Flutter and surge
 

Hey Gang,

I need some help with this. Full race car, partial throttle. Please listen to the vid here:

http://www.youtube.com/watch?v=Z1KZ-i3TdoE.

Tial 50mm BOV (new) , shimmed 8lb spring. A spec Precision BB 35/74. 44mm wastegate 14.5 lb spring. No boost/vacume leaks. Any ideas how to eliminate this?

So it's surging while the throttle is open? It's not just making a fluttering sound when you let off the gas? I did look at the video.

Throttle is open and gently being applied exiting the turn. Flutters as I continue to open up the throttle. If I go full, hard throttle, the flutter stops.

Have you pressure tested for boost leaks, just to be sure?

Yep, it's tight. I'm thinking the BOV is n't staying shut.

Surely its just compressor surge? This, but usually far more severe (ie at full throttle) is why many turbos have ported shroud inlets to help prevent.

And a boost leak from a pipe or BOV would lower that, not make it worse.

Off throttle surge does very little damage.
Part throttle surge does a little more, but still meant to be ok, if not ideal.
Full throttle surge can be pretty damn damaging to the turbo in no time at all.

Can sometimes be mapped out, but its just a slight engine/turbo/mapping missmatch if it only happens on part throttle.

Compressor is trying to force in more air than the engine can consume, air comes back out inlet, makes funny noise, and can do a little damage sometimes too.

How damaging it is at part throttle is debatable, but I'd not worry really.

When you say mapping, do you mean fuel curves or boost control or both?

What are the specs on your setup?

I think both can change it, but on rotaries maybe things are a little different.

On a car I had in the past with surge (but it was on throttle at low/mid rpm so very damaging to the turbo and performance- light throttle surge ive had on a coupla cars, inc my FC now, and it didnt affect me) I had to change to a bigger turbine housing to get rid of it. That also massively lowered response too mind you.

Is it affecting performance while its doing it? Ie is it juddering etc.

I would look in to the amount of damage this can cause to the turbo (IIRC, not much), but if its not affecting performance, you might be best off living with it, its a tuned turbo thing.