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Old 03-29-2005, 11:46 PM   #1
blackie
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Default Is their any way to get a MBC to taper the boost torwards redline?

Does anyone know you can get a MBC to taper down the boost torwards redline like the oem BCS does?
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Old 03-30-2005, 12:09 AM   #2
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nope, not unless the turbo itself tapers due to flow capacity.
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Old 03-30-2005, 01:11 AM   #3
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Biggest problem with the MBC. No control. It's a spring. It has no way to know you are approaching an RPM. It only responds to pressure on the boost side of the turbo. What you want is an Apexi AVC-R or EM like a UTEC or similar.
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Old 03-30-2005, 01:22 AM   #4
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Quote:
Originally Posted by 2phless
Biggest problem with the MBC. No control. It's a spring. It has no way to know you are approaching an RPM. It only responds to pressure on the boost side of the turbo. What you want is an Apexi AVC-R or EM like a UTEC or similar.
hmmm my mbc has perfect control, allittle to perfect it holds the set boost pressure perfect all the way to redline. but ive heard from a few people that this could be bad, but im also wondering if that stands only with the factory turbo, because my vf-34 is still efficent at 17psi.
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Old 03-30-2005, 07:13 AM   #5
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The problem with running full boost to redline is usually that turbos become inefficient, but also a lot of tuners will detune the very upper limit of the powerband because a knock incident up there could be EXTREMELY bad if you were running full boost/timing values.
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Old 03-30-2005, 09:15 AM   #6
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The original poster is trying to get it to taper. The way I interpret the title and first post is that he WANTS to lose boost (I know,I'm baffled too.) And as far as I understand the concept, the MBC can't do that.

A MBC on the right turbo can hit and hold a target boost. No big deal. But he isn't asking that. He want's the MBC to hit target boost, then slowly loose boost. I don't know why. Sounds silly to me. But I don't think it can be done at a pressure the turbo is capable of holding 'til redline, like 15 psi or whatever.

If he wanted to set the boost higher, above the PSI the turbo could hold with the wastegate line plugged, then sure, it'd have to taper off, like Timdog said in his first post. Set the boost for 20 (don't really! I'm just thinking out loud here.) And it'll hit 20 and come 4500 rpm or so it'll start falling off to it's happy-place at 15 psi or so.

So Blackie, if that is your real name... Why don't you tell us what you are working with (turbo, MBC, relevant mods) and what exactly you are trying to achieve. (Why do you want to taper boost off?)
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Old 03-30-2005, 12:37 PM   #7
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Quote:
Originally Posted by 2phless
The original poster is trying to get it to taper. The way I interpret the title and first post is that he WANTS to lose boost (I know,I'm baffled too.) And as far as I understand the concept, the MBC can't do that.

A MBC on the right turbo can hit and hold a target boost. No big deal. But he isn't asking that. He want's the MBC to hit target boost, then slowly loose boost. I don't know why. Sounds silly to me. But I don't think it can be done at a pressure the turbo is capable of holding 'til redline, like 15 psi or whatever.

If he wanted to set the boost higher, above the PSI the turbo could hold with the wastegate line plugged, then sure, it'd have to taper off, like Timdog said in his first post. Set the boost for 20 (don't really! I'm just thinking out loud here.) And it'll hit 20 and come 4500 rpm or so it'll start falling off to it's happy-place at 15 psi or so.

So Blackie, if that is your real name... Why don't you tell us what you are working with (turbo, MBC, relevant mods) and what exactly you are trying to achieve. (Why do you want to taper boost off?)
Heres the thing i really dont want the boost to taper off but everybody keeps telling me that its bad for my turbo and motor to run a full 17psi to redline??? i always thought it was a good thing to to maintain a constant boost pressure, but people are telling me otherwise saying that i will make more safe power if it tapers down torwards redline.
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Old 03-30-2005, 09:27 AM   #8
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Didn't notice that that middle post was by Blackie.

See, we are defining control in two different ways. The MBC has control over the boost. You do not have control over the MBC except to set one boost level.

