That's a good question.

I run at a very low RPM, so have a very low gas flow rate through the turbo. At 60 MPH (1450 RPM) my boost gauge is barely off the peg. At 70 MPH (1700 RPM) I'm showing 3-4 PSI. No black smoke at either speed, so I don't think I'm short of air.

With a smaller turbo, I'd probably get more boost, but it would impose more back pressure.

If I slow it down even more, I'd probably need a smaller turbo.

This is a part of the performance envelope that few guys think about.

 

Less psi seen by the map causes less fueling right? I would venture to guess that a larger exhaust housing on a stock turbo would net more MPG, maybe not a large gain.

Dave, have you tried messing with your wastegate at all?

 

In furnace they use about 10% excess air to make a complete combustion

 

Generally the rule of thumb is that "less boost while cruising will = better MPG", so long as your EGTs are under control. Something like an H2E, or a similar size turbo would be able to do both. In theory at 0lbs of boost with 500-600* EGTs, we should get our best mileage on the highway. That is as long as the tune your in doesn't start dumping extra fuel when you hit a hill or a strong head wind. But once again that part is left up to the tuner. There are plenty of guys out there with big injectors, big single turbos, & proper tuning that get great mileage on the highway with their setups.

 

I have read and seen that you can watch your egts. Lower usually means more complete burn. Anything that could aid better atomization of the fuel or more air. Diesel unlike gasser do not have to worry about detonation from to much air. They will eventually just stall as I understand it.

A centrifugal SC such as a Pro Charger might be interesting as even at cruising speeds its supposedly still is creating more power than what is lost from crank drag. The one I have seen numbers on with a 38R was showing 10 boost at cruising RPM and egts were crazy super low. Even in the most aggressive race tune EGTs never made it to 1000.

straight water injection may also show a small gain even with careful lite driving 1 mpg?

 

I makes since that a bigger turbo (less backpressure) would let the motor run a little more freely. BUT i think diesels like a little boost. Even tho you may not have any back pressure and letting the motor flow good i thnk a little bit of boost is needed for the extra power to keep the truck going with less skiny pedel. I think its just a fine line between it some one who knows alot about turbo sizing could tell you better than i can

 

These trucks are pretty slugish with out boost. I think you would find a decreas in MPG with a bigger turbo. the lag would be to much and you would find yourself on the skinny peddle more to light the turbo thus taking more fuel to get the truck rolling. A supercharger on the other hand would not require as much fuel and would create boost instantaly and you would not have to apply as much fuel to get the truck rolling. But than you are taking HP to turn the supercharger.

 

A bigger turbo will not flow more air necessarily, if sized right it will move an equal amount with lower backpressure.

This means for the same charge air less hp is required/lost in the turbine.

I got +1-2 mpg on stock injectors with a large frame turbo. The boost going down the highway was actaully lower most of the time. Backpressure was half what it was.

 

What turbo were you running with stock injectors?

 

That's a good question.

I run at a very low RPM, so have a very low gas flow rate through the turbo. At 60 MPH (1450 RPM) my boost gauge is barely off the peg. At 70 MPH (1700 RPM) I'm showing 3-4 PSI. No black smoke at either speed, so I don't think I'm short of air.

With a smaller turbo, I'd probably get more boost, but it would impose more back pressure.

If I slow it down even more, I'd probably need a smaller turbo.

This is a part of the performance envelope that few guys think about.

Using single shot style injectors and a larger turbo will "improve" fuel mileage with proper tuning and PROPER DRIVING habits.

At 60-65mph, I barely push more than 2psi and EGT's are below 600* on relatively flat ground. A lot depends on weight and gearing as well. As another general rule, keeping the engine under 2000rpm's helps out as well.

 

If you found yourself often having a fair amount of boost on the stock turbo (I'd gander anything over 6-7 psi) when you were cruising for mpg, then yes a larger turbo would reduce back pressure and therefore up effieciency. If you were looking for better mpg for your hot-shot rig that is usually loaded than there is no doubt that a larger turbo would improve efficiency and therefore mileage.

If your cruising boost is already very low (say less than 3 psi) than no, it would not help and could quite likely lower mpg to go larger on the turbo because of a lack of air flow and the need to push the skinny pedal more to get moving.

 

That's exactly my thinking.

 

Ebay GT45, the 70mm hybrid.

 

How did it run with stock sticks & an auto?

 

the only way it would is if you could stay outta the skinny pedal.
also, a few spooling mods (ceramic tubular headers, ported heads, etc) I think would help with that as well.

 

all things considered and equal(which our trucks are all ever so different) in perfect driving conditions the larger turbo would be more efficient in terms of fuel combustion. You'd not gain alot of mpg's so you'd have to do TONS of driving to justify it that way. Towing or in unperfect condition(most are) the larger turbo with stock inj would be beneficial. Throw in a tuner and good intercooler and the gains would be substantial. Not to mention all of these things would negate pumping losses(heat) which is our #1 enemy.

 

Oh and you'd have to adj your take-offs because turbo-lag would be noticeably more with the larger hairdryer. Clever waste-gate adjustment can overcome that too...higher holding pressure lets small boost build.

On an unrelated note, my 95 Eclipse got 25-30 in stock form regardless of the driving style. Once modded with inj, large turbo, intake, exhaust, and intercooler could get anywhere from 12-38 depending on how it was driven. So with the higher power comes the ability for high-efficiency or huge consumption. That said our right foot is our best mpg tool.

 

There is no wastegate adjustment one the larger turbos because it is not needed. oke:

 

Even if you have a wastegate that allows adjustment? So nobody around here ever looks at that as part of the overall performance picture?

 

On kill I am still at a 1:1 (or slightly better) 45-45.

So you tell me why I need to add a wastegate, there is not one in the system at all.

Of course they have their place, when you push the turbo out of its efficiency range. They are most important in the compound setups to control the boost pressure to a managable level.

 

Wastegates are used on small turbos because the manufacture installs a small turbo for good response and smoke control and the wastegate is needed so as not to overboost.

 

So, what Im getting at is, how big before a wastegate becomes a waste(not trying to be funny)? Besides doesnt a wastegate "hold" boost? No Im not being a smart@$$ either. Im going to attempt to make a point.