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Super Stock / Pro Street and Modified Discussion 55-56-57 Super Stock / Pro Street and Modified Discussion 55-56-57 Talk engines, race tracks , setup's etc

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Old 11-04-2019, 01:21 PM   #1
TN57bbc
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Default big block timing

Just recently installed big block and trying to tune it. At 900 rpm with vacuum line plugged off to dist. timing set at 20 degrees initial...... all in at 3000 rpm 38 degrees total.
With vacuum line hooked back up to the dist. timing stays the same as plugged off at dist.
At idle 900 rpm with vacuum line in the full vacuum port on the carb and hooked up to the dist. timing is at 36 degrees initial and 58 total. Starts fine with the full vacuum hooked up.
Question....does it have too much timing at 9000 with full vacuum?

I don't seem to have detonation and it starts fine and shuts off fine. It accelerates better with the full vacuum. Just concerned about the 36 degree initial.
I might be a little off on the readings ....trying to hold the timing light and use a remote tach to get the readings.
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Old 11-04-2019, 01:43 PM   #2
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It's hard to comment on your question without knowing more about the engine.

What octane fuel, what's the compression ratio, do you have a performance camshaft, etc.? What kind of vacuum does it make?

16 of vacuum advance at idle is a lot for a performance engine. 10 would be more typical.

Also 20 of initial timing is a lot.

Thing is though, if it's a low compression engine with a stock camshaft, the engine makes a lot of vacuum, and most important if it doesn't ping - then you may be fine.
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Old 11-04-2019, 02:16 PM   #3
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You should be on ported vacuum.
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Old 11-04-2019, 02:47 PM   #4
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Quote:
Originally Posted by Rick_L View Post
It's hard to comment on your question without knowing more about the engine.

What octane fuel, what's the compression ratio, do you have a performance camshaft, etc.? What kind of vacuum does it make?

16 of vacuum advance at idle is a lot for a performance engine. 10 would be more typical.

Also 20 of initial timing is a lot.

Thing is though, if it's a low compression engine with a stock camshaft, the engine makes a lot of vacuum, and most important if it doesn't ping - then you may be fine.
Engine is 496.... 10.5-1. Cam is 618/618 235/241 @.050. Running on 93 octane.
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Old 11-11-2019, 08:22 PM   #5
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Quote:
Originally Posted by TN57bbc View Post
Just recently installed big block and trying to tune it. At 900 rpm with vacuum line plugged off to dist. timing set at 20 degrees initial...... all in at 3000 rpm 38 degrees total.
With vacuum line hooked back up to the dist. timing stays the same as plugged off at dist.
At idle 900 rpm with vacuum line in the full vacuum port on the carb and hooked up to the dist. timing is at 36 degrees initial and 58 total. Starts fine with the full vacuum hooked up.
Question....does it have too much timing at 9000 with full vacuum?

I don't seem to have detonation and it starts fine and shuts off fine. It accelerates better with the full vacuum. Just concerned about the 36 degree initial.
I might be a little off on the readings ....trying to hold the timing light and use a remote tach to get the readings.
20 degrees initial seems fine to me. I like to run as much initial as I can as long as it will start and shut off. Your total timing “no vacuum hooked up” is a bit high. Big blocks with your cubes normally like 32-34. Not sure what heads you have but your combustion chamber will make the difference there. The vacuum attached reading seems very high. What Dizzy are you using. Is your vacuum pot adjustable? And full manifold vacuum is where you should be hooking it up so don’t change it.
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Old 11-11-2019, 08:27 PM   #6
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What is the LSA of your cam?
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Old 11-12-2019, 02:57 AM   #7
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You should be on ported vacuum.

Read the ignition posts on ported vs full vacuum here on the Forum for the last several years - full vacuum per those who seem to know the topic. Ported vacuum is the result of the early smog efforts by Detroit. My 327 w/MSD RTR certainly runs better and crisper since hooking up full vacuum (in direct contradiction to MSD instructions). Some of the old-timers here can probably cite the relevant posts.
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Old 11-12-2019, 10:42 AM   #8
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Quote:
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You should be on ported vacuum.
Sure, if you are running an early 70's smog car. Otherwise use manifold vacuum like they did most of the time.
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Old 11-13-2019, 10:44 AM   #9
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Sure, if you are running an early 70's smog car. Otherwise use manifold vacuum like they did most of the time.
Cam LSA is 110. Dist is MSD small cap (don't know the part #). Does not have adjustable vacuum can on it.
Heads are aluminum 300 cc intake runners....115 combustion chamber.

I have a HEI....but not much room between dist and firewall.
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Old 11-25-2019, 01:35 PM   #10
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You need to limit your total mechanical timing to 32-34 degrees by using a bushing in the dizzy. I run 22 degrees initial but I have a much larger cam. I use a 10 degree bushing for 32 degrees total. Your timing with the vacuum connected seems really high. You can adjust that as well by limiting the travel of the vacuum slot. A little more involved but not difficult.
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