[Amc-list] rpm, "cruise speed", manifold vacuum, spark advance

[Tom Jennings <tomj@xxxxxxx>]

Got a pleasant surprise this morning...

So my commute three days a week is very consistent, 45 miles each way of 
SoCal freeway, nearly always at full speed, rarely stop and go (odd 
schedule), consistent weather. It's a great test bed for basic 
(non-performance) tuning.

I drive my 195.6 OHV American at precisely 60 mph as much as humanly 
possible, and I'm quite careful with the mph. I've also been watching 
the vacuum guage since some AMClister (forget who!) was talking about 
driving-by-guage; I try to keep vacuum (arbitrarily) above 10 inches. I 
figure in there somewhere the main needle lifts and enriches.

I've been cranking spark advance a lot, and yesterday filed out the stop 
in the vacuum advance unit, haven't measure it in-car, but it's likely 
15 degrees total. Stock WOT advance at speed is 22 total, I'm running 
about 34 total and could use more. With vacuum on top of that, at light 
loads, it's up to 33 (stock) or 49 (mine). Zero pinging.

To maintain 60mph (like 58 - 62) cruise vacuum runs 10 - 13, more or 
less. But with all the additional advance the gas pedal has been 
"lighter", so I decided to cruise at 65mph instead of 60.

Wow, what a difference! Cruise vaccuum is now 12 - 15, the motor seems 
stronger, and it's only a 200 rpm difference.

Clearly I've been driving too low on the torque curve, but I'm surprised 
that it's so sharp. It's quite noticable, 60 to 65 mph.

The real test will be when I have 500 miles or so in the new regime, see 
if there's any mileage change.

RPM: I calculate 60mph=2050rpm (too low) but my tach says 2200,2300. 
65mph calculated is 2200rpm (still a bit low), tach says 2400,2500.

I've been going by the tach, but I think the spreadsheet is more 
accurate, but I guess I better reconcile this ASAP.


Since torque peak means highest cylinder-filling-firing-and-energy 
extraction then that's also got to be optimum cruise RPM. My software 
says torque peak is in the 2000's (that software runs at home, can't 
look now), same with the new six, even though TSMs like to say it's 
below 2000. Why they say that, I don't know, but it's wrong.
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