Higher Boost or higher RPM?

[Brian_B]

So, I do a lot of driving in the mountains with a 7MT. You kind of get to make the decision on where you want the engine sitting.

A lot of hills and the speeds you drive around here, you get to make the choice - do I sit in a lower gear with higher engine RPM, or a higher gear with more turbo boost?

Aside from the fact that the engine is more responsive in a lower gear - that isn't necessarily what I'm asking. Some of these are long stretches on a relatively decent grade - 5-10%. At 50mph I'm either at around 3k RPM in 4th (and 3-4psi boost) or sitting at around 15psi boost (at about 2.5k RPM) in 5th.

Which is better for the engine, or does it make much of a difference?

The manual recommends the following:

For best fuel efficiency when driving your vehicle, shift to the next gear by 3000 RPM.
Shifting at a higher RPM such as 3000-6000 RPM can increase the performance capabilities of the vehicle. However, higher engine and transmission speeds can increase wear and reduce fuel economy.
When downshifting your vehicle, always shift at the lowest engine RPM possible to match your driving conditions. This is usually under 2000 RPM for all forward gears, except for downshifting into the crawler (C) gear which you should do below 1500 RPM.

If I go by the manual, the engine drags badly at under 2K RPM - fine for cruise control on a long stretch of flat highway, but doesn't work at all in the hills/curves. I do usually shift around 3.5k RPM or so, but just cruising i'm usually closer to 2.5K RPM and rarely under 2K unless it's down in the valley on something long and flat.

Just curious what people with better aptitude for these things than I posses think about this.

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[PWillette]

I'm not 100% sure which RPM range is better or worse for the engine but IMO 2k rpm or under is probably more detrimental. My driving is basically same as you...cruising 2.5k to 3K, up-shifting bewteen 3k - 3.5k. IMO these 2.3s like to be wound up.

 

[colbert]

Let it spin.

 

[Boreal]

Running at 3k with 3psi is “easier” on the drivetrain than 2.5k @ 15psi.

At 3k/3psi The engine is naturally providing 95% of the power required with almost no turbo. This higher rev gets you a bit deeper into the power and torque curve, where the engine is most “efficient” with the fuel delivered.

At 2.5k/15psi, you are adding in almost twice the volumetric air compared to 3psi. To keep AFRs happy, that means you are doubling the amount of fuel injected. Like trying to power down on a bike in 10th gear; You are asking for more grams/rev, if we’re talking crank load.

My thoughts are that 3k/3psi is far easier on the drivetrain and it should even be more fuel efficient

 

[Techun]

higher rpm

 

[Brian_B]

Thanks for the quick replies - my intuition has been telling me to look at the boost gage for upshifts rather than listening for RPMs like I have done all my life on every non-turbo car I've ever owned.

No sense in upshifting if you are still sitting in boost. But figured I'd ask, I'm not terribly mechanically inclined and some of you have been running these Ecoboosts for a long time now.

Thanks again!

 

[Boreal]

look at the boost gage for upshifts rather than listening for RPMs
No sense in upshifting if you are still sitting in boost.

Yes and No!

Turbos also have a range for efficiency. That’s why down low in the revs you hear about the Turbo Lag - where there isn’t enough exhaust flow to power the compressor. (Turbo lag can also be described with the physical demand to get the prop shaft up to speed when you mash the pedal)

In the very high end of the revs, you’ll hear people say that it “Runs out of breath” - the turbo hits a point where it can’t flow enough air for the inlet and/or exhaust diameters.

OEMs prefer the turbo to be smaller, snappier in response to mitigate lag and reproduce the benefits of cylinder displacement as close as possible. These little snails are Twinscroll, meaning that instead of having a limited operating window from 2900rpm - 4500rpm, it’s expanded to say 2300rpm - 5500rpm. With more focus down low in the revs where the average consumer spends 99% of their time.

In short, it’s not so much as watching any particular gauge but butt feel. When the vehicle stops pulling hard, the turbo is out of breath. You’ll still show boost, still have room to redline, but the returns are diminishing. Go by feel and you won’t go wrong

 

[Ducati1098]

Higher the boost, higher the cylinder pressure.
In this scenario with minimal RPM difference, the higher boost pressure will be more harmful to the engine in my opinion.

 

[PWillette]

Thanks for the quick replies - my intuition has been telling me to look at the boost gage for upshifts rather than listening for RPMs like I have done all my life on every non-turbo car I've ever owned.

No sense in upshifting if you are still sitting in boost. But figured I'd ask, I'm not terribly mechanically inclined and some of you have been running these Ecoboosts for a long time now.

Thanks again!

I always have and still drive by listening to the RPMs more than looking at the tac or boost.

 

[thesocalexplorer]

Agreed with above. Even when cruising, I rarely go into 6th gear (especially after going with 33s) unless I'm pushing 75mph or higher. I'd rather be at 0-3PSI at 2,800 RPM than 5-10PSI at 2,000 RPM.

 

[Tricky Mike]

I found the fuel economy difference is stark between high RPM/low boost and low RPM/high boost. I haven't used 6th since I put 35s on.

Additionally, the non turbo versions of this engine often spun 3500+ on the freeway in top gear in the economy cars they came in. They can take it.

 

[Haze74]

I’ve got a Black Diamond non sas on 285/75/17s and I’ve been flipping back and forth between watching the boost and fuel consumption. On the highway going uphill, I will generally downshift to 4th if I’m doing around 100 km/h (60mph) and the fuel consumption goes above 20L per 100km (60% or more of the fuel consumption gauge). That generally puts the RPMs around 2700 RPM Which ends up putting the fuel consumption about the same or a bit more than being in 5th. Definitely feels a lot easier than the motor and we all know it goes above 3000 RPM comfortably.

Above 100 km/h on flat or downhill I will generally shift to 6th Keeping the fuel consumption quite low. I’ve noticed a big difference over a tank of gas driving like this.

I try to avoid putting a load on the engine and transmission for sure though.

 

[e34john]

Higer revs because I am scared of LSPI, also I think occasionally the good old "Italian Tune Up" does no harm.

 

[Haze74]

Higer revs because I am scared of LSPI, also I think occasionally the good old "Italian Tune Up" does no harm.

“Italian tune up” oh yes! I don’t actually know if it makes a difference anymore, but I remember doing that back in the day!

 

[e34john]

“Italian tune up” oh yes! I don’t actually know if it makes a difference anymore, but I remember doing that back in the day!

Even the BMW owners manuals in the mid-90s still mentioned it. But I'd give it a go just for the turbo noises and these DI engines seem to gunk up more even though I haven't had any issues with any of mine.

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