MPG turbo vs. non-turbo

Lots of sticky threads and info in the 6.2/6.5 tech forum, but doesn't seem like any thing new lately. I believe the general consensus (based on fact and data) is that the N/A 6.2 trucks get better mileage than the turbo 6.5's. Precups are different and could be (or are) contributing to this difference but what about the losses from the turbo and backpressure? On almost all non-race application turbo engines, the backpressure between the engine and the turbo is basically twice the intake boost pressure (nothing is free). So.. cruising down the freeway at 65 my truck is making 2-4 psi which means the exhaust pressure is 4-8 psi. What is the intake pressure doing for me in that situation?
Think about the path that all of the exhaust gases have to get pushed through to make it out. How much cleaner could you make it without a turbo. Remember I'm looking basically just at MPG's, not trying to pull or haul the world. I suspect that for a semi light duty (i.e. no towing) truck, in almost all situations you may be better off with a N/A engine at 6.5 litres instead of a turbo at 6.5 litres. Best solution would likely be a 3ish litre turbo but that's a really big can of worms. How much power do you need to pull around a 5000lbs truck?
Just wondering what opinions might be like.

Yep, you have it pretty much correct. Under light load / light fuel rate not a significant amount of excess heat is generated then wasted down exhaust and thus no "free" energy harnessed with the turbo for the IDI 6.5. Precup size is based on air flow, density, heat transfer, and basically "exchange air" etc so to optimize it they are different for turbo vs NA.

Where a turbo helps is emission to dilute polutants and when there is an appreciable amount of excess heat wasted on exhaust side.

I ran a non-turbo 6.2L diesel for about 9 years before installing a Banks Sidewinder turbocharger. The truck would consistently deliver 22-24 mpg while NA and "could" produce virtually the same after installing the turbo. I actually saw 1-2 mpg less with the turbo because having more power tends to corrupt... ;)

I believe there is a crossover point where a turbo can hurt/help fuel economy. It all depends on the load and speeds. If your truck weighs more than a certain amount or if you drive faster than a certain speed, a turbocharger can help to improve fuel economy. On the other hand, if you drive slower and lighter, not having a turbo could help to produce a little better fuel economy.

Jim

That makes sense. In an unloaded situation these trucks are actually overpowered with a turbo. If you take emotion out (which is hard, I've never driven anything with too much power) and look at the vehicle as a tool, up to a GCVW of 7000 or so you likely only need 125 hp. I think in society today we keep wanting more of everything. Anyway... back on point, does increasing flow potential through the pressure side of the exhaust would help part throttle economy? I would think yes. Problem could be what and how do you do it. If you could seal it up well enough you could put a valve between the crossover and the down pipe, basically allow the exhaust to take the least resistance in a cruise condition where boost is not required. The left bank manifold looks pretty good, the right bank is a bit of a kludge. If the flow potential through the waste gate was improved enough you might see less pressure differential between the exhaust and intake. On boost regulated race engines you'd see alot of work on exhaust flow through the wastegate. Have a look at some of the old champ cars, the sizing of the tubing on the wastegate side is easily 65-85% of what is going through the turbo. They ran less than 10psi.
Just stuff to think about.

Dave

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Originally Posted by DP

That makes sense. In an unloaded situation these trucks are actually overpowered with a turbo. If you take emotion out (which is hard, I've never driven anything with too much power) and look at the vehicle as a tool, up to a GCVW of 7000 or so you likely only need 125 hp. I think in society today we keep wanting more of everything. Anyway... back on point, does increasing flow potential through the pressure side of the exhaust would help part throttle economy? I would think yes. Problem could be what and how do you do it. If you could seal it up well enough you could put a valve between the crossover and the down pipe, basically allow the exhaust to take the least resistance in a cruise condition where boost is not required. The left bank manifold looks pretty good, the right bank is a bit of a kludge. If the flow potential through the waste gate was improved enough you might see less pressure differential between the exhaust and intake. On boost regulated race engines you'd see alot of work on exhaust flow through the wastegate. Have a look at some of the old champ cars, the sizing of the tubing on the wastegate side is easily 65-85% of what is going through the turbo. They ran less than 10psi.
Just stuff to think about.

Dave

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Sound principal, but only in part.

