High Rpm Boost Increase

[Wogan]

Was giving my car a bit of a squirt the other night and noticed something interesting. As my rpm gets to around 6000 the boost starts increasing and continues to do so until 7500 and i change gear. It's ok at the moment because i've only got it set to 12 psi, and it climbs to about 15 by the time i shift, but i do want to fix it because i plan to run 14 or 15 constantly.

I can think of 2 possible causes, and was wondering if anyone can rule out one or the other.

Possible cause #1:

When i fitted my JJR split dump pipe, i didn't check if the divider was effecting the wastegate's ability to open fully. It could be stopping it from reaching it's full open position, and thus my boost begins to increase at high rpm.

Possible cause #2:

A bit of high school physics here... When i put the rb25 turbo on, i had to weld in a nipple to feed pressure to the wastegate actuator (as the rb25 turbo doesn't have it on the turbo like a rb20 turbo).. I've welded it into the pipe just after the turbo, but on a little peice off to the side of the pipe (not in the direct flow of air) .. It could be that at high rpm, the fast moving air going past the section with the nipple on it is causing a pressure drop in that area. Same principle as an aircraft wing.

(sorry if i didn't explain the nipple position very well .. i can post a picture later.)

How can i test if the wastegate is able to open fully?

Should i move the nipple regardless?

[bubba]

low or high gears? mine does it in 4th and 5th but I think that's more a case of 25 turbo on 3L..

[R34GTFOUR]

what boost controller you running?

i would think option 1 is more likely, i doubt there would be a pressure drop big enough to affect the actuator. If you have a compressor handy, just pump some air on to vac line going to the turbo and watch the arm move, see if it is hitting at all.

[Wogan]

only noticed it happening in 3rd and 4th (coz the time between 6000 and 7500 is so small in 1st and 2nd)

just using a turbotech manual controller, but if the moving air is causing a pressure drop of 1 or 2 psi in that area, it would cause the actual boost to increase by that amount.

how can i tell if the wastegate is opening 90% or 100%?

[R34GTFOUR]

i would try and hear it and see if its hitting,

or

measure the amount of movement it has with the dump on, then take the dump off and measure again

If you think there actually is a pressure drop, take a vac line from the manifold and connect it to the actuator, take it for a quick spin and see if it creeps again

[Wogan]

thanks man i'll give it a shot.

[heller44]

be careful with a compressor hose ont actuator as you can damage the diaphram with the unregulated supply. use a regulator to cap it at 20psi or so.

[R31 POWER]

I'd say it's the jjr split dump. Lots of threads have been popping up involving those dumps with your same probs. I even had the same prob on my split turned atmo screamer pipe, chopped off the tiny screamer pipe and put a 2inch one and it's grand

[Wogan]

turns out the different turbo / exhaust combo was causing the actuator to hit the support for the heat shield.. took to the heat shield with an angle grinder and it's all good now.. wastegate was probably opening about 80% .. full range now. yay..

dial it up to 14 or 15 tomorrow, then gettin it dyno tuned monday.

[R34GTFOUR]

good stuff

[bubba]

yay I like happy endings

[Tezzah]

i had a 25 turbo and a jjr split dump/front with a 20 actuator.

my boost went straight to 9psi. then steadily creeped to 14.5psi @ 7000rpm

changed to a non split dump/front and now boost goes straight to 10 and slowly creeps to 12psi.

what exactly did you grind off?

[Wogan]

i had a 25 turbo and a jjr split dump/front with a 20 actuator.

my boost went straight to 9psi. then steadily creeped to 14.5psi @ 7000rpm

changed to a non split dump/front and now boost goes straight to 10 and slowly creeps to 12psi.

what exactly did you grind off?

There's like a plate which does a half loop around the dump pipe.. this plate is there to provide the 2 top points to screw in the original heat shield.

When the wastegate actuator was trying to fully open, it would hit the plate and not be able to reach the fully open position.

To test if yours is doing this, take off the heat shield and remove the circlip that holds the actuator's arm onto the wastegate control. You should then be able to move the little control peice of the wastegate.. turn it around with your hand and see if it hits the plate.

hard to describe.. if you like i can take some photos when i get home from work.

Edited September 25, 2008 by Wogan

[WA_R32]

it would be the dump pipe i would say Tezzah on the forums had the exact same problem and he replaced the dump pipe and it fixed his problem

[Tezzah]

There's like a plate which does a half loop around the dump pipe.. this plate is there to provide the 2 top points to screw in the original heat shield.

When the wastegate actuator was trying to fully open, it would hit the plate and not be able to reach the fully open position.

To test if yours is doing this, take off the heat shield and remove the circlip that holds the actuator's arm onto the wastegate control. You should then be able to move the little control peice of the wastegate.. turn it around with your hand and see if it hits the plate.

hard to describe.. if you like i can take some photos when i get home from work.

I dont have a heat shield on bro.

From what i remember of my engine bay, there is plenty of clearance for the actuator piston/arm to move freely.

And as i said, boost was 9-14.5psi, now its 10-12psi. So it isnt 100% fixed. but 12psi is what i want

Edited September 26, 2008 by Tezzah

[Tezzah]

Now that i think about it more, it could be a combination of both actuator and dump pipe.

Because i really have no way to tell if my actuator is working properly.

Edited September 26, 2008 by Tezzah

[Wogan]

hmm.. that's an interesting situation you have there then...

Not sure i can be much assistance then.. all i know is that in my case, the cause of the additional boost was the wastegate not opening fully.

Best of luck Tezzah!