Setting Up The Right Air-fuel Mixture (focussing The Co-values)

[Dandu 0]

Hi,

 

recently I got the idea to replace my throttle body with a bigger one from a 3.5l - BMW.

 

My original setup is as follows: 205 MI16, Gutmann Inlet Manifold, Gutmann Airbox, Jetronic, without catalytic converter. I've got another throttle body and another AFM. The attachment doesn't seem to be a problem. But I'm worried about the right air-fuel-mixture. In my view I can set it up in reference to the co-value. I can rich up the mixture via AFM and check out the corresponding co-value. But what are the right co-values at idle and wot?

 

Much thanks in advance!

 

Cheers Dandu

[DamirGTI 342]

Hi !

 

You can check Air-Fuel mixture with narrow band lambda sensor (or you can buy wideband 5 wire sensor if you have the money ..) and digital voltmeter connected to the sensor it's easy and cheap ..

 

Here is the list of the voltages referenced to Air Fuel ratio :

 

Volts - A/F ratio

 

0.1 - 17:1

0.2 - 16.5

0.3 - 16:1

0.4 - 15.4

0.5 - 14.9

0.6 - 14.4

0.7 - 13.8

0.8 - 13.2

0.9 - 12.7

0.985 - 12.1

 

 

As for fuel mixture : Don't touch the AFM spring tension you can't do anything good by loosing the spring tension believe me , if you must richen the mixture (if your car is running lean) buy an adjustable fuel pressure regulator and increase the pressure for 0.3 0.4 bars (5-7 psi) from standard settings .. then go for a ride and keep voltage readings from 0.87-0.88 at part load dropping to 0.85-0.86 at full load ( you can adjust these readings by increasing-reducing fuel pressure ) , after all this adjust the exhaust CO back to factory settings .

This worked like bloody miracle on my modified engine

 

Hope this helps

Damir

[Dandu 0]

Hi,

 

it was a pleasure to read your post. Much thanks for your efforts. Perhaps you might find the time to respond to the following questions too:

 

1. It appears a bit confusing to me, that the air-fuel-mixture is slightly leaner at full throttle than at partial throttle. Is it right?

2. At the end you told me to adjust the co-settings again. In my view both - checking the lamdba values and the co-values - give a clear hint for the air-fuel-mixture. After setting the engine up via lambda sensor, I shouldn't be in the need to do it again via co-tester. It doesn't make sense to me. Either the combustion works fine (correct mixture) or not. So, what did you mean?

3. Is there really a chance to increase the fuel pressure within the intensity you mentioned? I usually found regulators, that were able to increase the pressure in a range of +/-10%. That means +/-0.3bar - not more.

 

Bye Dandu

Edited June 29, 2007 by Dandu

[DamirGTI 342]

Hello !

 

Ok. i will try to explain that from my experience with this mixture adjusting :

 

1. Yes , same thing happened to me on my engine (after fitting ported head and rising the compression ratio) Ive got these readings from the lambda sensor :

0.85 to 0.90v - on cruising (2000rpm)

around steady 0.80V - on mid range (4000rpm)

0.78 to 0.82 - on full load (6000rmp)

 

So as you can see the engine was going increasingly lean especially on full load .

Why was this leaning out on full load happened i really don't know maybe this is something about air speed -gas speed swirl effects or just too much air for that amount of fuel to be burned on full load , but me thinks that you can't adjust the mixture just on one part of the rev range like just on mid range or just on top end and so that one doesnt affects the rest of the rev range . Well at least not with the OE ECU .

Anyway this lean mixture on top end isn't right it must not be like that for sure the mixture must be much more richer on the top end close to 12.7 or 12.3 (in between 0.85 to 0.90v) for the best power output !

 

 

2. Sorry if i didn't mention this : You must set CO mixture with exhaust gas analyser at idle speed , because of that what i've mention above , when you increase the mixture on top end then the mixture on mid range and idle will also be richer (well not as rich as on the top end but richer than before) so you must adjust this idle mixture because your car will bog down on idle also idle speed will be affected and the fuel consumption will be higher .

So this is for idle mixture correcting !

