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MG MGB GT V8 Factory Originals Technical - Fuel Injection Tuning

I have installed a hotwire fuel injected V8 in my MG and it runs quite smoothly. However, it is a tuned engine (10.1 pistons and Burgess Heads etc), and in comparison to my RV8 doesn't seem to have the same acceleration. Granted the RV8 uses a 3.9l lump although, maybe unrealistically, I expected roughly the same performance from my 3.5l set up.

I have started looking more thoroughly at the way I wired the 1990 air con Range Rover system into my car. I suspect that one of the following may be the answer. Any help or info would be much appreciated:

a. I do not use the road speed sensor (idle circuit), would this make the mixture leaner?

b. The system is closed loop and was removed from a cat converter car. I notice that cars without cat converters use a different value of tune select resistor. Is it possible to run the system in open loop and change the value of the resistor? I believe this method was used on some Range Rovers without cat converters(I have pins 23, 24 cat inputs to ECU currently pegged to 0 volts without a change of resistor)

c. By changing the tune select resistor for a variable resistor is it possible to tweak and change the fuel map in the ECU (14 CUX) to enhance performance.?


Any help and advise would be great

VMT
Stu Craggs
stuart craggs

Hi Stuart,

The lack of a road speed input will, contrary to some published information, activate the fuel injection warning circuit lamp, as found on this system in the R.R., but in practice I have used many R.R's with the speed sensor disconnected and there has been absolutely no effect on the performance of the vehicle.

In fact removal of the sensor does allow the R.R. (Classic) to achieve nearly 120 mph on the flat as experienced on a wide and windy ex airfield, now vehicle test facility. This clearly indicates that there is no inhibition to power production with this device not in circuit.

Only the finite idle speed control will potentially suffer, which can actually mean a quite high idle speed when your actually driving. Uncomfortable but since it is only going to be up to about 1200rpm, not earth shattering. The main influence to this control comes from high electric consumption circuits such as air con which when active at idle sap so much power that the idle speed may drop to stalling point.

Oddly enough the aspect of lambda inputs in lieu of the sensors being in circuit is an issue that I am trying to resolve for my own application. Having lost the best source of information, the PI lab at Lucas, I am trying to locate where the skilled men who actually designed and mapped the system have scattered too. The almost complete destruction of this area of Lucas has seen a very wide scattering of the people who used to work there. For example one has ended up in New Zealand and many I am told work for other car makers in Europe. Not much help to me!!

The best information to date indicates that if the correct tune resistor is used then the earlier ECU (like yours, and possibly later ones too) can be set up to work as a non cat spec so the lambda value of 0 volts would be correct.

Clearly this indicates that the tune resistor does affect the map that is used for the different applications. As your system is taken from a 1990 3.9 litre R.R. it was made at a time when the majority of UK market cars were still non cat cars. In addition the Discovery was available then with another version of the Hot Wire, but I have no information on this model to cross reference parts.

On the R.R. up to 1991 I see a total of 4 different tune resistors that were fitted dependant on the actual engine spec, and there are two specific 14CUX series ECU's, which were used on both cat and non cat cars. (From the start of vehicles with a GA VIN code and then from the start of HA codes.) This tells me that the use of a different resistor invokes a different map for the cat and non cat specs.

I note that the RV8 uses a different part number for the ECU there (two in fact with one being shown NLA) but their numbers have no bearing on the Land Rover part numbers. What is common is that the White Tune resistor is used on the RV8 and some 3.9 litre R.R's.

It is probable that your ECU is part number PRC 8702 or PRC 9061 and the optional part numbers for the tune resistors for that period are PRC 8005 Red tune, PRC 8007 Yellow tune and PRC 8172 Green tune. (PRC 8009 being the White tune resistor you already have) Clearly the path here is to use one of these alternative resistors and see which one works best. Unfortunately I have no information as to the value of each resistor so I can't provide an easy alternative route to going to an alternative source fixed or variable resistor, although if you find this out I will be very interested to know.

The problem with lack of performance is not one I would expect either, as with essentially the same set up as yourself, engine wise, I had a very, very responsive and powerful engine for years when using a modified Australian/Federal Airflow Meter system. The actual power output being an average 30 bhp (at the wheels) up on what the 3.9 litre gives. Only in the sub 2500 rpm area did the standard 3.9 have any real advantage. The change to Hot Wire should provide better control, slightly better airflow, (and by inference) slightly improved power/torque and fuel economy.

I wonder whether the problem with your engine is perhaps another problem or problems of just a very minor nature. As you have an RV8 you have the ideal opportunity to cross reference main parts from one car to the other to verify similar working performance. The only factor really different with your two ECU's is the fact that the RV8 unit has been modified to remove the speed limiter. (at between 110 and 112mph fuel is cut off) Otherwise the mapping should be the same.

When I was collecting the parts for my conversion from Airflow Meter system to Hot Wire I actually tested the main parts on several 3.9 litre Range Rovers, cat and non cat and noted no discernible difference in engine operation. That is of interest as the ECU I have is PRC 9610 and is (or should be) from a Discovery 3.5i and the testing was done on 3.9 litre Range Rovers. This indicates that the 14CUX is following in the footsteps of the 4CU series previously used on the Airflow Meter systems in that the standard mapping was well able to cope perfectly well with significant engine spec alterations and still maintain correct fuel control. The only internal mod I have had done (at Lucas before the axe descended) was to have the speed limiter disabled.

Really to check out the other engine functions you need the services of a rolling road where fuelling can be checked under load conditions. Peter Burgess is very adept with the V8 in whatever chassis it sits, and so there is a wealth of data and experience there to immediately show up any problems.


Rog
Roger Parker

Thanks Roger

Since posting my message I have found a possible source of information on this topic:

http://www.cryptontechnology.com/techtop.htm

I'm going to order the relevant articles and see if they are any help. I'll let you know how I get on.

Many thanks

Stu Craggs
stuart craggs

On a slightly different note. I have a Rover Vittese FI flapper system, which I have been looking into ways of tuning. The main restriction appears to be the flapper itself. Which has a flow rate of just over half that of the butterfly.

I have acquired what I believe is a Jaguar V12 flapper unit which is of the same type as the rover, with the same wiring. It is obviously much bigger, and has a potential flow rate of about 3/4's of that of the throttle butterfly.

Has anybody had the 28 pin eprom re-mapped for this unit, or does anyone know of a Midlands based facility that could.

Nick
Nick Smallwood

Stu,

Thanks for the URL.

Nick,

The 4.2 litre Jag airflow meter was often used on the Vitesse and the boys at the Lucas PI lab said it was an easy way to gain 10plus bhp. I am not certain of the V12 meter so I can't comment further.

Rog
Roger Parker

Nice tip!
Angus Davies

This thread was discussed between 08/02/2000 and 13/02/2000

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