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RE: 23D4 distributor

To: PilotRob@webtv.net
Subject: RE: 23D4 distributor
Date: Thu, 9 Jan 2003 13:06:49 -0600
Cc: spridgets@autox.team.net
Bob:
Thanks for such a good explanation of the factors involved in determining
what an advance curve might need to look at.  Once again, I've gotten more
than my money's worth from autox.team.net!
-John

> -----Original Message-----
> From: PilotRob@webtv.net [SMTP:PilotRob@webtv.net]
> Sent: Wednesday, January 08, 2003 9:23 PM
> To:   John.Deikis@med.va.gov
> Cc:   John.Deikis@med.va.gov; nosimport@mailbag.com;
> spridgets@autox.team.net
> Subject:      RE: 23D4 distributor
> 
> Hi, John...
> 
> I would go into equities now "but this
> is not important". :)
> 
> The Cooper "S" advance curve was designed for the "enthusiast". So it IS
> a WOT (wide open throttle) curve based
> upon a compression ratio of around
> 9.75 to 1, the Cooper "S" cam, intake
> valves of 1.4" and use of at least 100RON
> gasoline. This WOT curve was also found suitable for the stock 1275
> Series A which
> powers the Sprite with lower compression
> (8.8-1), smaller intake valves, and a slightly milder cam.
> 
> The operative words here are "WOT" and
> 100RON fuel.
> 
> The less throttle, the more advance you can run at any given rpm (as in
> "vacuum
> advance" on combo. distributors).
> 
> The higher octane you run, the more advance you can run, all other
> things
> being equal.
> 
> The stock Sprite 1275 Series "A" hi-compression ("H" designation) runs
> 8.8-1. For "enthusiastic" type of driving
> where consistently high output is the name of the game, 97-99RON fuel is
> recommended with this WOT mechanical
> advance curve in mind.
> 
> The best you can do with
> normal unleaded pump octanes is 93PON
> which conservatively equals 96RON, just
> outside the ballpark for the enthusiastic
> foot (consistently over time) though fine for part throttle operation.
> 
> If you have Sunoco 94PON available,
> that's even better for the enthusiastic
> foot. Now you are IN the ballpark for
> this advance curve, foot to the floor
> forever (in theory, anyway!).
> 
> In terms of WOT operation, all you will
> do by bringing the total advance in sooner
> (all other things being equal) is reduce
> your margin to pre-ignition detonation 
> when you put your foot in it at say, 3000rpm. If you drive gently,
> you'll
> get better fuel economy (this is what
> vacuum advance does for you on the
> street). Sure you will get more torque
> when you put your foot in it....for a short
> period of time...until pinholes
> appear in the piston crown, anyway!
> 
> With the mechanical distributor designed
> with a WOT advance curve, you cannot
> have your cake and eat it too.
> 
> Thats why the combo. mech/vac. advance
> distributors are recommended for the street. They give you a WOT
> acceleration
> curve and for light throttle cruise, use
> vacuum either in the carb or intake manifold to give you more advance
> under
> reduced load.
> 
> The only place to come up with a proper
> advance curve for your motor is a dyno.
> 
> However, if you have an air/fuel ratio
> set up installed and start with one "known"
> parameter, you can accurately set the
> other to some extent at least, using the former as a basis for doing so.
> 
> Fr'instance, if you know your carb.
> setting/calibration is correct for the
> engine's state of tune, you simply
> advance the timing a bit from stock
> and run WOT in third gear up a medium
> size slope (hill) of a mile or so...plenty
> of time to get the engine really hot.
> 
> Note the air/fuel ratio meter reading.
> 
> It should show on the rich side.
> 
> Go back and try it again with the timing
> advanced 2 degrees from stock. Where
> is the air/fuel ratio indication now?
> 
> You can see where we are going with
> this, obviously!! At some point you will
> be at the "bottom" of the "rich" zone.
> Retard the timing a bit from there to
> be on the conservative side in terms
> of combustion temperatures and that's
> about it.
> 
> If you had a dyno, you would do this for
> every 1,000rpm (or even 500rpm) and come up with an exact "power" curve
> for your motor.
> 
> There are variations on this also. A colder
> set of plugs will allow a little more advance than stock. Going from the
> stock
> "AN" needles in the twin 1.25 set-up to
> slightly richer "H6" or "M" types will allow a little more advance as
> well.
> 
> In a lot of cases, running more advance
> than the engine can safely use at whatever throttle setting you care to
> choose will result in a very quick engine
> for a relatively short period of time.
> 
> While you can hear "pinking", you
> probably won't hear "detonation" due
> to general engine and exhuast noise
> at high rpms, though you can see evidence of this on the spark plugs
> with a high powered magnifying glass
> which will show the metal particles
> coming off the piston crowns.
> 
> You never get something for nothing
> and so it is with the mechanical advance
> distributor in question. 
> 
> 
> 
> 
> Cap'n. Bob (Ret'd)
>       '60 Frog

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