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Old 03-17-2013, 07:34 AM   #1
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50 mpg-american style

Hello fellow hypermilers! My name is Andrew, 37 year old family man and automotive technician. Im a lifelong GM owner and diehard fan, I started playing with cars at age 16 buy geting my first vehicle running and road worthy. A $500 65 Chevy C10 shortbox, (I still have it) and I guess I just never stopped. After outgrowing my youth, (teenager with a heavy foot) I always had a knack for geting above average mpg. But I srarted really geting into it when i bought a $900-one owner-95 Saturn SL2 with a bad lung. (See "The American Dream" in the garage) The original owner had developed a valve issue on #3 cylilinder. He took it to a "professional mechanic". $1500 later they called him up and said they had fixed #3 but needed another $1500 to go back in and re-ring #1,2 and 4 because it uses oil now! He declined the work and drove it for a while adding (lots of) oil when needed. I saw the ad for it in the paper and bought it. After some diagnostic work I learned two things. The cylinder with the oil consumption issue was #3, and if you kept the rpm's under 2000 rpm it wouldnt smoke or use as much oil. This is where it gets interesting. Since I had to putt it around anyway, I started hypermileing by instinct. I used it around town and kept trying to improve my mpgs. My best to date in town is 38.6. So obviously the next challenge was longer trips. My parents live 320 miles away and on my fist trip at 50 miles an hour I found out pretty qiuckly how good of mileage you could get at 50 mph. (Saturn SL2 + 2000 rpm = 50 mph) How good of mileage you ask? 53.8 mpg is my best highway so far. Not bad for the sporty version of the S series. The SL2 has lower gears and a dual overhead cam. Since then i bought a Saturn SW1,(economy wagon) I gan get 48 mpg pretty consistantly at 60-65 mph. My next challenge is building a high mpg car. I already have the car (95 Saturn SL1) They weigh about 100 lb more than a Geo Metro and are more aerodynamic. My plan is to install a small diesel engine in it. 500-1000cc? The installation/fabrication is not a problem. I just cant find much information to know how big/how many HP I need. I dont care if if is gutless, what im looking is as high of mpg as possible in town. Highway driving would be nice but not neccicary. I can gear it way down and go with a smaller engine too. Any thoughts? If I can help anyone with autimotive questions, especially GM, Id be happy to! Thanks fellow hypermilers!
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Old 03-17-2013, 09:08 AM   #2
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Good story.

Quote:
Originally Posted by saturnfan View Post
My plan is to install a small diesel engine in it. 500-1000cc? The installation/fabrication is not a problem. I just cant find much information to know how big/how many HP I need. I dont care if if is gutless, what im looking is as high of mpg as possible in town. Highway driving would be nice but not neccicary. I can gear it way down and go with a smaller engine too.
I imagine you'll want to look at some European microcars, if there are any modern ones. I haven't seen any discussion of engines that size and the only ones I'm familiar with, for yard equipment, are probably not going to work. I vaguely remember one report on this site of using a smallish diesel from a farm tractor or mini-backhoe or something but I doubt I can find the thread, if it even exists and my memory isn't playing tricks on me.

It would definitely be a very interesting project.
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Old 03-17-2013, 03:14 PM   #3
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Thanks for the welcome!

Some of the small diesel engines I know about are- Yanmar, Kubota, Onan, Deutz, Perkins and Lombardini. It mould probably have to be a horizontal shaft style engine used in rototillers, generators and go-carts. On a horizontal shaft engine the crankshaft would exit the rear like an automotive engine to attach it to a flywheel/clutch/transmission. The most common horizontal shaft diesel engine would be used in a large generator. I think that would be the way to go. The biggest problem would be the expense of a big diesel generator! If I could find one with a good engine and a bad generator, that would be perfect! I htink I could get by with a smaller engine and drop the gear ratio way down.
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Old 03-17-2013, 04:48 PM   #4
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Depending on safety inspections in your area, this may be very difficult to do. I know in good ole North Carolina, they are very funny about what you do to your car. There again, South Carolina requires no annual inspections at all.

I know the EPA is very funny about diesels period. that is the reason you don't see many on the streets from the factory other than big trucks (they get an exemption).

as far as the horsepower thing goes, I have heard it takes 20-25hp to keep a car going 60mph on flat road. that is more of a rule of thumb answer but that is what I have heard. I would think that this figure would be weight dependent along with aerodynamic modifications to that figure as well. I would assume that the torquey diesel would need less horsepower given the torque figures.

Good luck to you on this. I have wondered why a car manufacturer hasn't already done something like this.
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Old 03-17-2013, 05:16 PM   #5
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Good point! The emissions would definately be worse than a stock 95 model year car. Worth checking into. I wouldnt mind going with a small gas engine. Again Im thinking of using it for to-and-from work. A max speed of 40-45 mph would be fine. By using the low geared sport 5-speed tranny and low profile 14" tires would bring the gear ratio way down. It would also lower the car. I should also be able to go with a full belly-pan. Both would help aerodynamics.
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Old 03-18-2013, 05:18 AM   #6
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Though they may be hard to find, My first geo metro came with 12" rims and my second came with 13" rims. I would probably steer clear of the 12s since it may be hard to find tires for them.

