Bigger turbo = better MPG?? - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 26 (permalink) Old 04-27-2010, 04:55 AM Thread Starter
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Bigger turbo = better MPG??

Hey guys not sure about this so lets talk about it. Would a bigger turbo on stock injectors give better Fuel Economy? In theory you are increasing the air in the combustion chamber offering a better chance of complete combustion... but would it really work like that? Maybe with compound turbos so that you dont have the lag? or Maybe a Supercharged/Turbocharged combo in order to get the extra air in (I dont believe the supercharger would help much since it uses a lot of the engine power to drive it...)

So what do you guys think?

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post #2 of 26 (permalink) Old 04-28-2010, 01:33 AM
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Re: Bigger turbo = better MPG??

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.

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post #3 of 26 (permalink) Old 04-28-2010, 01:55 AM
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Re: Bigger turbo = better MPG??

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?
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post #4 of 26 (permalink) Old 04-28-2010, 06:34 AM Thread Starter
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Re: Bigger turbo = better MPG??

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

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post #5 of 26 (permalink) Old 04-28-2010, 07:25 AM
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Re: Bigger turbo = better MPG??

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.


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post #6 of 26 (permalink) Old 04-28-2010, 09:34 AM
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Re: Bigger turbo = better MPG??

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?
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post #7 of 26 (permalink) Old 04-28-2010, 01:46 PM
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Re: Bigger turbo = better 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
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post #8 of 26 (permalink) Old 04-28-2010, 02:10 PM
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Re: Bigger turbo = better MPG??

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.

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post #9 of 26 (permalink) Old 04-28-2010, 02:16 PM
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Re: Bigger turbo = better MPG??

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.

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post #10 of 26 (permalink) Old 04-28-2010, 02:41 PM
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Re: Bigger turbo = better MPG??

Quote:
Originally Posted by INFOCUSHAULER View Post
Hey guys not sure about this so lets talk about it. Would a bigger turbo on stock injectors give better Fuel Economy? In theory you are increasing the air in the combustion chamber offering a better chance of complete combustion... but would it really work like that? Maybe with compound turbos so that you dont have the lag? or Maybe a Supercharged/Turbocharged combo in order to get the extra air in (I dont believe the supercharger would help much since it uses a lot of the engine power to drive it...)

So what do you guys think?
Quote:
Originally Posted by Dave Whitmer View Post
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.

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