I noticed something interesting as I was driving from Connecticut to Toronto, a good 800km drive for a healthy 7 hours.

Lately I have been driving in sport mode, using the paddle shifters. I noticed by watching the Instant L/100km gauge that I was getting better fuel economy cruising in 4th gear compared to 5th gear. I tested both gears out on similar terrain, going flat at 120km/h, going uphill and then downhill, and overall I'd say cruising in 4th gear wins.

I have a theory why that might be. I noticed that when cruising in 5th gear, the RPM sits somewhere around 2500 at 120km/h, and when cruising in 4th gear the RPM sits around 3200. What I also noticed was that the turbo is almost constantly being used when cruising in 5th gear. Even to accelerate 2km/h more, the turbo is maxed out. In 4th gear, however, the turbo is used minimally, if at all. I only noticed the turbo needle moving during very rapid accelerations or steep inclines.

I liked cruising in 4th gear because I felt like the car was right on the edge of the turbo threshold, where I had that power on demand should I have needed it, yet I wasn't quite consuming the extra gas yet. It seems that the extra gas consumed by the turbo is MUCH more than the extra gas consumed by cruising at a higher RPM.

It's kind of an unusual discovery for me, and I feel as though I may be doing something wrong here. Has anybody noticed similar behavior? The results are out, it was much cheaper driving back home in 4th gear than it was driving out there in 5th gear (about 60%). Ideas anyone? Thoughts?

 

No, you are more likely correct than not.

In order to allow for the boost pressure, "increased compression ratio", once the turbo comes on line the CX-7 engine normally (off-boost) operates in derated, detuned, mode, less than the optimal ~12:1 (with DFI) compression ratio. If you can get to the point wherein the turbo is just barely making up the difference, in this case raising the "overall" compression ratio to ~12:1, and maintain it at that level the "cruising" FE would undoubtedly improve somewhat.

Sounds as if you found that "sweet spot".

What was your actual MPG each way..??

To be fair overall I'd guess you were loaded up a bit heavier than normal putting your CX-7 out of range of the engine/transaxle ECU's operational parameters for achieving optimal FE.

 

RDX! Not CX-7!

I figured I'd found the "sweet spot", as you put it, because where I had my foot on the pedal, I would have to just barely push to feel a surge in the car (which I LOVE), but it was purely the engine unassisted by the turbo. When in 4th gear, the only time I saw the turbo working was when I really stomped on the gas to accelerate by more than 10 or 20 km/h (like coming out of those construction zones!)

My fuel economy was averaging about 11 L/100km on the way back in 4th gear, and close to 12-13 on the way down, if I remember.

There's a sweet spot in 3rd gear too. I call it my FDR gear, because I drove from the Triboro Bridge down to Houston St on the FDR in 3rd gear and was just zipping through all those curves and inclines. 3rd gear at around 40-50 mph is VERY zippy. It runs at a very high RPM, but it's unbelievably responsive to the pedal - I love it!

3rd gear is also called my Tappan Zee gear, and my BQE gear.

I really like zipping around in low gears at high RPMs, like close to 3000-6000 RPM. It's a nightmare for fuel economy, I'm sure, but in the city it really makes the car GO!

 

I used the paddle shifters for the first time in "S" mode. Wow! It transforms the RDX into a wanna be sports sedan. Me likes coming from a CL Type-S.

 

It's amazing, isn't it? Just keeping it in one gear lower than what it would be in in 'D' makes all that difference.

Funny, I was just in Connecticut with MY Carbon Bronze '08!

 

The RDX is a different car in S mode.

 

"It was purely the engine unasisted by the turbo.."

Not very likely...

I would guess that you were using the boost gage to make that judgment...

Does the "boost" gage only indicate positive manifold pressure in relation to atmospheric pressure or does it also show the "boost" level as a lack, "shortage" of a manifold vacuum...??

I doubt if the turbo ever stops spinning even at idle and so absent an open BOV some level of boost above a non-turbo will always be available in the intake. So the question becomes "just when does the gage begin to indicate a level of true boost..? Something above the local atmospheric pressure? Or rather simply a lower level of vacuum in the intake than you would have absent the turbo.??

Anyone know for certain..??