Question on Legacy B4 boost

[Andy_Mac]

i just ran a can of upper cylinder cleaner through the engine and was giving it a go to see if there was any noticable difference.

seems a fair bit more responsive in the low range so it looks like its done its job.

one thing i noticed, which i\'ve never tried before so don\'t know if its right or not, going up a bit of a hill on the motorway doing about 95 in fifth gear i floored to see what it would do. The revs went up to about 3400rpm and i noticed the boost go right up to ~15psi. Surely thats way too low for the secondary to be kicking in, or under full load will it open earlier as there would be enough exhaust gas for it?

i can\'t imagine the primary would be able to go that high without something catastrophic going wrong.

[Kiwiflyboy]

That\'s quite normal for the primary, once it\'s actually up and spooling it should be around that level. Most of the time you never notice it as the rev are going up quickly. Initially it hits around 10psi then around 3000rpm increases to its max.

[Andy_Mac]

i was under the impression ~10psi was the max for primary and then ~16psi when both are up and running

[Marky]

Nope, it seems to vary between cars a bit depending on what\'s been opened up on them. Stock 10psi is about normal

The pressure isn\'t directly related to if its one or both turbos working, 5th gear full throttle it\'s under a bit of load so may overshoot - especially if you have free-ed the car up a bit. A lot of them see around 13-15psi mark with air filter/exhaust etc done

Basically both turbos share the same wastegate - if you remove the factory boost control which is staggered (to give lower on primary than when both are active) and run a seperate boost control, you will end up with the same psi on single or twin operation.