2015 Cornerstone & energy management

[ken n deb]

Coming home to Atlanta from Thanksgiving in Winston-Salem on I-85 in the family car we were passed by a Cornerstone doing 80 mph thru some pretty hilly country. We kept up for a short while trying to get a better look but somebody was in a hurry! Constant speed up and down hills and they were soon out of sight. I'm not sure I'll drive that fast but it gave me lift to know I could

[Crippsfl]

Originally Posted by ken n deb

Coming home to Atlanta from Thanksgiving in Winston-Salem on I-85 in the family car we were passed by a Cornerstone doing 80 mph thru some pretty hilly country. We kept up for a short while trying to get a better look but somebody was in a hurry! Constant speed up and down hills and they were soon out of sight. I'm not sure I'll drive that fast but it gave me lift to know I could

It wasn't me, but it sure can do it if you are inclined to do it! Even in hilly country, on the freeway, I set the cruise to 65 - 68 and let the big girl run. She doesn't even breathe heavy. The only thing you have to watch is going downhill. You will be pushing 75 before you know it. Find my self tapping the brake in those situations.

One of the biggest joys is the power you have when getting on the freeway on ramps.

I swear I was spinning the dualies coming off a light once.

[Cruzer]

Normally I would say that a post like this is "off topic" because the topic is about Energy Management. However, when you talk about the ability of a Cornerstone to cruise effortlessly at 80 MPH - I guess it really is about "energy management".

[DSL417]

Actually, Mark, I was kinda surprised by that "energy management", We just travelled across the Southern end of the Southwest. A lot a hills and a few grades. We tried 65-70 and then for the heck of it and then did a day at 75. Actually got better gas milage at 75 than 65 (although we had a significant crosswind on the 65 day, and for some reason crosswinds seem to significantly affect milage). I am kinda thinking there is a sweet spot for the engine at the 1600-1700 rpm; but the 450 SL would have been cranking at 2000 rpm at that 75 speed. But I have found the same...all the "power" I want, for speed or entry onto the freeways. And, the electrical power management works pretty well too.

[Cruzer]

That does make sense. And that works for me too because we travel out west and that's the speed that what everyone goes in Montana, including 18 wheelers.


You want to get close to your peak torque for maximum fuel economy. The ISL puts its 1250 ft-lbs of torque at 1400 RPM and its peak horsepower at 2100 RPM. The ISX15 puts its 1950 ft-lbs at 1200 RPM and its 600 HP at 1800 RPM. The K3 chassis sports a 4.30:1 ring and pinion while the MMGT has a taller 4.88:1 axle.

The upshot of this is that the Cornerstone has more power down low in the RPM band and an axle ratio that lets the engine turn at slower revs. All that is a plus for fuel economy and can help counter the increased displacement of the ISX15 over the ISL9. Plus, when an engine doesn't have to work as hard, efficiency always improves and the air-to-fuel ratio leans out.

Many Tiffin owners found out that the Zephyr with the 500 HP ISM got equal fuel economy as the Allegro Bus with the 400 ISL. Most people assume that the 600 HP ISX is going to have a serious penalty at the fuel pumps but that's just not so. Unless you are constantly throttling it up mountain grades or trying to set the latest Bonneville record for heavy motorhomes.

[ken n deb]

Didn't mean to hi-jack the thread... Sorry!