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Big Dave 12-12-2012 02:15 PM

Forum Opinion: CNG Conversion
My read of the energy tea leaves is that natural gas will continue to be a bargain for at least the next 5-7 years.

One can again get a PHILL home gas compressor for about $4,700. For me, installation won't be too bad. I have a 240V circuit and a gas line handy in my garage. With projected residential gas and electric rate over the next 5-7 years, I could pump CNG for about $1.00/gge (gallon gasoline equivalent) which is less than a third of the price of RUG here.

I drive a good deal more than the average Joe - about 30,000 miles a year. I split between my F350 and my 96 Impala SS. the truck is mostly my cold weather vehicle. The Impala is a lousy snow vehicle and not Ziebarted so its afraid of the Midwest snow and gets parked in the garage mid-December to mid-March.

I like the Impala but simply cannot hypermile the darned thing. About 20 MPG is about all I can do with it. A full-on aero project might gain me a couple MPG and a stick might get me some more. But it already has reasonable 3.08 gears and I really don't see more than 25 MPG without converting it to a diesel.

I'm gonna do something with it anyway. The mid-90s electronics are buggy although the LT1 engine is OK. The 4E60E tranny is too frail. A very doable swap is a L92 (iron LS3) engine and a 6L80E transmission. Common truck/SUV drivetrain. The iron LS engine weighs about the same as my LT1 so it won't throw the weight distribution out of limits. The L92 has displacement on demand and variable cam timing. Six speeds gotta help MPG some. So this should get me at least 2 MPG improvement by itself. By doing this swap and a few other things, I've modernized the car for a fraction of what a new car costs.

BTW, a small car is simply out of the question. Aerohead (Phil) can confirm: I earn my moniker and prefer a big car or truck for very real reasons.

But why stop at a simple engine swap. I can get the LS engine converted to CNG for about $9,000 installed. At the rate I drive, it pays for itself in three years, including the PHILL.

CNG is much cleaner burning so maintenance is greatly reduced.

At this stage I'm thinking a bi-fuel conversion. If CNG gets more commonly available (for trips) I'll go fully CNG.

By getting a 10% increase in MPG and cutting fuel price by two-thirds I'll be driving for a little over a quarter of the per-mile fuel costs I have today.

Equipment is old tech and commercially available and not overly outrageously expensive. Project looks doable.

Forum opinions?

rmay635703 12-12-2012 02:39 PM

Your best bet is if you have a local fill station that you can drive to, it fills quicker at a higher price but you never need to rebuild or buy or jump through hoops to get a pump in your house.

This is of coarse unless you can locate an adsorbed CNG tank that also tollerates high pressure fills. Only foreign companies make them but then you can buy an antique cheap 500psi rated industrial unit for home use and when you need to drive further go to the high pressure fill at a gas station.

Most smaller cars and trucks have a fairly lengthy repayment period (aka 140,000 miles +)


shovel 12-12-2012 02:45 PM

I know tech has advanced a lot since 1992, but if my old Tempo was any indication your transmission will last a LOT longer behind a CNG powered engine because there's a lot less power available. I got very nearly the same MPG/MPGGE on CNG vs. gasoline in the Tempo, but horsepower was down by probably a third or half. It was still driveable barely in Phoenix, but wouldn't make it up any hills in a town with elevation changes - that was a 4cyl car in good mechanical shape.

Daox 12-12-2012 03:19 PM

There are plenty of vehicles that even a very large person can utilize that get much better than either of your vehicles and can be had for under $9K. I'd sell the Impala and get a more fuel efficient sedan. An Accord with manual trans comes to mind. Hypermiling it should get you in the 40s I'd imagine. They're also pretty good in the snow.

christofoo 12-12-2012 04:03 PM

Random thoughts:
  • Considering your mileage, I'd expect you'd find repair costs significant also, I'm really curious how CNG compares vs Gas in per mile maintenance, and I think that should weigh on your decision. (I find for my ooold Corolla at 45 MPG that gas and repairs cost almost the same, each in the $0.1 per mile range, although I expect my repair costs to come down since I'm doing more things myself now, and I think that Honda's and Toyota's are mostly in the $0.05 per mile range for most of their life (prior to 200k)).
  • (I'm just guessing, if you did a CNG conversion on the Impala that the Impala would end up with lower throttle loss and higher efficiency, corresponding to the reduced HP, so your overall savings with CNG may be noticeably higher than you'd predict just comparing cost per unit energy. If this is true, it would also open you up to more significant benefits available from aero mods.)
  • Can't seem to find front-seat specs, but if you want to look at replacing the Impala in search of better gasoline MPG, I believe that Ford Escort's are noticeably more roomy than Honda Accord's or Toyota Camry's, but are comparable in other ways. That being said, my personal belief is that Ford's cost more to maintain per mile, particularly in comparison with the Honda.
  • Also if you go in search of a different car, if you can put up with 2 doors, my impression is that coupe's usually have more aerodynamic rear windshield areas, so (short of a boattail), you can probably get more from hypermiling and modest aero mods in a coupe vs sedan or hatchback, especially if it's a manual.
  • EDIT: Even if you find that you can tolerate a mid-size car, considering your mileage it may still be favorable to do a CNG conversion. (I'll leave you to do the math though. :))

smokey442 12-12-2012 05:19 PM

I think its a winner. If your going to do the LS swap and run it on dedicated cng. kick the cr. ratio up at least one point. The motor octane rating of cng is something like 114

UFO 12-13-2012 11:45 AM

If you have read any of my postings, I do not favor CNG as a fuel for environmental and corporate welfare reasons. I don't think a cheaper cost per mile driving justifies the damage unregulated fracking is doing to our fresh water and air quality

Milwaukee 12-13-2012 05:03 PM


cRiPpLe_rOoStEr 12-26-2012 02:04 AM

CNG is widely used here in Brazil. Altough I'd be more favorable to a Diesel swap, if you would rather get a CNG conversion for the Impala I wouldn't try to discourage you.

Big Dave 12-26-2012 09:18 AM

"...the damage unregulated fracking is doing to our fresh water and air quality"

Big Dave says:
I thought we agreed to keep the discussion on the cars.

"...I'd be more favorable to a Diesel swap..."

Big Dave says:
Normally, I would be as well, but diesel is disproprotionately taxed, and EPA meddling has made suitable diesels a scarce commodity in the US.

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