Re: [Amc-list] Spirit Mileage re: was Where's my sign?

[Archimedes <Freedom@xxxxxxxx>]

I said:

>> AMC content: Yesterday, I finally found the
>> engine/trans setup I've been searching for for a
>> loooong time: a Jeep 4 cyl 5 speed 2wd setup,
>> complete, for $250.  It's going to go into a Spirit,
>> purely to see if I can turn one of those cars into a
>> 30mpg city car.  I figure if this motor can drag a
>> 4019-lb. Dakota's butt around and still get 21 to 23
>> mpg city, in a Spirit, which weighs +- 1200 lbs less
>> (2970 w/six) and has better aerodynamics, it might
>> be a great combo for MPG .
>> 
>> Any tips on what externals I'll need to do it?


JOE FULTON <piper_pa20@xxxxxxxxxxx> said:

>Having owned an Iron Duke Spirit in which I rebuilt
>the engine so I know it was running right, your
>project goal just "might" be doable.  
>
>I got a max fuel economy in the very low twenties with
>my set-up (Iron Duke (carbed) with 904 auto).  19-20
>mpg was more routine.


Compared to a early seventies design slushbox, the 5 speed ought to account for 5 highway mpg all by itself.


>I do recommend that you stay with (or go to) some
>narrower radial tires to meet your mileage goals even
>though they might look kinda "old lady".


Now this got me curious, so I read a few articles about rolling resistance:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=29

http://www.eng-tips.com/viewthread.cfm?qid=141306&page=1

http://www.greenseal.org/resources/reports/CGR_tire_rollingresistance.pdf

http://gaia.csus.edu/~grandajj/me143/ME143_Tires_1.pdf

The gist seems to be that tire rolling resistance is a minor factor in mileage.  Much bigger factors include rotational friction (ie engine and drivetrain friction (about 45%)

Points to consider:

1.  Low-rolling resistance tires don't save any money if they cost more than cheapies with higher rolling resistance.  They only pay for themselves if they're as cheap as the cheapest tires in the size you want.

2.  Proper inflation (or even extra inflation) affects mileage more than rolling resistance in most scenarios.

3.  Wide tires may be more efficient on long cruises at speed since they generally run much cooler than narrow tires -- if I'm reading the table on page  of the PDF link, tires become more roll-resistant as they heat up.

As far as this particular swap, I think the the 1200 lb drop in weight will probably be at the heart of any mileage increase over the engine's consumption rate in the 'kota.

Anyway, if I do this experiment, I will do an album to show how it turns out.

You sparked my curiosity, Joe.   Thanks.  It made me learn something new.

All that said, by "externals", I meant like computers, sensors, etc that I will need to connect to the engine.

-- Marc




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