I don't know anything about that particular boost controller, but I notice there is a thread in here about it.
That person had kind of a similar problem. He found out he had no preload at all on his actuator rod. So he added some preload and that helped. But still had a boost spike - then the thread ended. Anyway, it's worth a read.

patman said:

Hi guys!

I have a 2G GS-T and I installed a 16g turbo last year. I noticed some major cracks on the exhaust side so I changed the whole exhaust manifold and turbine housing to new, changed the wastegate and finally went for the test drive. So the situation is this:
-I have 16g with Fortran aftermarket wastegate (1.0bar spring)
-For boost control I have an oldschool Blitz dual-sbc (spec s)

I aim for 1.2 bar so I started setting up the boost level. Until around '78' it all goes linear and reaches about 0.7 bar. Then around '80' it has a spike at 1.0 then goes back to 0.5 then back to 1.0 in a never ending cycle. So the car is basically twitching under boost.

I noticed that '83' would be the sweet spot, I can reach 1.2 bar there, but then the oscillation between 1.2 and 0.5 start again. It's almost like the SBC is struggling to keep the boost.

Do you have any clue what I should try? I have my gain set to 9 (10 would be the recommendation, but it doesn't make a difference) and I have a warning at 1.3 bar which works but I am not really reaching it.

I would love to hear some input if anyone is familiar with the SBC. In theory it should be a good combo with the aftermarket wastegate and completely independent from the ECU.

Click to expand...

I'd start looking at the BOV

patman said:

I aim for 1.2 bar so I started setting up the boost level. Until around '78' it all goes linear and reaches about 0.7 bar. Then around '80' it has a spike at 1.0 then goes back to 0.5 then back to 1.0 in a never ending cycle. So the car is basically twitching under boost.

Click to expand...

patman said:

I noticed that '83' would be the sweet spot, I can reach 1.2 bar there, but then the oscillation between 1.2 and 0.5 start again. It's almost like the SBC is struggling to keep the boost.

Click to expand...

This sounds obviously something is not right.
You should run a boost leak test and triple check that there is no boost/exhaust leak and the controller is installed properly and the valve timing is set correctly first before playing with the controller.
The boost target number close to 100 means you are supposed almost maxing out the turbine through the controller but your boost pressure even doesn't reach at the actuator spring pressure.
The actuator is 1.0bar which means even you set the boost target at zero and the gain also at zero you should have a 1.0 bar steady boost pressure.

After making sure everything is as they should, go drive the car and make sure that you have a steady 1.0 bar boost with the controller off or set all zero. If you wouldn't get a steady 1.0bar boost, then you still have some issue with the car or boost controller installation.
Once you have a steady 1.0 bar boost with the controller off, then start to raise the boost target number little by little until it reaches at your desire boost pressure and start the gain from somewhere around 5~10. When you have the desired pressure, make the boost curve as you like by adjusting the gain. If you would like to spool quicker, raise the gain number as you like. Usually when you raise the gain number, you would have the spike higher. If you already have a hard spike, then try to lower the gain number.

Thanks for all the help guys, but something is still very much off. A checked every vacumm line, it's not too complicated, and everything is intact. I raised the wastegate preload a bit, and played with the settings on the boost controller but the waves are still on. Sometimes. After theese adjusments I found out that If I do a pull from a 5th gear, from lower RPM, slow spool, everything goes allright. The turbo spools slowly, but eventually it reaches 1.0-1.1 bar and the car goes well. But If I do a 3rd or 4th gear pull from higher RPMs, it stil does fluctuate between 1.6-0.8 bar. It's like something is leaking. I captured the thing on ECMLink you can clearly see the green line (boost) is going up and down under WOT



It's so periodic, it's weird. It tried attaching the log file but I don't know how. The knock was 0.7 so not that big thing as far as I'm concerned.

Next step I will start changing my vacuum tubes, maybe there is a crack somewhere, and I will check the membarne in the wastegate, maybe that is not set properly.

patman said:

Thanks for all the help guys, but something is still very much off. A checked every vacumm line, it's not too complicated, and everything is intact. I raised the wastegate preload a bit, and played with the settings on the boost controller but the waves are still on. Sometimes. After theese adjusments I found out that If I do a pull from a 5th gear, from lower RPM, slow spool, everything goes allright. The turbo spools slowly, but eventually it reaches 1.0-1.1 bar and the car goes well. But If I do a 3rd or 4th gear pull from higher RPMs, it stil does fluctuate between 1.6-0.8 bar. It's like something is leaking. I captured the thing on ECMLink you can clearly see the green line (boost) is going up and down under WOT

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It's so periodic, it's weird. It tried attaching the log file but I don't know how. The knock was 0.7 so not that big thing as far as I'm concerned.

Next step I will start changing my vacuum tubes, maybe there is a crack somewhere, and I will check the membarne in the wastegate, maybe that is not set properly.

Click to expand...

elg files can be attached like a photo