You can try adjusting the overall length of the actuator rod, so that it freely slips onto and off of the peg on the pivot arm on the flapper valve. More precisely, the hole in the turnbuckle's flat should freely slide on and off the peg. This will most likely give you more or less 1-12 PSI. This level is referred to as base spring pressure, base WG pressure, or just plain base pressure.

If your turbo is boosting to 14-16 PSI, then the spring is most likely "preloaded" a few PSI above base pressure. So, you could lengthen the rod a few rotations to the point where it just slides on a and off of the peg.

As we know this is adjustment point is called base pressure. This would be academic for you as what you actually need is to solve your boost creep issue. However, it would still be of interest to see if the turbo will creep when the wastegate is set to the base level.

Based on the way that you have explained the phenomenon, I suspect that you'll still be getting creep if you set the thing to base spring pressure.

So, what should you do about it? Yes a properly setup external gate would definitely solve the creep that you are experiencing. However, an external gate is costly. You would probably need a special, flanged tubular TS manifold in addition to the gate itself. So you'll be having to throw Dubloons and Ducats at it.

Creep is an inability of the internal waste gate to bleed off(bypass) exhaust gas flow from the turbine inlet area and on out through the bypass port. This allows excess exhaust flow to continue straight on through the housing impulsing the turbine wheel to spin the shaft and the compressor wheel which produces boost levels higher than the desired target level.

There are ways to eliminate creep that are far less expensive than setting up an external gate on your Evo:

1) Port the turn in radius to the bypass port located in the turbine inlet area of the turbine housing. Most peeps call the bypass port the wastegate hole. This is a pretty straight forward porting procedure and doesn't take more than 20 minutes, once you've removed the housing.

2) While you are at it you should also port the post exducer area or turbine outlet bell mouth on the turbine housing. Why, because sometimes the cause of the creep may not be in the bypass port but, instead a restrictive turbine outlet area may be causing three PSI of back pressure within the turbine housing and this might be the three PSI of creep the you are seeing.

3)Another option is to replace the 9.8 housing with a 10.5cm housing. The 10.5 housing is less restrictive than the 9.8 housing and thus will cause less back pressure. Also, the shaft speeds will be lower at any given engine speed and exhaust flow rate than with the 9.8. Lower shaft speeds reduce the propensity for the turbo to overshoot its target boost level because it spools slower. Finally, it is believed that the single flapper 10.5cm design is more efficient at controlling boost than the dual flapper 9.8cm waste gate design.

4)Replace the restrictive stock cast iron O2 housing for a more efficient tubular design. Furthermore, improve the flow of the upgraded housing by porting the crossover hole. Or alternately, replace the stock O2 housing with an even freer flowing O2 dump.

I would try any or all of the above methods for improving boost control while retaining the integrity of the stock internal gate before resorting to an external gate myself. But, I am a cheapskate.

 

Ok cool, thanks for the replies, I'll look into doing those.

Hm, I wonder getting rid of antilag would help as well. My antilag looks exactly like this (not the picture of my engine):

http://shell.world-net.co.nz/~momnitor/engine.jpg

 

i don't know if you will ever achieve linear boost on wastegate spring only. again, i have seen stock wga on 8 and 9 turbo do the same thing. i have run them in this manner for setting up meth injection kits. adjusting wga arm length will help smooth boost curve a little, but if you remove the pre load your turbo will be slow to spool. you may also find you will have a difficult time hitting boost targets. if you look at a tuned boost map, you will see you bleed more air before and after the rpms that your turbo will hit higher boost on its own.

 

I only wanted him to try running it at base spring pressure, as an empirical test, just to see if it creeps from 11-12 PSI base on up to 14 PSI. That is, I was curious as to whether it would still exhibit creep with zero spring preload. Thus, since he already has disconnected his EBC, then if he sets it at base pressure with the EBC disconnected, and it still creeps, we can rule out the WGA and the EBC as possible causes of his creep.

I pretty much suspect as you do that the problem is back pressure related, or a restrictive wastegate passage. Since he is running the stock restrictive O2 hsg., that alone can cause bypass restriction to produce 3 PSI of creep on a 9.8 housing. Either that or the 3 PSI of creep is actually 3 PSI of back pressure between the turbine inlet and the exducer, i.e. an increase in the pressure differential between turbine inlet and turbine outlet.

Keep in mind that the 9.8 housing is by nature more restrictive and creates more back pressure than the 10.5 housing. Also the dual flapper WG design and the 90 degree turn in on the 9.8 housing are more restrictive to WG bypass flow in general.

However, there should be no need for an external gate setup on a stock 16G/stock motor setup running stock boost levels as the OP is.