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M's Factory
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Discussion Starter #1
Well it seems we have touched on a subject this morning of detonation, so I thought it would be wise to start a subject on AFR ( Being as someone has been asking me about it on email also )

So here goes ;

My ideal AFR is 12.6:1, this condition is RICH NOT LEAN. Maximum power is at lambda 0.86 with an AFM of 12.6:1. This is taken from the "bible" on Bosch injection systems by Charles O.Probst, SAE in his book "Bosch Fuel Injection & Engine Management". Chapter 7 is on Tuning for Performance and Economy. On page 7-7 he specifically states, "Peak power is achieved with a slightly richer air fuel mixture (when Lambda is approximately 0.9). Minimum specific fuel consumption is achieved with a slightly leaner mixture (when Lambda is approximately 1.05). So, max power is on the rich side, not lean and the "ideal" value is 12.6. Note that the curve is relatively flat in this area so a small variation has little effect on the curve value.

There are two ways to monitor fuel/air (f/a) ratios: First, the function of the oxygen sensor is to monitor f/a ratios for the ECU. It produces voltage in proportion to the a/f ratio. High readings (~.9v) indicate a mixture which is conducive to long engine life. The reading should go to ~.9 and stay there as rpms increase. When you reach the limits of the fuel system, the voltage will start decreasing as rpms increase. Lower voltage = leaner a/f ratio = possible detonation, so back off the boost if this occurs. The factory thoughtfully provided a wire which can be used to monitor O2 sensor voltage in the passenger's footwell, in the wiring harness, so you can connect a voltmeter to it or an A/F meter e.g. Lumenition, and get a display reading given.



The A/F Ratio Meter is a voltmeter with a range of 0 to 1 Volt. The meter displays the output voltage of the vehicles oxygen sensor through 20 LED's. The first LED will come on at a voltage of .050V, the second at .100V, the third at .150V, etc.
The stoichiometric (STOICH) air/fuel ratio is the chemically correct ratio, theoretically all of the oxygen and all of the fuel are consumed. The mixture is neither rich nor lean. However, due to the fact that combustion is never perfect in the real world, there will always be a small amount of oxygen left in the exhaust. This small amount that is left is what the oxygen sensor measures. The smaller the amount of oxygen that is left in the exhaust, the richer the A/F ratio is, and the higher the oxygen sensor voltage is. The on-board computer monitors the voltage from the oxygen sensor. If the computer sees an oxygen sensor voltage greater than .450V, it immediately starts to reduce the amount of fuel that is metered into the engine by reducing the on time to the fuel injectors. When this happens, the A/F ratio starts to go in the lean direction, and the oxygen sensor voltage starts to go down. When the voltage drops below .450V, the computer immediately starts to increase the fuel metered to the engine by increasing the on time to the fuel injectors to produce a richer A/F ratio. This occurs until the oxygen sensor voltage goes above .450V. This repeating cycle happens very fast (many times per second). The computer is said to be in closed loop. It is constantly monitoring the oxygen sensor voltage and adjusting the on time of the fuel injectors to maintain a stoichiometric A/F ratio. This A/F ratio produces the lowest harmful exhaust emissions, and allows the catalytic converter to operate at peak efficiency, therefore reducing the exhaust emissions further. Although most people do not use Cats as standard this means results for maps would be different than a normal engine map ecu.

Since the oxygen sensor output is non-liner and very sensitive at the stoichiometric A/F ratio it will cause the A/F meter LED's to bounce back and forth rapidly. A very small change in A/F ratio causes a large change in oxygen sensor voltage as can be seen on the graph. This causes the A/F ratio meter LED's to rapidly cycle back and forth, and is normal operation when the computer is in closed loop and trying to maintain a stoichiometric A/F ratio.



The oxygen sensor is very accurate at indicating a stoichiometric A/F ratio. It is also very accurate at indicating an A/F ratio that is richer or leaner than stoichiometric. However it can not indicate what exactly the A/F ratio is in the rich and lean areas due to the fact that the oxygen sensor output changes with the oxygen sensor temperature and wear. As the sensor temperature increases, the voltage output will decrease for a given A/F ratio in the rich area, and increase in the lean area as shown on the graph.

