skids

Read enough posts on detonation, but after all is said and done, I still get detonation. Most noticeable in the 2800 rpm area, gear give or take, 4 th gear, other gears possibly also. Not always easy to hear with the side exhaust. It's a 68,327-350,stock internals, as far as I know. Msd with 24 in dist, vac can give 8 deg's, and initial is 16 deg's. Motor is snappy, and i'm sure there are other factors that could contribute, so am open to any suggestions. Maybe pull out 2 deg's in the can?

Tampa Jerry

I have a 66 and a 68 with the l-79 engines. The 68 like yours is mostly stock. Sixteen degrees as a base timing seens too high. I would back it down to 13 or so and road test the car. Are you using 93 octane for fuel? Are you still using the stock steel gasket for the heads? Jerry

IGO200

Too much advance. If the curve is adjustable pull some out down there.

Pop Chevy

I'll bet it's snappy ! You better back the timing down or you WILL destroy it. What is your total advance at say 3500 rpm?

AZDoug

6-10 initial, depending on the rest of the motor, is appropriate.

Doug

midyearvette

i would take six degrees out of the initial and give it a try..10 deg.
you may need to increase idle speed........

Westlotorn

I agree with those before me on this one, too much timing.
16 + 24 = 40 every time I add it. With Stock heads your max would be 35 to 37. If you are hearing detonation over side pipes I really hope you still have good rings and pistons.
The Vacuum advance can be fine tuned for good acceleration after you fix the total timing. Keep us posted on how this plays out.

skids

Thanks to all for your input. Changed oil yesterday for winter storage, so don't know if i'll get the car out anymore to report back any results.

Anyway, here's a chart that I get to play with with my dist. I'll have to see what I have in the dist now. My hunch is I have "E" with blue bushing. Anyone have a favorite on the chart to start with?

DansYellow66

No kidding. Took the words right out of my mouth.

The vacuum can is not the problem.

Mike Ward

This. The OP needs to map the mechanical curve.

65tripleblack

The L79 has the laziest mechanical advance program of all the mid sixties smallblocks! There is a good reason for this: the L79 has a lot of static compression based on intake valve closing event, as compared to all others. This means that it has the most dynamic compression (1962-63 L76/L84 excluded). Because of this, a more "leisurely" spark advance program is needed in order to quell detonation.

Because your mechanical advance is "all-in" at cruise RPM, and the original distributor WAS NOT designed with this in mind, then when you add the vacuum advance to the mechanical at cruise, it is too much. The typical place where detonation shows up first is during "tip-in" during highway cruising, where the engine is developing significant peak cylinder pressure (assuming that your engine cruises within the 3000-3500 RPM range).

There are a couple things you can do to help, ASSUMING THAT YOU WANT/LIKE THE ADVANCE TO COME IN SO SOON, which is making more torque, also assuming that your WOT advance is no more than 36-38 degrees BTDC:

1. Lower the intake manifold temperature.
2. Open the idle mixture screws to the maximum before engine idle drops. This will not only enrich the idle/off idle circuits, but will enrich the cruise mixture as well.
3. Limit the total vacuum advance in increments until cruise detonation stops.
4. Enrich the cruise A/F ratio by increasing pri main jet size.
5. Fully block the exhaust crossover in the intake manifold.

If you'd rather go back to the factory specified distributor setup, then here it is:

midyearvette

the op needs to leave in the blue bushing and use the slower curve, all in at 3500-4000
max advance at 36-38 degrees
it will be a happy running small block

as far as the "anointed vacuum can".....i will leave that to the vacuum can gurus......

skids

[QUOTE=midyearvette;1582517417]the op needs to leave in the blue bushing and use the slower curve, all in at 3500-4000
max advance at 36-38 degrees
it will be a happy running small block

as far as the "anointed vacuum can".....i will leave that to the vacuum can gurus......[/QUOTE

Can I ask then, your pick on my chart, post #8,would be?

midyearvette

[QUOTE=skids;1582517648]either a or b..blue bushing
you could go with a different bushing to limit advance from 24 deg. as you state if you want to run more initial for better get away.
msd makes a super unit and the tuning sheet is spot on for what you can tailor for a curve...
the key here is not to go crazy with advance all in at 2500 as the preferred wisdom here on the forum states....i always have preferred a slower curve on a street mill with todays slower burning fuels....later all in say between 3500-4000 but thats jmo......

hope this answers your pm to me, if not ,pm me again....good luck

65tripleblack

What in the name of Zeus and all the gods are you talking about!!!!!!!!!


Are you people stupid or can't you read!
The specs that he needs are in the chart in post #11.

If he likes his distributor as-is, the workaround is also in post #11.

midyearvette

too much coffee today guy??
chill out, it seems everyone has a different take on timing although we all agree the op is using too much, i say he is using too much too soon!
yes un leaded fuel is slower burning than leaded or race fuels.....
i am not stupid and i can read...please stop banging your head against a brick wall, might make you stupid........

65tripleblack

There is only one "take" on timing, and that is to have as much advance, ASAP, as the engine will tolerate without detonation. How much? Do some research on when in the cycle max cylinder pressure must occur for maximum down force on the piston (creating maximum torque). Once you have assimilated this, then you're welcome to comment further, otherwise you're not worth wasting my breath on.

The only difference between leaded/unleaded, E10, mogas, avgas, 87 PON, 89 PON, 91 PON, 92 PON, 93 PON, 100 LL, leaded avgas, and CAM 2 race fuel is the resistance to detonation, ie: ignition temperature. There is no difference in the burn rate of gasoline.

Mike Ward

Sorry, but Joe is correct. There is no variation in the speed that fuel burns attributable to it's type. Old myth.

JohnZ

You have 40* total timing (16* + 24*) right now, and that's WAY excessive; back the initial down to 12*. Forget charts and colors - map the actual advance curve with a dial-back timing light so you know what the distributor is really providing. 12* initial and 24* centrifugal (all in by 2800-3000 rpm) is a good starting point, and tune from there.

MikeM

You have it exactly right in my opinion.

Unless you're running competitive class events, there's no sense in pushing the envelope on ignition timing and taking a chance on breaking something.


Since everyone has an opinion, my suggestion would be to simply pull at least four degrees out of the initial and see what happens.