipe Freeze aerosol cans and several lengths of silicon urological catheter tubing borrowed from my Fathers medical cupboard...I`m sure he won`t mind the loan in the interests of power research and development. The pipe freeze will be squirted all over the air box and inlet trunking seconds before our blast up the hill. Paul will freeze the trunking down whilst I watch IAT fall to the lowest achievable temperature. Paul will slam the bonnet down, jump into the passenger seat bringing the pipe freeze with him. He will carry on squirting as the flag rises and we will go rocketing up the hill in an attempt to be the fastest car ever to climb the hill!! Its all done with safety and economy in mind and without significant loss of life.

Mike
X350 Co-ordinator

2004 XJR

irishjag
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Joined: Mon Mar 27, 2017 3:02 pm

Re: XJR Normal IAT value?

Postby irishjag » Tue Jan 09, 2018 11:24 am

Mike

Thanks for that very comprehensive reply.
Very interesting. So the IAT that i'm looking at must be the one after the supercharger and inter-cooler.
You're definitely dedicated to eking out every last drop of speed from your car.
Have you measured the 0-60 time after all your modifications? Standard is 5 secs I think. I wonder what yours is..
All the best
Chris
2003 XJR
1992 XJS

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J44EAG
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Joined: Sat Oct 20, 2007 9:57 pm
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Re: XJR Normal IAT value?

Postby J44EAG » Tue Jan 09, 2018 2:16 pm

No measured performance times yet or rolling road output. One has to consider that the gearbox doesn`t have a huge torque rating. For me, with now a quite chunky performance hike, that`s enough for this car.

Performance increases came progressively as finances allowed. With the benefit of hindsight, if I were doing the mods again, they would now occur in a slightly different order. We can talk about that some other time.This was the order they occurred.

K&N air filter fitted and small internal air box baffle removal mod.

10% small pulley mod with supercharger double V belt and up-rated tensioner and idler pulley mod to stop belt flinging due to extra torque load being applied to the belt and tensioner system by the small pulley load.

Gerald Morton/ Viezu Berkshire ECU up-rate.

Mina Gallery high flow aluminum cold air intake mod replacing restrictive, silenced OE unit.

Gas flowed air intake plenum above throttle body.

Major supercharger porting and near mirror finish polishing. Floor of supercharger inlet and throttle body elbow lowered by removal of 6mm of metal to open the ports. Further pressure plate and charger body re-profiling and polishing. Polished throttle body throat. Cowhorn pressure manifold gas flowed and re-profiled. Restrictive by-pass pipe neck opened up, ported and polished.

Standard 600 cell catalytic converters replaced with high flow 200 cell sports units to de-restrict exhaust system immediately after exhaust headers. Large resonator box with internal H-pipe removed but leaving the smaller rear box still in place to keep exhaust reasonably quiet. H-pipe replaced by high flow X-pipe but without large resonator box.

Telematica Sprint Booster adjustable throttle response improver chip unit. It is inserted between the cable loom socket and the throttle pedal plug. Function tested last year by PTJS1 and myself to see how it functioned. We found that speed of the throttle butterfly plate could be made to open up to four and a half times the standard speed of the OE set up. Generally, I have the unit set at level 2 for town work and up to level 5 for motorway or fast A-road work. The unit makes for very sharp response and instant throttle application. Set to level 9, the unit makes the car highly sensitive and unruly in town. It is great for the open road but when set at level 6-9, it can over face the ecu which then closes down completely as a total wipe out. One then has to clear fault codes to clear the ecu of a whopping headache!! At level 5 the unit is reliable but levels 6-9 can cause MIL lamp and engine check lights to illuminate or for the ecu to close down completely. Hence setting no higher than level 5. The unit makes for a very reliable and responsive car as long as level 5 is not exceeded.

High flow big Bosch inter-cooler pump fitted to increase coolant circulation speed through inter-cooler rad.

Methanol/water system installed to lower charge air temperatures and reduce propensity for detonation in high load situations. Can be used as an inlet cleaning system on just 99% pure de-ionised water or as a performance enhancer with 50/50% water/methanol cocktail in tank.

Original "all or nothing" performance has gone through the stages of better, much better, hugely better, seriously quick, phenomenally quick, ballistic, searing to finally insane! However the car still remains docile around town but can become a hooligan on the open road. Power is instant and you never run out of it. There is no longer a slow build up before the action starts. This can startle passengers and make them grab for the seats. Few have ever experienced such rapid acceleration and it comes as an assault to their senses. I`ve learned every foul language expletive in the book! One has to be very careful in wet conditions. The car can get very squirrelly at the back end if power is rashly applied. Others who have been allowed to enjoy this XJR can finish their drive and be lost for words. However, they all have one thing in common...a huge grin from ear to ear.

VVT equipped cars have even more potential with a tad more top end grunt but rather more torque than a standard non VVT XJR has at low engine speeds. How I wish I`d bought a VVT car, but doing so would have incurred higher road tax charges in the UK.

From slug to bullet in about eighteen months!

Mike
X350 Co-ordinator

2004 XJR


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