Boost Issue

[Skinon!]

Just installed a boost gauge in my 2000 BH5 legacy wagon, and i always thought that there was something up with the boost, as sometimes it would seem pretty flat when the secondry is supposed to be working. Anyway i found out my primary is boosting 11psi which seems normal. But when the secondary switches over it will drop right down to 3-5psi, and boost back up to around 11-12psi on the secondary...

Shouldn't the secondary be boosting around 15psi? Everything seems alright except for sometimes a bit of a 'jerk' is felt when you floor it while it's boosting on the primary.

So what should i start looking at? I spose i should do the codes? Im thinkin i would like to replace the O2 sensor and the water temp sensor and also check the AFM and knock sensor as these seem to be pretty common points of trouble...

Anyway any help would be appreciated! Would love to get to the bottom of this?

Actually also reading it could be the cat? It has the standard exhaust on it and done 120k so if the cat is factory it would be rooted anyway right? I have a 2.5inch exhaust from the bottom of the down pipes back, so ill chuck that on this weel maybe it will make a difference.... hmmmmmmmmmmmmmmmmmmmm

[Skinon!]

No ideas? or Advice?

[Guest]

sounds like std VOD

dependant on model but boost should be around 11-12 psi primary and 13-14 secondary

there are SOME models that boost to 16psi secondary from factory

This all varries dependant on model and mods done, ie exhaust on primary side will give up to 7psi gain - which often blows turbos

[Skinon!]

Ah yea thanks Ichi Ban, im pretty sure it's more than the VOD tho, as it sometimes goes good and sometimes feels pretty sluggish (not turbo lag) kinda like a intermittent problem. But i will try bring up any cels and go from there.

Also i have a exhaust i got off trademe, looks pretty good, 2.5 mandrel bent with two resonators, and it starts from the bottom of the turbo downpipes right to the muffler. I read that full exhausts on these are a bad idea, but i gathered that it wouldn't overboost signifigantly if the turbo downpipes were factory?

Thanks for your time Ichi Ban!

[Guest]

My old bg5 used to get confused in the middle of the transition when the secondary joins in... Never back off the throttle during the transition.

what your describing sounds fairly normal transition behaviour... The boost does back off for a short time. It has to as during the transition exhaust is bled from the primary to spool the secondary and in the first stage the secondary bypass valve is open. This should all happen in the space of a few seconds and full boost should resume soon after.

[Guest]

Some times if you are in the wrong gear and the planets aernt aligned you can trapped in the vod, but just grin and grab another gear and marvel at the the twin turbo wizardry. Get a good exhaust and a cold air intake and it should behave better.

[Skinon!]

Hmm yea after spending a bit more time with the car i know what your getting at... It sometimes seems to switch to the secondary turbo earlier than it should and then doesnt hit full boost.

Just strange how sometimes it only boosts to like 10-12 on secondary and others it hits a good 15psi on a same temp day doin the same driving, ie flooring it at 2k till 7.5k on a flat road boost seems to vary depending on how mr suby feels like on the day.

Also i read people getting 650kms a tank in a BH5? They either have a throttle stop after 5% throttle or they know a secret i dont. 470kms is what i get out of a tank at the mo, and thats open road. seems around 400km round town, which just seems odd to me.

Anyway gotta can of upper engine cleaner and 2.5inch exhaust to go on so will see what shes like after that lil bit of love!

[Guest]

check O2 sensor (s) , these can cause real bad fuel consumption when faulty

[Kiwiflyboy]

Yeah O2 sensor and knock sensor caused me crap power and 400km to a tank on open road when I first got my bh5

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[Skinon!]

Yea O2 sensor was first on my list, then water temp sensor. Hey Ichi, about my exhaust question, is that sorta sizing alright for my aplication? as in 2.5inch right through but keeping the factory downpipes? I also got a set of extractors with the exhaust but i cant see the benefit of installing these...

[Guest]

2.5 is fine and best of all it doesnt drone like 3"

Down pipes on primary side has cat in it, which can be removed - but care must be taken to check boost and lower as required

The primary is prone to blowing if boost not set correctly after down pipe change

extractors are a waste of time on TT

(I have assumed that you have a TT when you said about changing downpipes )

[Skinon!]

Awesome thanks for that!

Yea i can remember when these came out, they had issues when people were overboosting the primary turbo from changing the factory down pipe.

So assuming i keep a careful eye on the boost guage (which i do anyway). Changin to an exhaust like the one i have, keeping the factory downpipes on the turbos, the car will gain some boost but shouldn't overboost?

Yea i didn't think the extractors would do anything, factory ones are perfectly designed for what they do anyway...

I have a BH5 rev c, standard except for a boost gauge i put in. Pretty much want to keep it standard mechanically, brought it as a work/ski wagon but couldn't compromise tooo much haha.