Cruising RPM

[tk064]

Trying to work out gearing questions for a 4bt conversion. Looking for 40" tires with a . 75 OD. What is the optimum cruising RPM for the 4bt? I was looking at a 4. 56 or 4. 10 ratio. Appreciate any input.

 

[philb4]

ratios

Go to www.jeeptech.com and look at the bottom left ,hit Gear and Tire. This will calculate everything you ever wanted to know about this.

 

[JLEONARD]

I have specs for the marine versions of the 4bta.

The max rpm rating is 2800 for the 155 hp version, and 3000 for the 250 hp version.

That compares to 2600 rpm the 6bta. Therefore I would say that cruise rpm could be a couple hundred more than what the CTDs run.

 

[MAX A/C]

For best fuel mileage, you would want to run the motor at the rpm's where it consumes the least amount of fuel (it's "sweet spot") this is usually the same, or darn close, to the motor's peak torque. Look at the chart for BSFC (Brake Specific Fuel Consumption) and find what rpm's it consumes the least fuel at, which as I said will probably be peak torque. Remember, BSFC is pounds of fuel used per hour per horsepower at WOT. Of course, we don't cruise all day at WOT, but I have not seen a discussion of any improvements of the WOT number for part throttle conditions.



The chart for the 4BT can be found at the link below, and the 6BT can be seen also by going back to the site's main page. From the chart I gather that for the 4BT, 1600-1700 rpm's is most efficient. However, remember that if you are towing a load, or have a lot of wind resistance, you might not be making enough HP at 1700 to get the job done. So then you wind the motor up to make more HP, and suck more fuel!



Hope this helps! If I'm wrong (anybody) feel free to correct this info!

Scott



http://cgi2.x-pointcgi.com/~dieselrepower/Cummins_4B_Series_Engine_Specs.html