Basic Turbo Confusion

[coxpn2]

This may have (probably has) been discussed in depth before, and I have searched rather exhaustively prior to posting this. However, I have some questions regarding my turbo on the suburban.

This is more of an informative/inquiring post so I can better understand how the turbo/wategate work. My understanding (basically) is that:
1) Exhaust gasses from the engine spin a turbine, as engine speed increases, gasses increase, so it spins faster.
2) This turbine is connected to another turbine and compresses air in the intake of the engine. Again, the higher engine speed, the more air compressed. (obviously within capability of the turbo).
3) If left un-regulated, you would make in infinite amount of boost. (Well, not infinite, because the turbo loses efficiency at some point.) But anyway, this is where the wastegate comes in. When pressure gets to a certain point, the WG opens and bleeds of the pressure. (This point could be dictated by the stock vacuum setup or a turbomaster.

Question: Is my understanding correct above? If not, please help me understand

Next questions:
1) Since the turbo obviously creates 'backpressure' of sorts (blocking flow of exhaust gasses in order to compress the air) would it make sense that if the WG were held open, this would be the least restrictive scenario involving the turbo? That way no energy from the exhaust side would be needed to make pressure on the intake side, so the turbo would be allowed to "freewheel", correct?

2) Assuming turbomaster - what difference in wastegate operation does the spring make? When driving my suburban, it seems like it takes very little throttle/loading to make 5-7 psi of boost. I can very easily, without hammering on it or WOT, make 10 psi of boost just going up a slight hill. I guess my question is, if I could make 10 psi of boost under light loading, is that good or bad? If it is not needed, am I just burning more fuel unnecessarily? Or would I be better with no boost at light loading, and making boost when I really need it? (towing with load). It doesn't seem like I could get my last truck to make boost that quick, maybe because it was lighter? Or bad memory?

I'm not sure if this can be done with the TM and springs, but just wondering what an "ideal" boost curve would be.

Any thoughts? Hopefully this makes sense
Thanks guys.

 

[THEFERMANATOR]

Your close, the turbo is driven by the exhaust gases via heat. Since the exhaust is hotter than the intake air, the exhaust gases take up more volume(heat makes things expand), so there is a larger volume of gases going out than in since the gases going out are much hotter, and this is what drives the turbo. As for the turbo master, it opens via exhaust backpressure. As the pressure in the exhaust increases, it increases the pressure against the wastegate, and when it gets to the point the pressure on the wastegate is greater than the spring tension, it pushes open the wastegate. And yes, unneccesary boost is simply wasted fuel. Excess exhaust backpressure is extra work the engine has to do, and that equals wasted fuel. This is where the stock vacuum system is so good because it can drop the turbo out and minimize backpressure at will, and just as quickly bring the turbo back in. Ideally you want just enough boost to either clear the smoke, or have just a slight grey haze. Anymore boost is just wasted fuel and added heat(as you compress something, it gets hotter, so adding boost when not needed heats the intake charge as well as saturating more heat through the turbo). Not enough boost though and EGT's climb because of to rich of a mixture. On the other hand to much boost and you drive intake air temps up, and retain to much heat in the turbo, and run EGT's up as well(but not as much so as not enough boost). It's all a balancing act of boost VS fuel while keeping cylinder pressures in check.

The turbomaster is a rather crude design that many opted for becuase it eliminated the factory vacuum system that has been a rather failure prone system, but when it's working it is far and away better than a turbo master could ever hope to be.

 

[schiker]

I think you have the jist of it. The turbo is trying to harness the energy of the expanded gasses when the exhaust valve opens. But that is a widely variable amount of pressure and more importantly mass of air. The scale of variability can't be overstated and is I think where you really need to appreciate the complexity.

The turbo doesn't really run away and build pressure until the wastegate opens. It is driven by the exhaust gasses and can only make as much boost as its driven. So it then becomes a tade off to size the turbo for the amount of drive pressure and keep it efficient. Turbo's can't really freewheel because they are not positive displacement pumps. They are more centrifugal pumps and slip plus air is compressible so they need to be spun up and do some work. Then it is basically keeping them efficient with size. Wastegates are not very efficient ways to bleed off excess pressure hence the pursuit of variable (VGT) turbos.

 

[Will L.]

So, one thing I don't understand is the heat issue. I get the expansion from the heat and that the percect place to mount a turbo would be right at the end of a tuned header. But how much is lost when they do a rear mount turbo, say the vettes with it behind the rear axle. Or like or favorite van master here mounting the turbo farther away.

