Fellow Suburbanites... how's your mpg doin'?

[buddy]

Steve, how much boost are you running while cruising. A TM will set both the max and the min boost, and you dont want a lot of boost when cruising, will take away MPG, so 2-3psi at like 65mph would be good.

 

[Dave01]

Buddy, I'm not sure I understand that post. How does a TM set the min boost? Until the TM spring lets the waste gate open, the boost will be whatever the turbo can produce. If you have 4 psi at 65 mph on level ground, the only way to lower that is to let the gate open, which would set your max boost at 4 psi, or change the way the engine breathes by making other mods not related to the TM. Am I missing something (probably!!!).

 

[buddy]

Yes Dave you are missing what just about everyone does, so dont feel bad. When I say minimum I mean the lowest you get at anytime. If you cannot get less than 4psi at 65mph flat land with an empty truck then that is the minimum your TM is set to. If you loosen the TM the gate will open more and you will get less boost at 65 mph on flat land, but you will also have less maximum boost at 65mph going up an 8% grade with a trailer on the back. This is how people loose MPG with TMs, when setting them to reach 14psi on a stock program, and running 5-7psi boost cruising. A stock program has less dynamic range than a performance program because it has less fueling capability. So if 14psi is what someone wants max with 80mm3, then a stock guy that only makes 60mm3 sustained fueling should only reach 10psi and the vacuum controller probably cut boost to about nothing a lot of times, like when cruising. If you can manage to get the TM to reach 15psi about on your reflashed PCM, but still drop to 2psi when cruising then youre set up good and shouldnt loose any economy with the TM. Getting a spring to do that and still give you quick spooling and little smoke is the trick.

 

[dangerousdave]

A Haa said the blind man now I see;

3:73 & 4x4 may be your limiting factor Steve that and the burbs brick like aerodynamics ATT & a re-tweaking possibly of your Kojo chip to make the most of it, is your next upgrade

Ratio, tires, and 4x4 will avoid 20mpg easily. Lubrilon in the engine t-fer case and both axles couldn't hurt. The stuff really works. Field tested it, I did. May the force be with you... a hard runner, strong it seems you have built it.

 

[Dave01]

OK, thanks for the explanation, I definitely didn't think it worked that way. I assumed that at up to maybe 8-9 psi the spring pressure was strong enough to keep the gate all the way closed, not dump at all.

I'd like to experiment a bit after reading this. I set up my TM when I got it, haven't touched it since, and run the Heath GLE chip. As I commented earlier, my mileage is poor, and while I might blame the tires for some of it maybe I'm missing some mileage due to TM setup.

 

[buddy]

If you only set up the TM for 9psi then its probably good for mileage

 

[knkreb]

Ah ha... so the quested for 12 psi is way way too much for little ole me just pushin' the 'burb down the road... So, as long as I burn all the fuel in the cylinder, not smoke, I should be okay... weather at 8, 6 or even as little as 2 psi boost... I'm not pulling 26,000 lbs up at 27% grade.

Now the TM doesn't really do a minimum boost... minimum boost is only determined by the fuel and engine RPM. TM limits maximum boost, but can't dump boost below the max setting. Stock will dump boost at a variety of conditions. Dumping unneeded boost would be beneficial in reducing exhaust turbine restriction when you have a good enough air entering the cylinder to complete combustion.

So, from what I'm understanding here is: boost on a GM-x turbo is not really a linear relationship in the engine's needs? Meaning... the boost that is made by the turbo may be well above and beyond what is really needed. It raises the IAT's, increases exhaust turbine restriction, etc. etc. etc. which maybe counter productive. Higher boost is not always the answer. Giving the engine what it needs, it is what is desired... not just what the turbo can make for it.

Is my feeble little keyboard wrapping itself around the subject properly?

