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

[farna@xxxxxxx]

My 4.6L feels the same way -- cruising at 70 "feels" better than cruising at a lower speed, and I'm sure gas mileage is better. Vac gauge says the sameas yours (reads a bit higher), except when pulling long or steep hills, which need a bit more throttle to maintain the higher speed. I'm going to put slightly shorter tires on next time to effectively increase gear ratio and slow it down slightly -- a 215/65 instead of the current 205/70. The speed reduction in high gear is only 3 mph, so it won't make a lot of difference (or was that with a 215/60? will check before buying!). If you want to maintain a 60 mph cruise speed you might consider the same, but if mileage bears out the vac gauge you may want to just drive 65. Certainly not slowing anyone down or running over anyone! In my case 65-70 is just a bit fast for most of the rural roads around here, especially when you round a curve only to find a farmer pulling out with a large slow moving tractor and a semi in the on-coming
  lane! 

I bet your tach is correct. Calculations don't take into account wind resistance, tire size variances, and/or gear train loses. I've always found that tachs usually read 50-200 rpm more than calculations, depending on engine size, speed, and trans type.

I don't think the advertised torque is necessarily at the peak. Don't know why or how they pick the rpm that the rating is printed at, but it's obviously not necessarily the peak, at least not back in the 60s. Could be the middle of the torque curve? 

---------------
Date: Tue, 29 Jul 2008 13:59:18 -0700
From: Tom Jennings <tomj@xxxxxxx>

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.

--
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
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