OK, I have been going back and forth on this subject for a while and really have not come to a conclusion. At first, I figured the mechanical system of boost control would be the way to go since it eliminates the vacuum pump and related boost control equipment. I am all for simplicity as there are enough black boxes on my GM to to keep me busy. And, I reasoned that the boost curve would be linear due to regulation by exhaust volume, which I imagine is not as dynamic as the electronic control requesting boost.

But, every once in a while, I hear from one or two people, whom I consider well informed, that regulating boost pressure based on exhaust back pressure is not the best approach. However, the sentence usually ends there with no discussion as to why this is true.

Similarly, electronic control has been cited by other respected individuals as better performing without a real discussion as to why.

Now, my intention is not to start a flame war between the two camps of thought. I would like to know the pros and cons of each method of boost control with a heavy dose of theory and maybe have some experiences posted as well.

In the end, I will probably base my ECM upgrade decision on how it controls the boost with all else being equal.

BTW, I am not aware of any dyno data comparing the two methods. It would be interesting to see the following runs on a graph (boost actual and requested, hp and tq):
-Stock boost level electronic control
-Stock boost level mechanical control
-Enhanced ECM modified for mechanical control
-Enhanced ECM modified for electronic control

If by dynamic control you mean dumping boost as fast as you make it then yes it's in control. With a boost fooler and/or chip you can do better but pumps fail and it's just one more wheel to turn off the crank. I like controlling boost with intake pressure but I don't have any experience with it so I'll leave that to someone else.

Both popular types are the same type, simple, easy -

mechanical uses a pre-cal spring, sometimes adjustable, against exhaust back pressure

PCM uses digital control to actually regulate atmospheric pressure against exhaust back pressure.

Which is cool, because the vacuum pump lowers Baro on the control side of the diaphragm, while raw Baro forces the working side of the diaphragm against the rod, which holds the wastegate shut against exhaust back pressure.

PCM, MAP sensor, and WG Solenoid make up a true digital vacuum regulator - which, by default, is also a true exhaust back-pressure regulator.

Wanna see how much force is available, just from the air we breathe?

Multiply Baro at 15psia times the area of the diaphragm\piston, at about 2"dia, for the available force holding the wastegate shut.
The wg arm is 1:1 ratio so that force is directly applied to the actual wg flapper

PCM wastes as much exhaust as necessary to maintain desired Boost.
Whether you're stones-to-the-wall, pedal-to-the-metal exhiliarating rapidly, or just cruising, PM is always wasting exhaust past the wastegate.

Simple flick of the wrist, and 20psi Boost is easily available with the stock PCM on the stock fuel curves.

And, is easily controlled from inside the cab - try that with an adjustable spring, which holds the wg closed at all times.
Until ebp overcomes spring pressure holding the wg shut, that is.

Boost-controlled wastegates are similar, but do free up the exhaust path, where engine produces more than enough exhaust volume to drive a large turbine to make that prerequisite excess Boost necessary to control the wastegate.

A difficult task, indeed, in a low-output system.

PCM can handle the Baro/Baro-x task up to 20psi, so I let it..........:cool:

After running the Turbo-Master for over a year and a half, I can find no down side to it. Except to all the Duramax and 6.0PS I've dusted.

There is a third option that seems rather obvious to me, but I haven't heard of anyone trying it.
Instead of a vacuum pot or a spring, you can use a pressure system that operates the WG with manifold pressure. Most turbo wastegated applications use this system, its just a matter of fitting one to a GM-X turbo, which would be relatively easy.
There's tons of varieteys of shapes and sizes of Pressure operated WG actuators available, all you have to do is find one or modify one to fit and to open at the pressure you desire it to. Then, you just need to tap into the Manifold, anywhere, and run a hose to the new actuator to operate it.

They react immediately to varying load conditions, you won't get the spike like you do with a vacuum operated one.
And its also much more accurate than the spring vs WG pressure system.

If GM had done this from the get go, this forum would have a lot less threads ;)

Each system has it's advantages. Mechanical is far simplier. Electronic gives one last fail safe to dump boost in case of some other engine catestrophe where the computer says enough is enough. Electronic does have a few more parts to fail.

Moral of the story, if you go mechanical, getz yer gauges. You don't want to be using that little crying smiley :bawl: face guy if you don't have to on here.

