Hondata

I have a 99 Si and I was hearing this odd sound whenever I shifted the car at WOT. It almost sounds like a miniature blowoff valve. It only does this when I use the hondata and when I run it hard and shift in between gears.

My car although is n/a at the moment.....

One possibility is that it's air hissing past your throttle butterfly, which will briefly snap closed during shifting (assuming you let up on the throttle when you shift ) The engine generates quite a large vacuum at 8k rpm, and if you have an aftermarket intake you can really hear the air hissing past the butterfly at slight throttle openings.

If you're curious, you can try keeping your right foot planted on the accelerator while you shift (note that you'll likely hit the rev limiter). If you don't hear the noise anymore, then you've probably found the source of the sound.

-Andy

I am having this same problem, is it actually the air hissing past the TB butterfly at slight throttle openings.?

I never had this before I put hondata on my car.

Can also be an exhaust leak at the header flange when the engine becomes unloaded and the flange is distorted by the movement.

IT only happens with the hondata in the car. I took it out and put the stock ecu in and the sound was gone.

Could it be the purge solenoid?