Boost creep

[86lance]

What are the most common causes for boost creep? Car is an 89 daytona shelby with dual stocker intercoolers 3 inch exhaust dumping after the downpipe and a mbc with a vacuum black from mcmastercarr. I just put in a good quality boost gauge (autometer phantom) Turns out i was only running about 10 psi or so from going off the stock boost gauge. Anyways i went to up the boost a bit today to 12 psi. It spikes to 14 psi or so then settles to 12 where i want it then slowly creeps to cutout. So any ideas?

[Aries_Turbo]

the hole in the exhaust housing that bleeds off exhaust to control boost cant flow enough to prevent the creep. youll have to take off the turbo, port the hole (not too large or the puck cant seal) and put it back together.

or, socket the ecm and burn rob's 89 t2 cal over at D-cal http://autos.groups.yahoo.com/group/D-Cal/ to a chip and install... dont forget the +40's and a 3-bar map. I think thats easier than popping the head off to get the turbo off.

Brian

[86lance]

But you see it didnt boost creep before. I shouldnt have a problem running stock boost on a stock turbo. A vacuum leak can cause creep right?

[Aries_Turbo]

before what? the 3" exhaust? when you add a 3" exhaust the pressure differential is greater and requires the stock wastegate port to flow more to achive stable boost.

Brian

[86lance]

Yet some of the guys running mitsus can hold higher boost with a large exhaust and not creep? My garret must be possible. Also i have retained the stock 2.5 downpipe.

[Dave]

Mitsus have problems with creep. I've never had a problem when I used the good 'ol Garrett. It always hit the boost dead on. I ran 20psi on it all day long and it always hit on. Although mine was an '88 with the 2.25" SV and small can.

You could have a funky MBC. Or what I'm thinking, your WG can is weak. Try shimming the WG bracket and see if that eliminates or helps the problem.

In my opinion atleast 1psi spike is what I consider "normal." 2psi and more is excessive.

[86lance]

well if the can is weak i think i would get low boost correct? Im thinking it is my boost control so im going to try a couple more i have lying around. any idea if i can just hook the grainger valve directly between the wastegate and the intake source?

[Dave]

well if the can is weak i think i would get low boost correct? Im thinking it is my boost control so im going to try a couple more i have lying around. any idea if i can just hook the grainger valve directly between the wastegate and the intake source?

If the can is weak that could be why it's not holding the boost. It spikes because it simply can't bleed the boost fast enough through the WG, therefore it spikes, then it falls down to a lower boost point because the spring in the can just can't hold "14psiG" so the puck just flies open.

Try the boost controller and then try the WG. Changing MBC's is a little easier than shimming the can.

And yes you can hook the g-valve directly from the intake, to g-valve, to WG can. The nipple on the turbo is just a better source IMO.

[86lance]

Alright i can do that. But if i shim the can wont it open the wastegate can more?

[86lance]

alright well i went to the more reliable boost controller and the same thing. If not a little bit worse. Its connected as follows. Vacuum line from wastegate to a .025 mig welder tip. with a tee connecting to the spring side of the boost controller. Then the other side of the boost controller to the vacuum block on my intake manifold. Im pretty sure i have it connected correctly. Do I? So what whats left shim the wastegate?

[86lance]

Now if i shim the can with the some washers they have to go between the Wastegate can bracket and the compressor housing correct?

[86lance]

Anyone else?

[BadAssPerformance]

mitsu or garret?

[86lance]

garret

[BadAssPerformance]

The simple setup:

[Intake]----[Grainger]----[wastegate actuator]

If it creeps bad then you may have another issue/leak in the vacuum or exhaust somewhere as previously mentioned

[86lance]

hmm alright im going out to check everything else.....again

[BadAssPerformance]

see how it runs right off the manifold... we can look at it at Grove.

[86lance]

AH HA!!! found my problem jt. The damned vacuum line coming off of the compressor is torn.....and dangling on my transmission! LOL When i had the trans out for the clutch i knew it looked like it should be replaced and i guess so!

[BadAssPerformance]

Cool, glad you found the leak!

[86lance]

Yeah its tons better now. Still creep some but its about 1-2 psi. So im getting rid of a few more vacuum lines and shortening some other ones that could use a trim. Any other ideas? the creep is really slowly if that means anything. Also there is NO more leaks right now. Unless the hvac stuf is leaking inside of my dash. Everything else should be good as i used the propane test last night.