During wide open throttle (throttle opening greater than 80% as indicated by the throttle position sensor) the A/F ratio will be forced rich by the computer for maximum power. During this time the oxygen sensor outputs a voltage that corresponds to a rich A/F ratio. But the computer ignores the oxygen sensor signal because it is not accurate for indicating exactly what the A/F ratio is in this range. The computer is now in open loop, and relies on factory programmed maps to calculate what the on time of the fuel injectors should be to provide a rich A/F ratio for maximum power. The A/F ratio meter should indicate rich during this time.

Please remember also that all cars I know of have a narrow band lamda sensor ( cheap ) built into them which reacts very slowly from the ECU's point of view. If you are looking at tuning your car then a wide band Lamda sensor ( expensive ) is important.

The above settings of around 12 on the fuel/air ratio is where my personal preference lays after many discussions with other people.
 

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M's Factory
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Discussion Starter #2
My final comments to the majority of the population is you dont need to know this and get your money out and someone to map it for you like one of the reputeable tuners like Abbey etc who have all the equiment already and know how to use it properly to refine your parameters for your particular car.
 

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Busy day then Sarky :D
 

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M's Factory
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Discussion Starter #4
bombarded by emails so i thought id just post it.
 

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AFR

Sarky and others,

yes 12.6 afr is meant to be the idea afr for max power but you will find a the ratio you will start to get high exhaust gas temperature.
We find for a car producing up to approx 300 bhp at the axles you can get away with 12.0 afr at full throttle , but itiis the partially throttle afr that gives the car it,s sharpness off boost and fast spool up of boost pressure.
A big bhp car i.e above 600 bhp at the axles we will run approx 11.3/11.6 afr at full throttle to help cool the cylinders and keep the exhaust gas temperature at a safe level.
Afr is also a major contribution to detonation troubles, some cars will det is to rich and some car will do the opposite.

Mark
 

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Ben,

did they read the afr with the lambda sensor stuck out the back of the exhaust? fancy a free run on our dyno to back to back figures?

Mark
 
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Christ , first they try 2 get away with lower road tax ,
now free dyno runs.

Where will it end :)
 

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Ben,

the reason i ask about the lambda sensor is you get a ir-regular reading with the sensor fitted here,we fit a boss into the exhaust and screw the sensor direct in to the exhaust , normally on skylines this is where the cat used to be or in the front pipe if the car still has a cat fitted.
has your car got a cat fitted ?, and are you able to get time off during the week either early morning or late in the afternoon.
What horsepower did you car have at the wheels at Power engineering?

Mark
 

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Ben,

okay i will wait for you to sort out the wheel figures from power , we are based in Oxted in Surrey junc 6 on M25, but we are to busy to run your car this week sorry. What about this sat morning.


Mark
 

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Ben,

yeah not far from the station we are on the other side of the A25 towards Hurst Green Station, I know Ikea well just moving into my new house, Crotdon ikea is not far just up the A22 towards mitcham.

Mark
 

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Ben,

yeah sat is okay, would prefer you to be to us at approx 9-30 am
should be all done by approx 11-00 am , is that okay?

Mark
 

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Diug this up... I'm looking to fit a Lumenition AFR to my R32 GTR. The 'wire in the loom in the passengers foot well' that can be used to tap into the output from the lambda... how can it be identified, colour etc?

A pic of this would defo get a pint from me which is saying something from a tight Scotsman.

:cheers:
 

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Thanks for that. Sorry to be a complete idiot here but I can take the feed from this wire to my unit? How can I identify the actual wire / wires required? Anyone do this?

Thanks again
:cheers:
 

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You may want to get a second opinion on this but if you look at the pic in my previous post you'll see that there's a block in the center of the pinout that is staggered slightly too one side, so if you have alook at your ecu all should become aparrent by countung wires etc. I did notice you've got a post on sau about this as well so i'd imagine someone on there would know the exact colour of the wires you need. Hope that helps :D
 

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Cheers, had a wee look at the loom of the PFC... looking to fit next weekend so should have a good idea by then. Thanks again
 

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I think the wire loom plug connector is actually numbered...... Mind you when you are grubbing round down in there its a bugger to see owt :) Just be careful unbolting it from the ECU and reconnecting it again.
 
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