The FBC is controlled by the ECU. So it can "do" things. The EBC has another issue in that it is a bleedoff boost control and with the skinny little lines it can only bleed-off so much pressure before some of the pressure ultimately pushes open the wastegate and starts that taper as you approach redline (or maybe the ECU does it... all I know is thay my FBC under utec control, with much bigger bleedoff lines has to try hard to maintain boost to redline)

The MBC is inline with the wastegate control line. It interupts to flow to the wastegate and so yea, it has more control. But as far as what it wants to do with that control, you can't change "on the fly".

With the TurboXS dual stage MBC you might be able to setup a situation where both are activated (high boost set at 17) and when you reach a specific RPM a shift light could turn the circuit off (back to low, say, 15) dropping boost.
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Old 03-30-2005, 09:59 AM   #9
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You have pm back blackie. You don't need to taper the boost on your car. Your ECU is set up for 17 all the way to redline so run it.
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Old 03-30-2005, 12:12 PM   #10
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Ok you guys im going to insatll a wide band 02 sensor today in a few hours and run my car with the mbc set at 17 psi and check the real time afr's to see if this actually going to hurt motor if i dont taper my boost slightly like the FBC. what are some safe afr's with my set up? what would be considered to lean?
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Old 03-30-2005, 12:13 PM   #11
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also should reset my ecu after installeing the mbc or just leave it like it is when testing with the wideband?
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Old 03-30-2005, 02:20 PM   #12
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It can be done with at fixed bleed. The bleed off of the wastegate line is of a fixed size. As boost increases the resistance of air traveling through the hole also increases. There's you taper. Couple this to a solenoid control and you can tune for pretty much any boost peak and any taper by varying the size of the bleed hole and the duty of the solenoid. If you are using the stock solenoid and have no control over its activity level, you can still induce a taper by varying the size or number of holes, however it's a lot easier if you can adjust the both the hole size and the solenoid duty.

Just for a picture of what's going on. Imagine blowing through a straw. Blow softly and air flows nicely, not very hard to blow. Now try to blow very hard through the straw, yes more air comes through, but you really have to push hard. See how that "tapers" as more CFM comes off of the turbo outlet.

The resistance to air flow is inversely proportional to the size of the hole and it increases exponentially. So, very tiny changes in the size, number, or holes has a huge effect.

I did this for awhile by poking holes in a bit of plastic tubing. I had a bunch of tubes with different sized holes or more than one hole. You figure out very quickly what works by just driving. One has to just be careful to overboost if the hole is too big.

Hope that helps.
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Old 03-30-2005, 04:21 PM   #13
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I've been shooting for high 10s AFR at WOT. I run 93 octane and 20 psi though. You might be fine with low 11s at 17.
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Old 03-30-2005, 05:37 PM   #14
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Ok i put a wideband on my car earlier today and got some pretty good results with the mbc. set at 17 psi here is what happened average of three runs. during boost buildup say around 10-12 psi at around 3000 rpms the afrs averaged around 12.0-12.1 but by by the time full boost at 3800 rpms it would drop to 10.8-11.1 then it would contiue to get richer hiting around 10.3-10.5 by redline while hold full boost 17-18psi max! The car ran extremley well even in the texas heat. And check this out at one time i had the boost turned up allittle to much to where it would hit 20psi at redline and the AFR's stayed at 10.8 , but im not trying to go that high because the car wasnt specifically tuned for it, so i tunred it down a notch. im very happy with the very consistant 17 psi in every gear, and my egt's didnt seem to get any hotter as the rpms rose so im going to keep the mbc now with allittle peace of mind.
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Old 03-31-2005, 07:27 AM   #15
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you have seen firsthand the benefit to peace of mind that having a wideband afr meter can give you.

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Old 04-01-2005, 09:06 AM   #16
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Just a thought...instead of a MBC you could try to use TXS's ABC and the FBC will still be in effect. I used this when I was at Stage 1 to acheive target boost and it worked great.
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Old 04-11-2005, 02:02 PM   #17
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i dont know if you can taper it with a MBC but mine started to taper this past saturday also i couldnt hit peake boost of 15.9, it would hit 15 and back off to 13.5 or so, im thinking its the heat and hopeing its not something else!
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