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I have read on here somewhere that the best type of mbc is a ball and spring type which sounds fine to me but, why wont a needle valve work?

-Mike

It will probably work, but a Ball-and-spring will cost you like 3 bucks in parts, and it works very well.

I presume you're talking about connecting the compressor outlet and the wastegate actuator together with a tee, with the third fitting on the tee going to a needle valve that controls how much air gets bled off?

That'd work. It's a bleed valve boost controller.

A ball-and-spring boost controller works in a fundamentally different manner from a bleed-valve boost controller. Speaking generally, a ball-and-spring MBC does much better at reducing turbo lag, because the wastegate actuator sees no positive pressure until boost exceeds the preload on the spring. They're a little quirkier, though... it can be more susceptible to boost spikes and such. I think they're more sensitive to changes in temperature too.

ahh, I was thinking that you could put a needle vavle between the compressor outlet and the wastegate to control the flow and thus openening the wastegate later than sooner. if it was seeing less flow it would make more pressure before the wastegate would open. no?

-Mike

Not really.... a restriction between the compressor outlet and wastegate actuator would at best cause some boost spiking. The wastegate actuator is controlled by pressure, not by flow. You're right that the wastegate would open later, but the equilibrium condition is still the boost level determined by the wastegate spring.

True, but as boost pressure builds, so does the pressure going to the wastegate. So the wastegate starts to open as boost builds.

With a ball-and-spring, the wastegate is shut until full boost is acheived.

The longer the wastegate is shut, the faster the spool time.

ahhh I get it now. ok thanks guys, but feel free to keep this thread alive if you like lol

-Mike

Well, I'll add one more thing -- if you decide to set up a bleed, it helps to add a restriction to the line to make the leak effectively bigger. Otherwise there's just so much air moving that the leak has to be huge to have an effect.

A friend of mine from England is an STi/Prodrive tech, and when customers came in wanting more power from their cars they would put the bleed off valve in line, compressor to actuator, to thusly increase the boost. They would also use a brass restrictor, similar to a carbuator jet, in the line to stop the wastegate from fluttering on boost and smooth out its performance. It was a bulletin from Subaru to use the brass restrictor. He recommended the same thing for my car, seeing as it is basically the same turbo set up.

I would recomend the bleeder type, I've had a hard time with ball&spring MBCs. I found they are prone to boost spike. Harder to adjust, the bleeder I can mess with all the time. I use a butt conector in front of the T, just make sure it won't get sucked into the wastegate.

ok I'm confused. Do I put the ball and spring valve between the outlet and the wastegate or T it off? It sounds like it will work both ways. I will try the bleed type way and let it fly

Don't tee it off! That will either do nothing or eliminate boost control altogether!

It goes between the compressor outlet and the wastegate actuator.

Wait a minute... didn't you say that it would work with a tee? Listen, lets make this simple....I have a line going from the outlet to the wastegate right now. I have the vf-34 mind you so the waste gate is set to 8 or 10 psi or something. There is already a little brass restrictor in the line (it came that way) so tell me where to put the thingy and I'll do it lol

If the thingy is a ball-and-spring valve, just stick it in the line you have right now (ball end goes to compressor outlet, spring end goes to wastegate actuator).

If the thingy is a bleed valve, put a tee in the line and put the valve on the tee.

ooooooooooh ok, well the ball and spring valve I have is a festo flow control. We have tons of those things here at work. There is an arrow marked on it with flow direction.

looks something like this:



Sorry for the confusion, I swear I'm not usually this dumb

So looking at that, I follow the arrows like the flow of the air from the compressor to the wastegate right?

I don't understand that diagram at all. It kind of looks like a check valve in parallel with some kind of variable restriction...

I don't think a "flow control valve" is usually a ball-and-spring valve. Typically, the kind of valve we're talking about is a pressure relief valve.

If it is a pressure relief valve, then, yes, you plumb it so the arrows point from the compressor outlet to the wastegate actuator. But you also need a very small bleed on the outlet. I usually make these by drilling a 1mm hole in the base of the outlet barb, and making sure the hose doesn't cover it up.

I'll have one of the engineers at work tell me what that thing is. I can tell you that we use them on pnuematic cylinders to control their speed. it does not regulate the total psi like a regulator would but rather conrols the flow of said pressure. Maybe I'm trying to use the wrong thingy

Hm. It sounds like you could use it as a bleeder maybe then.

I might just buckle and order my apexi AVC-R and not deal with this crap

I think you'll be ok with that, it won't take much to see if it works. Just start with it unscrewed most of the way and watch your boost gauge. Keep tightining it a half turn at a time and see if your boost goes up above the wastegate setting.

I built an MBC based on the instructions on this webpage...

http://www.geocities.com/chmwatson/FAQs/mbc.html

...for like $10 in parts from Lowe's and 1 butchered ink pen (for the spring) and they can be made even more cheaply since I have enough spare parts to make a second for more like $5 instead.

The thing works great and has surprisingly little spiking. Boost spikes maybe a whole psi past the target before settling (which is better than many electronic ones do).

Keep in mind the installation instructions are for a DSM and don't really apply to the way the Subaru hoses are setup. Just take the hose that's going from the turbo to the wastegate, split it and put the MBC inline (with the ball bearing end facing the turbo, not the wastegate).

(My MBC is currently installed on the '86 RX I have, but will be moving to the Legacy (or I may just build a 2nd one) as soon as I have a boost gauge in place.)

BTW: As for the hole they mention drilling in the side. I didn't mess with it on mine and it works perfectly fine (though they do state that it's only an issue above 15psi). {shrug}

Oh yeah, almost forgot...also if I had to do it over again, I would not buy the male-to-male coupler and modify it like those instructions show. You can buy a part that is exactly what the end result is that's called a "bushing" instead and is right at the same price as well (though, IIRC they aren't in the air fittings, you'll need to look with the water fittings instead).

The MBCs I've made were actually a lot like those too.

What I usually do is just solder a nut onto the end of the tee, though, rather than cutting threads into something else. I solder the barbs on, too, rather than sealing them with Teflon tape, so there's no way they'll come apart.

Actually, a fine-thread tap of the right size (I think it's 1/2") can pretty cleanly turn 1/4" NPT threads into parallel threads too.

Without the relief hole I noticed I had a little more lag after shifts, so you might want to quickly just drill one into yours.

We are on my subject now: Home Depot Racing
My bleeder cost a little over $3
I also have a Home Depot Racing intake for my airbox

I have seen the apexi avcr for $375 on ebay. does that sound like some sort of a scam or something. I have seen them on regular performance websites for $500. For some reason I dont trust buying stuff on ebay

ok, I made myself one of those poor mans MBC's and installed it tonight. I played around with it until I got to about 12-13 psi. it does pull significantly better than before at the 8 psi but I must say that I'm not really impressed. Should I be dropping the 1600 on the CES tomorrow????? I dont know how to explain how the car feels. It just feels really "heavy" to me. I'll drive it more tonight with the upped boost and see if things change at all.