How to diagnose no boost on a 951?
 

I have an '86 951 for which I just installed a K27, new vacuum lines, new throttle seal, and new BOV.

Car cannot build boost. I have looked around and tried tests.

1. If I suddenly accelerate (while in garage) to 4k rpm, my stock and my aftermarket boost gauge zoom to 0lbs (=no vacuum nor boost), then falls back to -15 to -20.
2. If I slowly accelerate (while in garage) to 4k rpm, boost gauge stays at -15 to -20.
3. Does above mean my turbo is doing something (because vacuum goes away on sudden accelerate) but boost is being lost elsewhere?

What gives?

Visually double checked and all seems good. I tried to pressure test and I might be hearing air being lost around the throttle body but seal is new; can it be an exhaust valve is slightly open and I am hearing air through the exhaust to the turbo? How to decompose the issue?

I am very frustrated....

I you have a stock DME, you could start by putting the engine under load. Drive it out of the garage and see what happens.

I have Lindsey Racing M-tune, so kinda stock DME but different chip.

My wastegate is a newish dual wastegate.

Tomorrow I will try "pinching" the line to wastegate top...I think that means it should maximize ability to make boost, and will of course give gas carefully. Will also go for a drive to see if I hear any new noises.

I read something like a collapsed internally exhaust pipe....can that be? my exhaust from wastegate and back is new (however my exhaust is the Lindsey 3 inch exhaust).

Wont build boost without load. Go drive it and tell us how much boost..

Yup, what they said "drive it".
In my head I still struggle with this. I'm a heavy duty tech (diesel)… and the teacher tried getting the concept through my thick head by comparing turbo to weight lifting; Lets say bench press. If you were on the bench and had no weight, but going through the bench press motion, how much strength (torque... boost) are you exhibiting? Like none. But slap 100 lbs+ on the bar, and now you are building some torque.

I'm willing to make a small wager that if you can eliminate vacuum while flooring it in the garage, you'll be happy with the boost when you drive it under load.

All,

I changed throttle body seal since last drive. Before that I got ~5 lbs boost max. Now, i just drove and get 15 lbs (which is to what my MBC is set). GREAT!

However, I hear a strong WHOOSH sound as soon as I lightened the throttle, is that correct? Any slight reduction in throttle after a boost build makes the whoosh occur. Before with my stock 26-6 and original vacuum lines I never heard that. Boost seems to build quickly but I hear that whoosh. BOV is new and from LR. Is it the BOV? or do I have a small leak? My AFR is a bit offf but ok...still playing with FQS settings.

Thoughts?

Love IT! Everything is totally right... That Whooooosh is the boost venting out The BOV and the Turbo Spooling Down... I have a Cup Turbo on my 86 Street car and get that noise... I love to just "Roll" on the throttle low RPM and Build boost..and let off.. .....Cant help it... Its the juvenile in me that I just cant get rid of....:rockon:

Thoughts? Yeah. Hope mine turns out as good when it's back together. Haven't heard a WHOOSH in a while. Congrats.

Does a bear **** in the woods?

Thanks all! So before my turbo rebuild/upgrade, I did not hear such a whoosh.....at least if it did occur it was much quieter. Previously, my boost gauge did eventually hit 15lbs. What does that mean? It was building boost but barely? I think now it builds boost faster. I changed my vacuum lines, idle controls, venturi delete, and throttle seal change. Thus presumably significantly leak free.

The REAL Scary sound that I get sometimes is when Im barely on the throttle accelerating …. the WHOOOSh isn't there but there is a whirl.... and it sounds JUST like a Police car with its siren on approaching... Caused me to look in the rearview more than once....

Thanks all...boost (by a turbo) is great!!