Why do I experience turbo lag in VGT-equipped Montero Sport?

Our tech guru has the answers to your car questions
by Ferman Lao | Sep 23, 2013
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Hi, Ferman. I've read your articles online and in the magazine, and I am very impressed with the wealth of stuff you know and how you explain things in a way that we mere motoring mortals can understand. Keep it up and thank you for offering countless pieces of advice for us readers.

My father drives a 2012 Mitsubishi Montero Sport GLS-V. Like all V variants of Mitsubishi trucks, it's equipped with the 2.5-liter diesel engine with the variable geometry turbo (VGT). Correct me if I'm wrong but from what I understand, a VGT has the ability to change the angle of the turbine blades so that the turbo ideally gives the same amount of kick even if the engine is running at low revs. The angle at which the turbine blades are at will somehow compensate for the lack of exhaust gases being fed to the turbo by the engine running at low revs. So in theory, the turbo is kept happily spinning most of the time, and turbo lag is minimized if not eliminated.

The minimized turbo lag and torque at low rpms is what Mitsubishi promised when they equipped their 4D56 turbodiesels with a VGT. But in reality, with our Montero Sport at least, that is not the case. The engine has neck-snapping grunt (which I absolutely love), but it only comes in past 2,000rpm. When going full throttle from a standstill (and keeping the transmission in first), the acceleration is a bit underwhelming from idle up to 2,000rpm. Above the 2,000rpm mark, the engine comes to life and the car goes like a stabbed rat.

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Now, this isn't really a problem when driving within the limits of the metro since the engine will run below 2,000rpm most of the time. But in the provinces, where you need maximum thrust for overtaking, the engine takes its time to rev toward 2,000rpm and beyond even when the accelerator is pushed all the way to the floor. Unfortunately, the transmission is also a bit sleepy and takes a fair amount of time to shift to a lower gear. Time is wasted when charging down the opposite lane to overtake, and you sort of hope that the engine will soon come alive and give you that pulling power to pass slower cars and get back on the correct side of the road.

To overcome this problem, once the decision to overtake has been made, I put the transmission into manual and drop it down a gear myself so that the engine is kept spinning at 2,000rpm at least. When the time to overtake comes, the engine is kept in the powerband and is able to give me maximum power at the instant I need it.

Now, this isn't the first time we had a common-rail turbodiesel. We used to own another Montero Sport, a GLS SE, which had a 4M41 3.2-liter lump and generated roughly the same amount of torque and around 20hp less than the smaller VGT-equipped 4D56. It didn't have that turbo lag. Power delivery was smooth and linear from idle until the redline, and the transmission (which had one gear less than the current VGT Montero Sport transmission) eagerly drops down to the next lower gear--a big help during overtaking on provincial roads. We also used to own a Pajero Field Master that had the trusty 4M40 non-common-rail turbodiesel engine. Sure, it was underpowered by today's standards, but just like the GLS SE Montero Sport, power delivery was smooth, and the transmission was willing and able to keep the engine lively the moment you needed it.

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I don't know what is holding back the VGT engine until 2,000rpm. It could be turbo lag, but having a VGT, I don't think it's supposed to happen. It could probably be transmission-related. Maybe the transmissions put in Montero Sports these days cannot handle the gobs of power that the engine can hurl at them. That could possibly explain the hesitation that I experience during hard acceleration. Do note that our car is maintained to manufacturer standards and gets its maintenance checks at manufacturer-recommended mileage intervals. Our car is also equipped with a K&N drop-in washable air filter element, which I hoped would cure some of the supposed turbo lag but didn't.

Thank you in advance for taking time to read my long letter. More power to you and to the Top Gear Philippines team!

Miggi Solidum

 

Hi, Miggi. You're welcome, and thank you for the kind words.

A variable geometry turbo minimizes turbo on the onset of full boost time or "lag" by changing the effective "size" of the turbo to achieve better efficiency (or deliver "full boost" all of the time). It was developed to broaden the effective operating range of a turbocharger. A traditional turbocharger some 20 years ago would be most effective only after a certain engine rpm was achieved.

The exact rpm varies with engine type and manufacturer, but most factory-equipped petrol-fueled turbo cars usually achieve maximum boost at anywhere from 2,500rpm to 3,500rpm, and continue to do so until the turbo efficiency drops off at anywhere from 800rpm to 1,500rpm before redline. For diesels, it's usually a lower and narrower range of rpm. Which is why until the past decade or so, turbo diesel were primarily used for heavy equipment and commercial vehicles that carried heavy loads. These vehicles needed to pump out a lot of torque early and did not need to run at "high" engine speeds.

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I first encountered VGTs sometime in the very early '90s, but they were almost three times the cost of a standard turbo at the time. Nowadays, they've come down in cost, at "only" about 150% the cost of a regular turbo. They have also been called by many names, such as variable-area turbo nozzle and variable nozzle turbo. But essentially, they all do the same thing.

In the case of your Montero Sport, I don't think it's a VGT problem or transmission problem. It's possibly more a combination of occurrences that compound the perception of sluggishness or no response from the vehicle. I can't say for sure if there is a problem without driving your vehicle myself, but I have a theory:

The engine is rated to achieve maximum torque from 1,800rpm onward and is mated to an automatic transmission. Most automatic transmissions are usually programmed to start in second gear when coming from a standstill as well. They aren't also very responsive at engine speeds below 2,500-3,200rpm (depending on the vehicle). To help improve fuel economy, they are being programmed to upshift as soon as the conditions allow it.

That being the case, the only way to get more response out of your vehicle would either be to do what you are already currently doing: Perform the downshift yourself, and step on the throttle more to open up the throttle more, thereby signaling the ECU to tell the transmission to downshift. Or you can also have the vehicle tuned to develop more power at a lower engine speed. You've done the first solution. You might want to consider the second solution.

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Personally, when I am at cruising speeds and I need to overtake, I always plan ahead and usually drop down two gears if possible to be able to accelerate, overtake and get back into the proper lane quickly. If you choose to have your car tuned for more power, that certainly will help doing so apart from reducing the lag you're perceiving now. I also haven't had a car that's not modified one way or another in the last 20-plus years.

Many tuning solutions are available out there, and choosing one is entirely up to you. I would stay away from throttle controllers and plug-in diesel tuning "race" chips. The former doesn't really increase power as it only signals the ECU to open up the throttle bigger than what it would've been without the controller; the latter can't be optimized for the entire power band, making it no good in my book.

The whole point of tuning is optimization to get better efficiency and power. So to me, there isn't any value in either solution, given much better solutions are available out there for about the same amount of money.

You will probably want to consider a Unichip or a device with similar functions. At the moment, I think there are two or three other brands on the market that can do the same, but the Unichip will be the most cost-effective among them all--throttle controllers and "race" chips included.

 

Best regards,

Ferman Lao
Technical Editor

Do car problems keep you awake at night? Send questions to topgear@summitmedia.com.ph.

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