stock turbo + MBC = boost taper theory

[Kracka]

I have a theory I've been working on lately, and I'm wondering if a larger bleed hole in a MBC would lead to less boost taper in the higher RPM's. Obviously the taper is good for some of us on pump gas, but for the E85 guys or the guys running larger stockframe turbos on pump the extra boost up top may be beneficial.

Taken from a tech article by Perrin regarding their MBC:
"One key thing to our MBC (Manual Boost Controller) is our bleed hole. In the stem is a bleed hole that serves 2 purposes. One is to relieve pressure from behind the ball when the turbo is not making boost. If this wasn't there the wastegate would be stuck open after the first time it made boost. The second purpose is to bleed some of the volume of air flowing through the MBC. This helps with keeping a steady boost curve that doesn't drop off severely at higher RPM."

What would the drawbacks be for a bleed hole that's too large? More lag? Less precision? Has anyone tried enlarging their bleed hole?

[Kracka]

I definitely think each individual application has an ideal bleed-hole size; going too large could obviously hurt the MBC's performance.

My thinking is a slightly enlarged bleed hole would allow the wastegate to snap shut more quickly under high boost conditions, in turn allowing the turbo to run slightly more boost. I'm thinking the bleed hole may not be large enough to let enough air escape which ends up just holding the wastegate open longer causing the taper.

[Halon]

Dumb question, but how does a bleed hole help keep a steady boost curve at higher rpm's at all?

[Kracka]

Originally Posted by Halon

Dumb question, but how does a bleed hole help keep a steady boost curve at higher rpm's at all?

Boost control is achieved by the wastegate continually opening and closing, to close, the MBC (or 3-port) must be able to bleed off pressure.

[Archertsi]

Ha, this is a great topic. I have experimented with this myself. I do believe the bleed hole on each MBC is drilled to a correct size to keep a certain ratio with it's inlet/outlet port. But I also think it can vary with each cars set up. Different size vacuum hoses, the size of the hose barb fitting supplying boost to the controller, the hose barb on the wastegate, etc.

On my friends first gen with a E316g (stock WG flapper and actuator) we have been trying to solve boost issues. The car builds 30psi no problem, but tapers down to 23-25 psi by the end of the gear. We read about different theories like the wastegate being blown open causing it to taper off. Or that the turbo could be spiking the boost until the wastegate catches up.

Either way, we had to test to try and figure out what it was. My theory was that the boost was spiking to 30, and then tapering back to what the MBC is actually set to (23-25 psi) because the WG was opening too late. If you look at the boost inlet port on the MBC, there is a restrictor, the hole in the center is very small. By enlarging this hole, this allows more air to push on the ball and WG diaphragm, sooner . The down side is the additional air cannot be bled off through the stock size bleed hole. So when boost drops down, (WG open) it takes a little longer for the WG to close and raise boost back up.

So by enlarging the bleed hole (along with the hole in the inlet port) we allow more air to actuator sooner so it can work faster, opening the WG sooner and preventing a spike. No spike allows you to raise the pressure that the MBC is set to. But the holes need to be drilled in the correct ratio to each other.

Instead of 30psi tapering down to 23 where your MBC is actually set to, you set the MBC to 30 and leave it there. Testing what pressure your MBC is actually at can be verified with compressed air and a regualtor hooked up to the inlet port of the MBC.

The testing we have done so far has not yielded success yet (remember, these theories are just what we think)

Another thought I have yet to test is a WG actuator with a softer spring. A softer spring would take less air to open the WG acurately.

Think about it, for years people have stiffened WG springs to prevent them from opening early. But you loose some control that way.


Kracka, you asked if you get more lag. Well yes, you may. But to hold high boost through gears like 3rd, 4th and 5th is probably more worth it IMO.

Just my thoughts on the subject but I think this thread could lead to some great brain storming for those who are open minded enough and fully understand the theory of an MBC.

[turbotalon1g]

A little note to your 16G theory.

I was experiencing the same thing with no boost controller.

I'm interested in your softer spring experiment, no one has tried it.
Something that people have tried is a higher psi actuator and it has held a lot better, this will be the route that I will try out I think.

[Archertsi]

A higher psi WG actuator means a stiffer spring. People install these to prevent lag. Stiffer spring keeps the flapper closed until the last minute, that first few pounds of boost doesnt let it open early causing a slower spool up since some exhaust can escape.

But my theory is that it also causes the WG to open too late so the boost spikes until finally the WG opens fully and lets enough exhaust out and the turbo slows down.

I think it's a trade off either way with an MBC. Softer spring in the actuator may give you better control of peak boost, stiffer increases spool up but you have to deal with the spike.

Maybe Kracka could explain a little more about what Perrin is trying to claim. It looks like they are trying to say. My theories could be totally wrong if they have something figured out and it's proven to work?

[FattyBoomBatty]

Why not put another ball and spring screw where the bleed hole would have been, but let the second ball and spring vent to atmosphere. Maybe that would allow you to tune the boost taper better?

Just a guess, but easy to implement for experimentation.

[jrohner]

Increased exhaust backpressure is where the boost taper comes in mostly, but air seeping by the MBC does attribute to it as well. My 16G could only hold 22 psi, I gained a few psi by using a Holset WGA. If your wastegate spring is too light, you can also end up with the wastegate fluttering at certain RPMs; you'll see your boost gauge bounce around a lot.

[Archertsi]

Originally Posted by jrohner

Increased exhaust backpressure is where the boost taper comes in mostly,

If your wastegate spring is too light, you can also end up with the wastegate fluttering at certain RPMs; you'll see your boost gauge bounce around a lot.

Can you explain the back pressure theory a little more? Im not sure I understand what you mean, even if back pressure wasnt letting the turbo push air into the engine, the turbo can still make pressure in the intake plenum.

Also, is the flutter from the back pressure pushing the flapper open? Then boost drops and so does back pressure so the flapper closes again?

[EnjoyDriving]

Interesting topic, and how many people are seeing successful results from using the aftermarket WGA??

[Kracka]

Depends what you mean by successful. They will hold boost better in the upper RPM's, but you'll generally also see a larger boost spike depending it it's rated pressure. I'm thinking a 17psi one would be fairly effective for guys running 25+ but we'll really have to wait for someone with experience with one to chime in.

[CarPsyco84]

Internal gates are pretty well known to get blown open at higher rpms because the exhaust pressure pre-turbo is higher from the higher airflow through the engine. I used a hardware store spring on my wastegate back in the day, and It ran around 17psi with no boost controller and it seemed to hold the boost more stable than without it. Without the helper spring it would spike 2-3psi higher and then fall back.

[Shane@DBPerformance]

Food for thought:

A turbo can only flow so much air. When you hit that limit the boost will drop as you go up in RPMs.

Boost is a measure of restriction. You make things flow better and you make more power with less boost, because there is less restriction.

Sometimes trying to force more boost up top results in less power or a lot more knock due to excessive back pressure in the exhaust housing from overrunning the turbo like crazy. One of the jobs of the WG flapper is to open when exhaust back pressure gets too high.

[badinblack]

Another idea is a hole drilled at an angle so it would have a progressive port like a 2 stroke port?

[EnjoyDriving]

I'm not sure if the "tightening arm" will help the tapering "issue". In my thinking, the shortening is to reduce the amount of "opening" on the flapper and also make it shut tighter and faster, therefore improving the spooling?? But on the other hand, won't it make the wastegate easier to crack open by preloading it??

With a stiffer spring, like getting a Forge WGA, it would definitely hold the boost better, but won't it be more prone to the "boost spike" problem?

Please chime in on "tightening arm" vs "stiffer spring"