Boost Overshoot
 

I have a home-depot MBC, with a tiny bleeder hole drilled in the side. It's hooked up to reference just before the TB.

I get boost overshoot. I would like to be running between 12 and 13psi, but if I set it up to do that it will overshoot to 15 or 16, hitting the overboost fuel cut. I have it set up now to overshoot to about 13 and settle around 10, but I'm looking for ideas on how to get rid of the overshoot completely.

It's an internal wastegate and the actuator limits boost to about 7.5psi when left to it's own devices. I attached a log of the problem.

it's possible sourcing boost after the IC would help with the spike.

You could probably use a helper spring to help keep the wastegate closed with more rpms. what's happening now is, it's not opening enough then it's staying open too far.

I agree with brain, and just get a stronger wastegate, closer to your target boost. I tried running my "14psi spring" with no vac hose and wired closed...no change in spoolage.

Do you have a gt2860rs? The only way I could get mine to stop overboosting was raise target to 14.7psi. In the cold it overboosts to 17 or 18, but I straight don't give a fuck because I'm baller and bitches love me. Goose bottles left and right, bro.

I will look into the helper spring. Is there anywhere to get a replacement actuator can that will hold better?

atp or ebay.

I tried the helper spring with no luck. I couldn't find anything on here about the helper spring but the subaru forums have some stuff on it so I bought a spring and put it on, and stretched it pretty tight but I still get the exact same result. How strong does this spring need to be?

I bought the basic same mbc from NSX on ebay. Never had a problem with boost spiking. It is worth the $15.00.

Good Luck!

There's plenty of info here on helper springs- I posted some. You do need to make a run with the gate wired shut so you have a baseline to compare a gated run too. Then you'll know if there's anything to be had with the spring. If there's not, you won't waste your time.

if I wire the gate shut, it will just make more boost than I want to put into the engine. I have problems keeping the overshoot under the fuel cut as it is, with the gate wired shut the boost will climb until at least 18psi.

Helper springs, a different wastegate, etc. are all bandaids that don't address the real problem and won't help. Don't spend any more $ trying to fix this issue when you can set it up right for under $10 - you are not really having 'overshoot' - you are having drop off. As in, when your MBC is set for 15-16 psi and you are dropping off to 12-13 by redline because of pressure drop across the intercooler.
Read my thread here: https://www.miataturbo.net/forum/t32479/
Go do this and you will be able to set your MBC to 13 psi and you should drop at most to 12 psi at redline.
-Ryan

I did that before I started this thread, it says so in my original post. Thanks for the suggestion though.

FWIW, it does act more like boost drop than overshoot sometimes, but othertimes it acts more like overshoot than boost drop. It's a weird problem.

My reference was to the helper spring. Tying the wg shut will show you what you're spools like w/o the boost overpowering your internal gate and blowing by the valve on spool. If it spools better with the gate wired shut then you know it's leaking during spool up. Then a helper spring will help on spool up- even though it's a bandaid, it works and it's why they're used on gates that have a weak spring. Mine reduced spool up by something like 600rpm and with that I managed to nail target and hold boost to redline with only a .8psi variation from target.

Regarding your current problem- as mentioned above, moving your reference signal to the IC pipe right before the tb can help reduce (or even remove) the spike you're seeing.

I already did that. And I put in a helper spring. So far neither fixed it. Is there any way to know how stiff the helper spring should be?

There's a thread somewhere on here that discusses the spring strength. The second part is adjusting the tension on the spring with a thread rod. Once you know what the turbo spools at with the wastegate tied shut, then you aim for the same using your spring adjustment until you overshoot the mbc target, then go back to the previous setting.

Back when I was fussing around with helper springs I found a variety pack of different strengths at Home Depot. Use a threaded eye-bolt to attach it to the wastegate actuator's bracket and then you can adjust the tension of the spring properly. Then just adjust, test, adjust again until you find the sweet spot.

I'm still not entirely sure what I'm adjusting to. I got a much stronger spring and a threaded eye bolt to play with, but am I really just trying to match wired-closed spool to fix my overshoot/drop problem?

yea know...close the bleeder hole on the valve and see what happens.

