<VV> Octane

[Padgett]

>Our Land Cruiser actually lists different horsepower based on the Octane
>(actually based on using premium or regular).  I assume it adjusts timing
>primarily by detecting knock(?) and that the adjustment lowers power.

This thread is taking on a life of it's own, guess there is a lot of 
interest. Basically it comes down to this, every engine has a "sweet spot" 
where you want maximum cyl combustion pressure to take place. Generally 
this is between 7 and 10 degrees ATDC though the latest I've seen is about 
15 degrees. EVERYTHING concerning spark timing is an attempt to achieve 
this and why modern computer controls use a 3-D map table with inputs for 
mass air flow, coolant temp, air temp, throttle position, gear selection, 
RPM, and several other inputs  for timing selection. I suspect that the 
Land Cruiser has some means to detect octane (possibly relating to knock 
counts) and switches between several tables accordingly. No user 
intervention atol.

Now what the General had available in the '60s was much cruder. Initial, 
vacuum advance, and centrifugal advance.. For the 65 140 this is:
16-20 deg initial, vacuum starts at 6"  and 22 deg @ 14", centrif 0 @800, 4 
@1200, 18@ 4900

Warren mentioned that if you put a 95 hp distributer in a 140 it does not ping
4-8 deg initial,    vacuum starts at 6"  and 24 deg @ 14", centrif 0 @800, 
4 @1200, 28 @ 4200

so it starts out considerably retarded from the 140 but the final advance 
is about the same at hi rpm (18+22+18= 58 deg tot vs 6+24+28 = 58 deg tot 
while the advance curve is significantly lower (is assuming lower 
octane/faster burn) in the intermediate values.

Just as a rough correlation figure flame propagation at about 170 ft/sec 
and with the spark plug at the edge of the cyl, path length at about 3.3". 
At 4200 rpm this figures to 4200*360/60 (deg/sec) * 3.3/12/170 (seconds) = 
c.a. 40 degrees (you see why this takes a dyno and a test cyl to determine 
exactly and it is going to vary by rpm and charge).

The best way to tune without a computer is with a vacuum gauge and since 
the advance curve is pretty well fixed for any given situation, use the way 
you trim an aircraft engine for "economy cruise" - select a speed (load) 
and mixture (not much choice) and adjust the timing for maximum vacuum 
(least charge) while keeping cyl head temp within limits. Unfortunalely for 
a street car this can be a bit difficult. Access to a dyno helps.

The fun part is that when the timing is properly adjusted to hit the sweet 
spot, you also get the lowest cyl head temp for the load and are least 
likely to get detonation. Win-win.

BTW you can find the process in almost any piston aircraft -1 manual of and 
after the 1920's, is how Lucky Lindy made it to Paris. Nothing new.

Looks like once I find the right car, I am going to have to get my 
aunchient Sun Distributor Tester cleaned up.

Padgett