General engine theory, about 35 deg is the max for a "normal" car.
Beyond a certain point, the cylinder pressure starts building on the
compression stroke and pushes the piston backward, reducing power.
Maximum advance is not always a power adder. MSD makes a variable
timing control, gives you a dial on the dash to play and move the base
timing as you drive. They also make electronic advance boxes, you lock
out the timing and the box does the entire advance curve.
The race guys should know the upper limit. Jover's dyno results may be
an indication.
Here's what I have found on my U20. Don't know if you have covered
these items yet.
Alot can be done with carb oil, for example, mine pings badly with
Marvel Mystery, is fine with ATF. The SU's like less initial advance
than Solex, and a vac advance is said to help acceleration, and is not
really too active at WOT. Dean had a customer with high compression and
other mods, ran very little initial timing and then 20+ deg mechanical
advance. Odd, but worked for him.
Personal opinion, hopefully unbiased, first get a strong, accurate
spark, then play. I recommend an electronic ignition(anybody's, not
just mine) and/or a CD, like an MSD. Good wires and proper heat range
plugs are also a must.
If your carburetor shafts are at all worn, or mixture inaccurate, also
fix that first, lean and advanced is a bad combination.
Make small steps, keep us informed
Dave Brisco
-----Original Message-----
From: Nathaniel Leeds [mailto:nmleeds@mindspring.com]
Still tuning,
And now I'm starting to have evil urges. There's a little devil sitting
on
my shoulder which wants me to buy a crate of octane boost and crank out
my
advance (20+ degrees on my SU carb, Solex cam, U20). What is people's
experiance in this regard. How much maximum advance (mechanical +
vacume)
can the U20 take on 91 octane, and then how much more can it take with a
little street-legal octane booster. I know there are standard formulas
depending on the engine, I just don't know what they are.
Also, does someone know where I can find the advance charts for the U20
distributors (plural because I know there are a couple of available
curves,
and I don't yet know which mine is). I'd like to get a feeling for what
my
maximum theoretical advance is without pissing off the neighbors by
running
my engine at 6000 rpms.
Nathaniel Leeds
'69 2000 U20
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