Something like this would probably work well for us with GM-4 and earlier turbos, even with mechanical pumps.

the cummins use a manifold pressure system like this dont they?

Among countless others, yes.

And where did you find that @ TDG

Found the pic on Ebay, but we have piles of them at the shop.

the cummins use a manifold pressure system like this dont they?

yep, and practically every other wastegated turbocharger out in the world uses boost/manifold+ pressure to control the wastegate.. i've never seen any other engine with a vacuum coltroled system controling the wastegate [like the 6.5's have]... although i have seen vacuum controled VG/VNT turbochargers...

The vacuum system appears rather tolerant against failures --- which it needs to be because it's so much prone to them. If the pump, tubing, solenoid or wiring fails, the wastegate remains open, and it seems pretty unlikely that the actuator can get sticky and keep the WG closed. That's some assurance against too much boost or overspinning the turbo.

How do they deal with leaking tubes on the pressure systems?

The vacuum system appears rather tolerant against failures ---
How do they deal with leaking tubes on the pressure systems?
Tolerant against failures? This system is THE Most Failure prone ever invented. Vacuum pump, fragile plastic hoses, WG solenoid are all just waiting to die on you.

How do they deal with leaking tubes? There's only one tube, and its all but unheard to have one rupture.

Found the pic on Ebay, but we have piles of them at the shop.
Want to get rid of one of them ??? Or just a mechanical one would be great?

:exactly:

Tolerant against failures? This system is THE Most Failure prone ever invented. Vacuum pump, fragile plastic hoses, WG solenoid are all just waiting to die on you.

How do they deal with leaking tubes? There's only one tube, and its all but unheard to have one rupture.

TDG, QM,

with 'tolerant against failures' I mean that it is unlikely for something very bad to happen if something fails. If something fails, the wastegate stays open and you loose boost, but it is unlikely that you get too much boost or that you overspin the turbo. It is _tolerant_ against/in case of failures, but prone to them.

In case my vacuum pump fails again, I'll go mechanical. The vacuum setup imho doesn't make much sense, but there are people who claim that it is better than mechanical control because it adjusts better to varying operating conditions, mainly varying athmospheric pressure. But I'm not an expert to tell.

I only wonder what reason GM had to use the vacuum system ...

Pressure operated WG actuators are not very prone to failure, and when they do, you normally get overboost, I'll agree with you on that.

But having a boost is a MUST before you do any mods to the turbo charging system, and a REALLY good idea even if you don't.

Thus, the way to go would be a pressure controlled system for the 6.5, eventually including a blow-off vent (and having gauges, of course) to be on the saver side.

Is that doable?


PS: What levels of boost should you have with that? I. e., would such a pressure system just make sure that boost doesn't become more than a max. value by opening the wastegate once the desired level is reached or by keeping it somewhat open to keep it at max. level, or does it also do something else?

Is it a good idea trying to always get to the max. level? The vacuum system turns off boost or lowers it as the PCM thinks is needed. But a mechanical system would keep boost up as long as the charger can provide it. The PCM could do the same, thus I wonder what the reason might be that it doesn't.

Pressure system is VERY simple. the actuator is just a spring loaded pot, the spring rate and preload will determine how the actuator will respond to manifold pressure.

On a 6.5, you would probably want to set it so the actuator starts moving around 7 psi, and fully opens by 12. This way you will keep your boost close in the 8-10 psi range, and never spike above 12.

I'm just picking numbers, the range the actuator will work in can be tuned to your liking by simply changing the spring or linkage arm length.

You've been quicker than my edit :) Any negative effects of keeping boost up to max. level as long as charger can provide?

Thats what the stock system attempts to do, but as I'm sure everyone here has noticed, the stock system has a bad habit to overspool then drop down to desired boost levels, this system will eliminate that.

Overspool? I didn't know that ... I'm not getting more than 6 PSI with the stock setup, shouldn't be overspooling?

I think your more likely to damage the 6.5 with low boost than high boost.

Cruise control more or less turns off boost. If I go about 60 mph on level road, I've got 2--3 psi boost, and when I turn on cruise control, boost drops to 0 or almost 0.

Going up a hill at that speed with cruise control engaged increases boost the more the steeper the hill is.

My idea is that this is normal and the PCM has been intentionally programmed that way, but I don't know.

If it could damage the engine, why would they have made it so?

