Some deep thinking on getting Best MPG in a VAN

[racprops]

OR:

Years ago I planned on using the whole old Borg Warner 3 speed with OD transmission, without first gear, but using the 3rd and 2nd gears as a under drive, with 3 at 1:1 being no under drive and 2nd as a under drive...with these transmissions you can engage or disengage the OD as it is its own transmission at will so I could split all gears...
They had a feature called freewheeling, it was a one way sprang that without power to lock it up allows the engine to drop to idle, and just a little push on the gas and it would spin up and lock.

This will allow me to coast (hypermileing...) without really taking it out of gear (which I believe is illegal) but as it would be just idling keep all my power extras like steering, A/C and brakes all working...

On these old overdrives have a pull or push control cable would either free it up to free wheel or to lock up and give engine braking.

SO I would have an 8 speed with a 2 speed rear end. OR another way to look at it a 16 gear system. 8 low and 8 high.

I still have two of the OLD Borg Warner 3 speeds with OD in storage. One was rebuilt without the first and reverse gear.

One major problem is building a new front end and either a direct connection to the output shaft of the front transmission OR a yoke and with a support bearing (Perhaps) and then mounting it a midships as a brownie with a short drive shaft with the correct off set for the u-joints.

BUT building a shifter and mounting it the floor and the cable control and then adding a modern VSS sensor kind makes it a bit much, and adding that old cast iron tranny is another pain. AND a weight cost as well.

On the other hand perhaps I can rig up an electric powered shifter to shift from third to second and back again…as I have learned with these old over drive tranys, with it in freewheeling even shifts into the non-synchronized first gear was a possible due to the lack of any power (load) on the drive shaft…

The over drive is cable and electric control plus a manual lock out by the shifter so a powered shifter to lock out the OD by shifting into reverse, note there is no reverse gear, but this could be a control to be sure anytime I shift into reverse automatically lock out the over drive, and perhaps as an on/off control as well.

I will look into the gear massup with a 4L80 transmission.

Rich

I found out I maybe wrong about the freewheeling feature of the old OD, seems one in OD that is locked out by the overdrive, it is only in use in gears before OD is ingaged, it is needed to allow it to shift INTO OD.

[racprops]

I am looking for anyone that can show me where I might be wrong with these ideas and perhaps show me a better way to do what I am trying to do:

LOW RPMs torque as in around 1000 to 2000 in a gasoline engine. NO Diesels.

Build or install a transmission(s) that will give me great take off power and allow me to find a gear for nearly every speed and load even hills.

Thanks for you interest and what help I have already received and any future help.

Rich (The Madman)

[Ecky]

As long as your gearing is tall enough that you can keep the larger engine near 100% load, it should work.

Diesel have three major advantages, one of which you can offset.

First is that diesel fuel has (I believe) around 15% more energy per gallon, nothing you can do about that. A gallon of diesel will always take you farther down the road.

Second is that diesels have much higher compression, which improves efficiency a lot.

Third is that diesels have no throttle plate, which means no vacuum losses. You can mitigate this in a gas engine with tall gearing which keeps vacuum very low. If you can’t get tall enough gearing to do that, you should downsize the engine a little, to get load higher.

[racprops]

That seems counter to most of my understanding, 100% load should mean full throttle and thus full fuel use.

I am aware of the so called advantages of the diesel but feel it is somewhat off set by the not so much better gas mileage and much higher repair costs.

I believe the much higher compression rations really help them make torque and low RPMs.

I also believe the caming and low RPMs power curve is also a major reason for that.

I think I can get much of that advantage with my engine.

Rich

[Ecky]

High load is well documented to improve efficiency. That’s a major reason why taller gearing helps, too.

Consider you have two engines, a 3.0L and a 6.0L. The 6L can produce X horsepower at 2000rpm with 50% load, while the 3.0L can produce the same amount of horsepower at 2000rpm at 100% load. Which do you suppose will deliver better economy?