 

[buddy]

Yes, you are on the right track there. I consider that the spring set the min and max, because its tightness determines how little the waste gate will open, and so for the same fuel and air and rpm it can make boost higher unnecessarily. If you can loosen the TM and get lower psi boost at the same conditions then you obviously are lowering the minimum boost at those conditions. You are also lowering the maximum.

Naturally Aspirated 6.2s did well on fuel one because they didnt dump as much and two because when cruising it had no exhaust restriction. They had the same camshafts, so same valve events and everything else. Similar programming on the electronic 6.5 NAs. They had retarded timing early on although didnt retard it with fuel and RPM like the stock turbo programs, they advanced timing through the curve which is good, but still way lower timing at idle and cruise.

You must understand that the turbo is parasitic, and not everyone gets that part either. If it requires exhaust backpressure then that means the piston has resistance during the exhaust stroke. That makes the other pistons in the combustion stroke need to push a little more to drive the turbo.

 

[knkreb]

You know... there is an idea here. Once I post my how-I-did-my-turbo-master thread... I threaded the stock rod that came out of the actuator. The threading goes all the way down below the bottom of the vacuum actuator housing. It is possible to place two nuts below the housing to create a limit stop to hold open the wastegate a fraction to create the minimum opening on top of the spring maximum boost limiting. Setting that minimum would be nerve racking with only slight turns on the nuts to create a slight opening in the wastegate.

 

[Dave01]

If you only set up the TM for 9psi then its probably good for mileage

Sorry, didn't mean to indicate that was my setup. I'm at about 12 psi max with the quiet muffler, was 14 psi max with the Diamond Eye at same TM spring setting. So, there is room for me to drop it down a bit to see what happens.

I wondered about the dynamics of the more open Diamond Eye muffler I ran for a while. Turbo spooled much faster and higher, clearly could feel more power. So, more open exhaust creates less back-pressure on the turbo increasing boost and response. But, more boost has to mean more fuel. So, the more open exhaust should be less fuel efficient at steady highway speed, right? Not sure I have all of this straight.

I wonder if anyone ever tried a turbo bypass system. Just tooling along on the highway, flip a switch and 2 Y valves cause exhaust to go straight out bypassing the turbo, and the engine to breath ambient air. Then you have one engine, choice of turbo or N/A. Just thinking aloud.

 

[knkreb]

I wonder if anyone ever tried a turbo bypass system. Just tooling along on the highway, flip a switch and 2 Y valves cause exhaust to go straight out bypassing the turbo, and the engine to breath ambient air. Then you have one engine, choice of turbo or N/A. Just thinking aloud.

Now that's dreamin'... Just need a check valve for intake. Draw air when no boost is present, and snaps shut when turbo spools up. How much back pressure does the GM offer with WG open?

We've really gone off topic from Suburban MPG to reworking of the turbo haven't we?

 

[buddy]

Its part of getting MPG, I really just wanted to say you can get more MPG if you set the TM up for 2-3psi at your cruise speed, rather than setting it up for a maximum on acceleration. And if you need more boost at some point just tighten it up a little. You dont need more fuel to make more boost all the time. The more open exhaust allows you to make the same boost with less work, since it has lower pressure on the back side. The turbine spins from pressure differential. Lower pressure on exhaust side means less backpressure at engine to make the same boost. So DE muffler set to 12psi max would be more efficient than silent muffler at 12psi max.

 

[buddy]

You know... there is an idea here. Once I post my how-I-did-my-turbo-master thread... I threaded the stock rod that came out of the actuator. The threading goes all the way down below the bottom of the vacuum actuator housing. It is possible to place two nuts below the housing to create a limit stop to hold open the wastegate a fraction to create the minimum opening on top of the spring maximum boost limiting. Setting that minimum would be nerve racking with only slight turns on the nuts to create a slight opening in the wastegate.

Thats an interesting idea, so its held open at cruise enough to get low boost, but tight enough to get high pressure at high fuel rate. You would unfortunately lose some spool performance with it not all the way shut at idle.