 

[Missy Good Wench]

Some turbo systems do not use a waste gate, as in the Banks system that was sold as an add on for the 6.2 back in the 80's

The turbo was limited to about 10 psi simply by the size of the unit.

The turbine is the exhaust driven or HOT side wheel, and the compressor or cold side is what delivers the intake air to the manifold under pressure.

The vacuum controlled waste gate system, though effective had more issues than anyone needed to put up with.

The vacuum system was put in place to hand off boost control to the ECM and maintain a good balance of boost to fuel ratio.

This entire system was due mostly to the addition of the DS4 electronic injection system in 1994.

The "fly by wire" DS4 has computer input on the fuel solenoid and this is balanced/controlled by the manifold pressure.

Mashing the throttle would only give X amount of fuel, keeping a clean stack, and as the turbo spooled and manifold pressure increased the fuel delivery would increase too.

The stock setup would max out at approx 7 psi give or take.

Another issue with turbo charging or for that matter huffing by any means increases intake air temperature as the pressure increases.

Above 10-12 psi the intake air temp starts into the arena of needing an aftercooler to get rid of the heat.

Heat added to the intake directly translates to increasing Exhaust gas temps.

The stock system will maintain a safe exhaust temp throughout the entire throttle range (with minimal exception)

The early 6.5 with the DB2 injection system used a simple spring pod the was very similar to the turbo master.


The issue of a little smoke on acceleration was normal with these and cleared as soon as the turbo spooled up.

Back to the Vacuum system.

Very much prone to issues.

The factory chip in the ECM (OR PROGRAMMING ON 96 +) will complain loudly if the boost passes about 10 psi for more than a few seconds and will set a DTC turn on the SES light and go to defuel (low power setting)

Using a turbo master is great, but you need to install an aftermarket chip that has the programming to allow higher boost.

12 psi is a great spot to shoot for with the 6.5, and is at the edge of needing an aftercooler.

15 psi is into the area where some form of intake charge cooling is needed.

We need to remember that the 6.2/6.5 was never meant to be a horsepower brute, but instead a mileage getter.

The early 6.2 in a 2500 Burb 4x4 with 4.10 gears and totally show room trim could easily get low to mid 20 mpg fuel economy.

For a rig this size, these fuel numbers were fantastic.

Along came the quest for MORE POWER

The 6.2 was made a tad bigger (bore size increased) and a turbo was added and more fuel was made available to the beast.

The power increased some, but so did the incidence of failures as well.

In 1994 the electronic engine controls came along and the entire game was changed.

Emissions were the tune, and the path that GM chose was a poor one, as we all know by this late date.


The 6.5 IDI engine has serious flow limitations in the exhaust system as well as other issues that affect it's ability to produce more than modest power levels.

The Turbo master or similar type of device is about as simple as it gets when it comes to controlling drive side pressure.

Set up with a good control chip and a turbo master along with a good free flowing exhaust system the 6.5 can do well in the 250 hp arena.

Some folks have gone to 300 hp, but overheating can be an issue.

With a turbo master and a chipped ECM and the other goodies, YOU are in control, you need gauges to run the thing by, monitoring the EGT AND BOOST

When the boost reaches 12 psi and EGT gets to 1000 F pre turbo you need to level off the fuel and or get your boot out of it.

Selecting a different gear range to lessen the load also helps too under hard pulls.

The factory vacuum system was a system that anyone could drive. A no brainer, just drive it system that the ECM looked out for and protected.

The factory can't train every driver to moderate the fuel, boost and EGT and thus all the automated controls.

Was the vacuum system the BEST ?? well, sort of, hell no, ahhh, it depends on you're point of view.

You want all the little engine that could can give, NO WAY. You want a house car that Mom and the kids can run day after day with no issues, most certainly (Providing everything stays working)

The turbos on the 6.5 evolved over the 93-2000 life of the engines tenure.
Several changes were made to the the air intake system in these years with various changes in the turbo and air inlet components, including a better air filter box.

The OBD2 system of 96----> saw several changes to the controls.


I could go on for another page or 2 but I digress.

Missy

 

[JayTheCPA]

So, one thing I don't understand is the heat issue. I get the expansion from the heat and that the percect place to mount a turbo would be right at the end of a tuned header. But how much is lost when they do a rear mount turbo, say the vettes with it behind the rear axle. Or like or favorite van master here mounting the turbo farther away.

At just bare basics, definitely some loss from heat leach into the larger amount of metal that the exhaust gas must pass through prior to hitting the turbine. So, the greater the exposure to an exhaust pipe, the more heat loss of the exhaust gas and less 'punch' at the turbine as compared to a system that has the turbine closer to the exhaust port. Am sure there is software out there that can calculate the effects.

