MBC risks?

[greg donovan]

what are the risks to using a MBC, if any?

[vrg3]

The turbo spins faster at part throttle than it does with stock boost control, so you get hotter intake air even when not really on boost. On my car I've noticed this sometimes causes the car to run a little weird at the threshold between vacuum and boost, probably because ignition timing control is weird. The intercooler probably complicates this.

Some MBCs vary boost level with temperature, so you might not always get the level you want.

If the MBC isn't built right, and it malfunctions and/or fails, you might end up with uncontrolled boost.

[greg donovan]

so detonation is a pretty serious risk then?

is an EBC worth it if you can afford one?

[Brat4by4]

Not really worth it without exhaust. You're just stressin the turbo with all the backpressure.

Should be used very sparingly if you aren't going to get engine management. It is the last thing I am touching personally. If my car can be quick with 9lbs, it will be screaming by time I decide to turn it up.

[vrg3]

I don't know how much of a risk detonation really is.

I don't know much about EBCs, except that I think they're really really expensive. It would be hard for me to believe that most of them are worth it. I could design and build a new EBC from scratch for less than what most of them cost.

I've been using manual controllers for a while and I'm very happy with the bang for the buck; my current incarnation cost 40 cents.

I think I'm gonna try to rig together a really simple electronic dealie like Vishnu's EMI adapter, to make them work a little better. I'm thinking maybe an oil pressure switch can be used to trigger a change in the TPS signal or something.

[Warp3]

Several years ago Sport Compact Car did a comparison of various EBCs and used a ball-spring style MBC to benchmark them. Though a few were a good bit better at controlling boost, several actually performed WORSE than the MBC and the rest were about even with the MBC.

Unless you have a need for "scramble" mode or need to alter the boost curve to change when boost comes on (or any of the other specialized features like that where EBCs have a distinct edge), I personally don't see the big advantage of an EBC for straightforward boost control.

[Kelly]

I don't know how much of a risk detonation really is.

.

Detonation puts huge stresses on the motor, and its usually the cause of grenading motors. When the eplosion happens while the piston is still coming up, somethings godda give.

I prefer the MBC for its simplicity. Fewer parts in the chain to go bad.

[vrg3]

My understanding is that detonation isn't normally combustion during the upward stroke -- that would be really really really bad for a motor of course -- but just pressure spikes caused by uncontrolled combustion ATDC. But anyway, that's off topic.

Sorry, what I meant was that I don't know how much you risk detonation by increasing intake air temperatures at medium load.

[Kelly]

In my experiance, too much boost at half throttle isnt a problem. It doesnt seem to build full boost unless I really get on it. Ofcorse I dont really have a way to watch my intake temps, but I certianlly never get any knock at half throttle. In fact I dont really get any knock until about 15/16 PSI, and thats probably because my fuel pump is getting tired.

What are you using to control boost Vikash?

[vrg3]

I'm using a spring stretched between the wastegate actuator bracket and the wastegate flapper rod. You can see it in this picture:

http://www.graphics.cornell.edu/~v/pics ... legacy.jpg

[BAC5.2]

They run the line off the manifold/BOV instead of off of the turbo outlet.

I wonder if there's an advantage using the line from there, instead of the line from the turbo outlet?

[vrg3]

If you consider part throttle full boost an advantage...

[scottzg]

It is the last thing I am touching personally. If my car can be quick with 9lbs, it will be screaming by time I decide to turn it up.

Second.


...Unless it's a junker, or you're planning to rebuild eventually anyway.

[free5ty1e]

V.... you change a spring when you want to adjust boost? Man... that's ultimate -- so what does that particular spring in the photo result in for pressure regulation, how much boost?

BAC - who are you referring to that uses that setup? I've always seen boost control done from the compressor's outlet, not from the manifold tap. That doesn't seem right....what other effect could that have except to have no chance to push the wastegate open at partial throttle? (or is that the desired effect?)

