SpeedOfLife

iTrader: (16)

I bought a bleeder type mbc from a forum member and I finally got around to installing it today. I ran about 3' of silicone hose for it. It only raises my boost pressure when nearly completely closed (or all the way closed), and when not all the way closed the raise in boost really only shows up as a spike on the onset of acceleration (for example I'll romp on it in 2nd or 3rd gear, it'll get up to 10psi real quick then die back down to 5psi after a couple seconds, all the while I'm still flooring it), so I disassembled it to see if the direction of flow could make a difference. I discovered that it may make some difference, but doubtfully enough to make it act this way... Btw I didn't have the nerve to go WOT with the mbc completely closed, spiking to 10-11psi so quickly was enough to scare me a bit for the sake of my motor.

I have some questions that may help me figure out what's wrong with this controller, or my car*. I have listened and looked for vacuum leaks in the mbc install and it seems sealed up just fine (and besides, if there were leaks I ought to be seeing higher boost...). I also tried using a new air pressure regulator (for air compressors) I had lying around, I made sure it was all sealed up and that the intake was connected to its respective place, and I could completely close that makeshift mbc and never saw a rise in boost pressure...

Now, I know my wastegate could be stuck open, which I will check on soon, BUT if it were, would I be able to hit the full 5psi often before 3k RPM? My exhaust isn't that free flowing, I have a 2.5" dp with a 2.5" Thrush Turbo muffler between that and the y-pipe that goes to two cans.

-Does the wastegate actuator vent pressure as it's opening/open or is it sealed?
-Has anyone else even seen this issue before? I read a bunch of threads on mbc's and saw a lot of problems/questions, but never this...

Also, once in a while (maybe one out of 30-40 stop light launches) I will hear/feel something awful when under boost, the car shudders while still accelerating. It feels a lot like wheel hop and it seems to boost normally, but I suspect my BOV or turbo is pulsing for some reason. This is under normal, stock boost pressures and seems to happen most often in first gear. So it could be wheel hop, as it's winter and the roads can be messy, but normally when I lose traction when I'm gunning it the tires just plain spin instead of hopping... I'm pretty sure this isn't caused by the mbc, as it's happened before I installed it, but I want to get that issue resolved, too.

*'87 TII w/ Rtek 1.7, 720cc secondaries, 255lph fp, ported wg, some exhaust work w/ downpipe, pulled emissions, pulled OMP, 91k miles, turbo is in good shape, external BOV

I really want to get this resolved, because having 8-10psi of boost available at a moment's notice would ROCK. This has basically just given me a taste of what this car can really do, it's so frustrating that the boost falls away like that... I can't wait to see my wife's face when she gets sucked into the passenger seat.

RETed

...and there's your answer.


-Ted

SpeedOfLife

iTrader: (16)

Even though I'm able to jump to 10 psi but can't seem to sustain anything above 5psi? In my mind that's certainly within the realm of feasibility, but an unfortunate PITA, as I just built this exhaust and now it looks like I'll have to change it again...

SpeedOfLife

iTrader: (16)

I'd still like an answer to this question and my other acceleration issue...
-Does the wastegate actuator vent pressure as it's opening/open or is it sealed?

arghx

iTrader: (3)

first question: do you have the 3800 stumble elimination on your Rtek 1.7? it's a fix they just released a little while ago. It is recommended for all cars I believe.

second: you're really expecting way too much precision from a manual boost controller and a stock wastegate and hotside that just don't flow well.

I had a full 2.5" turboback with no cat. I disconnected my wastegate (had race gas in) and spike to 15 psi but still fell off to about 6 or 7 at redline.

MaczPayne

iTrader: (5)

The wastegate actuator is sealed, there is no venting of pressure.

You might want to invest in an EBC for better control of the wastegate. I had initially thought about an MBC during my build, but I was glad I went with the EBC since I can fine tune boost consistency.

