Last week my car was ‘missing’ - I cured this by cleaning the MAF with spray, and replacing the plugs and air filter. It’s been divine all week, then started doing the same again. It is now cured with cleaning the MAF again - I wonder if there is an air leak letting dirt get to the Hotwire of the MAF.

I tried changing back to my Worldcarparts MAF and took some readings - graphs below. It maxes out just over 2.8 on the OBDII, which x100 is about the 284g/min or so that Andy has told us is the max airflow for the XJR MAF , LNA1620AA

I then swapped back to my genuine LNA1620AA MAF after cleaning it, and that also maxes out at 2.8 (or 284 g/min).
With my throttle fully open (96% on the graph), I reach max measurable airflow of 2.8 at about 4000 rpm with the following modifications:-

1. Andy’s reprogrammed EPROMs
2. Powerhouse 10% larger supercharger drive pulley

That max airflow at 4000rpm with throttle wide open corresponds to an unpleasant squeal from the engine bay which sounds like pulley slip, but may be something else. It only squeals at full throttle over 4000rpm, so now I know that is max airflow with the pedal fully down , there is no point exceeding 4000rpm with full throttle.

 

Since the above post, a week ago, I’ve had to clean my MAF twice, each time it runs beautifully again.
Because I’ve had the MAF out so often over the past few years, the rubber hose (part number NBD7764AB) has now got a split in it which I have just found, and I think only happened today. Luckily this is one XJR-specific part that is still available, so as usual I have ordered two when I really only need one.. coming from Moss , found on UK ebay

I assume there is some air leak letting dirty air in, I’ll have to check it out more thoroughly

 

This is exactly what happens if the engine is modified to flow more air. I have remapped the fuelling for my own car fitted with an Eaton M112 supercharger (24% bigger than standard M90 unit fitted to XJR6 engine) running at standard 2.5:1 drive ratio.

I think the Powerhouse pulley is no more than 10% bigger than standard, so when fitted, the engine can't flow more than 10% extra air. At worst, this can only make the engine run 10% less rich than standard.

The standard fuel map is already very rich in an effort to reduce max exhaust gas temperatures. I would therefore still expect the engine to run richer than necessary for best power. If you want to know for sure you would need to get a boss welded into the exhaust and fit a wide band oxygen sensor. If it never reads less rich that lambda=0.85 (12.5;1 air:fuel ratio) then you don't need to consider enriching the fuel map to offset the MAF "maxing out".


Your engine will still make more power if you rev it beyond 4000rpm. It will just get less rich in the ratio of actual engine speed to the speed at which the airflow "maxed out". This is only true up to the point of max airflow. Above about 5000rpm the airflow will actually start to reduce due to reducing volumetric efficiency. The error in the airflow measure will also reduce and with it any error in the mapped air:fuel ratio

 

Is there any advantage to reducing the the richness to improve fuel economy and emissions? I'm actually surprised that exhaust gas temps are that important, is it to extend valve life at high loadings?

 

Andy,
My car already has your reprogrammed EPROMs, fitted by the previous owner On Edge.
How much leaner does that make it run, and would that increase the concern about being too lean at/above maximum measured airflow?
Thanks

 

My reprogrammed EPROMs are intended to stop the engine running less rich than optimum, when the airflow is increased beyond what is measured by the MAF. I'm aiming for the engine to run at 12.5:1 air :fuel ratio

Jagbio64.
Typically, and engine of this performance doesn't spend enough time at these high engine speeds and loads for the level of enrichment to have much effect on real world fuel economy. Hydrocarbon and carbon monoxide emissions both get worse the more rich the engine runs. However, every engine that I've worked on in the last 30 years needed to run richer than optimum on gasoline at high speeds and loads in order to limit exhaust gas temperature. Excessive temperatures affects the durability of everything into which the exhaust gas comes into contact;- valves, exhaust manifolds, turbocharger, catalysts, oxygen sensors. In future, emissions legislation may require engines to run at the optimum air:fuel ratio for catalyst conversion efficiency (stoichiometric). This will be one of the great challenges for the whole automotive engineering industry.

 

XJRengineer ,

What is the sustainable EGT target for the different metallurgy's of the NA and SC valves . I'm assuming the valve seats re the same for manufacturing simplicity unless the upper chamber is different negating this simplicity .

 

Perhaps I have misunderstood. Is the 12.5 AFR target under full load, or an across the board target? I understood it to be an across the board target, which would include light load cruise conditions. 12.5 under high load/high RPM situations makes perfect sense.

 

After 24 years I'm struggling to remember the maximum continuous exhaust gas temperature limits that we used at Jaguar when I mapped the XJR engine, but it was probably 950degC for both SC and NA engines. The AFR target of 12.5: is only under high load. At light loads, the target is stoichiometric to ensure good catalyst conversion efficiency. For instance, I remember once noticing on a test track that the car was still operating in closed loop fuelling control mode while cruising at 120mph.