In order to keep my engine RPM above 1500 at a reasonably safe speed, I can never use 6th gear. In 6th gear above 1500 RPM I would be at 80 MPH or faster and I am not comfortable driving that fast. Anyone else have this problem?
Hmmm. 1500 in mine is right at 65
2000 u-320 4010
I am just over 1500 RPM at 70 MPH in 6th.
I don't like to let the RPM drop below about 1700 because it feels like it's lugging in 5th at that RPM.
Royce,
Wonder if your tach is accurate?
Could have it verified.
Could use this formula to verify: (Engine RPM x 60) / (Rear Axle Ratio x Tire Revolutions per Mile x Transmission Ratio) = MPH
96 U295. C8.3 Cummins. Allison 3000 series. 1800 rpm=62 mph 6th gear
1700 rpm seems a little high, max torque is at 1200 rpm, believe cummins recommends 1500 rpm for best all round performance.
At 1500, I am going 70.5 mph by silverleaf and gps.
Royce, we same set up. We right at 1375 rpm at 65 mph. No need to be 1500 rpm, IMHO. My reading on rpm not by digital though so tach a little hard to read very closely but the 1375 seems it.
I remember seeing a chart somewhere indicating optimal fuel economy is at 1400 on the ISM so that is where I keep it. Either 55mph in 5th or 63-64mph in 6th.
That is closer to what I found. To keep 1500 rpm I have to be over 71 mph.
Different coaches will have different rear end ratios so different speeds for a given RPM. To verify tachometer, use Silverleaf or Pro-Link 9000 and it will give you a digital readout to compare to the analog tach reading.
The Allison should sense a "lugging" condition and shift down to keep RPM within specs for the load.
Pierce
Cummins will have a spec that says "above XX RPM full throttle operation allowed". Now, this was mostly for mechanical engines. The computer will prevent overloading (aka "lugging") of the M11.
For flat land driving, I'd aim for around 1300-1400rpm at cruise of 70-75mph. Just kick it down to 5th for the hills.
I'm re-gearing my c8.3. Going to go from 5.13 to 4.33. There's a thread I made about it.
Pierce said:
So from what I am reading here is 1500 is not too low an RPM for a ISM450. Correct?
I think it is peak torque for that engine, the effiecency sweet spot
1500 rpm, best combination of power and efficiency.
I went to find the chart that I found before regarding ISM fuel consumption vs rpm and all I can find the is the one for the C83 which does indicate 1500 is the sweet spot. I could swear what I found before was that 1400 was the sweet spot for the ISM. Either way I don't think it has nearly the effect that cruise speed does (55mph vs 65mph).
Sweet spot is always max torque rpm per cummins RV mileage publication, as far as engine efficiency goes. Mind you that may not be a speed/top gear sweet spot depending on gearing
Agree. Going from 55mph to 65mph requires more HP. Making more HP requires burning more fuel!
What a engine fuel efficiency curve shows is how efficient the engine is at making each of those HP. Brake Specific Fuel Consumption (BSFC).
So, your engine may consume more fuel if it is putting out more HP, but actually operating more efficiently.