In an unloaded situation, the turbo is a liability. It creates a restricted airway, which is a parasitic drain on available power. Opening the exhaust to flow freely (bypass the turbine housing, and WG plumbing), is only a part of what is needed to free up the airway. The turbo, even with the turbine bypassed, is still a liability. To realize a full gain of free air, you'll also have to bypass the compressor. This would essentially revert the engine to a N/A state, offering the best of both worlds. A free flowing intake and exhaust could yield the greatest economy, when the additional power (fuel) is not needed for cruise speeds or mild acceleration. The exhaust backpressure can be significant under heavy (boosted) load, but is offset by the boost and additional fuel. The parasitic loss from the friction of air alone uses much of the power provided by the additional fuel. However, great care should be considered to prevent a loaded turbine and an unloaded compressor at the same time. Turbo overspeed can get costly, real quick.

Does their really have to be a crossover point? If turbo'd engines produce boost even at idle, doesn't this mean that they can create the same power level with less fuel (given the higher intake pressure)? Does this hold true at idle and lower power bands? Or, does a turbo only impact the top end by allowing more fuel to be added with a larger air charge?

I, too, am interested primarily in maximize MPG and have a rebuilt 6.5TD in the garage about 60% complete with 6.2 pre-cup loaded heads finally back from the machinist. It'll be another month before it gets swapped in as I'm converting from gas so it'll take quite a bit longer to swap harnesses and fuel supply, but I'm hoping to see that "30%" increase (over stock 6.5TD) that was theorized in the MPG article.

A TD engine does not develop boost at idle and lower RPM's. However, the parasitic effect of the turbo is present at any RPM, more profound at the RPM extremes (lower and higher). Beneficial boost isn't available until sufficient fuel and air is combusted, and exhaust gas volume is present, to overcome the parasitic loss. A given amount of fuel will always be spent (and not available for crankshaft power) to offset the parasitic effect of the turbo.

My thoughts about a hiflow bypass became much clearer during my drive home today. Why not put a real wastegate on the passenger side of the crossover pipe? Why push all of the exhaust from the left bank all the way through the right manifold? Then why push all of the exhaust down the downpipe? Wastegate control should be pretty simple to have a manual adjustment or two step adjustment in order to minimize boost and backpressure, bypassing a large volume of exhaust directly into the exhaust down by the gearbox (or if you're not too concerned with noise, just dump it straight down towards the ground).
As far as the drag the compressor will introduce, I'm not sure how much it would hurt the flow. Even if the wheel isn't turning, air will still flow easily through it and during part throttle cruising I can't see it being a huge loss.

The compressor is a very significant restriction. The turbulence alone is a significant restriction. It isn't designed to free flow. It won't make a difference at idle, but at cruising power demands, it will be. Guaranteed.

It might be possible to rig a turbocharger bypass for both the exhaust and intake sides of the engine, but I doubt it would be worth the effort from a fuel economy position.

Many 6.2L NA diesels in the 80's pickups and Suburbans would deliver low to mid 20's for fuel economy - when geared (3.42 gearing and an OD) and driven correctly. I've owned three 6.5TD's. One a 1994 1500 series Blazer and 2500 & 3500 series pickup. None could approach the routine fuel economy produced by the 6.2L NA I owned (24-mpg) - which is about a 30% advantage over the 6.5's (an average of about 17-mpg).

Jim

I obviously have too long of a commute and too much time to think about stuff.
Remember I'm talking about low hp stuff, no towing or major league hauling, stock hp (190) or maybe a bit less is the target. What about an asymmetrical turbo? Saab is doing this on their new V6, taking exhaust from one bank and pressurizing all 6 intakes. They're running very low boost, something <5psi, the object was to significantly broaden the torque range of the engine. Mechanically it would be a piece of cake to implement on a 6.5. Plump the left bank from the crossover to the downpipe and just cap the entrance from the crossover to the right bank. Obviously boost potential would be significantly reduced, but I wonder how much exhaust is bypassed by the wastegate at 3, 5, 7, 9, etc psi? In my truck ('93, stock except for downpipe and exhaust) the only time I'd see over 5 psi is while towing, which I don't use if for now. Any body have any ideas about how much the wastegate is moving around under different conditions? I may try making a fixed wastegate rod/actuator thingy. If the wastegate is fixed in some open position and the engine will still make some boost (3-5) under full load, it may add some credence to this thought. I envision that if you were to try the asymmetric turbo, the wastegate passage would end up with a pipe plug in it and the turbo would be free floating (the Saab has no wastegate). Are the GM turbos completely proprietary or are they based on someone else's? It would be nice if this would work without changing the turbine housing, but if a smaller housings are available I know it could be made to work. You could also change the turbo and make it work but that would cost money!
Might make a pretty big difference in MPG's by reducing all backpressure from one bank and significantly reducing flow restrictions through the turbine.
Opinions welcomed.

