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Old 04-11-2019, 09:26 AM   #11 (permalink)
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My 2 cents:

Going larger is more likely to hurt economy than to improve it. Building for low end torque (generally speaking) means both that you're going to be able to put more load on the engine before downshifting, but also that you're going to be running at a lower percent load, all else equal. Lower load = worse economy.

I agree with others, you need to carefully balance engine power with gearing such that you're around 75% loaded at your desired cruising speed. Every engine has a peak BSFC, and there's an ideal power output for peak economy. For this reason there's also an ideal engine output for your needs. Maybe it's a 3L engine at 3,000rpm, and maybe it's a 6L at 1,500rpm. It's really hard to say without having driven your vehicle and knowing whether the engine is over- or under-loaded, and whether your target RPM is going to be below peak efficiency and start to hurt economy.

One thing you can say for sure is that, all else equal, a larger engine has more parasitic losses. So, going bigger is only a good idea if everything else lines up perfectly.

Adding a second overdrive to reduce RPM will also increase parasitic losses. I'd probably start by trying to cut those, rather than add to them. Get roller cams, minimize brake drag, synthetic fluids, reduce accessory load, do your aerodynamic optimizations and weight reductions. Maybe after that your existing engine isn't loaded enough, so decrease the final drive to reduce RPM.

Another alternative if you're feeling really adventurous is to de-stroke the motor, which will both move peak BSFC up the rev range *and* cut down on friction losses, since piston speed will be lower. Stroking it combined with a taller rear end may also have the desired effect.

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Old 04-11-2019, 09:37 AM   #12 (permalink)
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Here's a real world example:

I'm currently in the process of replacing my 1.0L Honda engine with a 2.4L. With the existing 1.0, peak efficiency is around 2000rpm. At 55mph in 5th gear it turns around 2000rpm at 75% load. At this speed I can get 100mpg on a flat road. It's almost perfectly optimized for those conditions.

However, if I wanted to drive at 75mph instead, it's way outside of peak efficiency - spinning too fast, and there isn't enough reserve power for me to gear it taller. Likewise, if I'm climbing a hill (and I live in a mountainous state) I have to downshift, which also hurts efficiency. Normally the hybrid system adds torque with virtually no penalty for the periods where I'm climbing hills, but without it the ideal engine size for my car in the environment it's used is probably a little bigger than 1.0L - maybe 1.2-1.5L. Finding the correct balance of all these factors is a moving target.

The 2.4L engine I'm installing is mostly for fun, but I'm still doing my best to make it efficient. I'm gearing it extremely tall to hopefully get load up, though this will be at the expense of often operating *below* its peak efficient RPM. I suspect it will do worse than the 1.0L engine on flat ground, but might do nearly as well or even a little better if combined with engine-off coasting when driving through mountains and/or at higher speeds.

Efficiency improvements to the 2.4L include significantly reducing rotating mass, helping it to warm up more quickly, deleting accessories where possible, deletion of balance shafts to cut down on parasitic drag, and playing with a lean burn tune to get load up during level ground cruising. I don't think I can out-engineer Honda when it comes to camshafts, stroke, cam angles and that sort of thing. I expect an efficiency loss overall, but I'm trying to minimize it while gaining 400% more horsepower for some laughs.

Last edited by Ecky; 04-11-2019 at 10:05 AM..
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Old 04-11-2019, 09:44 AM   #13 (permalink)
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I forgot why the mission statement is what it is but if it has to be an 80 mph van it would be cheaper and easier to start with a Transport with the 3.8; owners claim 30 mpg at reasonable speeds and I think that's believable.
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Old 04-11-2019, 02:19 PM   #14 (permalink)
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Quote:
Originally Posted by racprops View Post
Hope that covers it.
HP & TQ necessary to move a 6k lb van of X-sq ft frontal area at 60-mph in Direct at sea level. Level ground and no wind.

Thatís a basic calculation. Starting point. Dead minimum.

(No, I donít think you ďhave itĒ)

As before, plenty tried it in the 1970s. Iím guessing you werenít there.
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Old 04-11-2019, 03:46 PM   #15 (permalink)
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If you're going to do that much work to your engine, you might as well bump compression too to increase thermal efficiency. If you still want to run 87 octane fuel, you can probably set up your engine to do that by doing various things like using colder plugs, retarding timing, etc... Or if your area has e85 you can use that and not worry about all that. The benefits of raising compression start to diminish once you get to about 12.0:1 CR.

You will also need to do some drastic aero mods to your van if you want a "super mpg" vehicle at highway speeds.
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Old 04-11-2019, 05:10 PM   #16 (permalink)
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Chevy has already did all the work you are thinking is hidden for some reason in a 350 small block designed in the 1950s they tweaked it all the way into the 90s and made it better but completely leapfrogged it with the LS motors. Go find one of the millions of good 5.3s in the junkyard pay $1500 for it, get a standalone computer and harness to run it, and be done. You still will be lucky to get 20 mpg at 65, 16-17 at 80, but it will be a good 20% better than anything you will be spending 3 times as much building designed in the 50's.
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Old 04-12-2019, 05:53 AM   #17 (permalink)
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Quote:
Originally Posted by Hersbird View Post
Chevy has already did all the work you are thinking is hidden for some reason in a 350 small block designed in the 1950s they tweaked it all the way into the 90s and made it better but completely leapfrogged it with the LS motors. Go find one of the millions of good 5.3s in the junkyard pay $1500 for it, get a standalone computer and harness to run it, and be done. You still will be lucky to get 20 mpg at 65, 16-17 at 80, but it will be a good 20% better than anything you will be spending 3 times as much building designed in the 50's.
Looks like excellent advice.

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Old 04-19-2019, 01:38 AM   #18 (permalink)
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Quote:
This is a three step plan.

First is a 350 or 383 engine built for LOW RPM operation and MAX torque.

Second using a 80's TPI system.

Third adding a second overdrive transmission behind the stock 4 Speed automatic transmission.
You've gotten some good advice. Look for the Caprice wagon thread https://ecomodder.com/forum/showthre...mpg-33961.html. It has a good summary in the first post.

I assume you're attached to the vehicle. My van had a 1600cc flat four, 3/4 ton axles, with sway bars and suspension mods. It got 30 mpg, topped out at 85 and would top the Coast Range at 75, empty.

My idea of a super MPG van would be a Previa with an EV conversion.
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Old 08-27-2019, 12:51 AM   #19 (permalink)
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Hadn't it been for the sake of emissions certification being a PITA to deal with, I'd be tempted to work around a Cummins 4BT, even if it would involve a conversion to spark ignition just like Cummins did to the CNG/LNG or LPG versions of the 6BT mostly fitted to shuttle buses and some school buses.
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Old 08-28-2019, 02:25 PM   #20 (permalink)
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This is where I would start for a fuel efficient van build.

Quote:
Originally Posted by cRiPpLe_rOoStEr View Post
Hadn't it been for the sake of emissions certification being a PITA to deal with, I'd be tempted to work around a Cummins 4BT, even if it would involve a conversion to spark ignition just like Cummins did to the CNG/LNG or LPG versions of the 6BT mostly fitted to shuttle buses and some school buses.
I have a 2003 Dodge Sprinter. I accidentally won it in an auction for 595 dollars. Didn't run well. A few hundred in parts and my labor and I had a short, low roof van that got 28 mpg on the freeway at 65 mph.

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