 

[Dave01]

We've really gone off topic from Suburban MPG to reworking of the turbo haven't we?

Not at all! What Buddy has brought up is that someone like me focuses on tires, IP health, etc. but the truth is that the turbo is at the heart of the power of our rigs, and also the efficiency. All these things impact mpg, but had I not read this I never would have thought of adjusting my TM to gain mileage. Very interesting discussion!

 

[ak diesel driver]

conversely if you tow you can get better mileage with a properly set up turbo. sometimes more power tends to be more efficient.

 

[knkreb]

Thats an interesting idea, so its held open at cruise enough to get low boost, but tight enough to get high pressure at high fuel rate. You would unfortunately lose some spool performance with it not all the way shut at idle.

Are we in great need of boost at idle?

 

[buddy]

Are we in great need of boost at idle?

No, you dont make boost at idle, but the wastegate remains closed at idle on either the stock vacuum system or a manual spring actuator. That allows it to build boost fast, for better acceleration off the line. So the turbo will spool faster. It will still work with the wastegate open a little, but what will happen is youll get a big puff of black smoke if you push the pedal a fair amount and then boost will build like 2 seconds later than normal off the line. I accidentally had run my GM4 a little bit like that before, and it certainly did put me down to almost no boost while cruising. I think the GMx turbo is efficient enough at 2-3psi though that its not a big burden in restriction, so in that range it may help economy while cruising due to the added O2 in the cylinder.

 

[Turbine Doc]

Kev the GM turbo is good for what it was designed for, a middle of the road turbo good for light tow work supplying 195 flywheel HP, when we go beyond that you start to see the performance of the GM turbo fall off, once you get hard into the turbo you for lack of a better way to say it "hit a wall" engine is still accelerating but much more engine effort to do it than with a better flowing/bigger turbo.

A fully loaded burb with aerodynamics of a brick (& family stuff), or truck pulling a big load benefits with a better turbo in both power & mpg, there is a slight trade in that the bigger turbo is not at same boost psi off line with a turbo sans WG, the non WG turbo is still moving air so actual off line delta IMO is negligible, in a snapshot, the GM turbo probably will hole shot the ATT, but half to full truck length the ATT will tromp it, as this is where the GM turbo begins to struggle 2000+ rpm, the ATT comes alive, and all you are limited by for most part is how much fuel is available.

Diesel power is all about airflow, more of it produced in an easier format (less flow restriction) is going to result in more power when appropriate fuel is applied to go with the air http://www.bankspower.com/techarticles/show/36-Airflow---The-Secret-To-Making-Power

 

[buddy]

You arent necessarily going to get better economy with an ATT over a GMx when cruising higway miles if you just set up your GMx to not boost much while cruising. Maybe thats why some people get better econ when switching to ATT, because their TM was setting a minumum boost that was too high and causing too much of an exhaust restriction while cruising. And an ATT will certainly do better economy wise when towing.

The ATT makes minimal boost at non-towing cruise, like none to 2psi because its not enough of a restriction at low fueling. Make GMx do the same thing and will be comparable.

 

[Turbine Doc]

Even at minimal boost settings running empty with vac or when with TM at it's low boost spring setting my ATT'd burb gets better mpg than with the GM turbo I never ran the TM hogged down for max boost or vac fooled for max boost on that vehicle.

I can't say with the truck with the same certainty that the ATT was the sole mpg improver (but IMO it was/is a major component) as so many variations on a theme with that vehicle and me hard boosting and hard right foot often, I ran stock vac/PCM at 1st, then fooled vac and stock F PCM many years, before going "fooled" stock F PCM & TM with light & max spring tensions, and then flashed and TM, until now ATT & flashed with & without IC. But all those configurations I did see some mpg improvement in my current configuration, how much more I can't equivocably.

I'm always 2+ psi boost when cruising, I'm 7100# with 4x4 & 3:73 gears, maybe that is another datum point your experiences vs mine Buddy, as IIRC you are "light" by comparison.