930 BOV/Bypass Valve Question

scmguru · 07-08-2009, 02:14 PM

I've been noticing a boost spike when I shift while on boost.

For example, If' I'm at .8Bar when I shift, when I let off the gas, the boost will spike for a split second to 1-1.1 Bar.

I'm running stock turbo w/ factory bypass/diverter valve (new seals) and my boost gauge is tapped into the wastegate input.

The tach boost gauge doesn't spike and I havent hit the boost cutoff switch.

Is this something I should be concerned about or is it normal?

Thanks!

Mike

MBruns · 07-08-2009, 06:04 PM

Is your waste gate feed plumbed into the manifold or the intercooler ?
Mike

billjam · billjam's Garage

If you shut the throttle while you've got the turbo working hard, the inertia of the turbo and the inertia of the air passing through it will cause pressure to increase in the intercooler in the short time interval that it takes for the BOV to open.
You can easily see this at each gear change on the log below.

I have since upped boost to 1 bar (with a manual boost controller) and changed my gauge connection to the manifold side of the throttle body, so I don't see this spike any more because I'm measuring on the downstream side of the throttle, but I'm sure it still exists on the upstream side. Physics!

scmguru · 07-09-2009, 06:17 AM

Bill,

Thanks for the explanation!

Mike- The WG input line comes from the intercooler (stock)

Mike

MBruns · 07-09-2009, 07:36 AM

Thats what I thought, as Bill explained you will get better readings without the spike before the throttle, Just move it to the intake manifold, closer to the middle the better that way its not influenced by a single cylinder.
Mike Bruns

scmguru · 07-09-2009, 04:44 PM

Mike,

I've tried the brake booster line but I may just run a dedicated line for the gauge..

In in other news, I'm pretty sure my A/C compressor kicked the bucket this week.. ACK!

Thanks for the input guys!

1100sport · 07-10-2009, 07:31 AM

I am also experiencing this on my 930.

If I am not mistaken, this is called "compressor surge" and can be cured with a more responsive BOV, like a Synchronic valve (at least that is what they are aiming for). I have not tested them but thought I should report.

http://synapseengineering.com/cart/products/Synchronic_BOV_Weld_Steel_Silver_Black-7-2.html

Mathieu

scmguru · 07-10-2009, 03:16 PM

Interesting development.. I replaced the manual boost controller that runs inbetween the wastegate input and the wastegate vent with a 15PSI pressure regulator (vs. the original 60PSI regulator) and the compressor surge spike seems to have been reduced by about 75% (per the gauge).

A930Rocket · 07-10-2009, 06:10 PM

Sounds like the new controller gives better control.

Quote:

Originally Posted by scmguru

Interesting development.. I replaced the manual boost controller that runs inbetween the wastegate input and the wastegate vent with a 15PSI pressure regulator (vs. the original 60PSI regulator) and the compressor surge spike seems to have been reduced by about 75% (per the gauge).

mark houghton · 07-10-2009, 06:51 PM

Quote:

Originally Posted by MBruns

Just move it to the intake manifold, closer to the middle the better that way its not influenced by a single cylinder.
Mike Bruns

That's where mine is hooked to...manifold side of the throttle body. Very little spike if at all. Note that the stock boost gauge sender is located dead-center in what I think is referred to as the secondary intake manifold (the one with the boost recirc valve incorporated into it).