Eric.
The 270 GMC stock advance is 20 degrees centrifugal with another 24 to
28 degrees from the vacuum advance, so you are correct with 48 degrees total
advance, but there are many other factors that affect the spark advance
curve - camshaft profile, piston design, fuel/air ratio, cylinder head
temperature, compression ratio, vacuum signal, etc. If any of these are
different from stock you have to use trial and error to get an approximate
baseline. Spark timing is all about efficient combustion and increasing
torque.
Most performance-modified engines want more initial advance (stock 270
is 5 degrees). If using domed pistons and an aggressive camshaft, another 7
to 10 degrees will usually wake up the motor and deliver crisper response
and more torque. But you risk detonation if you go too far. Short of using a
dynamometer or dragstrip testing, the best rule of thumb is to increase
initial advance until you get pinging under load, with the grade of fuel you
plan to use. Then retard the timing until the pinging goes away. This will
give you a stab at total advance but the centrifugal weights in the
distributor, and the vacuum can used, may still have to be changed to
achieve best driveability and fuel economy. Mostly, we just live with the
compromises.
When you realize how many variables go into optimizing a spark advance
curve, you can really appreciate modern electronic engine controls.
Jack / Winter Park FL
----- Original Message -----
From: "Eric Johnson" <j.eric1@worldnet.att.net>
To: "OleTruck list" <oletrucks@autox.team.net>
Sent: Thursday, December 23, 1999 12:37 PM
Subject: [oletrucks] Ignition advance
> Hi to all, Thanks for the answers to my hei ?s. I've got a couple more .
> Does anybody know what the total advance should be for my 56 270GMC. It's
no
> where near stock. Is your advance charactoristics related to what kind of
> cam you have. And is it normal practice to stamp the cam grind on the end
of
> the shaft incase I need to know that to set up my advance. I'm not sure
but
> I think my total advance should be 48 degrees, with 24 degrees vacume
> advance. Do advance curves differ from stock to high perf. If they are
> different where could I get that info. I have set my new dist. up with my
> old vacume advance. The one that came with it made my face collapse to get
> it to move. My Gramps built this motor himself back in the early 60s in
> Burbank Ca. He was a master machinist for lockeed. I got the truck 7yrs
> after he passed so thats why I dont know any of this stuff. The one thing
I
> do know is that if everything isn't just right it leads to total failure.
> Enjoy the holidays.
> Eric 48 carryall
>
> oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
>
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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