Setting timing with a Vacuum gauge..by Ear? Bad Idea. - Chevy Nova Forum
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Old 5th-September-2006, 09:32 PM   #1
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Setting timing with a Vacuum gauge..by Ear? Bad Idea.

Greetings,

I learned a new tuning technique over the weekend...Get a vacuum tester and put it inline with manifold vacuum. Advance the distributor until you get the maximum amount of vacuum reading on the gauge. Then turn the distributor back exactly 1 degree on the vacuum gauge. Tighten everything down and adjust your idle if necessary. For grins I did it on mine, and I must admit it does run a little bit better than doing the traditional advanced timing!

You learn something new every day,

Jason
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Old 5th-September-2006, 09:37 PM   #2
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Quote:
Originally Posted by Jason628 View Post
Then turn the distributor back exactly 1 degree on the vacuum gauge.
My vacuum gauge doesn't read in degrees.
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Old 5th-September-2006, 09:39 PM   #3
Jason628
 
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Greetings,

One mark in Hg...LMAO

Jason
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Old 5th-September-2006, 09:48 PM   #4
Paul Wright
 
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This one floats around the internet but it's not a good trick.
Use a vacuum gauge to adjust idle mixture and use a timing light to adjust timing.
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Building a small, high rpm engine
with the perfect bore, stroke and rod ratio is very impressive...
like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar.

Cubic inches is like Indiana Jones with a cheap pistol.
.
 
Old 5th-September-2006, 09:51 PM   #5
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Sure, but can you adjust your idle mixture with a timing light???
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Old 5th-September-2006, 09:53 PM   #6
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Quote:
Sure, but can you adjust your idle mixture with a timing light???

Now that was funny
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Old 6th-September-2006, 12:10 AM   #7
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This "method" just plain doesn't work, didn't work in the first years of the gasoline engine, and it doesn't work on today's engines, or anywhere in between.

Set paramters are the ONLY way to do timing correctly, and not with a vacuum guage.

Man, I am tired to the max hearing this one, over and over again.
 
Old 6th-September-2006, 12:16 AM   #8
Paul Wright
 
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Originally Posted by IgnitionMan View Post
Man, I am tired to the max hearing this one, over and over again.
So am I. Unfortunately the Internet easily perpetuates urban myths like these.
At least in this small corner of the virtual world we try to keep the information accurate.

Jason, where did you hear or read this? Another web site? If so, please let them know it's false.
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Quote:
Originally Posted by Paul Wright
Building a small, high rpm engine
with the perfect bore, stroke and rod ratio is very impressive...
like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar.

Cubic inches is like Indiana Jones with a cheap pistol.
.
 
Old 6th-September-2006, 08:44 AM   #9
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Greetings,

No, not another website...a 60 year old friend of the family told me this who's being doing Corvette restorations for about 30 years. So tell me this: What damage am I doing by having my engine set up this way?

Jason
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Old 6th-September-2006, 09:40 AM   #10
Paul Wright
 
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Jason, this is just another variation on the "turn the distributor until it pings and then back it off". Don't these people own a timing light?
The trick is:
First set your base timing with a timing light, then tune the idle mixture with a vacuum gauge to get the highest reading at the lowest steady idle.

Somehow your friend got the timing light part mixed up with the idle mixture. If you advance the base timing "by ear" or with a vacuum gauge you'll over advance the total timing. This is not good for an engine. The pressure before TDC is like hitting the rising pistons with a hammer.

I know a lot of old timers that do it by ear. I've posted this before but a friend of mine who did it that way wound up with trashed main bearings from the detonation.
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Quote:
Originally Posted by Paul Wright
Building a small, high rpm engine
with the perfect bore, stroke and rod ratio is very impressive...
like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar.

Cubic inches is like Indiana Jones with a cheap pistol.
.

Last edited by Paul Wright; 6th-September-2006 at 09:45 AM.
 