SpeedOfLife

iTrader: (16)

Just the other day when I installed a couple gauges I finally got to the ECU to get the serial number for them, they wouldn't send me a chip without that number. I'll have that soon.

I don't think precision is the issue, but I have strong evidence that agrees anyway. I changed up my exhaust today so it wouldn't be quite as restrictive and I've already seen some moderate improvement without even having to close the mbc more than halfway. That is pretty crazy about your old exhaust setup, but IIRC don't mildly modded 7's often lose a little boost near redline?


Maczpayne, thanks for the info. And I do want an EBC, but I need to focus my money on some other pressing issues. I probably would have bought a straight pipe to replace this old muffler instead of gutting it if money wasn't so tight. We're doing ok, we just have to be aggressive with our debt right now, and I already had the welding wire (...and the electricity in my garage doesn't cost us...).

Pics of what I did today:

SpeedOfLife

iTrader: (16)

I decided that wasn't enough for me, and I felt like cutting/grinding/pounding/welding some more. So I did.



Whoever built my y-pipe was sloppy, because while they cut the angles in the exiting pipes, they twisted one of them so bad that it cut off about 30+% of the other pipe; terrible for flow even if the pipes had been bigger. To employ the eternal words of Bender: "Needless to say, I was mortified." I remedied this, and now I'm actually seeing some boost creep. I'm hitting around 7psi normally and I'll see about 9 psi, possibly more if I would keep going in fourth (the lower gears don't get enough time to see 9psi). I used a piece cut out of the muffler to cover the y-pipe's wounds.

I have a question about boost creep though, say I creep to 9psi at a maximum and it runs fine there, if I set my mbc to push me to 8 or 9 psi, could I creep above 10? My logic tells me 'no', but I wanted to ask to be sure. If the answer is 'yes', I would appreciate an explanation.

I'm also considering welding a resonator onto my exhaust, and I did some calculations at work having to do with the resonant frequencies of my exhaust coupled with the known noisy/buzzing RPM and discovered the buzz at 3k RPM is caused by exhaust pulses around 100Hz (which must be shaking the exhaust creating some high pitched buzzing, because I can hear the low tone then when it hits resonance it also buzzes). I figure the resonating wavelength is about the whole length of the exhaust (versus twice the length or 1.5 times the length), so a good place for the resonator would be right at the middle shouldn't it? I would probably weld my own resonator again using scrap pulled out of that muffler and maybe some other junk.

arghx

iTrader: (3)

I don't have a hard and fast answer as to whether setting the controller to 9psi will then raise the max boost even further due to creep. You get creep any time you're flowing more exhaust than the wastegate can effectively move. Thus the biggest thing about boost creep is that you can pretty much guarantee it will fluctuate based on ambient conditions, and get pretty bad in the winter. I had a 20psi boost spike in 30 degree weather, and that was with a ported s5 wastegate.

SpeedOfLife

iTrader: (16)

Well it has been quite cold here lately, some days with highs of ~5 degrees. Perhaps in the summer I won't even be able to hit the boost pressure I want... I'll just have to see I guess. Hopefully by then I'll have a motor rebuilt, too, probably a good time for some more changes.

SpeedOfLife

iTrader: (16)

I was considering building an MBC with a ball bearing and spring, or modifying my current one to be that way, but I stumbled upon one detail that I figured would probably do the trick and take a lot less trouble. I simply drilled a tiny hole into the turbo side of the MBC so it would relieve the wg system pressure down to a minimum of 0psi. I used a 1/16" drill bit JUST TO START the hole then finished with a tiny diamond tip on my dremel, I knew 1/16" would be way too big to control effectively. Anyway, now I can have the MBC mostly closed and hit 9psi and stay there. I'm pretty happy with that and figure that 9psi gives me a little room to spike just in case that happens. If the hole ever gets clogged then I'll simply boost less, meaning that it's completely safe unless A) crud gets into the motor (unlikely, and if it did it wouldn't be much or very big) or B) it plugs the entire hose (also very unlikely).