Cummins N14 best cruise RPM

Wait...what? What do you mean doing the bearings at the first 200k?

 

When I was at Stevens, we had N14s running through direct 10 speeds and 2.85 rears. My truck was governed at 68 and the engine was running at 1650 rpm and I always got at least the fleet required 6.2 mpg.

 

Wow.... it sure getting deep around here, LOL 8 mpg pulling a loaded flatbed with a N14 ? Hahahaha



N14 peak torque is 1200 rpms, peak HP is at 1600 rpms. (Ever notice when your heavy and pulling a hard hill that you can't seem to get past 1600 rpms ?)

According to Cummins, 1500 for peak performance, but then again they were talking about the ISX...

N14 - 525hp (1850 trq) 13 spd w/ 3:42 rears = 68 mph @ 1350 .... 70 mph @ 1400 rpms. = 5.8 to 6.5 (no idle time)
I've tried 1450 @ 60 mph in 12th and never could get 6 mpg. Funny how 100 rpms can make such a difference.
Best fuel mileage I've ever gotten was running 55 mph @ 1300 rpms with a light load across Ohio (flat land)

I've had my truck for 8 yrs and trust me when I say I've tried #### near everything from switching over to SS, full flow exhaust and a APU
The best ever was bobtail 8.4, best loaded 7.5 worse loaded 3 mpg.... You just can't make a rolling brick (Western Star) more fuel efficient.

 

I'm thinking about buying a reman N14 next week to replace a 475 ISX (egr). I average 8.2 loaded on that ISX. Do you think I will be disappointed in the N14 fuel economy? I know a lot of variables are involved to answer that, but just generally speaking; do you think the ISX egr motor will out perform the N14 in the economy category?

 

Hello everyone

I bought 379 Pete with Cummins N14 red top 525 18 speed 39 rears and 22.5 tires, rpm goes 1500 at 59 mile with 22.5 tires. On the very first trip my fuel economy was 5.4 mpg. Please suggest me what should I do to increase fuel economy. Should I change tire as 24.5 or go with valve setting. Please suggest.

Thanks

 

If you're on interstates, slowing down isn't an option for you. 24.5s aren't going to do much for you either. I suggest you swap your rears for 3.55s or lower. Expensive yes, but you'll get more miles per day to offset the cost.

 

If you knew which series of N-14 and the horsepower rating, it would help a lot.
The problem is, there's probably 20 different hp and torque ratings for these engines depending on the year, yours is an early engine and I suspect you don't want to cruise that engine too much under 1450 or 1500 rpm with a load on.
If it's not an econo engine it probably makes hp up to at least 1800 rpm where the curve goes flat, time to shift gears.

I drove one N-14, a 460 hp "Red Head" in a year old 1996 IH 9400 Eagle, 24.5 rubber, 3:70 rears and Super 13 spd, truck ran a steady 65 mph at around 1500 - 1550 rpm.
This truck had the full fairing package except for the roof fairing.

That's where the sweet spot was on that particular N-14, the year round average fuel mileage was close enough to say 6 mpg US, 6.6 CDN.
The truck was specced to pull a van at a constant cruise speed of 65 mph with loads of varying weights, about 70% of the time loaded close to max.
Sometimes it went a little slower, sometimes a little faster when conditions were favourable, tailwind, rolling hills, down a mountain with a light load or no brakes.

If you want to increase fuel mileage with your 3:90 gears and 22.5 rubber, you need to slow down, period, gonna say 55 to 62 mph.
Your truck is geared for mountainous terrain, off road or heavier loads over 80,000 lbs.

As someone suggested, a 3:55 gear with your 22.5 rubber would be a good general purpose choice IMO, and give you nearly same road speed and rpm as 3:70's with 24.5 rubber, likely better fuel economy if you keep it at 65 mph or under and will still pull decent enough in the hills or a heavy wind.

If you want to cruise at 70 mph plus with your engine in the sweet spot, likely a 3:42 or 3:36 would be your gear, but I wouldn't do anything unless I knew for sure which engine and power curve I had or was going to program into.
(sometimes more becomes less)

 

I used to own an N14, 525 HP. It only had 1850 torque, and 3.73 gears, 24.5s. But my runs were in the mountains of BC Canada. I averaged 5 mpg. That sounds low, but given that 95% of my time was in real mountains with constant grades of 6% or more pulling 104,200 lbs, it wasn't too bad.

But, I owned another truck not long ago with 700 hp, 2250 torque, and 3.21 gears with 22.5s. I averaged 7.2 mpg pulling the same weights in the same mountains, and I don't drive for fuel economy. Yes, some 13 liters can get better mileage, but I saved myself over an hour and half compared to them. Time is money is also money is time. I get paid the same no matter how long it takes. To each their own. But I pulled the grades like a freight train, and on the flat I could cruise at 1375 rpm and do 75 mph. Also, I put the lie to you need 3.90+ for mountains.

 

Listen, I'm really happy for you and your super truck, hoorah!

But...., I told no lie, 3:90 is a gear found in a lot of trucks that run mountains all the time or pull heavy all the time.
If you look at what truck dealers are speccing to pull super B's you'll see a lot of 3:91. 4:10 and even 4:30's with Cummins X-15's at 500 to 600 hp.

By the way, I don't believe you average 7.2 mpg driving 75 mph with a 700 hp engine and 3:21 gears pulling 104,000 lbs in the mountains all the time, sounds like absolute BS to me!
That may be your best mileage, but definitely not your year round or monthly average.

 

I'm not in the habit of lying. That 7.2 was an average, based on mountains and a small amount of time each day on the flat. In fact, my yearly calculation was 7.5, but I also had a block and bunk heater installed, so there wasn't much wasteful idling. And if you ever watch Highway Through Hell, that featured hill is called The Smasher. It used to be 14% but somehow the government has now decided it's actually 8%. Probably to avoid paying for so many plows during winter. Anyhow, with a maxed tridem, 104,200 max GVW, I could be in 7 over all the way to the top. But then again, I have almost 40 years on the road and know how to get maximum performance here in BC.