<VV> Economics Re: Regular Gas and Fuel Economy in a 110hpCorvair

[Shaun McGarvey]

OK, I defer to your greater knowledge. It still FEELS like it's getting 
better HIGHWAY mileage to me with the timing retarded a few degrees. It 
floats along at far less throttle opening. Of course accelleration suffers 
greatly. Good for the smog check too ...

yea, Vairily ... Shaun

----- Original Message ----- 
From: "Jim Burkhard" <burkhard at rochester.rr.com>
To: "Shaun McGarvey" <shaun_mcgarvey at shaw.ca>
Cc: <virtualvairs at corvair.org>
Sent: Sunday, January 06, 2008 9:22 AM
Subject: Re: <VV> Economics Re: Regular Gas and Fuel Economy in a 
110hpCorvair


> You are definitely wrong on count #1 on count #2, it is "depends"
>
> Count#1:  Best highway fuel economy results from peak thermal
> eifficency. This occurs when you time the spark event right at MBT
> (minimum advance for best torque).  If you over-advance, the thermal
> efficiency will gradually drop off, but over-advancing typically isn't
> a concern since the engine will knock like hell first and let you know
> not to run there.  On the other side of the coin, If you *retard* the
> spark timing from MBT, efficiency drops off more quickly. You slow the
> combustion process and more combustion occurs later and later in the
> expansion (power) stroke. The slowed combustion means more heat gets
> transferred to the head and cylinder walls and out the exhaust and
> less is doing useful work on the piston. You lose performance and fuel
> economy AT THE SAME TIME (!) and get everything a lot hotter. Bad idea.
>
> Count#2:  In a world of pure regular gasoline against pure premium
> gasoline, yes the premium gasoline often does have a very slightly
> lower volumetric enegry content on account of the specific
> hydrocarbons in teh blend.  The effect is pretty slight, and more
> importantly basically gets lost in the wash when both premium and
> regular fuels get adulterated with oxygenates such as ethanol, or
> until a few years ago MTBE.  When said oxygenates are added, the
> relative energy differences are very puny. Still, it makes sense to
> set the spark timing to where it should be and run the minimum octane
> needed to keep the car from knocking. There is no benefit to running a
>  typical 95hp Vair engine on premium (it doesn't need it). In
> contrast, it really dumb to take a 110hp Vair engine that typically
> *requires* premium and retard the spark to keep it running (hot, weak,
> and inefficiently) on regular.  Better to just buy a 95 engine...
>
> QED:
> a. It is best to run spark timing right at MBT (approximately by the
> factory setting if you don't know better). If you can't get there, get
> as close as you can without knocking.
>
> b. Retarding the timing so that you can run on regular to save $ is a
> pretty dumb thing to do. If the engine was designed for premium, run
> the recommended timing with the recommended fuel. By retarding the
> spark advance, you lose power, hurt the thermal efficiency, and run
> the heads hotter which makes burned valves, dropped valveseats,
> pre-ignition and knock all the more likely.  Doing so in an emergency
> when you need to get home and can't find premium is one thing, but
> purposefully doing this to "save money" is a fool's errand.
>
> Jim Burkhard
>
> Shaun McGarvey wrote:
>> I believe retarding timing gives better hwy mileage.
>> I also believe that regular fuel gives better mileage.
>> Am I wrong?
>
>
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