The answer is almost always the 3L engine. Half as many cylinders, fewer bearings, far less friction, but more importantly it also is wasting no energy at all creating vacuum.

You could gear the 6L to run at 1000rpm, of course. That way it would also be at 100% load, making the same horsepower, just by spinning half as fast. Or in other words, a 3L engine is often very equivalent to a 6L engine spinning half as fast. Smaller engines are like taller gearing.

What happens though, when you can’t slow the engine down anymore? When load is low, but you can’t cruise at 500rpm or whatever is necessary to get load up? For this reason, there is an optimal engine size and gear set for every vehicle.

Don’t take my word for it though. Read up on base specific fuel consumption. Here’s a sample chart showing engine efficiency vs rpm vs load.

[aardvarcus]

Engines typically have higher BSFC at moderate or higher loads for a given RPM, as outlined by the map Ecky posted. That doesn’t mean less fuel useage, but that they are producing a given amount of power using less fuel. It would be preferable to just require less power, but absent that if you make your gearing taller your engine will be turning less RPMs. As power is a function of RPM, if power requirements (load) are fixed, the lower the RPM the more torque you will require at that RPM to produce the same power. This is good as your engine loading (torque required @ RPM/available torque @ RPM) is increasing and your BSFC is improving. If you continue down that path, eventually you will go to far in that your engine will not produce enough torque at that RPM to keep up or you will cross over your BSFC sweet spot by over lugging the engine. I believe based on your posts you have already figured this out, as you want a torque down low engine combined with tall gearing.

[aardvarcus]

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.

[racprops]

OK Now we are talking, just wish I fully understand what is being said....

So now I am still kind of up a river without a paddle:

As I do not know how heavy my van IS, nor how much wind resistance it has at say 60, 70,80 MPH so I cannot really know what torque/hp I need to make those BSFC ratings...

I am afraid I need a little more help understanding that chart.

I am shooting for between 1000 RPMs and 1400 RPMs at 80MPG (I do a lot of my driving at that speed, many of the highways around here have 75MPH limits...)

So I can build both the transmission and rear end for those numbers.

BUT the engine is a unknown, so far EVERY 383 I look up is not built with any stock cam, all are Hot Rods...their power ranges are 2000/3000 to 6000.

Rich

[Ecky]

My advice is to get it built, then replace the rear end as needed. If your load is very low at your desired speed, you need a lower numerical rear. With an engine as large as 8.1L you may not be able to get a rear end that's tall enough. But, maybe you will. Worst case is that the engine is simply too big to get the load you want for good efficiency.

Conversely, if you find you can't reasonably use your top gear, you'll need a less tall rear end, a higher ratio.

[aardvarcus]

He doesn’t want to use an 8.1L, that is just my example engine I mentioned, he wants to custom build a 6.3L by stroking a 5.7L.

If you knew the basic torque curve of your old motor, your rpms at your old crusing speeds, and roughly how much throttle you were giving it (e.g. ¼ throttle) to maintain speed you could probably make an educated guess as to the HP. HP=Torque*RPM/5252=[(Max torque @ RPM)*(% Throttle)]*RPM/5252. For example, if you were going 80 at 2500 RPM using 25% throttle and look up that you motor has 300 ft-lbs torque @ 2500RPM, you could estimate HP=[(300)*(25%)]*2500/5252=35.7 HP.

To be clear, ecky’s chart is just an example. Every engine would have a different chart. Most people do not have BSFC charts for their engines, they are fairly rare. Ideally you would be in the lowest region of the chart, which for this example is in the “eye” at 2000-2500 RPM and 120-140 ft-lbs. Absent this information, you end up having to make educated guesses such as lower RPM and over 50% throttle is probably where you want to be.

To custom build an engine such as this, since you can’t find a build to copy, you would probably need to work with an experienced builder or purchase simulation software. Guys I know that raced always modeled engines on software that would spit out torque curves based off of all the components/specs.

Ecky’s idea of building it and then dialing in the gears would work as well.