Hum from air filter/turbo inlet
 

It's been slow progress, but I've got the FD up and running. Seems to work OK, but it has a very loud hum from the air filter/turbo inlet.

I'm guessing that it's some sort of resonance and the 4"pipe between the air filter and turbo inlet is tuned just right to sound off.

Question is, has anyone else run into this?

And is there a solution?

Here's a link to a video I posted on youtube.


Victor

OK, I think, just maybe, that I am on to something.

Being an automotive technician I've heard similar noises associated with idle valves, usually when the airbox is not secured.

So, knowing that the BACV is set to operate at 244hz, I went online and found a tone generator and and listened to a 244hz tone then compared it to the noise from the car. As best my ears can discriminate, it's spot on.

Here's a link to the tone generator, set it to 244hz and compare this to the hum in the video. tell me what you think.

I had originally discounted this because I can't really envision a noise generated in the intake manifold making it all the way back to the turbo inlet. But with the matching frequency it's nearly impossible to discount it now.

I'm guessing that the resonant frequency of the intake plumbing must be about 244hz. So maybe if I change the frequency of the valve I can move it out of that frequency range.

I'll give that a try tonight and report back.

How far off 244hz can I go without creating a problem for the valve?

Very likely the Bac valve.

If using aftermarket ECU, change the Bac frequency to see if the sound changes.

it's a haltech ps1000 and thats the plan

do you know what range of frequencies the BACV on a 94 FD will work with?

Changing the frequency of the idle valve is the trick.:icon_tup:

As I moved it away from 244hz the hum changed pitch and diminished to the point where it's tolerable.


Right now I have it set to 300hz and if you listen to the air filter, you can hear it but nothing like it was.

I still would be interested in feedback about the actual range of operation as far as frequency settings for the BACV. I haven't tested to see if the valve is still responsive. I guess it could need to have the PID re-adjusted.


I guess it's not too likely that many people will have this issue, but hopefully this thread will help someone out.

Thanks Jetlude for your support.

Victor

I have see it posted that the stockFD runs on 244hz, but thats very noisy on my car. I run at 220hz which is much quieter.

I'm sure with every variation in pipe length and diameter it would change the resonant frequency of the intake system, and cause noise if the valve is set to operate at that frequency. If you think about it from this perspective, it makes me wonder how much an influence this resonance played when the engineers at Mazda choose 244hz?


And one other thought, for tonight,,,,
If this system is tuned for 244hz, does that create a Helmholtz effect at (244x60)/2= 7320rpm?:scratch:

I'll be looking at the dyno charts when it gets tuned

Victor,

I appreciate your post!

My Rx7 has a loud "hum", particularly on start up, from the air filter/turbo inlet which is quite annoying!

I'll look into the BACV and report back in due course.
(Custom garrett single turbo/motec etc etc etc)

Regards,

Roger

Where is the return discharge pipe from the ACV unit.

Flow is air pump to ACV with excess air coming back forward with the other pipe.
That one make sounds including the stock seq moo-fart sound.

If you are talking about the sound in the video that I was experencing, the air pump is currently not hooked up, and there is no ACV.

I've heard the sound you describe and it's not really like what you can hear in the video.

My plan is to put together a video that shows the settings and resulting different tones.

Don't talk about it 'Just do it':):)

Beat me with a stick