[azn2nr]

i just recived my mbc in the mail and am probaly going to put it in this weekend.

one thing im wondering though is if you plumb it in to the compressor and the wastegate acutrator will it go to full boost at part throtle. or would you have to plumb it into manifold?

i was under the impression that if you have an mbc thats ball and spring type that it would always be to full boost no matter the throtle position.

after further driving i found that this is the reason why the 39 felt faster than the 16g. with the 39 it would always hit full boost nomater the throtle position. it probaly had someting to do with the teh control lines not fitting sung on the nipples but its an outcome i hope to achive

[dzx]

That's strange, I would think you would still have to push the peddle down far enough to put enough load on the engine to spin up the turbo.
With my manual boost controller, I can control how much pressure the manifold sees by pushing the gas further down or letting it up a bit. Mine is connected to the nipple on the turbo tho.

[vrg3]

Chris - Well, not exactly. I picked a spring and made it work by adjusting it. You can increase tension by cutting coils off to shorten the spring (or by just hooking the spring further down). And you can decrease tension by inelastically deforming it to make it longer. But I don't adjust it. I just got it to where I like it and am leaving it that way. I get about 10ish psi peak boost through the Saab 15-channel intercooler.

A lot of people take their pressure sources for MBCs from the manifold instead of the compressor outlet. It promotes fast spoolup, since the wastegate doesn't open until the manifold is at the desired pressure, which they think is good. Unfortunately, that allows the compressor to overspeed, generating lots of excess heat, and producing excessive boost in order to achieve the set boost even when the throttle plate is partly closed, essentially making the throttle control air density through heat rather than pressure. This is where the complaints that MBCs cause poor driveability and high EGTs come from. The fact is that most of it is due to the way the controller was installed, not the type of controller.

So, Jason, plumb it to the compressor outlet. That way, you will always have full boost at the compressor outlet. The throttle will continue to function correctly, like dzx's. You don't want full boost in the manifold at part throttle.

[free5ty1e]

That makes sense. As expected.

So - does your spring control your boost steady at 10ish psi throughout the gears? Or do you still get a spread based on engine load...? I'll get me a spring if that's the benefit!

[vrg3]

I do still get less boost in lower gears and more boost in higher gears. In first gear I might only get 8 psi, and in 5th gear I might get as much as 10 psi. The spread seems to be less than what I had with ball-and-spring valves, but it's still there. I don't quite understand it.

[simonasaur]

My boost differances are much more drastic

1st-11lbs

2nd-14lbs

3rd-16lbs

4th-18lbs

5th-21lbs

I have a one peice header-up pipe and a downpipe, but ive always wondered why the gears differ so much. Is that the case w/ all turbo cars?

[azn2nr]

woah. thats weird. i just put mine in and its getting full boost 13 psi thorughout the gears.

maybe if you pumbed it into the manifold instead. its not good for the car but at least you wouldnt have the seperation in pressures in different gears.

[free5ty1e]

1st-5th = 10-15 psi with my mbc. I dont know about regulating it like that, I'm not cool with the extra stress that puts on the engine under normal driving conditions.

[vrg3]

Jason - Why would connecting it to the manifold "fix" this "problem?"

Chris - Hehe, well, you could always rationalize it by saying that at higher speeds your intercooler works better, and the strain on the transmission is lower, so higher boost levels are reasonable . But, yeah, a 5 psi spread is a lot. In that kind of situation I'd have to recommend picking a conservative boost level for the lower gears.

Oh, I forgot to mention, another thing I did was to put a restrictor in the hose that I have going from the compressor outlet to the wastegate actuator. It's got a roughly 1mm orifice. I think that might have helped reduce the different-boost-in-different-gears problem, but I'm not sure. I put it in figuring it might help keep the wastegate from flying open so eagerly in low gears.

[azn2nr]

it wouldnt fix the problem persay but only give another one that just might be more livable. 20+psi nomatter how you look at it is pretty dam high unless some sort of tuning and fuel system upgrades

[vrg3]

What I meant was, why would taking the signal from the manifold make boost constant irrespective of gear?