My first geo metro had the 1 liter 3 cylinder motor. I think it was rated 55hp and it got me 46 mpg the last time I checked it before I got rid of it. that was back when gas was around a buck a gallon so I drove it like a race car (yes, they laughed at me). Given what you have said about your goals and the gearing you are considering, I would think this motor would even be too big for you.

Belly pans are fun, aero mods can get creative. I heard recently that they sell coroplast (corrugated plastic) at home depot now by the sheet.

One other consideration would be to just buy a ready made vehicle that would meet your needs rather than build it yourself. I have been looking at this one for a while and am very curious what the end product will look like. ( Ultra High Mileage Car : Elio Motors ) though I will say there is a certain level of satisfaction building something yourself.
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Old 03-18-2013, 12:12 PM   #7
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Good points, Beef. I was thinking 14" wheels because that is the smallest wheel Saturn used, besides donut wheels. (also an option in my mind- skinnier tires=less road contact=better mpg) I agree 55 hp would be more than i need, but would make it more highway worthy. Im looking into middle sized horizontal shaft air cooled small engines 20-25 hp now (like in a large snowblower or a generator). As far as buying an Elio or a ready made, I am definately a "build it yourself ' kinda guy. Thank you for you input.
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Old 03-18-2013, 03:50 PM   #8
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Quote:
Originally Posted by saturnfan View Post
besides donut wheels. (also an option in my mind- skinnier tires=less road contact=better mpg)
Not necessarily true. Wider may be better. Much of the loss from rolling resistance is sidewall deformation. Contact patch is approximately a function of pressure and load; with 100 pounds per square inch and 1000 pounds of load you'd be looking at 10 square inches of contact patch. A narrower tire at the same pressure requires more of its sidewall to deform to make that contact patch. Every good explanation I've read supports this, however I have seen some reasonable arguments against it. There are certainly other factors involved.

I do recall that somebody tried four donuts and, besides the severe consequences to safety, ride, and handling, the fuel economy result was awful. I did a search and found mixed results. One person got a 9% improvement in fuel economy, another reported an unspecified improvement, another used a rolldown test and measured at most a 1% improvement in rolling resistance, and another found the results worse.
Thread: http://www.gassavers.org/f6/donut-ti...tml#post127201
All reports of handling were awful. Besides safety and fun, good handling is important for fuel economy so you don't have to slow down and reaccelerate as much for turns.

You could potentially try donuts for a much larger vehicle. I have a donut from a Chevy S10 Blazer that's pretty large. It would still be narrower than anything else you'll find, and the extra weight capacity should translate into a slight reduction in risk of failure in extended use. Of course it would probably be difficult to find such a large donut with the right bolt pattern.

Also, donuts would get expensive. I'm sure they don't wear for long and fail easily. I know that one I have from that Blazer hasn't weathered well, it's been outdoors for a few years and shows pretty bad dry rot.
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Old 03-18-2013, 06:48 PM   #9
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Good points too. Ive never actually ran a donut except for an emergency, and I have no idea how long one would last. I just happen to have 4 S-series Saturns so I have a set that I could run for a couple of tanks and see if made a difference. I was thinking more like putting like a 175/185 width tire on a donut wheel. I noticed on your Bessy (nice truck by the way) you run like 80 psi in the tires. My question is if you put a tire on a "little bit to skinny" wheel wouldnt that be similar to going to max air pressure? I think it would kinda give it the "donut effect" and lower the contact area. You know how when you run max psi it will wear tha center of the tire. Anyway thats a ways down the road. Ive thought about it before but never tried it because a donut is smaller in diameter and would drop the final gear ratio down. I think lowering the gear ratio would be counter-productive.
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Old 03-19-2013, 03:21 AM   #10
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Assuming that my understanding of pressure vs. load vs. contact patch is correct, mounting a tire on a too-narrow wheel won't help with rolling resistance/fuel economy.

I've been running 80psi in this truck for 150,000 miles (and increased/maximum PSI in other vehicles for hundreds of thousands more) and the center wear has been barely noticeable. The center wear issue is severely overstated. It just doesn't happen as bad as people think it does -- and when it does, it's harmless. The tread in the center isn't anywhere near as important as the rest of the tread; you need the rest to channel away water, but a bald center acts like a motorcycle tire's canoe-shaped contact patch that expels water effectively without tread...at least that's been my analysis and experience.

I agree about gear ratios, although in your major project where you're going to be doing major gearing changes anyway it could easily be figured in, and if you found taller donuts then it wouldn't be an issue.
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