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Discussion Starter · #1 ·
I wanted to understand this. Am I right to say....

The EFI ECU dispenses fuel by varying the duration the injectors are open. To get more fuel in than maximum permissible, you fit higher flow injectors, and/or increase the fuel pressure. However, the ECU has no idea of the actual quantity of fuel being dispensed - it only knows the duration the injectors are opened. This means that the ECU map, the injector fuel flow AND the FPR needs to be "matched" to give the optimum performance.

Fitting higher flow injectors and increasing the fuel pressure might make the air-fuel mixture too rich, unless an O2 sensor is fitted, and it varies the mixture to remain optimum. There is a limit to the O2 sensor operating band, hence a "wide-band" O2 sensor allows for more variations.

Am I right?
 

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If I remember correctly James you have the Lucas system, which I like. I don't have much experience with the Bosch, and opinions only by hearsay. Back to your question; what you say is generally true for Lucas and I suspect Bosch too, regarding injector size and fuel pressure. The ECU has no idea how much fuel is going in if there is no O2 sensor. If an O2 sensor is fitted it will generally override any variation in injector size and fuel pressure increase to correct the mixture back to the optimum programmed into the ECU.
 

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Discussion Starter · #3 ·
If I remember correctly James you have the Lucas system, which I like. I don't have much experience with the Bosch, and opinions only by hearsay. Back to your question; what you say is generally true for Lucas and I suspect Bosch too, regarding injector size and fuel pressure. The ECU has no idea how much fuel is going in if there is no O2 sensor. If an O2 sensor is fitted it will generally override any variation in injector size and fuel pressure increase to correct the mixture back to the optimum programmed into the ECU.
Yes, I do have a Lucas.

If you don't have an O2 sensor, would "unmatching" the ECU with the FPR and injector flow rates (such as installing bigger ones) only waste fuel?
 

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Yes, I do have a Lucas.

If you don't have an O2 sensor, would "unmatching" the ECU with the FPR and injector flow rates (such as installing bigger ones) only waste fuel?
If you have no O2 sensor, fitting larger injectors and increasing fuel pressure will almost certainly waste fuel if the fuel air ratio is increased above the ideal stoichiometric ratio for petrol, which is around 14.7 IIRC. There is often a small power increase by enriching the mixture above this, but it is not fuel efficient. Also with a turbo, which I think you have, boost pressures can be increased with richer mixtures, as the extra fuel cools the combustion process and prevents knock. Saab did this on the early 900s with a switch on the throttle which enriched the mixture at WOT and allowed higher boost pressure and CR.

Many T900 power junkies fit a fifth fuel injector for this reason. Do a search or go here; http://www.bluesedan.com/fifthinjector/index.htm
 

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Doesn't the O2 sensor only control off boost fuel settings, whereas on boost are controlled by ECU ,or am I thinking of T5, because I know that's how T5 works, closed and open loop modes.
 

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Doesn't the O2 sensor only control off boost fuel settings, whereas on boost are controlled by ECU ,or am I thinking of T5, because I know that's how T5 works, closed and open loop modes.
Shoot me down if I am wrong, but with the Lucas system, I don't think the fuel ECU knows if you are on boost or not. Don't think there is an ECU input signal for it. So the O2 does control through boost. I don't have my reference to hand, so can't confirm 100%. T5 is a different animal.

PS - Just checked. There is no open loop facility on the Lucas CU14.
 

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Discussion Starter · #7 ·
Shoot me down if I am wrong, but with the Lucas system, I don't think the fuel ECU knows if you are on boost or not. Don't think there is an ECU input signal for it. So the O2 does control through boost.
The Lucas EFI is totally separate from the APC. As far as the EFI is concerned, it's just more air being pulled through when it's boosting, and hence it needs to give more fuel, for a given RPM, until the O2 sensor is happy. Right?
 

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The Lucas EFI is totally separate from the APC. As far as the EFI is concerned, it's just more air being pulled through when it's boosting, and hence it needs to give more fuel, for a given RPM, until the O2 sensor is happy. Right?
Couldn't have put it better myself. ;ol;
 
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