I need a bigger turbo myths and how to drive a vgt turbo and turbo lag - Ford Powerstroke Diesel Forum
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Old 09-06-2013, 10:34 AM
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I need a bigger turbo myths and how to drive a vgt turbo and turbo lag

Well I have decided to make a post about this because i have seen many people talk about this and many people misunderstand the info. This has been from my own personal experiences with other people teaching me about turbos. These are also many of my own opinions and can be debated. Feel free to teach me because I love learning.

I am hoping to teach/learn stuff about the 6.0 turbo and how you drive it. I also want to point out a few things that people do not understand. Most of this applies to people who have bigger tunes and modified trucks. This is not to be compared to how a stock truck with stock or low hp tunes run.


Rolling into the throttle: Many people probably already do this but many people might not know what it means and why it is important. Rolling into the throttle is the process of helping the turbo spool faster and not shoot out so much smoke in the process. This is accomplished by slowing rolling your foot into the accelerator pedal instead of just mashing it to the floor. The reason for this is you are helping the turbo to build up drive pressure/boost so that when you do mash the pedal the turbo is able to keep up with the amount of fuel that is about to be dumped into the truck. This will help you getter better fuel mileage, better acceleration, less turbo lag, less smokey truck and much more. As many have probably experienced that if you just mash the pedal to the floor it will spit out of ton of smoke out the exhaust and then rip the tires loose when the turbo finally catches up. This is kind of the same theory as rolling your foot out of the clutch with a stick shift. You don't want to just dump the clutch because the vehicle will stall. There is also no need to go super slow. After a while you will get a feel for it and get do it fast with no problems. Press the pedal about 1/4 throttle, wait 1/2 sec. Then press 1/2 throttle, wait 1/2 sec, then just floor the throttle. Hope that makes sense

Is it normal for my truck to be smokey after loading a hot/race tune?: Tuners will often add more fuel down low to help with turbo lag. This should help minimize turbo lag and make your truck feel lighter on its feet. One side affect is that you will get a little puff of smoke on take off that should go away after a few seconds as long as you don't have the pedal past 3/4 throttle. That is just the extra fuel helping the truck to spool faster. Some tunes are worse than others but many would consider a slight puff of smoke on take off to be normal. Now if your truck smokes bad at idle, while cruising down the freeway at a steady speed, or if the smoke is excessive upon normal acceleration then there might be something else wrong with your truck. Most really hot/race tunes will smoke upon full throttle but should clear up to a slight haze after a few seconds. Rolling into the throttle at all speeds should help to minimize the smoke

Is it true that VGT turbos spool slower and flow less than non vgt turbos: Yes and No... The vgt system is actually designed to decrease turbo lag, but it is a double edged sword. What is cool about the vgt system is that it can manipulate the back pressure/drive pressure to help with spooling... but those same vanes that can help with turbine drive pressure can actually get in the way. By rolling into the throttle you will build up drive pressure/back pressure and when you finally floor the accelerator pedal the vanes can angle all that built up back pressure right against the tip of the turbine and really build up the boost fast. The double edged sword comes into play because those vanes are in the way and can cause increased back pressure. I am going to make a generalization about vgt vs non vgt turbos... a vgt turbo has the ability to spool much faster than a non vgt turbo of the same size, but a non vgt turbo of the same size has the ability to flow more air up top than the same size vgt turbo. Also the vgt vanes will get in the way at initial mash of the accelerator pedal and open up all the way. This makes you loose most of your drive pressure and loose the ability to build boost quickly. This is the feeling you might have if you were coasting at 45mph and mashed the pedal to the floor, the truck would bog down for a second, probably down shift, shoot out black smoke, and then take off like a bat out of hell. Rolling into the throttle would keep the truck from bogging down and make you build boost much quicker. These generalizations get exaggerated as the size gets bigger.

So with the same size turbo...

