Hey guys question.....my boost drops a soon the vtech engages...2nd gear and 4th will hold just fine...but when the vtech engage ar 5500rpm the boost starts to drop! Peak 18 and then at 7000rpm I'll drop to 13.8! Then in 4th it holds nicely at 17ish....I just do not understand the reason behind this! I have done datalogs with both with vtech and with out vtech and the nonvtech log the boost holds great...every gear is perfect but with vtech it dropps....driving me crazy!!

 

turbo size?
wastegate vacuum source?
wastegate size?
motor?
intercooler size?

 

^^ Also, what camshafts and engine? Sometimes this can be caused by extra camshaft overlap on the larger VTEC lobes. I've experienced that in a few setups where VTEC would activate and a loss of 1-2psi would be logged briefly. In your case that doesn't sound as likely. More likely related to the turbo setup.

 

blowoff valve could be opening up also? He never said if it’s always done this or just started the problem

 

Both of you guys could be right...the blow off valve is on the really really solf setting and it could be opening under pressure...this has always happen

 

The logs you sent me don't look too bad. I do see the steady drop in boost from 17 psi down to 13 as you're revving it out.. If you have a restrictive exhaust manifold and downpipe setup it could be the back pressure causing the boost drop as it cannot get out enough exhaust gasses but I doubt it since you're boosting less than 20 psi.

 

Hey OP, what Cams are you running.

 

Stock z6 cams

 

Well That throws my theory out the window. Let us know what you end up finding on it. I wouldn’t know what would cause a drop in vtec only

 

Stock springs and such too? What's your exhaust setup?

 

Yea bro...stock head...just bottom is built

 

What engine management? Also, what are your VTEC settings for low/high load. Screenshot and put them up if and when you can.