EFI & Backpressure

[martin]

I was told today that fitting a higher flowing exhaust would not be a good idea because the resulting lower backpressure would enable the engine to breath more air but the efi computer would not add more fuel resulting in the car running lean and breaking stuff...

i thought that the point of O2 sensors and airflow meters was to conquor this particular problem and help meter the fuel etc... am i wrong?

My car does backfire sometimes under heavy deceleration (ie dong 90 and dropping it into 3rd foot off the gas) which means its running rich on overrun... is a faulty 02 sensor to blame?

cheers

[Ben Wight]

Hey Martin, this is true, if your car runs on a MAP sensor, then allowing the engine to breathe more air will also make it run a bit leaner due to the MAP sensor not knowing the differences between the airflows.

An airflow meter on the other handle will add extra fuel to complement the extra airflow of a better system as it measures the amount of air entering the engine, not just the engine load as a MAP sensor measures.

An O2 sensor will counter the effect of a bigger exhaust with a MAP sensor equipped engine whilst cruising and light load, however, when applying a throttle of over 80% or so, the output of the oxy sensor is ignored and only the maps stored within the computer are referenced, with engine load as one of the variables, not airflow (once again, only with a MAP sensor based system). Thus, when flooring it, thus generating heavy load on the engine, a MAP sensor based system has the ability to run lean with a modified engine, (ie exhaust). In my experience though, the change in a/f ratios has been minimal and nt much to worry about unless the original exhaust system was completely horrible.

You dont have to worry though do you, isn't your car equipped with a flow meter?

If your car backfires when the engine is on overrun like you describe, it is actually running lean. The lean mixture has trouble combusting, and as a result, the unburnt fuel passes into the hot exhaust system where it collects and combusts, giving the bacfire and popping sound.

This is actually a good thing, as the engine needs zero fuel while on overun, as there is no load on the engine, in fact its negative load, the momentum of the car is driving the engine. I actually cut fuel completely in this instance on my car.

I take it you have a different exhaust already, as a standard OEM exhaust system should disguise this popping trait.

Ben Wight

[martin]

thanks for that ben!

yeah its got a single pipe... all of the ZZ imports have twin pipes from the headers all the way to the end. I take it that in its 20 odd years it rusted out or broke or something.

Every exhaust shop i have asked has said 'get another single'

at the moment it is hardly a pea shooter but its still reliatively small , so i was looking at upgrading the pipe so even if i do go turbo i can still use it from the turbo back (maybe 2.5 inch?) as the turbo thing wont happen until i have plenty of funds in case i go over budget!

cheers
Martin

[Tooleeda]

It also depends greatly on what sensors you run. Basically the more the merrier as the ECM can make better decisions. For example a speed sensor as well as the TPS and MAP or MAF sensors would help with correct fuelling when the engine is 'braking' so to speak. I dont think I would be cutting the fuel *completely* during this though. (Is that what youre doing Ben?)

[Ben Wight]

Hey Tooleeda,

A speed sensor is a crank angle sensor that measures engine speed, all EFI systems need one of these.

Why not cut fuel completely, this is what I am doing, injectors turn off when throttle is closed, and engine above 2000rpm. There is no real reason why this is an unsafe practice, the engine still gets lubed by the oil, but as there is no load on the engine (the car actually completely unloads the engine), there is no need for it to burn ANY fuel at all. It doesn't overheat, as there is no fuel burning in the chambers.

A lot of brand new cars do this by the way.

Ben Wight

[Tooleeda]

I was talking vehicle speed sensor that attatches to the speedo drive cable output on the gearbox. Its not a necessity but it helps the ecm distinguish between doing 60 in second gear, or doing 100 in 4th gear. The engine load may well be the same but the fuel enrichment needed in acellerating from these points is very different.

If no fuel is needed when coasting, for example down a hill in top gear, why do carbs bother with enrichment solenoids and all that extra crap. You would find the car actually slowing down on a hill when you take your foot of the fuel?

It may be something to do with the lower compression on the old leaded engines (a new ULP engine will run alot leaner anyway) but I always thought you needed some fueling in this condition.