VGT- faster spooling, less flow up top
non vgt- slower spooling, more flow up top

My truck has turbo lag so I must need a bigger turbo this is just not true. There is so much that can come into play with turbo lag. You could have a boost leak, exhaust leak, bad ebp sensor, sticky vanes, etc.. etc.. But even if you truck was running perfect, adding a larger turbo will make turbo lag worse in 90% of all cases. Some people have cured other problems by claiming a bigger turbo fixed their turbo lag. It is much more likely they fixed something else in the process like a leak, or the turbo was bad, or the up-pipes got replaced at the same time etc... in general bigger turbos have more lag, so if you want less lag, then get a smaller turbo.

My truck does not build enough boost, a bigger turbo will fix this This is not true. Low boost is not caused by your turbo being too small. Similar to turbo lag you could have a boost leak, exhaust leak, bad ebp sensor, sticky vanes, etc.. etc.. By adding a bigger turbo you could possibly increase boost pressure but still not fix your problem. It is true that adding a bigger turbo can create more boost but many many people have reported stock turbos making 30+ psi on a powerful race tune. If you are only hitting the low 20s then there is probably something else wrong besides the size of your turbo.

More boost means more power Yes and No... Flow is what is important. If you have a ton of boost, a ton of back pressure, and your turbo is surging then it will be pretty obvious that you are not making a ton of power even though you were making big boost numbers. Many people think that you just need to turn up the boost to make more power. This is not true, although it can help to a certain extent. You need to make sure the air gets in and out of your motor quickly! Not just create a ton of pressure before and after the motor... For example (this is not possible but might help explain my theory) if you were to weld your exhaust closed and make everything air tight. Then pressurize the intake and the exhaust using an air compressor... you could have super high boost numbers and super high ebp numbers but you would be making 0 power. If you have a ton of boost building up and you are not getting the flow through the motor then you are not making more power. The pressure of the air built up before the motor is not important, it is the amount of air going through that makes the power.


There is a big difference between peppy, fast, powerful, etc...

A small vgt will feel peppy and feel fast driving around town but will not make much power on the dyno and wont win any races at the track. Great for a DD and towing, not for racing.

A medium sized vgt or non vgt will acquire a little bit of lag but it will be very powerful and very streetable without too much smoke. You may be able tow with it but is not usually ideal. Those peppy honda civics will beat you off the line for the first 15ft but you will pull on them fast and beat them.

A giant non-vgt turbo will feel super slow on the street, will not beat anyone from a stop light, smoke like crazy until spooled, but... they will make huge power on the dyno and do really well on the track.



This info is dumbed down and very generalized... it was just to give the basic idea. Fell free to add anything or correct my mistakes

Last edited by peixinho; 09-07-2013 at 12:23 PM.
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  #2  
Old 09-06-2013, 12:19 PM
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Took me a day to read thru all the turbo info on the Garrett web site. I'm no expert by any means after that day but I'm sure TONS smarter about turbos. Plus when you figure out how to read compressor maps the whole picture becomes quite clear.
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Old 09-06-2013, 02:33 PM
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Great write up with awesome info!
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Old 09-07-2013, 12:24 PM
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I added a little bit on smokey trucks

Is it normal for my truck to be smokey after loading a hot/race tune?: Tuners will often add more fuel down low to help with turbo lag. This should help minimize turbo lag and make your truck feel lighter on its feet. One side affect is that you will get a little puff of smoke on take off that should go away after a few seconds as long as you don't have the pedal past 3/4 throttle. That is just the extra fuel helping the truck to spool faster. Some tunes are worse than others but many would consider a slight puff of smoke on take off to be normal. Now if your truck smokes bad at idle, while cruising down the freeway at a steady speed, or if the smoke is excessive upon normal acceleration then there might be something else wrong with your truck. Most really hot/race tunes will smoke upon full throttle but should clear up to a slight haze after a few seconds. Rolling into the throttle at all speeds should help to minimize the smoke
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Old 09-07-2013, 05:42 PM
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Thanks for posting! Good read for understanding our turbos...
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Old 09-08-2013, 11:56 PM
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Old 09-09-2013, 06:07 AM
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Thanks for the post
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