Dave

An asymmetrical turbo might work (to what advantage escapes me) on an engine designed for it. The balance factor alone would probably kill a 6.5L in short order. And you couldn't use a stock 6.5 turbo, any of them. You'd have to use a turbine/housing capable of spooling on 50% of the exhaust gas, and a compressor capable of providing air for 200% more cylinder volume.

A better approach, IMO, is a symmetrical turbo, which the 6.5's are not. The Duramax, on the other hand, is (with the exception of the restricted driver side manifold). The system is volumetrically balanced. (and no reason it couldn't be adapted or applied to a 6.5L)

If your goal is MPG, with no significant power requirements, don't discount anything. Under this guideline, anything that gains 1/4 MPG is worth considering. You need only to take into account the ROI (return on investment).

One of the guys here at work today thought the same thing about a balance problem, but I don't know. At 170hp each cylinder is contributing 21hp to the total. Assume 5psi of boost, gives you 10psi of backpressure on the right bank and what, maybe 1 or 2 psi of backpressure on the left bank. How much hp difference do you get just from the backpressure? Everything else will be the same. Maybe the right bank is making 20hp and the left is making 22hp. Any engine, gas, diesel, whatever is going to some imbalances between cylinders.
As far as the turbo goes, it might work. I'll try to rig up a full open wastegate and see how the truck drives and what kind of boost it makes (or doesn't make!). Even if it only makes 3-4 psi max, just think how much better the airflow through the engine will be.
More input, what do you guys think?

Dave

A full open wastegate will produce zero PSI boost. Whatever backpressure is present will not be against the turbine. The intake will be driving the spool, not the other way around.

And, a 10% power differential with an asymmetrical system (according to your supposition), will translate into a 20% parasitic loss (according to my supposition). The cylinder imbalance will translate into a net fuel consumption increase for a given BHP. In other words, lower economy. Economy is simple. The more fuel energy you can put to the flywheel, the greater the economy. Anything done that decreases net HP at a given fuel consumption rate will decrease economy. Internal combustion engines are inherently very inefficient. The best we can do is attempt to make them less inefficient. The BTU rating of a fuel is constant, while the efficiency rating is the only variable. An engine with a 100% efficiency rating would require no cooling system, make no noise, and never wear out. What we end up with is something between that, and a brick (no matter how much fuel you put into a brick, it will never develop any HP).

I agree with Dmax along the lines of the compressor causing drag on intake air and causing ineffeciency with little drive pressure.

As the name implies a wastegate wastes possible drive pressure. Under light demand it wastes away enough so the turbine drive pressure is not high but does push the compressor instead of it dragging. So its a compromise. Under light fuel rate there is no real reason to have boost. But when fuel rate is there you want to harness the wasted heat so I think the variable turbo is the key to effeciency. There is just too much variation in load and heat during all driving accelerating and hills etc plus rpm variation to size a turbo small and then waste away excess turbine pressure under load. At least if you want to build 190 hp. Thats kind of the problem with the gm X it's a touch small and spools quick but gets ineffecient at about what 2600 rpm and higher fuel rates.

I think the variable turbo, compounded, or maybe gated to different systems little or big and have a switch for power or economy might be one way to increase effeciency but would it pay back ??? doubtful unless fuel is $5.00 gallon.

