Changing MAF tube diameter

[Evoluzione]

Anyone any experience of this to pass on?
Short story:

If I change the internal diameter of the MAF sensor housing to one 10mm bigger diameter will It cause any issues?

Long story:

I'm building up an N/A track toy bit by bit, chassis mods are done and the original (seemingly healthy) engine is still in so i'm going to do a couple of trackdays in it to check what has been done so far is ok whilst I build the new (bigger and more powerful) engine.
In order to give it a bit more more power/response/suitable noise I've fitted a tubular manifold and de-catted it. I then looked in my box of bits and found a K&N and various bits of 75mm tube so am going to ditch the original airbox with associated 65mm piping and MAF tube and replace it with a straight tube, and bigger MAF tube. Car is equipped with knock sensing and two Lambda sensors. They were before and after cat, but are now just one after the other and seem happy with that (no management light).
I can guess air speed will go down due to larger tube, but not sure if this will cause any issues, maybe it will overfuel a bit perhaps or would the O2 sensors adjust?

I suppose I could put a wideband sensor on and do a before and after test perhaps....

[stevesingo]

I think you will have issues.

The MAF works by measuring the current required to maintain a constant temperature of a wire filament. The more air speed over the filament, the more current required to maintain the temp.

As air speed is related to mass air flow through an office, enlarge the orifice will reduce air speed. So, for a given mass airflow, the cooling effect will be less and the current will be less. The ECU will think there is a lower air flow than is actually present and it will run lean.

The difference in cross section is 33%, that will be a large change in air speed.

Can you re calibrate the ECU?

[rottenegg]

Been there, done that, it's painful. For an easy life, use the original MAF housing, or get a standalone ECU and use a MAP sensor instead.

[Evoluzione]

stevesingo said:

Can you re calibrate the ECU?

Yes it will have a full remap when the new engine goes in, I was just wondering how it would run as it is now. It's not a problem to keep the old set up on for a while if it won't be happy.

[rottenegg]

It will run lean as he said. Injector on time is proportional to air mass voltage. Bigger injectors would get around it as a temporary fix, but it's far simpler and cheaper to stick the factory MAF size.

[stevesingo]

Do you know the g/sec of the 65mm MAF and the expected engine output?

(value MAF g / s) / 0.82 = hp

[Evoluzione]

stevesingo said:

Do you know the g/sec of the 65mm MAF and the expected engine output?

(value MAF g / s) / 0.82 = hp

I don't know the G/sec, whether you mean existing engine or the new one or why you're asking.

[Krikkit]

If you're putting in a new engine with accompanying remap why not keep the 65mm pipework for now? If it's OEM it'll be nicely optimised anyway...

[stevesingo]

Evoluzione said:

I don't know the G/sec, whether you mean existing engine or the new one or why you're asking.

The MAF part numbers and some Googling will likely get you the respective g/sec. If you know the power of the engine, you can decide if the 65mm is big enough.

[stevieturbo]

10mm is a pretty big difference and will cause big changes in running. Even altering the pipework can affect this with the same diameter.

So it will absolutely need re-tuned to suit.

[E-bmw]

I don't know what effect you're aiming for with your change but just a quick recap from my experience.

I had a 328 e36 track day car a couple of years ago & with standard (apart from CAI) inlet pipework & M50 inlet manifold it made 224 bhp.

After fitting a BBTB & "de-restricting" the inlet where possible, it was later dyno'd at 238 bhp, all of which was on the standard MAF which had a much smaller bore than the rest of the inlet after I had finished, so I guess the question I would be asking is will you really need to actually change it?

[Evoluzione]

I'll leave it as it is for now or get some 65mm tube, thanks for the replies.

[Steve_D]

Evoluzione said:

I'll leave it as it is for now or get some 65mm tube, thanks for the replies.

Even changing to 'tube' may upset it. The airbox on many cars look very strange and defy logic as to why they were designed that way but i'm told it is to do with smoothing oscillations (and other things I don't understand).

Steve

[rottenegg]

Probably to quieten induction roar. Every time I've replaced a factory airbox with a bit of pipe with a cone on the end, it ran like crap. Just not worth it for the small gain you might get at high rpm.