Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Some deep thinking on getting Best MPG in a VAN

aardvarcus · 09-25-2018, 03:08 PM

Here is what I would do if I were trying to do your project with your constraints (no major drag reduction mods, sticky tires, no diesels, etcetera).

You are set in that you will have moderately high power requirements for a given speed. It is now all about making that amount of power in the most efficient manner. I would do that at the lowest RPM possible. The 8.1L power plant I am familiar with makes 400+ foot pounds of torque at 1000RPM, which is almost 100HP. Thus, if we assume a 50-60HP cruising requirement, that would imply at 1000RPM we are loading the engine at over 50%, putting it in decent BSFC territory, but still have enough reserve HP to climb small hills without downshifting.

You seem to want to custom build an engine, so if you can custom build an engine to hit similar sorts of torque numbers at similarly low RPMs that would work the same way. In this case I would suggest dyno runs on the completed engine so you know what you have, preferably with fueling information if possible.

To get anywhere near that crusing RPM, I would be using a Tremec 6 speed manual transmission with the 2.97 first and 0.50 OD. Pick your tire diameter and you can reverse engineer what your rear end ratio needs to be.

You seem to have a different transmission picked out, so adjust accordingly based on the OD ratio. I wouldn’t get too focused on being perfect at every speed and RPM.

If you do this project, but you don’t get your RPMs low enough to be moderately loading your engine at your cruising RPM it isn’t going to work out the way you think it is. Fundamentally it would still be better to try to reduce the load along with trying to produce power more efficiently.

09-25-2018, 03:08 PM	#67 (permalink)
aardvarcus Master EcoModder Join Date: Apr 2012 Location: Evensville, TN Posts: 676 Deep Blue - '94 GMC Suburban K2500 SLE 90 day: 23.75 mpg (US) Griffin (T4R) - '99 Toyota 4Runner SR5 90 day: 25.43 mpg (US) Thanks: 237 Thanked 580 Times in 322 Posts	Here is what I would do if I were trying to do your project with your constraints (no major drag reduction mods, sticky tires, no diesels, etcetera). You are set in that you will have moderately high power requirements for a given speed. It is now all about making that amount of power in the most efficient manner. I would do that at the lowest RPM possible. The 8.1L power plant I am familiar with makes 400+ foot pounds of torque at 1000RPM, which is almost 100HP. Thus, if we assume a 50-60HP cruising requirement, that would imply at 1000RPM we are loading the engine at over 50%, putting it in decent BSFC territory, but still have enough reserve HP to climb small hills without downshifting. You seem to want to custom build an engine, so if you can custom build an engine to hit similar sorts of torque numbers at similarly low RPMs that would work the same way. In this case I would suggest dyno runs on the completed engine so you know what you have, preferably with fueling information if possible. To get anywhere near that crusing RPM, I would be using a Tremec 6 speed manual transmission with the 2.97 first and 0.50 OD. Pick your tire diameter and you can reverse engineer what your rear end ratio needs to be. You seem to have a different transmission picked out, so adjust accordingly based on the OD ratio. I wouldn’t get too focused on being perfect at every speed and RPM. If you do this project, but you don’t get your RPMs low enough to be moderately loading your engine at your cruising RPM it isn’t going to work out the way you think it is. Fundamentally it would still be better to try to reduce the load along with trying to produce power more efficiently.