I've been thinking about this for a while. Visualize what is happening at 1900 rpm at cruising throttle. You've only got X amount of fuel going into the engine, enough to produce let's say Y amount of HP to maintain 1900 rpm. At this point the engine in my truck is making about 1.5 to 2.5 psi of boost which means there is 3 - 5 psi of backpressure pre-turbo and the wastegate is likely completely closed. I think it's a reasonable assumption that if no black smoke is visible then you're getting adequate consumption of the injected fuel. Using the same parameters as above but doubling the boost to say 5 psi will make exactly the same HP. Since the backpressure will have increased to 10 psi and the turbine will be pulling more power from the engine. In order to get Y HP at the flywheel, you'll have to inject more fuel. If you could get the boost for free, it doesn't make any more power. I think the goal would be to get as free of an airflow as possible, make no boost at cruise, and insure you've got no black smoke and/or strive for as low of an egt as possible.
I'm going to likely try one of two possibilities in the next couple of months.

1. Use an electric exhaust cutout between the crossover and the downpipe. Fab the transition so as to good flow out of the crossover, not just a simple 'T' junction. This would be paired with a 2.5 or 3 inch diameter low cracking pressure flap valve between the air filter housing and the intake manifold plenum. This flap valve will open at about .5 psi on one side and they're rated to hold almost 100 psi the other way. This would allow the turbo to be completely bypassed on both the turbine and compressor side but re-engaged at the flip of a switch.
The bypass is about $210, exhaust mandrels will cost $40, flap valve's are about $80 for that size, and intake plumbing could run $50 in mat'l. Figure $400 or so. Now.. IF we assume that we see an increase from 18 mpg to 22 mpg (22%) and $4/gal fuel, the payback is in 10,000 miles. Not great but not too bad.

2. Rig up an asymmetric turbo by dumping the left bank into the downpipe and capping the inlet into the right manifold. Could either use the stock GM-3 turbo, possibly swap the turbine housing to a smaller A/R housing from a T3, or swap the whole turbo to a T3/T4 hybrid. This gets more interesting... Keeping the GM-3 may end with a system that produces almost no boost, would of been better off just completely removing the turbo. From what I've seen, I believe a T3 turbine housing MIGHT fit onto this turbo. If yes, then the turbine could be matched to the output from one half of a 6.5. Or.. the whole turbo could be swapped. Going to the asymmetric turbo might be more interesting, but I don't know what might happen with the mileage. If it increased by 10% or so and you could keep the cost to less than $250, you'd still get a payback in less than a year. The $250 might be a pretty low ceiling if you replace the turbo. Stuff in that price range has been used up by the ricer crowd. Regardless of what is done with this scenario, it's likely that the max HP will be lower than stock.

Anyway... lunch is over... I'm getting back to work.

You are correct: the wastegate is closed until boost reaches a preset level. Why not put the switch on the boost solenoid? Let the wastegate open unntil you want to add boost.

My feeling is that unloaded, cruising down the highway, any boost is a waste. Until the fuel rate starts to exceed what can be burned completely by a NA engine, any boost just adds to the backpresssure unnecessarily.

'93 truck. No solenoid, just a spring.

Even with the wastegate open, all of the exhaust has to go to the turbo, then either make a 90 deg turn an squeeze through the 1" (?) wastegate hole or through the turbine.

At low power demands, the exhaust gas volume is minimal. With an open wastegate, the gasses pass through the gate, as well as the turbine. Any mods you attempt to reduce the back pressure at low load should start at the manifolds, not the turbine/WG. Once power demands increase, then the back pressure is necessary to drive the turbine. By passing the WG (X-over to downstream) is not going to gain anything, except more investment expense, weight, and complexity. If you are going to modify the exhaust in any way, consider a complete redesign. Use HMMWV headers, and employ the exhaust gates you brought up earlier (electrically controlled), dumped into a large exhaust with free flowing junctions. The "gates" can be controlled manually, or an electrical pressure switch in the intake (default/no power to open gates). This would open your options to many different turbos, and not just GM wastegate turbos. Another thing to keep in the equation is the actual cross sectional area of the exhaust path. Even though the WG is open, gasses still pass through the turbine. The wastegate just removes the ability to build back pressure. At low power demands, the need for additional exhaust gas escape is not significant, especially with an exhaust system designed to operate this way. I don't think you'll gain anything without a complete redesign of the entire system. If you are after the most efficient airway path, you'll have to design a symmetrical intake and exhaust system.

I still recommend abandoning the whole asymmetrical charge-air idea. No matter how I pencil it out, the logic is too fuzzy, and the inherent risks don't disappear. The 6.5L engine is externally balanced. Without extensive R&D, you will not get the engine balanced, especially dynamically, to be efficient in either operating mode.