Old 6th-September-2006, 09:52 AM   #11
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Even if it doesn't detonate, most SBCs don't like 43 or 44 degrees of total timing, which is quite possible if you tuned it by the vacuum gauge. I'd stick a light on it and find out what it's actually doing. Tune for 38 degrees total at first and then get the idle to work from there.

Kev
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Old 6th-September-2006, 10:21 AM   #12
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Most engines idle smoother with advanced base timing, but at some point they make less power when the total advance is too much.
Most people assume that as long as they can't hear "pinging" then it's ok to keep adding advance but that's not always true. Over advanced timing can cause excessive combustion pressure before TDC (which the piston has to fight) which isn't always audible. A modern knock sensor can detect inaudible detonation that the human ear can't.


The piston is still rising when the spark plug fires and the air fuel mixture begins to burn. Combustion pressure is building at the same time it's being compressed by the piston. This BTDC pressure is negative crankshaft energy until the piston goes past TDC and it becomes positive.

Use the least amount of total advance that gets best power and no more.
A SBC engine that makes best power with 34 degrees of total advance is more efficient than one that requires 40.
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Quote:
Originally Posted by Paul Wright
Building a small, high rpm engine
with the perfect bore, stroke and rod ratio is very impressive...
like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar.

Cubic inches is like Indiana Jones with a cheap pistol.
.

Last edited by Paul Wright; 6th-September-2006 at 10:45 AM.
 
Old 6th-September-2006, 11:48 AM   #13
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Quote:
Originally Posted by Paul Wright View Post
Use the least amount of total advance that gets best power and no more.
A SBC engine that makes best power with 34 degrees of total advance is more efficient than one that requires 40.
So is there anything inherently wrong with running 40 degrees total timing? Seems like there are a lot of guys doing just that around here. I had my total dialed to 34 degrees and was about to try bumping it up some. Should I not be doing that? Or are we just discussion theory here?

I suppose ideally I should put the car on a dyno and get the facts on the changes rather than guessing or using the seat of the pants method. That of course is a hassle for some of us, I have an hour and half drive to the nearest shop with a dyno.

Since the posted tuning suggestion turned out to be bunk, do you guys have any other tips or tricks to share regarding getting timing and idle mixtures set as close to optimum without dyno time?
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Old 6th-September-2006, 12:08 PM   #14
Paul Wright
 
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Every engine is different so don't assume that what one guy runs is what you should run.
Most, but not allhot rod small block Chevy's run best with 36-38 degrees of total timing unless they have fast burn or Vortec style heads. They like around 32-34 degrees.

the only way I know of to determine what timing is best for a particular combination is to test.

In lieu of dyno testing there's always the drag strip dyno. Try averaging the MPH for 3 runs each for 2 degree timing increments 36, 38 & 40.
Then maybe try 37 and 39 on the way back down.

You could even graph the results in Excel to show how the MPH curve goes up and then back down. Your best total timing is at the peak of the curve.
You could even experiment with octane vs timing the same way.

Here's a sample to show what a timing test might look like. Advancing the timing helps up to a point.
See how over advancing the timing causes the MPH (power) to drop off then plummet?
Every engine is different, but if your buddies didn't test, then they may just be "guy thinking" that if 38 is good then 40 must be better. Think for yourself and test.
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Quote:
Originally Posted by Paul Wright
Building a small, high rpm engine
with the perfect bore, stroke and rod ratio is very impressive...
like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar.

Cubic inches is like Indiana Jones with a cheap pistol.
.

Last edited by Paul Wright; 6th-September-2006 at 12:46 PM.
 
Old 6th-September-2006, 12:16 PM   #15
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Quote:
Originally Posted by Paul Wright View Post
You could even experiment with octane vs timing the same way.
Also make sure to use the same fuel. Changing fuels/octane will have an effect on how much timing your motor wants. In other words, don't introduce two variables at once.

Dave:
I was building a graph when you posted that clarification but you are correct.
There will be different results when you change variables such as octane.
This is why I keep a log book when testing and tuning. Change one thing at a time. ~Paul
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Last edited by Paul Wright; 6th-September-2006 at 12:57 PM.
 
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