Low Boost at cruise, or any other time, indicates power requirement is low, and engine is running closer to naturally aspirated.

As loading increases, as in up-hill, power requirement goes up, Boost and fuel go up.

Yes, I understand that, but I keep the power requirement the same. I can maintain 60 mph on level road and have boost 2--3, but once I turn on cruise control under same conditions, even on the same road, boost goes down.


BTW, what about the overspooling? Does the stock system overboost and then drop down so quickly that the gauge displays the average pressure, in a continous cycle? If it does, I'd think that it will considerably shorten the live of the charger, besides other bad things.

PCM assumes cruise to mean steady power requirement, where the pcm between your ears has a habit of demanding power at its own volition.
Your engine will pul your truck at hiway speeds quite satisfactorily, normally aspirated, but the turbo PCM has increased fuel rates\curves for turbo demand.
If you let the PCM call the shots - cruise - Boost and fuel will be managed for best economy.

Turbo doesn't over-speed - Boost rises quickly to prevent black smoke at heavy fuel demand, then PCM begins to waste excess exhaust gas to maintain desired\required Boost level, as dictated by oem programming.

About 6-8psi under PCM control, uncontrolled Boost can rise to 20psi, until something bad happens, to the unwary, the un-gaged, the un-wise..........

PCM assumes cruise to mean steady power requirement, where the pcm between your ears has a habit of demanding power at its own volition.
Your engine will pul your truck at hiway speeds quite satisfactorily, normally aspirated, but the turbo PCM has increased fuel rates\curves for turbo demand.
If you let the PCM call the shots - cruise - Boost and fuel will be managed for best economy.
Ahhh..you give the PCM way too much credit.

Cruising boost pressure is determined by engine load and is bi-product of PCM programming, not the direct product.
Tire size and type, tire pressure, final drive ratio, vehicle weight + load, wind direction, aerodynamics, speed, RPM, condition of your turbo, size of your exhaust, anything that effects the load the engine will have at a given speed or effect the overall performance of the turbo combined will determine boost pressure while cruising.

Boost pressure will vary according to engine load (fuel rate) / RPM. If you set your cruise @ 60 you might only be making 2-3 psi average, but if you set the cruise on 70 you might get 4-5psi. If you put on tires a size smaller, your also get more boost while cruising. Not because the PCM has determined this to be the most economical setting, your wastegate is fully closed either way, you just don't have enough load to drive the turbo to make higher boost pressure than that, reguardless of what the PCM wants.
Turbo doesn't over-speed - Boost rises quickly to prevent black smoke at heavy fuel demand, then PCM begins to waste excess exhaust gas to maintain desired\required Boost level, as dictated by oem programming.Again, this is not directly because of PCM programming, the pressure spike is because of a quick increase in fuel rate, which results in high engine load and drives the turbo faster. The PCM sees the higher than desired boost pressures, and decrements WG Duty cycle until it gets desired boost pressure.

In addition...
Anyone out there who frequently uses Cruise control, and has changed from vacuum to spring operated boost control can sure vouch that it made no difference in boost pressure while cruising at the same speed.
But, those who upgraded to a bigger exhaust or just hooked up to a trailer and cruised at the same speed will definitely note a change in boost pressure while cruising.

Just a thought - could an existing "inverted" wastegate control system be converted to a normal pressure system (by replacing a wastegate) and an existing wastegate solenoid be used to control pressure signal from manifold to wastegate, this time though it will be "wired" with boost lines, instead of vacuum lines?
Seems quite doable, with leaving PCM still in charge.

Do I understand it right that the only purposes for the PCM to control boost pressure are to prevent too much pressure and cutting off boost in emergencies?

And that could mean that either my exhaust is rather plugged or my turbo charger is deteriorated because I have to be unusual heavy on the pedal to get boost up to 6 psi (if it gets there at all)? I'm always afraid of the tranny and the engine when I do so ... Never floored it in the almost two years I own it, and never needed to.

Air filter is still clean enough, I checked it recently. Or the WG solenoid is somewhat rusty and I should see more boost otherwise?

Hmmm ...

intake or compresionn leak right behind vac system leak.

Hmmmm. I thought my boost levels are pretty normal, as the exhaust is stock. It goes up to 6 or almost 6 on hard (but not too hard) acceleration. Should I see more?