The way I rationalize it is (presuming WOT): as the exhaust cools down, its velocity slows down as the gas is effectively collapsing within itself. So the more internal cooling of the exhaust prior to the turbo, the less 'punch' for spinning the turbine.

 

[schiker]

Having the turbo farther away mainly causes lag as the bigger volume to fill to create pressure. The temp loss could be insulated and or coated against. With good flow it would be ok. I imagine the extra volume is in the ball park of the lag from adding a significantly large intercooler maybe. No not the same because intercoolers are on compressor side and helps to cool but similar in volume increase of amount of air to compress to raise pressure so probably in ball park. I would say its relative. I'd rather have a turbo mounted farther away than none at all.

 

[schiker]

Let me say it like this you are kinda sorta are correct in thinking you just regulate the backpressure to create 12 psi boost max.

When we set a turbo master if I remember correctly its at 2400 rpm with a load or uphill with max fuelrate. We then Adjust waste gate to limit 12 psi max boost by adjusting spring force to allow I don't know 25 psi back pressure to run the turbine and waste away what ever else.

So anywhere blow 2400 rpm and less than max fuel the waste gate should be closed and the turbo will make what ever boost it can and back pressure will be variable with boost up to a max of 12 psi boost (and 25 psi drive pressure). Works fine and you see fairly linear boost with fuel pedal and load.

Anything after 2400 RPM and max fuel wastes away a lot of compounded air through the wastegate. Remember its compounded air and not a linear relationship. You are wasting several HP or watts of energy from burning fuel just allowing the air pump (motor) to blow air down the wastegate.

The vacuum system had some logic to it to limit boost but can fail more easily and tended to be problematic. The max boost was set lower and it had some safeties to limit boost by wasting away drive pressure say if IAT became too high. The operation is the same but the force of the wastegate is different depending on vacuum controls logic.

 

[schiker]

So no you can't achieve optimal boost with just a spring (TM.) It won't reference fuel and RPM to control boost. I don't think you can really freewheel a turbo because the engine doesn't want to draw air through it very well. You can allow it make little to no boost and keep drive pressure to a minimum but that is not the point either. You want to harness the left over heat expanded air energy after the exhaust valve opens. But making boost for boost sake is not optimal. So that is a bit of a hard question.

Optimally I guess you want to stay in the efficiency range then after its say over 2 or 3 to one drive pressure to boost you can waste it away. But I am not sure that is optimal either.

 

[WarWagon]

The small GMx turbos are designed to spool and provide boost at low RPM. Turbo's are supposed to work off heat, but, the GMx is so small that it will spool up to 6 PSI at 2500 engine RPM from airflow alone with zero throttle if the wastegate is closed. Turbo's are not supposed to... The back pressure becomes extreme at higher boost PSI on a GMx like 3 to 1. This keeps heat in the engine and wastes power on the exhaust stroke. Better turbo's deliver boost with less backpressure for example a HX40II wastegated medium size turbo at 24 PSI boost delivered nearly the same power as a huge A Team Turbo at 14 PSI on my engine when it was put to the dyno rollers. the ATT is less than 2:1 for back pressure as I recall.

The wastegate simply limits the amount of work a turbo does. Some are referenced off boost. In any case it keeps the small turbo from overspeeding and becoming more of a heater than compressor.

So with your Vacuum operated Wastegate the computer is opening it with low throttle. If you are at 2500 RPM and shut the wastegate you have nearly instant 6 PSI from airflow. Very little fuel makes say 10 PSI of boost and then the computer opens the wastegate to keep the boost aka turbo work and IAT heating down. Otherwise the turbo would run off the efficiency map. With a 1993 spring pot or turbomaster you have boost at 2500 RPM with or without fuel going in the engine.

ECT is more important than EGT on this engine at least at higher RPM. I have faithfully tried to "melt my engine down" with ECT below 210 running 1400 EGT pulling grades with a GM3 turbo and running out of numbers on the EGT gauge at 1550 EGT sustained towing with a 6.2 military longblock and A Team Turbo. The only concern was running out of numbers on the EGT gauge. The turbo blanket was scorched a little on the inside at 1550. The paint on the inside of the cab floor was brown from 50K miles of 1400 EGT grade climbing pulls towing as we noticed after wrecking and salvaging that pickup. Solution wrap the downpipe past the bend in the cab. Of course don't lug the engine at low RPM, but, aside of that EGT is not a relevant number to worry about for damage on the 6.2/6.5 engine.

 

[coxpn2]

Well, you all have given me some reading to do. Thanks for the info - I'll sort my way through it all!