I am beginning to get very frustrated with this. I have tried everything that has been suggested and none of it seems to make any difference. I've got a giant spring on the WG and I loosened the MBC way up and it still overshoots. If I tighten the spring, it just does the same thing, 3psi higher.

Hustler, I tried covering the hole in my valve and it doesn't seem to make any difference either.

How about porting the wastegate?

What DP and what exhaust are you using?

Porting the wastegate is something I really don't want to do, and I can't imagine that's the problem since my boost doesn't creep.

My downpipe is a custom 2.5" piece, and the exhaust is 2.5 straight through with a "high-flow" cat.

just get yourself a boost controller. I have one made by blox and it kicks ass. you can turn up the psi too 55 if you want to

tanx 4 reedin teh thred.

As for your problem, I think at this point, we need some pics of the specific parts. Something that you think is fine and normal may actually be a problem, however unlikely it is.

One thing I havent seen mentioned is the boost controller spring. Home depot MBCs are definitely the best thing short of an EBC, which I personally dont care for much anyways...but they still have to work to begin with.

Outside of that, I have no idea. Wouldnt hurt to post pics though :P

stfu dumbfuck queer.

Hmm, today I experimented with the MBC I got from Egay. I run a 14psi actuator. Withouth MBC I spool around 3300rpm and have a nice 14 psi boost, no overshoot. I run the MBC on minimal and I spool by 3100-3200rpm but it will overshoot if I apply throttle quickly from an off-throttle situation. My basis conclusion is that the MBC cannot bleed quickly enough... so I will probably revert to the simple 14 psi wastegate actuator run of the compressor housing and be done with it for this year.

Can I tune a helper spring to get me 13psi without an MBC? I'll take it right out if I can and suffer the 100rpm hit on spool.

I believe this is the cheap way SRT4 guys and such raise boost in their cars. There's an assortment of info floating on the internet about "wastegate shimming." It works, but its cheap and really not the way to go.

I have been trying different things for the past couple weeks and have some conclusions and I'd like some advice on what to do to fix the problem.

1. When my boost signal is sourced just before the throttle body and run through my home-depot MBC, the boost spike is very severe.
2. When boost signal is sourced just before the throttle body and run directly to the the wastegate actuator, there is boost spike but it is not as severe as with the MBC.
3. When boost signal is sourced from the compressor outlet and run through the MBC, there is about the same amount of boost spike as with scenario 2, but I get boost drop as the RPMs rise.
4. When boost signal is sourced from the compressor outlet and run directly to the wastegate actuator, there is no boost spike, but there is boost drop with RPM rise.

My conclusion is that as the turbo spools, it pressurizes the charge piping and by the time the pressure wave makes it to the throttle-plate, the first part of the pipes are overcharged. The overshoot is much worse at higher RPM, and nearly non-existent at lower rpm.

I have pretty much concluded that I need to purchase a real MBC from ebay or something, and that should fix the MBC-related spike, but how do I fix the drop with RPM without introducing overshoot based on the pressure wave?

I hope this makes sense outside of my head...

Good findings, and somewhat to be expected. I havent dont preTB yet, only comp. housing. You didnt log your 4 tests now did you?

I have some logs of 4th and 5th gear runs and i THINK my boost drop off is minimal... how much do you see? I run 14psi/195kpa.

I don't have any logs of any of these runs, so I don't even know how much boost drop off I have. I know there is some, and it's not negligible, but it is probably only a pound or two.

Do you have a fair amount of length in your MBC to WG signal lines? If you do, try to shorten up the signal lines so there's less delay for that air to reach the WG. I reduced my boost spikes by about 1 psi by cutting about 2 ft of excess tubing.

The signal line runs straight across from the TB nipple it's sourced from (in the charge pipe), to the MBC, to the wastegate. The first line is maybe 2.5 feet long, and the second is maybe 8 inches.

Just read through your thread again and don't see any mention of adjusting the actuator arm itself for preload. Is it adjustable? Every actuator I've had responds dramatically to the adjustments - not that it provides stable boost, but always shows improvement over a first try fitting.

I don't think it's adjustable. How would I find out for sure?

Most if not all "new" internal wastegates come with a threaded actuator arm nowadays. Theres a retaining nut on it, usually closer to the wastegate diaphragm.