 

3. Well , i don't know which is the maximum pressure that you can set with these aftermarket regulators anyway i think that they can go up to 4.0 bars must check that really i didn't experiment with that , but it is not advisable to raise the pressure more than 0.4-0.5 bars (7psi) from the factory pressure settings for your engine . You can go up to around 4.0 bars but never push the pressure past 5.0 bars (especially not with the standard pipework and standard fuel pump) because the injector control will be lost and your fuel pipes might start to break-leak , life of the OE fuel pump will be seriously shorter etc. However if you fit performance fuel pump and stronger fuel pipes you can run with high pressure but for road car that is useless me thinks .

 

I've found the best combination for fuel mixture adjusting is with a little bigger injectors from the standard size (not too big , but around 20-40cc more than standard injector size) and up the pressure a little bit with an adjustable pressure regulator ! You can tight the AFM spring tension a little if you notice that the mixture is too rich with these bigger injectors .

I've also fit modified ECU that "famous" Superchips DIY ECU and this helps also for the fine tuning of mixture on mid range .

 

It's the best that you try this yourself and see how this works on your engine

Try adjusting a little bit and go for a drive and listen the engine response !

 

Have you check the colour of your spark plugs ? are they white , shiny-white ?

 

And also one note - you want get precise readings with the narrow lambda sensor but it's roughly good for checking mixture especially for checking if the engine is running lean as it's very precise when it's about lean fuel mixture , so i can advice you if you can afford it that you buy an wideband 5 wire lambda sensor , they are very useful and precise sensors for checking-adjusting the air-fuel mixture - it's worth to invest in one wideband sensor !

 

Have fun !

 

Cheers

Damir

Edited June 30, 2007 by DamirGTI

[Dandu 0]

Hi Damir,

 

much thanks for your elaborate response. It has been a pleasure for me to read it. I'm going to see, what I prefer to do in order to set up the engine. I'll let you know ...

 

Bytheway I feel pretty interested in the reasons, you don't prefer to enrichen the mixture via AFM? Did you get involved into any kind of trouble, as you made the attempt to?

 

Your idea of bying a wideband lambda sensor is very convincing. Do you know of any kind of sensor, I can easily attach into the exhaust system without drilling a hole into it? I can't imagine another way, but perhaps you can.

 

Bye Dandu

[DamirGTI 342]

Hello !

 

I'm glad if i can be of some help to you !

 

Well all aspects/effects of AFM spring tension adjusting are not so much familiar/known to me and to other people , there's too little information about that , especcialy about adjusting the spring tension on wide rmp. range for someone who doesn't have the necessary equipment because this is the job for the "Rolling Road" workshops only .. all people who advice me (some of them are experienced race engine builders !) on my engine mixture setup sad that i must not do that in order to richen the air/fuel mixture !

 

If i had the wideband lambda sensor i would try and experiment with that all thou i have try some tests with various spring tension settings and I've discover that it has very limited area for adjusting suitable Air/Fuel ratio , area of around 4-5 tooth (maybe a little more 6-7 on the end..) on each side and when i try and adjust the spring tension more than 7 tooth (loosen the spring anti-clockwise so this means richer mixture) on the rich side i have found that mixture goes back to lean ! voltage on my lambda sensors suddenly starts to fall down and become very unstable

So me thinks that this is best to left alone , go and adjust that properly on rolling road and after that mark original position on the toothed wheel then you can try and adjust the spring tension if you wish or must do that , but i can advice you that you don't go more then 4 tooth on each side from that original position ! 1-2 tooth make sense if you must reset that spring tension for some reasons ...

But also you must try that for yourself and see the end result

 

Yes buy an 4 wire lambda sensor ( i don't know if the 5 wire wideband lambda sensor has heater element but me thinks that it has ), this sensor has heater element inside so it doesn't need to be fitted close to the engine/exhaust mainfold because the heater will heat the sensor tip .

You can make one short piece of pipe with this sensor fitted in between and stick that in your tailpipe , see this :

http://autospeed.drive.com.au/cms/article.html?&A=2055

 

(also you have some tips regarding AFM spring adjusting in this article but unfortunately it's just for the idle setup)

 

 

Cheers !

Damir

[Dandu 0]

Hi Damir,

 

I don't know what to say as ... it's amazing. I cherish your help. Much thanks again. Now I feel prepared to start the conversion. I'll let you know, how it worked.

 

Cheers Dandu

[DamirGTI 342]

Hi !

 

No probs mate !

 

I wish all the best results for you and please report when you finish that adjustment .

 

Regards

Damir

[Dandu 0]

I'll do that. Promised. Bye Dandu